optimization of polystyrene properties using reversible addition...

Nathalie Ghyselinck

addition-fragmentation chain transfer polymerizationOptimization of polystyrene properties using reversible

Academiejaar 2012-2013Faculteit Ingenieurswetenschappen en ArchitectuurVoorzitter: prof. dr. ir. Guy MarinVakgroep Chemische Proceskunde en Technische Chemie

Master in de ingenieurswetenschappen: chemische technologieMasterproef ingediend tot het behalen van de academische graad van

Begeleiders: ir. Pieter Derboven, dr. ir. Dagmar D'hoogePromotoren: prof. dr. Marie-Françoise Reyniers, dr. ir. Dagmar D'hooge

Optimization of polystyrene properties using RAFT polymerization I

Preface Before introducing the aim and the actual results of this research project, I would like to

express my gratitude to some people in particular who supported me in completing this

Master thesis. First of all, a word of thanks goes to Professor Guy Marin for making this

research possible, and for providing the necessary accommodation. In addition, a special

thanks goes to my supervisor Professor Marie-Françoise Reyniers for giving me the

opportunity to perform research on this fascinating subject, and for providing me excellent

guidance and feedback through the completion of this Master Thesis.

Secondly and especially not in the least, I would like to thank my other supervisor and coach

Dr. Dagmar D’hooge for making me extremely enthusiastic to carry out research on this

challenging subject. He gave me the opportunity to gain new insights, to learn more about

extensive modeling, to have instructive discussions, and to improve my communication

skills. Without exaggerating, he supported me in a way that I will never forget, by

contributing to my current future vision.

Furthermore, a well-deserved word of thanks goes to my coach Pieter Derboven for

supporting me at any time throughout my research. Thanks for the useful discussions giving

me extra insights in the subject, to extend my experimental and modeling skills, and for the

extensive contribution to this Master thesis. Just thanks for being a fantastic coach.

In addition, thanks to Paul, Carolina and Gilles who were always prepared to help me.

Especially thanks to Carolina for the help during the experiments, to share her knowledge

regarding the several analysis techniques, and for making me laugh, even in the most

difficult periods.

An extra word of thanks goes to the technical staff of the Laboratory for Chemical

Technology to make my experimental study possible. Special thanks goes to Michaël Lottin

for assisting me with the GC instrument. Additionally, I would like to thank Jan Goeman of

the Department of Organic Chemistry to guide me in the search of a perfect GC column.

Finally, I would like to thank my mother, my family and my friends for their mental support

during the completion of this Master thesis.

FACULTEIT INGENIEURSWETENSCHAPPEN

EN ARCHITECTUUR

Laboratorium voor Chemische Technologie • Krijgslaan 281 S5, B-9000 Gent • www.lct.ugent.be

Secretariaat : T +32 (0)9 264 45 16 • F +32 (0)9 264 49 99 • [email protected]

Laboratorium voor Chemische Technologie

Verklaring in verband met de toegankelijkheid van de scriptie

Ondergetekende, Nathalie Ghyselinck

afgestudeerd aan de UGent in het academiejaar 2012-2013 en auteur van de

scriptie met als titel: Optimization of polystyrene properties using reversible

addition-fragmentation chain transfer polymerization

De auteur(s) geeft(geven) de toelating deze masterproef voor consultatie beschikbaar te

stellen en delen van de masterproef te kopiëren voor persoonlijk gebruik.

Elk ander gebruik valt onder de beperkingen van het auteursrecht, in het bijzonder met

betrekking tot de verplichting de bron uitdrukkelijk te vermelden bij het aanhalen van

resultaten uit deze masterproef.

(Datum)

(Handtekening)

Vakgroep Chemische Proceskunde en Technische Chemie

Laboratorium voor Chemische Technologie

Directeur: Prof. Dr. Ir. Guy B. Marin

D

Optimization of polystyrene properties using RAFT polymerization III

Optimization of polystyrene properties using reversible

addition-fragmentation chain transfer polymerization

Nathalie Ghyselinck

Supervisors: prof. dr. Marie-Françoise Reyniers, dr. ir. Dagmar D’hooge

Coaches: ir. Pieter Derboven, dr. ir. Dagmar D’hooge

Master Thesis submitted for the achievement of the academic degree of

Master in Engineering Sciences: Chemical Technology

Department of Chemical Engineering

Director: prof. dr. ir. Guy Marin

Faculty of Engineering and Architecture

Academic year 2012-2013

Abstract

A combined experimental and kinetic modeling study is presented for the bulk styrene

reversible addition-fragmentation chain transfer (RAFT) polymerization initiated by cyano

isopropyl dodecyl trithiocarbonate (CPDT) and azobis(isobutyronitrile) AIBN. Different

targeted chain lengths (TCLs) and initial CPDT to AIBN molar ratios are covered at

temperatures ranging from 343 K to 403 K. The measured conversion, average chain length

and dispersity data are being used to determine Arrhenius parameters for the RAFT specific

addition-fragmentation reactions. Both an increased polymerization temperature and initial

initiator concentration yield a less control over chain length and a lower degree of livingness

of the polymerization. In addition, a kinetic model was developed for the styrene RAFT

polymerization in miniemulsion, allowing in a later stage a comparison with the bulk case.

Keywords

reversible addition-fragmentation chain transfer polymerization, polystyrene, kinetic

modeling, miniemulsion

Optimization of Polystyrene Properties using Reversible Addition-Fragmentation Chain Transfer

Polymerization

Nathalie Ghyselinck1

Supervisors: Prof. dr. Marie-Françoise Reyniers1 and dr. ir. Dagmar R. D’hooge1 Coaches: dr. ir. Dagmar R. D’hooge1 and ir. Pieter Derboven1

Abstract: A combined experimental and kinetic modeling study is presented for the bulk styrene reversible addition-fragmentation chain transfer (RAFT) polymerization initiated by cyano isopropyl dodecyl trithiocarbonate (CPDT) and azobis(isobutyronitrile) (AIBN). Different targeted chain lengths (TCLs) and initial CPDT to AIBN molar ratios are covered at temperatures ranging from 343 K to 403 K. The measured conversion, average chain length and dispersity data are being used to determine Arrhenius parameters for the RAFT specific addition-fragmentation reactions. Both an increased polymerization temperature and initial initiator concentration yield a less control over chain length and a lower degree of livingness of the polymerization. In addition, a kinetic model was developed for the styrene RAFT polymerization in miniemulsion, allowing in a later stage a comparison with the bulk case.

Keywords: reversible addition-fragmentation chain transfer polymerization, polystyrene, kinetic modeling, miniemulsion

I. INTRODUCTION

In the last decades, free radical polymerization (FRP) has become an indispensable technology enabling the production of polymeric materials that have enriched the lives of millions of people on a daily basis. However, well-defined polymeric architectures, such as block copolymers, star polymers or polymer brushes, are not accessible by FRP, in which termination events are unavoidable and the incorporation of functional end-groups is very difficult.

Therefore, the incentive to synthesize complex macromolecular structures has led to a huge scientific interest in controlled radical polymerization (CRP) techniques, such as reversible addition-fragmentation chain transfer (RAFT) polymerization. These polymerization techniques are ideally suited to optimize polymer properties, in particular in light of high-tech applications, such as coatings, adhesives, drug delivery systems, etc. Among the CRP techniques, RAFT polymerization has been put forward as the most industrially promising one, due to its strong resemblance to FRP, and its high monomer flexibility.

Scheme 1: Basic mechanism of reversible addition-fragmentation chain transfer polymerization as proposed by the CSIRO group:[1] I, initiator radicals; M, monomer; CTA, chain transfer agent; Pn, dead polymer molecule; Rn, macroradical; Rn-X, dormant polymer molecule or macro-CTA.

1 Laboratory for Chemical Technology (LCT), Krijgslaan 281 (S5), B-9000

Ghent, Belgium; www.lct.ugent.be. E-mail: [email protected]

The RAFT polymerization technique relies on the degenerative transfer principle, and its basic reaction scheme is presented in Scheme 1.[1] Livingness and efficient control over the molar mass distribution (MMD) are obtained by reversible addition-fragmentation reactions involving a (macro) RAFT agent, superimposed on a conventional radical chain-growth polymerization. After initiation, the formed macroradicals can be captured by the precursor chain transfer agent (CTA) while releasing a R0

• radical that efficiently reinitiates the polymerization. Since the Rn-X dormant species act as a macro RAFT agent, i.e. releasing a macroradical upon addition of a radical onto the carbon-sulphur double bound, the control is maintained during the polymerization.

In this work, polystyrene properties are being optimized via RAFT polymerization initiated by azobis(isobutyronitrile) (AIBN) and with cyano isopropyl dithiobenzoate (CPDB) and cyano isopropyl dodecyl trithiocarbonate (CPDT) as CTAs in bulk on one hand, and in miniemulsion on the other hand.

A systematic combined experimental and kinetic modeling study was carried out in order to assess lacking temperature dependent intrinsic kinetic parameters for the RAFT polymerization specific reaction steps of the aforementioned bulk systems. It is shown that the level of livingness and control decreases with increase in polymerization temperature and initiator concentration. Additionally, a kinetic model was developed for the RAFT polymerization of styrene in miniemulsion, using the assessed intrinsic bulk kinetic parameters, allowing to compare the homogeneous RAFT polymerization of styrene and the corresponding heterogeneous miniemulsion system in a later stage.

II. KINETIC STUDY OF BULK STYRENE RAFT POLYMERIZATION

A. Identification of the optimal RAFT CTA

In RAFT polymerization, the choice of the RAFT CTA is crucial

to ensure a high control over polymer properties. In this work, based on literature reports,[2,3] first RAFT polymerization experiments were performed using CPDB as RAFT CTA and AIBN as conventional radical initiator. However, when comparing the polymerization rate to that of the corresponding FRP (i.e., same initial initiator concentration, no RAFT CTA) significant rate retardation is observed as shown in Figure 1a in which the red points correspond to the FRP reaction and the green ones to the RAFT polymerization with CPDB. Simulations revealed that this retardation can be attributed to slow fragmentation and significant cross-termination of the RAFT intermediate radicals. In addition, an moderate control over chain length is obtained (Figure 1b-c). Further research pointed out that CPDT is a better RAFT agent as negligible rate retardation (Figure 1a, blue dots) and a good control over polymer properties can be obtained throughout the polymerization (Figure 1b-c).

Figure 1: Comparison between RAFT polymerization with CPB (green dots), CPDT (blue dots) as RAFT CTA and the FRP using the same initial initiator (AIBN) concentration (red dots). (a) conversion vs time profiles. (b) number-averaged molar mass versus monomer conversion. (c) dispersity versus monomer conversion.

B. Kinetic modeling study of the RAFT polymerization of styrene initiated by AIBN/CPDT

In order to be able to determine approximate temperature dependent values for the kinetic parameters related to the RAFT specific addition-fragmentation reactions, an experimental study has been performed covering different TCLs (50-400) and initial CTA to initiator ratios for temperatures between 343 and 403 K. Diffusional limitations on termination were accounted for using literature data and the simulations were limited to a conversion of 0.8 to avoid significant interference of diffusional limitations on the addition/fragmentation process. Adjustment of the rate coefficients for addition and fragmentation to the experimental data at different temperatures yielded Arrhenius parameters for kadd and kfrag as shown in Figure 2a. Moreover, the corresponding thermodynamic parameters (Figure 2b) are in agreement with literature.[4]

Figure 2: (a) Arrhenius plots for the addition (kadd) and fragmentation (kfrag) rate coefficients. (b) Derivation of thermodynamic parameters from the temperature dependency of the equilibrium coefficient.

Using the parameters presented, a reasonable agreement between simulations and experimental data is mainly obtained at a TCL of 200 as is clear from Figure 3. This figure shows the conversion versus time profiles and the evolution of the polymer properties with monomer conversion for a TCL of 200 but different initial initiator concentrations. Clearly, a better control and livingness are reached when lower initial initiator concentrations are applied. However, it is found that lowering the initial CTA to initiator ratio below two is only beneficial for the end-group functionality (EGF) in spite of the increasing polymerization times. Similar results were obtained for the effect of polymerization temperature: a higher polymerization temperature induces a higher radical flux into the polymerization process and leads to less control and a significantly lower EGF. However, further model optimization (both intrinsic and diffusion parameters) is necessary to obtain a good description over the complete range of polymerization conditions.

III. RAFT POLYMERIZATION OF STYRENE IN MINIEMULSION

After the assessment of the RAFT specific rate coefficients in bulk, the kinetic model was extended to describe the miniemulsion styrene RAFT polymerization, in which the RAFT polymerization is assumed to occur in small ‘nanoreactors’ stabilized by surfactant in an aqueous phase under ideal conditions (cf. Figure 4). Four-dimensional Smith-Ewart equations were derived,[5] including the possible compartmentalization of initiator radicals, leaving radicals originating from the RAFT agent, macroradicals and RAFT macroradical intermediates involving two polymeric chains. This kinetic model was then implemented in computer code, starting from the LCT in-house code for NMP miniemulsion of styrene.[6]

Figure 3: Comparison between experimental data for the RAFT polymerization initiated by CPDT with a TCL of 200 for different initiator concentrations (green: CPDT:AIBN = 1:1, blue: CPDT:AIBN = 2:1 and red: CPDT:AIBN = 5:1). (a) conversion vs time profiles. (b) number-averaged molar mass versus monomer conversion. (c) polydispersity versus monomer conversion.

However, the obtained computer code is still in its testing phase in

which optimal integration parameters have to be selected. Furthermore, non-idealities such as exit and entry phenomena and possible diffusional limitations on reaction steps are not implemented yet. Therefore, simulations results corresponding to the styrene RAFT polymerization in miniemulsion are not included in this Master thesis.

Figure 4: Schematic representation of styrene RAFT polymerization in miniemulsion.

IV. CONCLUSIONS

Based on experimental data for the RAFT polymerization of styrene initiated with AIBN/CPDT covering different TCLs and initial CTA to initiator ratios at different polymerization temperatures, Arrhenius parameters were assessed for the RAFT specific addition and fragmentation reactions. The simulations capture in particular the experimental observation that both an increase of the initial conventional radical initiator concentration and the polymerization temperature lead to a lower control over chain length and livingness. Additionally, a kinetic model for the ideal miniemulsion styrene RAFT polymerization was derived including four- dimensional Smith-Ewart equations. However, the obtained computer code is still in its testing phase in which optimal integration parameters have to be selected.

NOTATION

RAFT: reversible addition-fragmentation chain transfer, FRP: free radical polymerization, CRP: controlled radical polymerization, CTA: chain transfer agent, MMD: molar mass distribution, TCL: targeted chain length, EGF: end-group functionality, CPDB: cyano isopropyl dithiobenzoate, CPDT: cyano isopropyl dodecyl trithiocarbonate, AIBN: azobis(isobutyronitrile)

REFERENCES

[1] J. Chiefari et al. Macromolecules 1998, 31, 5559-5562 [2] M. L. Coote, E. H. Krenske, E. I. Izgorodina, Macromolecular Rapid Communications 2006, 27, 473-497 [3] J. Chiefari, R. T. A. Mayadunne, C. L. Moad, G. Moad, E. Rizzardo, A. Postma, M. A. Skidmore, S. H. Thang, Macromolecules 2003, 36, 2273-2283 [4] M. L. Coote, Macromolecules 2004, 37, 5023-5031 [5] V. S. Wendell, H. E. Roswell, J. of Chemical Physics 1948, 16, 592-599 [6] L. Bentein, D. R. D'Hooge, M. F. Reyniers, G. B. Marin, Polymer 2012, 53, 681-693

Nederlandstalige samenvatting: ‘Optimalisatie van Polystyreeneigenschappen via Reversibele Additie-

Fragmentatie Ketentransferpolymerisatie’

Nathalie Ghyselinck1

Promotoren: Prof. dr. Marie-Françoise Reyniers1 and dr. ir. Dagmar R. D’hooge1 Begeleiders: dr. Ir. Dagmar R. D’hooge1 and ir. Pieter Derboven1

Abstract: Een gecombineerde experimentele en kinetische modelleringsstudie wordt toegelicht voor de reversibele additie-fragmentatie ketentransfer (RAFT)-polymerisatie van styreen in bulk, geïnitieerd door cyanoisopropyldodecyltrithiocarbonaat (CPDT) en azobis(isobutyronitrile) (AIBN). Verschillende beoogde ketenlengtes en initiële CPDT tot AIBN molverhoudingen werden bestudeerd bij temperaturen tussen 343 K en 403 K. De gemeten conversie, gemiddelde ketenlengte en dispersiteit werden telkens aangewend ter bepaling van de Arrhenius parameters voor de RAFT-specifieke additie-fragmentatiereacties. Zowel een toename van de polymerisatietemperatuur als de initiële initiatorconcentratie leiden tot een verminderde controle over de ketenlengte en een afname van het levend karakter van polymerisatie. In een tweede luik werd een kinetisch model ontwikkeld voor de RAFT-polymerisatie van styreen in mini-emulsie, hetgeen in een verder stadium een vergelijking met de bulk polymerisatie toelaat.

Kernwoorden: reversibele additie-fragmentatie ketentransfer-polymerisatie, polystyreen, kinetische modellering, mini-emulsie

I. INLEIDING

Gedurende de voorbije decennia heeft vrije radicalaire polymerisatie (FRP) zich ontpopt tot een onmisbare technologie voor de productie van allerhande polymeermaterialen die dagelijks deel uit maken van de levens van miljoenen mensen. Nochtans kunnen goedgedefinieerde polymeerarchitecturen zoals blokcopolymeren, sterpolymeren of polymeerborstels, niet vervaardigd worden via FRP, omwille van de onvermijdelijke terminatiereacties en de moeilijkheid om eindgroepfunctionaliteiten te introduceren in de precursorpolymeren. Het hoog geachte potentieel van de synthese van complexe macromoleculaire structuren heeft een enorme wetenschappelijke interesse opgewekt voor gecontroleerde radicalaire polymerisatietechnieken (CRP), waaronder RAFT-polymerisatie. Deze polymerisatietechnieken zijn uitermate geschikt om polymeereigenschappen te optimaliseren, vooral met het oog op hoogtechnologische applicaties zoals deklagen, adhesieven, geneesmiddelafgiftesystemen, etc. Van al deze CRP-technieken is RAFT-polymerisatie naar voorgeschoven als de industrieel meest belovende techniek wegens zijn sterke gelijkenis met FRP en zijn hoge flexibiliteit voor monomeertypes.

De RAFT-polymerisatietechniek is gebaseerd op het degeneratief transferprincipe en zijn basismechanisme wordt voorgesteld in Schema 1.[1] Levend karakter en efficiënte controle over de molaire massadistributie (MMD) worden verkregen door een reeks reversibele additie-fragmentatiereacties met een (macro-) RAFT- agens, gesuperponeerd op een conventionele radicalaire ketengroeipolymerisatie. Na de initiatie kunnen de gevormde macroradicalen ‘gevangen’ worden door een precursor ketentransferagens (CTA) waarbij er een R0

• radicaal wordt afgesplitst dat de polymerisatie doeltreffend reïnitieert. Aangezien

1 Laboratorium voor chemische proceskunde en technische chemie (LCT),

Krijgslaan 281 (S5), B-9000 Gent, België, www.lct.ugent.be. E-mail: [email protected]

ook de slapende ketens (Rn-X) zich gedragen als een macro-RAFT-agens, i.e. er vindt afsplitsing van een macroradicaal plaats bij additie van een radicaal op hun koolstof-zwavel dubbele binding, blijft de controle onderhouden gedurende de polymerisatiereactie.

Schema 1: Basismechanisme van reversibele additie-fragmentatie ketentransferpolymerisatie zoals voorgesteld door de CSIRO-groep:[1] I, initiatorradicalen; M, monomeer; CTA, ketentransferagens; Pn, dode polymeermolecule; Rn, macroradicaal; Rn-X, slapende polymeermolecule of macro-CTA.

In deze studie worden polystyreeneigenschappen geoptimaliseerd via RAFT polymerisatie geïnitieerd door AIBN met cyanoisopropyl-dithiobenzoaat (CPDB) en cyanoisopropyldodecyltrithiocarbonaat (CPDT) als ketentransferagentia zowel in bulk als in mini-emulsie. Een systematische gecombineerde experimentele en kinetische modelleringsstudie werd uitgevoerd om de tot dusver ontbrekende temperatuurafhankelijke kinetische parameters te bepalen voor de RAFT-polymerisatiespecifieke reactiestappen van de bovenvermelde bulksystemen. Er werd aangetoond dat het levende karakter en de controle van de ketenlengte afnemen met toenemende polymerisatietemperatuur en initiatorconcentratie. Bovendien werd een kinetisch model ontwikkeld voor de RAFT-polymerisatie van styreen in miniemulsie, waarbij de intrinsieke bulk kinetische parameters aangewend werden. Dit zal in een verder stadium toelaten de homogene RAFT-polymerisatie van styreen te vergelijken met de corresponderende heterogene RAFT-polymerisatie in mini-emulsie.

II. KINETISCHE STUDIE VAN DE RAFT-POLYMERISATIE VAN STYREEN IN BULK

A. Selectie van het optimale RAFT-CTA

In RAFT-polymerisatie is de keuze van het RAFT-CTA cruciaal

om een goede controle over de polymeereigenschappen te garanderen. Gebaseerd op rapporteringen in de literatuur[2,3] werden in deze studie eerst RAFT-polymerisatie-experimenten uitgevoerd met CPDB als RAFT-CTA en AIBN als conventionele radicalaire initiator. Wanneer de polymerisatiesnelheid echter vergeleken werd met die van de overeenkomstige FRP (i.e., zelfde initiële initiatorconcentratie, geen RAFT CTA), werd een significante vertraging vastgesteld zoals blijkt uit Figuur 1a. Hierin

corresponderen de rode punten met de FRP-reactie en de groene met de RAFT-polymerisatie met CPDB. Simulaties hebben aangetoond dat deze vertraging te wijten kan zijn aan trage fragmentatie en terminatie van de intermediaire RAFT-radicalen. Bovendien wordt een matige controle over de ketenlengte verkregen (Figuur 1b-c). Verder onderzoek heeft uitgewezen dat CPDT een beter RAFT-agens is aangezien de vertraging ten opzichte van FRP verwaarloosbaar klein is (Figuur 1a, blauwe punten) en bovendien een goede controle over polymeereigenschappen verkregen kan worden doorheen de polymerisatiereactie (Figuur 1b-c).

Figuur 2: Vergelijking tussen RAFT-polymerisatie met CPB (groene punten), CPDT (blauwe punten) als RAFT-agentia en de FRP-reactie met dezelfde initiële initiatorconcentratie (AIBN) (rode punten). (a) conversie versus tijd. (b) aantalgemiddelde molaire massa versus monomeerconversie. (c) dispersiteit versus monomeerconversie.

B. Kinetische modelleringsstudie van de RAFT-polymerisatie van styreen geïnitieerd door AIBN/CPDT

Om de bepaling van benaderde waarden voor de temperatuurafhankelijke kinetische parameters gerelateerd aan de RAFT-specifieke additie-fragmentatiereacties mogelijk te maken, werd een experimentele studie uitgevoerd waarbij zowel de beoogde ketenlengte (50-400) en de initiële CTA tot initiator molverhouding (1:1 – 5:1) werden gevarieerd bij verschillende temperaturen tussen 343 en 403 K. Diffusielimiteringen op terminatie werden in rekening gebracht via literatuurwaarden en bovendien werden de simulaties beperkt tot een conversie van 0.8 om aanzienlijke interferentie van diffusielimiteringen op additie- en fragmentatiereacties te vermijden. Aanpassing van de snelheidscoëfficiënten voor additie en fragmentatie aan de experimentele data op verschillende temperaturen resulteerde in Arrheniusparameters voor kadd en kfrag, zoals weergegeven in Figuur 2a. Bovendien zijn de corresponderende thermodynamische parameters (Figuur 2b) in overeenstemming met de literatuur. [4]

Figuur 2: (a) Arrhenius plots voor de additive- (kadd) en fragmentatie- (kfrag)snelheidscoëfficiënten. (b) Bepaling van thermodynamische parameters op basis van de temperatuurafhankelijkheid van de evenwichtscoëfficiënt.

Gebruikmakend van deze parameters wordt een redelijke overeenkomst verkregen tussen simulaties en experimentele data voor een beoogde ketenlengte van 200, zoals blijkt uit Figuur 3. Deze figuur toont de conversie versus tijd profielen en de evolutie van de polymeereigenschappen met monomeerconversie voor een beoogde ketenlengte van 200 maar verschillende initiële initiatorconcentraties. Het is duidelijk dat een betere controle en levend karakter verkregen kan worden wanneer lagere initiële initiatorconcentraties worden toegepast. Een afname van de initiële CTA tot initiatorverhouding tot lager dan twee is echter enkel voordelig voor de eindgroepfunctionaliteit (EGF), terwijl de polymerisatietijd sterk toeneemt. Gelijkaardige resultaten werden verkregen voor het effect van de polymerisatietemperatuur: een hogere polymerisatietemperatuur induceert een grotere radicaalflux in het polymerisatieproces en resulteert in minder controle over de ketenlengte en een aanzienlijk lagere EGF. Verdere model-optimalisatie (zowel intrinsiek als diffusieparameters) is echter noodzakelijk om een goede beschrijving over de hele set polymerisatiecondities te verkrijgen.

III. RAFT- POLYMERISATIE VAN STYREEN IN MINI-EMULSIE

Na de bepaling van de RAFT-specifieke kinetische parameters in bulk werd het kinetisch model uitgebreid om de RAFT-polymerisatie van styreen in mini-emulsie te kunnen beschrijven, waarin verondersteld wordt dat de RAFT-polymerisatie doorgaat in kleine ‘nanoreactoren’ gestabiliseerd door een surfactant in de waterige fase onder ideale condities (zie Figuur 4). Vierdimensionale Smith-Ewart-vergelijkingen werden afgeleid,[5] waarbij mogelijke compartimentalisering van de initiatorradicalen, de R0

• radicalen, de macroradicalen en de RAFT intermediaire radicalen tweearmige macroradicalen in rekening werden gebracht. Dit kinetisch model werd vervolgens geïmplementeerd in een computercode, waarbij gestart werd van de LCT-code voor de nitroxide-gemedieerde polymerisatie van styreen in mini- emulsie. [5]

Figuur 3: Vergelijking tussen de experimentele data voor de RAFT-polymerisatie met CPDT als RAFT-agens en een beoogde ketenlengte van 200 voor verschillende initiële initiatorconcentraties (groen: CPDT:AIBN = 1:1, blauw: CPDT:AIBN = 2:1 en rood: CPDT:AIBN = 5:1). (a) conversie versus tijd. (b) aantalgemiddelde molaire massa versus monomeerconversie. (c) dispersiteit versus monomeerconversie.

De verkregen computercode is echter nog steeds in haar testfase waarbij optimale integratieparameters geselecteerd moeten worden. Bovendien zijn niet-idealiteiten zoals ‘exit’- en ‘entry’-fenomenen en mogelijke diffusielimiteringen op de verschillende reactiestappen nog niet geïmplementeerd in de code. Simulatie-resultaten voor de RAFT-polymerisatie van styreen in mini-emulsie zijn daarom nog niet weergegeven in deze masterthesis.

Figuur 4: Schematische voorstelling van RAFT

polymerizatie in mini-emulsie.

IV. CONCLUSIES

Op basis van de experimentele data voor de RAFT-polymerisatie van styreen geïnitieerd door AIBN/CPDT voor verschillende beoogde ketenlengtes en initiële CTA tot initiator molverhoudingen bij verschillende polymerisatietemperaturen, werden Arrhenius-parameters bepaald voor de RAFT-specifieke additie- en fragmentatiereacties. De experimentele trend dat zowel een verhoging van de initiële conventionele radicalaire initiator-concentratie of de polymerisatietemperatuur leidt tot een verminderde controle over de ketenlengte en verlaging van de EGF, kan gereproduceerd worden via het kinetisch model. Bovendien werd een kinetisch model voor RAFT-polymerisatie van styreen in ideale mini-emulsie afgeleid via vierdimensionale Smith-Ewart-vergelijkingen. De verkregen computercode is echter nog steeds in haar testfase waarin optimale integratieparameters gezocht worden.

NOTATIE

RAFT: reversibele additie-fragmentatie ketentransfer, FRP: vrije radicalaire polymerisatie, CRP: gecontroleerde radicalaire polymerisatie, CTA: ketentransferagens, MMD: molaire massadistributie, EGF: eindgroepfunctionaliteit, CPDB: cyanoisopropyldithiobenzoaat, CPDT: cyanoisopropyldodecyltrithiocarbonaate

REFERENTIES

[1] J. Chiefari et al. Macromolecules, 1998, 31, 5559-5562 [2] M. L. Coote et al., Macromol. Rapid Commun. 2006, 27, 473-497 [3] J. Chiefari et al., Macromolecules 2003, 36, 2273-2283 [4] M. L. Coote, Macromolecules 2004, 37, 5023-5031 [5] L. Bentein et al., Polymer 2012, 53, 681-693

Optimization of polystyrene properties using RAFT polymerization VIII

Table of Contents

Chapter 1: Introduction .......................................................................................... 1

Chapter 2: Literature survey .............................................................................. 3

1. Controlled radical polymerization.................................................................................. 4

1.1. Fundamentals ......................................................................................................... 5

1.1.1. Criteria for a successful CRP ............................................................................. 5

1.1.2. Basic reaction schemes .................................................................................... 7

1.1.2.1. Deactivation/activation process................................................................. 7

1.1.2.2. Degenerative transfer process ................................................................... 8

1.2. Main techniques ..................................................................................................... 8

1.2.1. Nitroxide-mediated radical polymerization ..................................................... 9

1.2.2. Atom transfer radical polymerization .............................................................. 9

1.2.3. Single electron transfer – living radical polymerization ................................. 10

1.2.4. RAFT polymerization ...................................................................................... 11

1.2.5. Advantages and limitations ............................................................................ 11

2. The RAFT polymerization process ................................................................................ 13

2.1. Mechanism and kinetics ....................................................................................... 14

2.1.1. Reaction scheme ............................................................................................ 14

2.1.2. Average molar mass ....................................................................................... 18

2.1.3. Initiation systems ........................................................................................... 19

2.1.3.1. Conventional initiation systems ............................................................... 19

2.1.3.2. Thermal self-initiation of styrene ............................................................. 20

2.1.3.3. Microwave-enhanced RAFT polymerization ............................................ 22

2.1.4. Reactions leading to rate retardation and inhibition ..................................... 23

2.1.4.1. Slow fragmentation (non-stationary state model) .................................. 25

2.1.4.2. Irreversible termination of intermediate radicals .................................... 25

2.1.4.3. Controversy between the different theories ........................................... 26

2.1.4.4. Other side reactions ................................................................................. 27

Optimization of polystyrene properties using RAFT polymerization IX

2.2. RAFT mediating agents ........................................................................................ 28

2.2.1. Criteria for effective RAFT agents .................................................................. 30

2.2.2. Role of Z-group ............................................................................................... 30

2.2.3. Role of R0-group ............................................................................................. 31

2.2.4. Types of RAFT mediating agents .................................................................... 32

2.2.4.1. Dithioesters .............................................................................................. 32

2.2.4.2. Trithiocarbonates ..................................................................................... 33

2.2.4.3. Xanthates and dithiocarbamates ............................................................. 33

2.2.5. Choosing the right RAFT agent ....................................................................... 34

2.3. Monomers ............................................................................................................ 38

2.3.1. RAFT polymerization of styrene ..................................................................... 38

2.3.1.1. Styrene RAFT polymerization mediated by commercially available RAFT

agents………………………………………………………………………………………………………………….38

Cyanomethyl dodecyl trithiocarbonate (VII) ........................................................ 39

2-cyano-2-propyl dithiobenzoate (VIII) ................................................................ 39

4-cyano-4-(thiobenzoyl) sulfonyl pentanoic acid (IX) .......................................... 40

2.3.1.2. Intrinsic kinetic and thermodynamic parameters for the RAFT specific

reaction steps in polymerization of styrene ............................................................ 42

Determination of kinetic and thermodynamic parameters by kinetic modeling 42

Determination of kinetic and thermodynamic parameters by computational

chemistry .............................................................................................................. 43

2.4. Future outlook ...................................................................................................... 48

Chapter 3: Kinetic Study of Styrene RAFT Polymerization in

Bulk ...................................................................................................................................... 49

1. Experimental procedure .............................................................................................. 50

1.1. Materials .............................................................................................................. 50

1.2. Experimental conditions ...................................................................................... 51

1.3. Experimental setup .............................................................................................. 53

1.4. Temperature monitoring ..................................................................................... 56

1.5. Analysis ................................................................................................................. 57

Gravimetric analysis ............................................................................................. 57

Gas chromatography ............................................................................................ 58

Optimization of polystyrene properties using RAFT polymerization X

Size exclusion chromatography ........................................................................... 59

2. Kinetic model ............................................................................................................... 60

3. Results and discussion.................................................................................................. 63

3.1. Experimental study of the RAFT polymerization of styrene initiated by

AIBN/CPDB ....................................................................................................................... 63

3.2. Experimental study of the RAFT polymerization of styrene initiated by

AIBN/CPDT ....................................................................................................................... 66

3.3. Kinetic modeling study of the RAFT polymerization of styrene initiated by

AIBN/CPDT ....................................................................................................................... 69

3.3.1. Assessed Arrhenius parameters and related thermodynamic parameters ... 69

3.3.2. AIBN conversion profile .................................................................................. 72

3.3.3. Influence of temperature ............................................................................... 73

3.3.4. Influence of the initial CPDT to AIBN ratio ..................................................... 78

4. Conclusions .................................................................................................................. 80

Chapter 4: RAFT Polymerization of Styrene in Miniemulsion 82

1. (Mini)emulsion polymerization: principle .................................................................... 83

1.1. Ab initio emulsion polymerization ....................................................................... 83

Interval I: Nucleation stage .................................................................................. 84

Interval II .............................................................................................................. 84

Interval III ............................................................................................................. 84

1.2. Seeded emulsion .................................................................................................. 86

1.3. Miniemulsion ....................................................................................................... 87

2. Kinetic modeling of ideal miniemulsion RAFT polymerization .................................... 89

2.1. General assumptions............................................................................................ 89

2.2. Reaction scheme .................................................................................................. 90

2.3. Macrospecies types .............................................................................................. 91

2.4. Four-dimensional Smith-Ewart equations ........................................................... 91

2.5. Implementation ................................................................................................... 93

3. Conclusions and future recommendations .................................................................. 94

Chapter 5: General Conclusions and Future

Recommendations .................................................................................................... 95

1. General conclusions ..................................................................................................... 95

Optimization of polystyrene properties using RAFT polymerization XI

2. Future recommendations ............................................................................................ 96

Appendix ......................................................................................................................... 97

Appendix A: General Kinetics of RAFT Polymerization ............................................................ 97

1. Transfer coefficients..................................................................................................... 97

2. Determination of kinetic parameters .......................................................................... 99

2.1. Experimental measurement of transfer coefficients ........................................... 99

2.2. Kinetic modeling to estimate rate coefficients .................................................. 101

2.3. High-level quantum chemical calculations to predict rate coefficients ............ 101

Appendix B: RAFT Polymerization of Methacrylates ............................................................. 103

1. Methacrylate RAFT polymerization mediated by commercially available RAFT

agents……. ........................................................................................................................ 104

2-cyano-2-propyl dithiobenzoate (VIII) .............................................................. 104

4-cyano-4-(thiobenzoyl) sulfonyl pentanoic acid (IX) ........................................ 107

2-cyano-2-propyl dodecyl trithiocarbonate (X) ................................................. 107

4-cyano-4-[(dodecylsulfanylthiocarbonyl) sulfanyl] pentanoic acid (XI) ........... 108

2. Intrinsic kinetic and thermodynamic parameters for the RAFT specific reaction steps

in polymerization of methacrylates ................................................................................... 112

Appendix C: Experimental Conditions.................................................................................... 113

Appendix D: Kinetic Model of Styrene RAFT Polymerization in Bulk ..................................... 115

Appendix E: Parameters of the Composite Model ................................................................ 123

Appendix F: RAFT Polymerization with CPDB: Kinetic Modeling ........................................... 125

Appendix G: Smith-Ewart Equations for RAFT Polymerization of Styrene in Miniemulsion . 128

References .............................................................................................................................. 140

Optimization of polystyrene properties using RAFT polymerization XII

List of Abbreviations and

Symbols

List of Abbreviations

ATRP Atom transfer radical polymerization

ca 2+2-cycloaddition reaction

CLD Chain length dependent

CRP Controlled radical polymerization

CSIRO Commonwealth Scientific and Industrial Research Organization

CTA Chain transfer agent

dim Diels-Alder dimerization

er Ene reaction

ESR Electron spin resonance

FID Flame ionization detector

FRP Free radical polymerization

GC Gas chromatography

HMM High molar mass

HO Harmonic oscillator

IUPAC International Union of Pure and Applied Chemistry

LMM Low molar mass

LRP Living radical polymerization

MADIX Macromolecular design via the interchange of xanthates

MAOS Microwave-assisted organic synthesis

MMD Molar mass distribution

NMP Nitroxide-mediated radical polymerization

RAFT Reversible addition-fragmentation chain transfer

RAFT-CLD-T RAFT chain length dependent termination

rd Retro Diels-Alder dimerization

Optimization of polystyrene properties using RAFT polymerization XIII

SEC Size exclusion chromatography

SET-LRP Single electron transfer - living radical polymerization

th Monomer-assisted homolysis

List of Symbols

̅ [g mol-1] Number-averaged molar mass

̅ [-] Number-averaged chain length

̅ [-] Average number of initiator radicals

̅ [-] Average number of R0• radicals

̅ [-] Average number of macroradicals

̅ [-] Average number of RnTRm intermediate radicals

[-] Number of particles having i initiator radicals, j R0

• radicals, r macroradicals and s RnTRm intermediate radicals

[L

mol-1 s-1] Intrinsic addition rate coefficient

[s

-1] Intrinsic fragmentation rate coefficient

[L mol-1 s-1] Termination rate coefficient between two 1-mers

[L

mol-1 s-1] Chain length dependent termination rate coefficient between two i-mers

[mol Lp

-1] Zero order moment corresponding to the distribution of macroradicals

[mol Lp

-1] Zero order moment corresponding to the distribution of RAFT intermediate radicals containing two polymer chains

[mol Lp

-1] Zero order moment corresponding to the distribution of RAFT intermediate radicals containing a R0-group and one polymer chain

[mol Lp

-1] Zero order moment corresponding to the distribution of RAFT intermediate radicals containing a I-group and one polymer chain

Optimization of polystyrene properties using RAFT polymerization XIV

[mol Lp

-1] Zero order moment corresponding to the distribution of dead polymer chains

[mol Lp

-1] Zero order moment corresponding to the distribution of cross-termination products involving three polymer chains

[mol Lp

-1] Zero order moment corresponding to the distribution of cross-termination products involving one polymer chain

[mol Lp

-1] Zero order moment corresponding to the distribution of cross-termination products involving two polymer chain

[AIBN]0 [mol L-1] Initial concentration of AIBN

[CTA] [mol L-1] Actual concentration of CTA

[CTA]0 [mol L-1] Initial concentration of CTA

[I]0 [mol L-1] Initial concentration of a ‘CRP initiator’

[I2] [mol L-1] Actual concentration of conventional radical initiator

[I2]0 [mol L-1] Initial concentration of conventional radical initiator

[M] [mol L-1] Actual monomer concentration

[M]0 [mol L-1] Initial monomer concentration

[Rn•] [mol L-1] Actual concentration of macromolecules with chain length n

[Rn-X] [mol L-1] Actual concentration of dormant chains

[Rn-X]0 [mol L-1] Initial concentration of dormant chains

[kJ mol-1] Gibbs free reaction energy

[kJ mol-1] Reaction enthalpy

[J mol-1 K-1] Reaction entropy

Al [L mol-1 s-1]

or [s-1] Pre-exponential factor for elementary reaction step l

CM,0 [mol L-1] Initial concentration of monomer

CRAFT,0 [mol L-1] Initial concentration of RAFT agent

Ctr [-] Transfer coefficient (Notice that an additional subscript indicates the considered reaction)

d [-] Number of chains produced in a radical-radical termination event

DPn [-] Degree of polymerization or number-averaged chain length ̅

Ea,l [kJ mol-1] Activation energy for elementary reaction step l

EGF [-] End-group functionality

Optimization of polystyrene properties using RAFT polymerization XV

f [-] Initiator efficiency

i [-] Chain length i

igel [-] Chain length at the onset of the gel effect

iSL [-] Cross-over chain length between short and long chain behavior

kact [s-1] Rate coefficient of activation

kadd [L mol-1 s-1] Rate coefficient of addition reaction step (Notice that an

additional subscript indicates the actual elementary reaction)

Kchem [L mol-1] Equilibrium coefficient of the reversible chain transfer reaction steps

kcomp [L mol-1 s-1] Rate coefficient of comproportionation in SET-LRP

kd [s-1] Rate coefficient of dissociation of conventional radical initiator

kdeact [L mol-1 s-1] Rate coefficient of deactivation

kdisp [s-1] Rate coefficient of disproportionation in SET-LRP

kfrag [s-1] Rate coefficient of fragmentation reaction step (Notice that an

additional subscript indicates the actual elementary reaction)

ki [L mol-1 s-1] Rate coefficient of actual chain-initiation step

kp [L mol-1 s-1] Rate coefficient of propagation

kre-in [L mol-1 s-1] Rate coefficient of re-initation

kt [L mol-1 s-1] Rate coefficient of termination

ktherm [L mol-1 s-1] General rate coefficient of thermal initiation of styrene

MMCTA [g mol-1] Molar mass of CTA

MMM [g mol-1] Molar mass of monomer

Np [-] Total amount of particles

PDI [-] Polydispersity index

R [J mol-1 K-1] Universal gas constant

t [-] Ratio of the number of chains produced in a radical-radical termination event to the number of chains involved in the termination reaction

T [K] Polymerization temperature

vp [Lparticle] Volumetric size of a particle

VSTY,0 [L] Total initial volumetric amount of styrene

Optimization of polystyrene properties using RAFT polymerization XVI

wp [-] Polymer weight fraction

x [-] Conversion

αgel [-]

αL [-] Power law exponent in composite model

αS [-] Power law exponent in composite model

Δ[M] [mol L-1] Concentration of consumed monomer

τ0 [mol Lp-1] Zero order moment corresponding to the distribution of dormant

polymer chains [-] Partition coefficient

List of species

Dimer radicals originating from thermal self-initiation of styrene

Styryl radicals originating from thermal self-initiation of styrene

ACHN Azobis(cyclohexanenitrile)

AIBN Azobis(isobutyronitrile)

AMIB Azobis(methylisobutyrate)

AX(A)(A) Cross-termination product

CB 1,2-diphenylcyclobutane

CDB Cumyl dithiobenzoate

CPDB 2-cyano-2-propyl dithiobenzoate

CPDN 2-cyano-2-propyl dithionaphthalate

CPDT 2-cyano-2-propyl dodecyl trithiocarbonate

D Dimer (1-phenyl-1,2,3,9-tetrahydronaphthalene)

DCM dichloromethane

DEC n-decane

DMSO Dimethyl sulfoxide

IiIj Conventional radical initiator

Ij• Initiating radicals originating from dissociation of a conventional radical

initiator, or from thermal self-initiation of styrene

Optimization of polystyrene properties using RAFT polymerization XVII

L Ligand of a transition metal complex

M Monomer

MA Methyl acrylate

MMA Methyl methacrylate

Mtm Transition metal with oxidation state m

Mtm/L Transition metal complex in oxidation state m

Pn Dead polymer of chain length n

Rn• Macroradical of chain length n

RnTRm RAFT intermediate radical with two polymer chains of chain length n and m

Rn-X Dormant macromolecular species of chain length n

SG1 N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethyl-propyl)]

STY Styrene

THF tetrahydrofuran

VAc Vinyl acetate

X-Mtm+1/L Transition metal complex in oxidation state m+1 coordinated with halide ligands

XRn Dormant macroradical with chain length n including the thiocarbonylthio group

Optimization of polystyrene properties using RAFT polymerization 1

Chapter 1

Chapter1

Introduction

In the last decades, free radical polymerization (FRP) has become an indissoluble part of our

society enabling the production of polymeric materials that have enriched the lives of

millions of people on a daily basis. However, well-defined and more advanced polymeric

architectures (e.g. block or star copolymers) are not accessible by FRP, in which termination

events are unavoidable and the incorporation of functional end-groups is very difficult.

Therefore, the incentive to synthesize complex macromolecular structures has led to a huge

scientific interest in controlled radical polymerization (CRP) techniques, such as reversible

addition-fragmentation chain transfer (RAFT) polymerization. These polymerization

techniques are ideally suited to optimize polymer properties, in particular in light of

high-tech applications, such as coatings, adhesives, drug delivery systems, etc. Among the

CRP techniques, RAFT polymerization has been put forward as the most industrially

promising one, due to its strong resemblance to FRP, and its high monomer flexibility.

The main principle of RAFT polymerization is presented in Scheme 1. Growing macroradicals

denoted by Rn• are temporarily captured by a RAFT chain transfer agent (CTA), while at the

same time releasing new macroradicals Rm• which can undergo further polymerization.

Scheme 1: Main principle of reversible addition-fragmentation chain transfer polymerization.

In reality, several side reactions can influence the RAFT chain-growth process leading to the

synthesis of less well-defined polymer molecules. In order to fully exploit the industrial

potential of the RAFT polymerization technique, a thorough understanding of the RAFT

polymerization kinetics and the influence of diffusional limitations at the laboratory scale is

of crucial importance. Such knowledge will allow to determine the optimal polymerization

conditions as a function of the desired polymer product properties, and to achieve a

successful scale-up to industrial scale. In particular, it is not clear if homogeneous

(bulk/solution) polymerization or heterogeneous polymerization is favored.


Introduction

Therefore, the aim of this Master thesis is the optimization of RAFT polymerization

considering both homogeneous and heterogeneous polymerization. As model monomer

styrene is used. The RAFT polymerization is initiated by azobis(isobutyronitrile) (AIBN) as a

conventional radical initiator with 2-cyano-2-propyl dithiobenzoate (CPDB) and

2-cyano-2-propyl dodecyl trithiocarbonate (CPDT) as CTAs. First, an extensive screening of

literature reported experimental and kinetic modeling studies on RAFT polymerization in

homogeneous and heterogeneous media using commercially available RAFT agents was

carried out. The most relevant results of this literature survey are written down in Chapter 2.

Reported temperature dependent kinetic parameters for the specific RAFT polymerization

addition-fragmentation reaction steps are scarce. Therefore, a systematic combined

experimental and kinetic modeling study was carried out in order to assess these intrinsic

kinetic and thermodynamic parameters for the bulk RAFT polymerization of styrene initiated

with AIBN using CPDB and CPDT as RAFT agents, as described in Chapter 3.

Furthermore, in Chapter 4 a kinetic model was developed for the RAFT polymerization of

styrene in miniemulsion using the intrinsic kinetic parameters of the RAFT polymerization

specific reaction steps as determined in Chapter 3, allowing in later stage to compare the

homogeneous RAFT polymerization of styrene and the corresponding heterogeneous

miniemulsion system. This heterogeneous system ideally takes advantage of

compartmentalization, i.e. the RAFT polymerization can in principle be carried out with

smaller termination rates, leading to higher end-group functionality and thus, allowing a

controlled synthesis of complex polymer architectures.

Finally, the most important conclusions are summarized in Chapter 5, including some

recommendations for future research on RAFT polymerization of styrene in homogeneous

and heterogeneous media.


Chapter 2

Chapter2

Literaturesurvey

Polymeric materials obtained by free radical polymerization (FRP) have become an inherent

part of the society and have taken an indissoluble position in everyday live. Millions of tons

of vinyl (co)polymers are yearly being processed via FRP into commodity and high-tech

products. This broad industrial exploitation of FRP relies on some key features, such as its

easy implementation, the low production of off-spec polymers and the possibility to carry

out the FRP in benign solvents such as water. However, well-defined polymeric

architectures, such as block copolymers, star polymers or polymer brushes, are not

accessible by FRP, in which termination events are unavoidable and the incorporation of

functional end-groups is very difficult, i.e. FRP produces solely ‘dead’ polymer molecules

with significantly different chain lengths.

The addition of a mediating agent, which allows reversible deactivation of the active radical

species, has enabled the polymer chemist to synthesize well-defined block and star

copolymers at laboratory scale, i.e. living radical polymerization (LRP) characteristics can be

reached under less stringent conditions. Depending on the type of mediating agent, one can

distinguish between different polymerization techniques, all of which are typically covered

by the term ‘controlled radical polymerization (CRP)’. The associated market value of these

CRP products is anticipated to be over 20 billion dollar per year,[1]

invoking a huge scientific

interest as evidenced by the great number of papers published. For CRP polymers,

applications are envisaged for the production of automotive paints and coatings, adhesives,

sealants, cosmetics, health products and electronics.[2]

Unfortunately, only very few

industrial applications are available so far, as multiple hurdles still have to be overcome. The

lack of a universal mediating agent and the required extra purification or end-group removal

steps afterwards are the main constraints for the industrial development of CRP.

Nevertheless, the few low polydisperse (co)polymers that made it to the market, have led to

high-solid coatings,[3]

pigment stabilizers,[4]

and dispersants [5]

with niche applications.


Literature survey

In this chapter, first the most important CRP techniques are briefly discussed. Afterwards,

attention is focused on the technique studied in this Master thesis, i.e. reversible addition-

fragmentation chain transfer (RAFT) polymerization.[6]

Various aspects of the RAFT

polymerization process are highlighted, such as a general reaction scheme and the selection

of an appropriate RAFT mediating agent. An overview of the most important successful

experimental conditions and kinetic parameters reported in literature is also included for the

RAFT polymerization in homogeneous (bulk, solution) media of two important monomer

types, i.e. styrenes and methacrylates.

1. Controlled radical polymerization Today’s most frequently applied CRP techniques are nitroxide-mediated radical

polymerization (NMP), atom transfer radical polymerization (ATRP), single electron transfer -

living radical polymerization (SET-LRP), and RAFT polymerization. Some examples of possible

complex macromolecular structures, which can be prepared with these polymerization

techniques are shown in Figure 1.

It is important to stress that there exists some controversy on the use of the terms living and

controlled in describing processes for radical polymerizations, such as NMP, ATRP, SET-LRP

or RAFT polymerization. According to the International Union of Pure and Applied Chemistry

(IUPAC),[7]

a living polymerization is defined as a chain polymerization in which chain transfer

and chain termination are absent. With this definition, strictly speaking NMP, ATRP, SET-LRP

and RAFT polymerization cannot be referred to as living polymerizations since termination

reactions are only suppressed, but not completely avoided. Furthermore, the use of the

term controlled is also in contrast with IUPAC recommendations. This term can in principle

not be used to indicate a specific polymerization process, because the word has an

established, much broader usage. Recently, an IUPAC task group recommended the term

reversible deactivation radical polymerization to describe these polymerizations.[8]

However,

in this Master Thesis, CRP will be used to designate radical polymerizations such as NMP,

ATRP, SET-LRP, NMP and RAFT polymerization, while LRP can be seen as an idealized form of

these polymerization techniques.

In this section, first the fundamentals of CRP and the requirements for CRP to approach the

characteristic properties of LRP are discussed. At the end of this section, a brief description

of the four most frequently applied CRP processes is provided.


Literature survey

Figure 1: Examples of controlled functionality, composition and architecture via controlled radical (co)polymerization.

1.1. Fundamentals

In order to obtain insights in today’s known CRP techniques, and in particular the RAFT

polymerization technique, it is necessary to gain confidence with the main principles of CRP

processes. In this subsection, a discussion is provided on the most important concepts of

CRP. In addition, a clear distinction between LRP and CRP is given.

1.1.1. Criteria for a successful CRP

The precise macromolecular synthesis via CRP is based on the concepts of LRP. The

livingness of the LRP process was initially described by Szwarc [9]

as a chain-growth process

without transfer and termination reactions. However, to synthesize by CRP well-defined

polymers characterized by a narrow molar mass distribution (MMD), and with the provision

of end-group control, some extra requirements are necessary. First of all, there should be an

efficient reversible deactivation of the active radicals using a mediating agent in a way that

this exchange is fast in comparison with propagation and termination. A growing species

should ideally react only with a few monomer units, within a few milliseconds, before it is

deactivated to a dormant state, where it remains for several seconds to minutes.[10]

Secondly, the ‘CRP initiator’ should be consumed at the earliest stages of the polymerization

to ensure a uniform growth of all polymer chains. When these additional criteria are

full-filled, the polymerization can be called a controlled radical polymerization.[11]

However,

in the case of RAFT polymerization it is not important to have an immediate dissociation of

the conventional radical initiator. On the contrary, it is preferable to have a relatively slow

dissociation to avoid rapid creation of propagating radicals and hence, keep the probability

for termination reactions to occur low.


Literature survey

In the most idealized case of CRP, polymers are obtained with degrees of polymerization

(DPn, or number-averaged chain length��) predetermined by the ratio of the concentration of the consumed monomer (Δ[M] = [M]0 – [M], with [M]0 the initial monomer concentration

and [M] the actual monomer concentration) to the initial ‘CRP initiator’ concentration [I]0

(Figure 2b), and with end-functional groups for every polymer chain. Furthermore, since an

ideal case is considered, i.e. termination reactions are absent, ln([M]0/[M]) should increase

linearly as a function of time, as can be seen in Figure 2a.

In Figure 2a-b, non-ideal cases are additionally presented. A rate acceleration may indicate

slow CRP initiation, while a deceleration is typical for the presence of termination. In

addition, higher chain lengths are obtained when the initiation is rather slow, whereas lower

chain lengths are an indication for chain transfer reactions.

Under controlled conditions, the polydispersity index (PDI) should decrease with conversion

for systems with slow initiation and slow exchange, while on the other hand an increase of

PDI with conversion is typical in the presence of chain transfer.

In the case of slow initiation and exchange, end-group functionalities may remain

unaffected, but when chain transfer and termination reactions are present the amount of

end-group functionalities can decrease significantly.

Figure 2: Effect of slow initiation, termination, and transfer on; a) kinetics, b) number-averaged chain length (ideal cases:

full lines, in particular no termination).[11]

Note that to minimize termination, the radical concentration should be kept sufficiently low.

This is possible since propagation is a first order reaction in the propagating radicals, while

termination is a second order reaction. Consequently, the relative contribution of

termination reactions will diminish to a greater extent if the concentration of propagating

radicals is reduced. In CRP, the latter is minimized by means of a dynamic equilibrium

between active and dormant species favoring the dormant form.


Literature survey

1.1.2. Basic reaction schemes

As already mentioned, the requirement to establish a dynamic equilibrium between the

propagating radicals and the dormant species is essential in CRP. This can be realized either

by a deactivation/activation process, as can be seen in Scheme 2 (NMP/ATRP/SET-LRP), or it

can take place as a reversible transfer, i.e. a degenerative exchange process, as in Scheme 3

(RAFT polymerization). According to those mechanisms, the majority of polymer chains are

in their dormant form. Rapid equilibration between the active and dormant species ensures

that all polymer chains have an equal chance for growth and thus, will grow intermittently. It

is under these conditions that the number-averaged molar mass increases linearly with

conversion, and that a narrow MMD can be obtained.[12]

Scheme 2: Controlled radical polymerization: deactivation/activation process (NMP case).

[10]

Scheme 3: Controlled radical polymerization: degenerative exchange process (RAFT polymerization).

[10]

1.1.2.1. Deactivation/activation process

The success of the deactivation/activation process in Scheme 2 (NMP case) is based on the

occurrence of the persistent radical effect, a typical kinetic feature in certain CRP systems

such as NMP and ATRP. In the deactivation process (with a deactivation rate coefficient

kdeact), propagating radicals Rn• rapidly add to certain types of species X, typically persistent

radicals, e.g. nitroxides in NMP. Next, the dormant species can again be activated (with an

activation rate coefficient kact) either spontaneously or thermally, in the presence of light, or

with a catalyst as in ATRP. The radicals released in the activation reaction can propagate

(with a propagation rate coefficient kp), but also terminate (with a termination rate

coefficient kt). In addition, the persistent X species cannot terminate with each other, as they

can only reversibly interact with the growing species. Hence, every termination is coupled

with an irreversible accumulation of the persistent radicals favoring the deactivation

reaction. The concentration of growing radicals and thus, the probability of termination,

decreases therefore with time.[10]


Literature survey

1.1.2.2. Degenerative transfer process

Systems that are making use of a degenerative transfer, like in Scheme 3, do not rely on the

persistent radical effect. These systems, e.g. RAFT polymerization, follow the same typical

kinetics as for FRP, but are making use of chain transfer agents, of which the concentration is

much higher than that of the conventional radical initiators (typically ten times). In this case

the transfer agent is playing the role of the dormant species.[10]

1.2. Main techniques

The most important CRP techniques that are receiving much of attention today are NMP,

ATRP, SET-LRP and RAFT polymerization. In this subsection, these techniques are briefly

discussed and compared.

The NMP technique was invented by Solomon et al. [13]

in 1986 and has been exploited

extensively for the synthesis of low polydispersity homopolymers and block copolymers of

styrene and (meth)acrylates. However, the NMP technique is still restricted to a rather small

monomer domain. The ATRP technique can be seen as a more versatile method, but it

requires unconventional initiating systems that have often poor compatibility with

polymerization media, and catalyst removal.[14-16]

In addition, it is impossible to polymerize

non-activated monomers that generate stable alkyl halide dormant species, such as vinyl

acetate and vinyl chloride. However, the recently introduced SET-LRP technique provides an

opportunity to polymerize activated and non-activated monomers containing an

electron-withdrawing functional group. SET-LRP is considered to proceed via an

outer-sphere electron transfer mechanism, while ATRP is made possible by an inner-sphere

electron transfer process.[17-19]

The RAFT polymerization process is one of the most recent

CRP techniques and was invented in 1998 by the Commonwealth Scientific and Industrial

Research Organization (CSIRO) in Melbourne, Australia, by a team of researchers.[20, 21]

This

technique is probably the most versatile CRP process.[12]

It should be mentioned that these CRP techniques are not as ideal as LRP processes, e.g.

anionic polymerization reactions, for which a very high end-group fidelity can be obtained

due to the absence of termination reactions. In CRP less stringent reaction conditions have

to be applied, however there is a continuous increase of dead polymer molecules with

increasing monomer conversion, and hence a loss of functional end-groups.[22]


Literature survey

1.2.1. Nitroxide-mediated radical polymerization

The NMP technique is based on the above described deactivation/activation process

mediated by nitroxides. A simplified scheme of the NMP process is presented in Scheme 4.

The nitroxide mediating agent is a persistent radical that can react with NMP initiator and

propagating radicals with the formation of an alkyl hydroxylamine, i.e. dormant species.

When the polymerization is carried out at a sufficiently higher temperature, this

hydroxylamine derivate can split again homolytically in the nitroxide radical and an active

species. This active macromolecular radical can then be involved in a number of propagation

reactions until it is again captured by the nitroxide mediating agent.[15]

It should be

mentioned that there is still a certain probability for termination of the propagating radicals.

An example of a common-used nitroxide is N-tert-butyl-N-[1-diethylphosphono-

(2,2-dimethyl-propyl)] nitroxide (SG1).

Scheme 4: Simplified nitroxide-mediated radical polymerization mechanism.

1.2.2. Atom transfer radical polymerization

Similar to the NMP technique, ATRP is also based on the deactivation/activation mechanism

including a persistent radical effect. It is a catalytic process and can be mediated by many

redox-active transition metal complexes, e.g. a redox system based on copper. The ATRP

process, as shown in Scheme 5, is controlled by a pseudo-equilibrium between propagating

radicals and dormant species, initially in the form of initiating alkyl halides and at higher

conversions as dormant macromolecular species (Rn-X). The dormant species periodically

react (with an activation rate coefficient kact) with transition metal complexes in their lower

oxidation state (Mtm

/L, where Mtm

is an indication for the transition metal and L is a ligand)

which can act as activators to intermittently form growing radicals Rn•. Consequently, in

competition with propagation (and to a lesser extent termination), deactivation is possible

by interaction with the transition metal complexes in their higher oxidation state,

coordinated with halide ligands (X-Mtm+1

/L). These species can react with the propagating

radicals in a reverse reaction with a deactivation rate coefficient kdeact to again form a

dormant species and the activator.[23]


Literature survey

Scheme 5: Simplified atom transfer radical polymerization mechanism.

1.2.3. Single electron transfer – living radical polymerization

Scheme 6 shows that in the presence of an electron-donor catalyst such as Cu(0)

(including an

excess of ligand) in an appropriate solvent (such as dimethyl sulfoxide; DMSO), an

ATRP-related process can be obtained, i.e. SET-LRP. Full lines in Scheme 6 indicate the

dominant reaction paths. In the SET-LRP process, the activation/deactivation occurs with

Cu(0)

as major species, since the ‘activator’ of ATRP (Cu(I)

X) is in principle immediately

converted into Cu(0)

and deactivator Cu(II)

X2 via disproportionation (with a disproportionation

rate coefficient kdisp). However, in case that comproportionation (kcomp) is dominant, again an

ATRP technique is obtained.[17-19]

One of the advantages of the SET-LRP technique is the

possibility to utilize a copper wire which can again be removed afterwards.

Scheme 6: Simplified single electron transfer - living radical polymerization mechanism.


Literature survey

1.2.4. RAFT polymerization

The main principle of RAFT polymerization is presented in Scheme 7. Growing macroradicals

(Rn•) are temporarily captured by a RAFT chain transfer agent (CTA) (with an addition rate

coefficient kadd), while releasing new macroradicals (with a fragmentation rate coefficient

kfrag) which can undergo further polymerization. This RAFT polymerization mechanism will be

discussed more in detail in the next section (Section 2).

Scheme 7: Main principle of reversible addition-fragmentation chain transfer polymerization.

1.2.5. Advantages and limitations

The common feature of all CRP techniques is the dynamic pseudo-equilibrium between the

propagating radicals and the various types of dormant species. Radicals are involved in

propagation reactions and exchanges with dormant species. To a lesser extent, they can also

undergo termination, chain transfer reactions and other side reactions. Each CRP system has

its own advantages and limitations compared to the other techniques, as shown in Figure 3.

The following discussion will mainly focus on the most important advantages of the RAFT

polymerization technique in comparison with NMP, ATRP, and SET-LRP.

Figure 3: Relative advantages and limitations of ATRP, NMP and RAFT polymerization as applied to the synthesis of low

molar mass (LMM) and high molar mass (HMM) polymers, the range of polymerizable monomers (Monomer Range), the

ability to produce block copolymers (Blocks), the incorporation of functional end-groups (EGF), the synthesis of various

hybrid materials (Hybrids), the ability to carry out polymerization in aqueous media (Water) and some environmental

issues (Env. issues).[24]


Literature survey

The ATRP technique is mainly suitable for the synthesis of low molar mass polymers, with an

efficient incorporation of end-group functionalities, and hence very appropriate for the

preparation of block copolymers or post-modification reactions.[15]

An important

disadvantage is the usage of metal complexes as catalyst, which have to be removed after

the polymerization. As SET-LRP is characterized by comparable advantages and limitations as

for ATRP, the SET-LRP technique is excluded from Figure 3.

Good control over the macromolecular structure, without the need for metals, can be

obtained by the NMP method. On the other hand, this CRP process is only applicable to a

small range of (co)monomers, i.e. styrene and (meth)acrylates. Furthermore, the large costs

of the nitroxides should be taken into account.[15]

The RAFT polymerization technique operates via a degenerative chain transfer process and

does not make use of the persistent radical effect to establish control. While the

polymerization rates for NMP and ATRP reactions are significantly smaller than for the

conventional FRP process,[22, 25]

the RAFT polymerization process does not inherently suffer

from rate retardation in case the mediating agent is chosen well. In this way, the RAFT

polymerization can be carried out with similar kinetics to those for conventional FRP, apart

from effects of chain lengths and side reactions.[12]

However, the presence of inhibition and

rate retardation effects cannot be ignored when using less suitable RAFT agents. A careful

choice of the RAFT agent is therefore absolutely necessary. An explanation about rate

retardation and inhibition phenomena is addressed to in Paragraph 2.1.4.

In addition, RAFT polymerization can be carried out by simply adding a certain amount of a

suitable RAFT agent to a conventional FRP process, to achieve polymers with narrow MMDs.

The same monomers, initiators, solvents and temperatures can be used as in FRP.[12]

The

technique can successfully deliver control over polymerizations in bulk, solution (both in

organic and aqueous phase), as well as in heterogeneous systems (e.g. (mini)emulsion). Also,

the tolerance of the RAFT polymerization process to functional end-groups is relatively high.

This property allows the production of well-defined complex polymer architectures with

almost any functionality, demonstrating the versatility of RAFT polymerization.[22]

There are however some major drawbacks that have to be taken into account regarding

RAFT polymerization. First of all, the thiocarbonylthio end-groups attached to the polymers

can lead to an undesirable coloration of the end-product. This color may range from violet

through red to pale yellow, depending on the type of controlling agent. In addition, the

polymers may also release an odor over time due to decomposition of the end-groups.[26]

Furthermore, the reactivity of the thiocarbonylthio end-groups towards amines can be seen

as a problem, since under appropriate conditions secondary amines can react with the

end-groups to form carbamates, leading to a deactivation of the controlling agent.

Therefore, it is rather difficult to perform RAFT polymerization of amine monomers.[22]


Literature survey

2. The RAFT polymerization process The most recent CRP technique, which nowadays dominates the literature on CRP processes

along with NMP, ATRP and SET-LRP, is reversible addition-fragmentation chain transfer

(RAFT) polymerization. Among these CRP techniques, RAFT polymerization has been put

forward as the most industrially promising one, due to its strong resemblance to FRP, and its

high monomer flexibility. The RAFT polymerization methodology was invented in 1998 by

the CSIRO group in Melbourne, Australia, by a team of several researchers led by Ezio

Rizzardo.[20, 21, 27]

Although the terminology of RAFT can be used in a more general sense, it is

now closely being associated with radical polymerizations mediated 1 by thiocarbonylthio

compounds as chain transfer agents which react by a reversible addition-fragmentation

chain transfer.

Almost simultaneously, a process called macromolecular design via the interchange of

xanthates (MADIX) was patented by Rhodia in France.[28]

The only difference with the

CSIRO-reported RAFT polymerization process is that this polymerization technique makes

use of xanthates as controlling agents.[28]

The acronym MADIX is nowadays mainly used to

describe RAFT polymerizations mediated by xanthates.

The RAFT polymerization process relies on the degenerative transfer principle, as already

highlighted above. In order to minimize termination reactions and to approach LRP, the

RAFT polymerization makes use of thiocarbonylthio compounds as mediating agents to

capture the propagating radicals reversibly in dormant species. The details of this

mechanism will be discussed in the next subsection.

With an appropriate choice of the RAFT mediating agent and polymerization conditions, the

RAFT polymerization can be very efficient to synthesize polymers with a narrow MMD and a

number-averaged molar mass predetermined by the monomer conversion and CTA

concentration. Furthermore, these polymers can be used as precursors to block copolymers

by the addition of other monomers. Due to the high degree of control in RAFT

polymerization, it is also possible to produce star polymers and other complex architectures.

In this section, first a description of the RAFT polymerization mechanism and its kinetics is

provided, followed by some guidelines to choose the right mediating agent. Next, an

overview is given of the most important literature reports on the RAFT polymerization of

styrenes and methacrylates in bulk and in solution. Finally, a small impression is provided

about the future outlook of RAFT polymerization.

1 In this Master Thesis, mediated/mediating is commonly used to denote the use of a RAFT CTA agent in the

RAFT polymerization process. However, this terminology is not fully correct, since only the thiocarbonylthio

group provides the control throughout the polymerization reaction, both when being part of the initial CTA and

the macro-CTA.


Literature survey

2.1. Mechanism and kinetics

Various research groups have attempted to obtain insight in the mechanism and kinetics of

the RAFT polymerization process through direct observation of intermediate species,

computer-based modeling strategies or high-level ab initio molecular orbital calculations. A

brief description of these insights is given in the following subsections, additionally covering

different types of initiation systems. In particular, the outcome of those different studies has

initialized a scientific debate on the behavior of the intermediate (macro-)RAFT radicals that

are formed in the addition-fragmentation steps. The discussion is based on the observation

of a significant rate retardation and inhibition for the latter RAFT polymerization compared

to the corresponding FRP. Two main contradictory explanations and a handful of derivative

reasonings have been put forward by different groups during the last decade, all being in

agreement with some experimental data or high-level ab initio calculations. However, so far

none of them covers the whole picture. Ideally, the RAFT mediating agent should not induce

any inhibition or rate retardation phenomena during the polymerization. This retardation

topic is addressed to in Paragraph 2.1.4. It is worthwhile noting that computer-aided

modeling has an important role in this debate, as it offers a fast and flexible way to test

newly proposed features of the RAFT polymerization mechanism to experimental data.[29]

For completeness, in Appendix A a short elaboration on the RAFT polymerization kinetics is

presented in the way it is often encountered in literature.

2.1.1. Reaction scheme

The basic RAFT polymerization mechanism mediated by thiocarbonylthio compounds, as

provided by the CSIRO group,[20, 21]

is presented in Scheme 8. In this scheme, there are two

important reversible reactions (r5-6 and r8 in Scheme 8) that are superimposed on a

conventional radical chain-growth polymerization, consisting of radical initiation,

propagation, and termination. At the start of the RAFT polymerization, radicals are formed

by the typical initiating steps (r1-3 in Scheme 8). These Ij• radicals can be generated by the

dissociation of conventional radical initiators IiIj (r1 in Scheme 8 with a dissociation rate

coefficient kd and an initiator efficiency f), or in the case of styrene RAFT polymerization, by

the thermal self-initiation of the styrene monomer (r2 in Scheme 8 with a corresponding

rate coefficient ktherm). Different types of initiation systems will be discussed more in detail in

Paragraph 2.1.3. These radical generation steps are then followed by the actual

chain-initiation step (r3 in Scheme 8 with a rate coefficient ki), involving the reaction of the

monomer (M) with the Ij• radicals, to form R1

• radicals.


Literature survey

The key feature of the RAFT polymerization mechanism is a sequence of reversible addition-

fragmentation steps (r5-6 and r8 in Scheme 8), involving a RAFT mediating agent

((macro-)CTA in Scheme 8). These mediating agents are compounds with a reactive C=S

double bond and a stabilizing Z-group which invokes an appr

optimization of polystyrene properties using reversible addition...

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