Green polycarbonates from orange oil : synthesis,functionalization, coating applications and recyclabilityLi, C.

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Citation for published version (APA):Li, C. (2017). Green polycarbonates from orange oil : synthesis, functionalization, coating applications andrecyclability Eindhoven: Technische Universiteit Eindhoven

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https://research.tue.nl/en/publications/green-polycarbonates-from-orange-oil--synthesis-functionalization-coating-applications-and-recyclability(17fc9acb-3dc8-4244-9ffc-08ec356d38f7).html

Green Polycarbonates from Orange Oil: synthesis, functionalization, coating applications

and recyclability

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op

gezag van de rector magnificus prof.dr.ir. F.P.T. Baaijens,

voor een commissie aangewezen door het College voor Promoties, in het openbaar te

verdedigen op woensdag 15 februari 2017 om 16:00 uur

door

Chunliang Li

geboren te Anhui, China

Dit proefschrift is goedgekeurd door de promotoren en de samenstelling van de

promotiecommissie is als volgt:

voorzitter: prof.dr.ir. R. Tuinier

1e promotor: prof.dr.C.E. Koning

copromotor(en): dr. R.J. Sablong leden: prof.dr. C.K. Williams (University of Oxford)

prof.dr. M. Johansson (KTH Royal Institute of Technology)

prof.dr. ir. E.J.M. Hensen

prof.dr. R.A.T.M. van Benthem

prof.dr. G. de With

Het onderzoek of ontwerp dat in dit proefschrift wordt beschreven is

uitgevoerd in overeenstemming met de TU/e Gedragscode

Wetenschapsbeoefening.

Nature does not hurry, yet everything is accomplished.

Lao Tzu

Chunliang Li

Green Polycarbonates from Orange Oil: synthesis, functionalization,

coating applications and recyclability

Eindhoven University of Technology, 2017

This research has received funding from the European Unions Seventh

Framework Program for research, technological development and

demonstration under grant agreement no.289253 (REFINE). This research

forms part of the research programme of the Dutch Polymer Institute (DPI),

project #796p.

A catalogue record is available from the Eindhoven University of Technology

Library

ISBN: 978-90-386-4214-7

Copyright 2017 by Chunliang Li

Cover design by Huiying Ma and Chunliang Li

Printed by Ipskamp, Enchede, The Netherland (www. ipskampprinting.nl)

Table of Contents

Chapter 1. Introduction

1.1 Polycarbonates ...................................................................................................... 1

1.2 Conventional APCs .............................................................................................. 3

1.2.1 Poly(propylene carbonate)............................................................................. 3

1.2.2 Poly(cyclohexene carbonate) ......................................................................... 4

1.3 Functionalized APCs ............................................................................................ 5

1.3.1 Functionalized epoxide monomers ................................................................ 6

1.3.2 Properties and Applications of Functionalized APCs ................................... 7

1.4 Aim of this study ................................................................................................ 21

1.5 Outline of this thesis ........................................................................................... 21

Chapter 2. Hydroxy-functional poly(limonene carbonate)s

2.1 Introduction ........................................................................................................ 28

2.2 Experimental section .......................................................................................... 30

2.2.1 Materials and general considerations .......................................................... 30

2.2.2 Characterization........................................................................................... 31

2.2.3. Copolymerizations of limonene oxide with CO2 ........................................ 32

2.2.4. Transcarbonation reactions of PLCs with polyols ...................................... 32

2.2.5 Thiol-ene modification ................................................................................ 32

2.2.6. Solvent casting and curing of hydroxyl-functional polycarbonates............ 32

2.2.7. Evaluation of the cured coatings by the acetone double rub test and the

reverse impact test ................................................................................................ 33

2.3 Results and discussion ........................................................................................ 34

2.3.1 Synthesis of ,-dihydroxyl-terminated PLC and the initial coating

evaluation ............................................................................................................. 34

2.3.2 Post-modification of PLCs with mercaptoalcohol: curing and coating

properties .............................................................................................................. 43

2.4 Conclusions ........................................................................................................ 53

Chapter 3. High glass transition temperature thiol-ene networks

based on poly(limonene carbonate)s

3.1 Introduction ........................................................................................................ 60

3.2 Experimental section .......................................................................................... 61

3.2.1 Materials and general equipment used ........................................................ 61

3.2.2 Methods ....................................................................................................... 62

3.2.3 Preparation of PLCs .................................................................................... 63

3.2.4 Solvent casting and curing of PLCs ............................................................ 63

3.2.5 Coating evaluation ....................................................................................... 64

3.3 Results and discussion ........................................................................................ 64

3.3.1 Preparation of the PLC prepolymers ........................................................... 64

3.3.2 Thermal curing ............................................................................................ 65

3.3.3 UV curing .................................................................................................... 80

3.4 Conclusions ........................................................................................................ 85

Chapter 4. Novel poly(limonene-8,9-oxide carbonate)s: synthesis,

post-modification and application in alkyd paints

4.1 Introduction ........................................................................................................ 90

4.2 Experimental section .......................................................................................... 91

4.2.1 Materials ...................................................................................................... 91

4.2.2 Characterization........................................................................................... 92

4.2.3 Copolymerization of R-limonene dioxide and CO2 ..................................... 93

4.2.4. Typical post-modification of polymers ...................................................... 93

4.2.5 Synthesis of the PLOC-derived alkyd resins ............................................... 94

4.2.6 Film preparation. ......................................................................................... 95

4.2.7 Evaluation of the films ................................................................................ 95

4.3 Results and discussion ........................................................................................ 96

4.3.1. Synthesis of PLOCs .................................................................................... 96

4.3.2. Modification of PLOC with thiols, carboxylic acids and amines ............. 104

4.3.3. CO2 insertion into PLOC .......................................................................... 107

4.4. Fatty acid-modified PLOC as alternative alkyd resin ...................................... 109

4.4.1. Introduction .............................................................................................. 109

4.4.2. Results and discussion. ............................................................................. 110

4.5 Conclusions ...................................................................................................... 113

Chapter 5. Network formation based on poly(limonene-8,9-oxide

carbonate)

5.1 Introduction ...................................................................................................... 120

5.2 Experimental section ........................................................................................ 121

5.2.1 Materials and general equipment ............................................................... 121

5.2.2 Methods ..................................................................................................... 122

5.2.3 Preparation of PLOC ................................................................................. 122

5.2.4 Solvent casting and curing of PLOC ......................................................... 123

5.2.5 Coating evaluation ..................................................................................... 124

5.3 Results and discussion ...................................................................................... 124

5.3.1 Preparation of the PLOC prepolymer ........................................................ 124

5.3.2 Network formation .................................................................................... 125

5.3.3 Thermomechanical properties of the networks using DMTA ................... 133

5.3.4 Formulation and coating properties ........................................................... 136

5.4 Conclusions ...................................................................................................... 138

Chapter 6. Application of limonene-based polycarbonates as powder

coating resins

6.1 Introduction ...................................................................................................... 144

6.2 Experimental section ........................................................................................ 146

6.2.1 Materials and general considerations ........................................................ 146

6.2.2 Methods ..................................................................................................... 146

6.2.3 Preparation of the polymer resins .............................................................. 147

6.2.4 Synthesis of 1,5,7-triazabicyclo[4.4.0]dec-5-enyltetraphenylborate

(TBDHBPh4) ..................................................................................................... 147

6.2.5 Powder coating preparation and curing ..................................................... 148

6.2.6 Coating evaluations ................................................................................... 149

6.3 Results and discussion ...................................................................................... 150

6.3.1 Thiol-ene system ....................................................................................... 151

6.3.2 Thiol-epoxy system ................................................................................... 153

6.3.3 Carboxylic acid-epoxy system .................................................................. 156

6.4 Conclusions ...................................................................................................... 157

Chapter 7. Depolymerization of limonene-based polycarbonates

7.1 Introduction ...................................................................................................... 162

7.2 Experimental section ........................................................................................ 163

7.2.1 Materials .................................................................................................... 163

7.2.2 Characterization......................................................................................... 164

7.2.3. Depolymerization-related experiments ..................................................... 164

7.3 Results and discussion ...................................................................................... 165

7.3.1 TBD-initiated depolymerization of PLC and PLOC ................................. 165

7.3.2 Inorganic base-initiated depolymerizations of PLC .................................. 169

7.3.3 Metal-assisted depolymerization of PLC................................................... 172

7.3.4 Metal-assisted depolymerization of PLOC ................................................ 179

7.4 Conclusions ...................................................................................................... 180

Chapter 8. Epilogue

8.1 Highlights ......................................................................................................... 183

8.2 Technology assessment .................................................................................... 185

8.3 Outlook ............................................................................................................. 188

Glossary ............................................................................................... 191

Summary ............................................................................................. 193

Acknowledgement .............................................................................. 196

1

1

Introduction

1.1 Polycarbonates

Poly(bisphenol-A carbonate) (BA-PC), the most common aromatic polycarbonate, is an

important engineering polymer with superior properties such as optical clarity,

excellent thermal and flame resistance and high impact strength.1, 2

It has been widely

used in electronic devices, household appliance parts, automotive applications and

containers. The consumption of BA-PC was estimated to be 3.3 Mt in 2008 and to

increase by 7% per year.3 Currently, there are two main routes for the synthesis of high

molecular weight BA-PC, i.e. the interfacial reaction between phosgene and the sodium

salt of bisphenol A in a heterogeneous system and a melt-phase transesterification

between bisphenol-A and diphenyl carbonate (Scheme 1.1). However, the production

of BA-PC requires hazardous dichloromethane and phosgene, which is

environmentally undesirable, or very high temperatures to remove phenol, which

consumes high amounts of energy.4 Recently, an phosgene-free production process

from CO2 has been reported.5

Scheme 1.1. Synthetic routes to poly(bisphenol-A carbonate).

Aliphatic polycarbonates (APCs) have gained increasing attention due to their good

biodegradability and biocompatibility.6-8

Many efforts have also been dedicated to

improve their properties as potential alternatives to BA-PC. Generally three (catalytic)

methods are employed to prepare APCs (Scheme 1.2), namely ring-open

polymerization (ROP) of cyclic carbonates, polycondensation between dimethyl

carbonate and diols and catalyzed copolymerization of epoxides and carbon dioxide

(CO2). The first approach is mainly used to prepare functionalized APCs for

biomedical applications, during which the reaction proceeds under mild conditions and

gives no side product. However, the functional monomers, mostly six-membered cyclic

2

carbonates like, e.g. trimethylene carbonate, need to be specially designed and prepared prior to the ROP reaction.

6, 8-12 In comparison, more monomers are available for

condensation polymerizations and thus APCs with various structures and properties can

be readily obtained.13

Nonetheless, several drawbacks are limiting the use of this

method. First, phosgene is used in the production of dimethyl carbonate. Then, many

factors are essential to achieve high molecular weight (MW) polymers, such as strict

stoichiometry control, high conversion of the monomers and high reaction temperatures

or vacuum for efficient removal of the small molecules generated during the

polycondensation. Besides, high temperatures are also required to maintain a

homogeneous mixture in the presence of diols with high melting points, in particular

for the production of high MW polymers.

Scheme 1.2. Preparation of APCs: a) ROP of cyclic carbonate monomers. b)

Polycondensation of polyols and dimethyl carbonate. c) Copolymerization of carbon

dioxide and epoxides.

The epoxide/CO2 copolymerization approach was first reported by Inoue et al. in 1969

and has been widely investigated in the following decades.7 This approach presents

several advantages over the previous two. First, the chain-growth mechanism of the

reaction allows a good molecular weight control and high molecular weights can be

reached at low monomer conversion at low catalyst loading. Another advantage lies in

the use of less harmful monomers which avoids the safety hazards related to processes

using phosgene. Furthermore, the comonomer CO2 is readily available, cheap, non-

flammable, non-toxic and renewable. In the light of those features, the work described

in this thesis is exclusively based on the APCs prepared via this strategy. Catalysts for

epoxide/CO2 copolymerizations have been systematically described in recent reviews.11,

13-16 This chapter will focus on the current state-of-the-art related to the properties and

applications of the PCs produced by catalyzed epoxide/ CO2 copolymerizations, paying

special attention to functionalized APCs.12, 14-17

3

1.2 Conventional APCs

Scheme 1.3. Synthesis of polypropylene carbonate (PPC) and polycyclohexane

carbonate (PCHC) by epoxide/CO2 copolymerization.

Propylene oxide (PO) and cyclohexene oxide (CHO) are the most commonly used

terminal and alicyclic epoxides, respectively, for APCs synthesis in academia and

industry (Scheme 1.3).

1.2.1 Poly(propylene carbonate)

Poly(propylene carbonate) (PPC) has been the most successful APC so far. A general

introduction to its properties and applications is given below. Detailed information on

PPC can be found in several reviews.18-22

1.2.1.1 Properties

PPCs are biodegradable polycarbonates with glass transition temperatures (Tgs) ranging

from 25 to 45 oC, depending on molecular weight and microstructure

(regioregularity).24-25

They are typically thermally stable until 180 oC.

18 The fracture

toughness of PPC increases with molecular weight (9.1 kJ/m2 at Mn = 29 kDa and 12.6

kJ/m2 at Mn = 141 kDa). The Youngs modulus was determined to be 830 MPa, with an

elongation at break of 330% and a tensile strength of 21.5 MPa for a purified PPC (Mn

= 50 kDa). A tensile modulus of about 680 MPa was reported for a thermally stable

PPC (Mn = 260 kDa, PDI 5).23-25

1.2.1.2 Applications

In spite of the low rigidity and Tg, PPC has been commercialized in different countries.

The industrial production of PPC has recently reached 3,000 tons/year. New PPC

plants with a total capacity of 10,000 ton/year were commissioned recently by China

BlueChemical Ltd and Inner Mengxi High-Tech.19

4

As mentioned above, one favorable property of high molecular weight PPC is the large elongation at break, which allows the enhancement of the elastic modulus by using

(inorganic) fillers or blending with other polymers without severe loss of toughness. By

blending polyhydroxybutyrate with PPC, Siemens and BASF have developed an

alternative material to styrene-based acrylonitrile-butadiene-styrene (ABS) plastic.

Empower Materials is producing PPC on a pilot scale, which is used as sacrificial

binder for high quality products in advanced ceramics, powder metals and sealing

glasses.7

PPCs can also be utilized as functional polymers via terpolymerizations of PO,

functional epoxides and CO2 or end-functionalization.26

In polyurethane (PU) industry,

low MW PPCs have been successfully used as polyol components, acting as soft blocks

in PUs, particularly for rigid and flexible foams production.29-30

Other potential applications are packaging material and adhesives,27

organic fillers in

containers due to the good compatibility with other polymers, flame-retarding materials

via incorporation of phosphorous groups as a green alternative to halogenated flame-

retarding polymers,28

passive electronic components,29

and medical dressings and

tailored tissue scaffold materials due to good biocompatibility.30

1.2.2 Poly(cyclohexene carbonate)

CHO is frequently used in academia as it can be readily copolymerized with CO2 under

mild conditions. However, in spite of many efforts made to improve the polymer

properties, the applications of PCHC are still limited because of its brittle character.

1.2.2.1 Properties

PCHCs usually have Tgs varying from 65 to 115 oC, depending on the content of ether

linkages and the molecular weight. They are also thermally more stable than PPC and

unzipping of PCHC starts only at 250 oC. PCHC is a brittle polymer with an elongation

at break of 1-2%. The tensile modulus of PCHC (3,600 MPa) is much higher than the

corresponding value for BA-PC (2,400 MPa). The brittle behavior of PCHC can be

explained by the relatively low plateau modulus in the melt, implying a low

entanglement density.31

A comparison of the general properties of BA-PC, PPC, PCHC

is shown in Table 1.1.

5

Table 1.1. General properties of BA-PC, PPCand PCHC and PLC. Property BA PC PPC PCHC

Tg, oC 145 30-41 112-115

Modulus (MPa) 2,400 993 3,600

Tensile strength (MPa) 65 33.2 43

Density, g/cm3 1.20 1.275 -

Dielectric constant (103 Hz) 2.96 3.0 -

Elongation at break (%) 80 330 2

Refractive index, n 1.586 1.463 -

Burning heat, 103 kJ/kg - 18.5 -

Water up-take, % (23 oC) 0.12 0.397 -

Td-5%, oC 458 218 240-280

1.2.2.2 Applications

So far there is no report on the scale-up of PCHC. Koning and coworkers described the

synthesis and subsequent curing of ,-dihydroxyl-terminated PCHCs. The resulting

coatings showed promising properties such as high transparency and high scratch

resistance.32

1.3 Functionalized APCs

As described above, PCHC and PPC as such have limited commercial applications

because of their unsatisfactory physical and mechanical properties, like low rigidity (in

case of PPC) and low elongation at break (in case of PCHC). Moreover, the lack of

additional functionality in the corresponding epoxides makes it rather difficult to

enhance the properties by chemical modification. On the other hand, the selective

polymerization of functional epoxides, namely epoxy monomers with an extra

functionality like an alkenyl, carbonate, or hydrophilic group, leads to functional

polymers of interest for many applications such as reactive substrates, coating resins,

polymeric nanoparticles, electronic and biomedical materials. A review related to

functional epoxide monomers, functional APCs, their properties and potential

applications is presented in this section.

6

1.3.1 Functionalized epoxide monomers

Figure 1.1. Structures of functional epoxide monomers.

Functionalized epoxide monomers are epoxy monomers bearing either a functional

group that is stable under the polymerization conditions (oxirane, ester, carbonate,

alkenyl) or that is protected with a protective group, allowing the introduction of

various functionalities such as hydroxyl groups, vicinal diols and carboxylic acids after

deprotection. The post-modification of the resulting APCs can be further realized via

various chemistries to produce new materials for specific applications. Figure 1.1 gives

an overview of different functional epoxides discussed in this chapter.

7

1.3.2 Properties and Applications of Functionalized APCs This section is dedicated to the synthesis, properties and potential applications of the

functional APCs. This discussion is divided into six parts, based on the nature of the

monomer functionalities, viz.: (i) alkenyl/alkynyl, (ii) ester/carbonate, (iii)

hydroxyl/carboxylic acid, (iv) hydrophilic/ hydrophobic, (v) epoxy/cyclic carbonate

and (vi). side-chain liquid crystalline (SCLC).

1.3.2.1 APCs with pendant alkenyl/alkynyl groups

The functionalized epoxides bearing an extra alkenyl or alkynyl group have been

studied for various purposes, e.g. for improving the thermo-mechanical properties of

the polycarbonate or for enhancing the hydrophilicity. First, the extra alkenyl/alkynyl

groups typically show high stability during the copolymerization with CO2, so the extra

functionality is retained in the copolymers. Moreover, the versatility of ene/yne groups

permits the post-modification of the resulting polycarbonate via different chemistries

such as radical polymerization, thiol-ene coupling, epoxidation, azide-alkyne or Diels-

Alder addtion. This strategy favors the modulation of the properties of the APCs and

thus is able to extend their potential application fields.

ukaszczyk et al. described the synthesis of the first functionalized APCs bearing

pendant allyl groups in 2000.33

The copolymerization of allyl glycidyl ether (AGE) and

CO2 was catalyzed by a ZnEt2/pyrogallol (PG) system. The resulting poly(AGE

carbonate) (PAGEC) was further oxidized with m-chloroperbenzoic acid to give an

epoxy polycarbonate (see Scheme 1.4, Route a), which degraded much slower than the

parent polycarbonate while exposed to an aqueous buffer. A tendency of gelation of the

parent and epoxy polycarbonates during storage illustrated the high reactivity of the

pendant functional groups, leading to the homopolymerization.34

Tan and coworkers

also prepared PAGECs with high carbonate content using Y(CF3COO)3 as catalyst.35

The pendant allylic groups reacted with 3-(trimethoxysilyl)propyl methacrylate in the

presence of benzoyl peroxide, generating alkoxysilane-containing copolymer

precursors for a subsequent sol-gel process, which effectively produced PAGEC-SiO2

nanocomposites. (Scheme 1.4, Route b) The thermo-mechanical properties of the

nanocomposites including Tg, the thermal stability, the tensile strength and the

elongation at break were much better than that of the parent PAGEC, without

sacrificing the transparency. Similarly, Tao et al. synthesized UV-crosslinkable PPCs

by terpolymerization of PO, CO2 and a low level of AGE, which has limited effect on

the yield of the copolymerization. However, the crosslinking significantly enhanced the

thermo-mechanical properties of PPC and extended the application temperature

window of the polymer accordingly (Scheme 1.4, Route c).36

8

Scheme 1.4. Several examples for synthesis and post-modification of AGE-based PCs .

Darensbourg and coworkers also contributed significantly to the preparation of alkenyl

functionalized APCs and their post-modification, aiming in particular at surface-

functionalized films and hydrophilic/amphiphilic polycarbonates for biomedical

applications, Random terpolymers were prepared from PO/AGE/CO2 under ambient

conditions in the presence of a Salen cobalt catalyst.37

The terpolymers were then

partially crosslinked via thiol-ene chemistry using polythiols and modified further with

N-acetyl-L-cysteine and 2-(Boc-amino)ethanethiol to produce films with carboxyl or

amine-functionalized surfaces (Scheme 1.5, Route a). In spite of some observed

degradation of the films during the deprotonation process, this strategy leads to

materials for potential biomedical applications such as coatings on biomedical devices

with good biocompatibility and mechanical properties. Amphiphilic polycarbonates

could also be prepared from these monomers. First, ABA triblock polycarbonates were

obtained through a one pot ,two-step strategy, in which PPC diols were produced as

macro-initiators and used in subsequent AGE/CO2 coupling reactions. Secondly, the

triblock precursors were modified by thiols with different functional groups, which

were then converted into anionic or cationic groups, affording amphiphilic

polycarbonates (Scheme 1.5, Route a). Nanoparticles with high uniformity were

formed by self-assembly of these polycarbonates in water, whose size could be tuned

by controlling the length of the blocks. These charged nanoparticles could be

potentially used in drug delivery systems.

Similarly, poly(2-vinyloxirane carbonate)s (PVICs) with more than 99% carbonate

linkages were prepared using the same catalyst system (Scheme 1.5, Route b). The

subsequent quantitative thiol-ene click modification afforded the amphiphilic

polycarbonates (PVIC-OH and PVIC-COOH) bearing pendant hydroxyl and carboxyl

9

groups, respectively.38 Further modifications, e.g. by ring-opening of the hydrochloride salt of L-aspartic acid anhydride for PVIC-OH and deprotonation by aqueous

ammonium hydroxide for PVIC-COOH, yielded two water-soluble polycarbonates,

showing an average hydrodynamic diameter of 32 nm while dispersed in water. These

amphiphilic/water-soluble materials were expected to form a powerful platform for

bioconjugation and being ideal candidates for polymer therapeutics.

Scheme 1.5. Synthesis and post-modification of AGE-based PCs.37-38

Geschwind et al. reported the terpolymerization of PO, 1,2-epoxy-5-hexene (EH) and

CO2 catalyzed by a cobalt-based catalyst system under mild reaction conditions (25 bar

CO2, 30 C, 2 h).39

The resulting polycarbonates were highly alternating terpolymers (>

99% carbonate linkage) with random structure with Mns of up to 34 kDa and Tgs of up

to 29 oC. The pending double bonds were then quantitatively converted into

functionalities such as thiols, alcohols and carboxylic acids via thiol-ene reactions

(Scheme 1.6). The hydroxyl-functional polycarbonates were used further to synthesize

graft polymers with well-defined structures and variable grafting densities via ring-

opening polymerization of L-lactide through the grafting-from strategy. These

polymers may be useful for low temperature processing of poly(lactic acid) (PLA) due

to their lower melting temperature compared to that of linear PLA. The full

degradability also makes them promising materials for biomedical use.

10

Scheme 1.6. Synthesis and post-modification of EH-based APCs.39

Vinyl cyclohexene oxide (VCHO) is also an interesting monomer, exhibiting similar

reactivity as CHO. Hsu and coworker synthesized block copolymers based on a living

PPC macro-initiator by the subsequent copolymerization of VCHO with CO2, catalyzed

by a Y(CF3COO)3ZnEt2m-hydroxybenzoic acid coordination system. The resulting

block copolymers showed moderate thermal and mechanical properties, lying between

PPC, PCHC and PVCHC. The PVCHC block could be further modified to expand their

applications.40

Cherian et al. reported the transformation of a linear CHO/VCHO/CO2

terpolymer into nanoparticles of controlled dimensions via cross-metathesis (Scheme

1.7, Route a). The intramolecular crosslinking of the vinyl-containing polycarbonate

under dilute conditions caused an increase of Tg from 114 to 194 oC and a decrease of

the hydrodynamic volume. The resulting nanoparticles are potential materials for

electronics applications.41

Recently this terpolymer has also been evaluated in our

laboratory as a potential new generation coating resin. This polycarbonate was

successfully cured with a trithiol by UV or by thermal curing, generating coatings

showing promising properties for (powder) coating applications (Scheme 1.7, Route

b).32

Similarly, Taherimehr et al. reported the crosslinking of PVCHC obtained from

VCHO/CO2 copolymerization, catalyzed by an iron pyridylamino-bis(phenolate)

catalyst. The reaction with 1,3-propanethiol resulted in crosslinked polycarbonates with

high Tgs and good chemical resistance, which may be suitable for high performance

applications.42

In another study, poly(vinyl cyclohexene carbonate) (PVCHC) was

quantitatively modified with 2-mercaptoethanol via thiol-ene chemistry by Zhang and

coworkers.43

The resulting hydroxyl-functionalized PVCHC-OH was used as the

initiator in the ring-opening polymerization of -caprolactone, affording brush

copolymers with well-defined structures and nearly 100% of grafting density (Scheme

1.7, Route c). The full degradability and large space between the side chains, favorable

for the accommodation of small molecules, make the brush copolymers potentially

useful materials for biomedical applications.

11

Scheme 1.7. Synthesis and post-modification of VCHO-based APCs.40-43

All summarized reports described potential applications for each type of polycarbonate.

The great dependence on the petroleum sources might however limit their production

and use in the future, when the non-renewable fossil feedstock will dry up. Thus, the

development of new PCs based on bio-renewable resources is not only promising but

even necessary as a solution to this problem.

Scheme 1.8. Synthesis and post-modification of 1,4-cyclohexadiene-1,2-oxide

(CHDO)-based APCs.44-47

12

1,4-cyclohexadiene-1,2-oxide (CHDO) is a potentially biobased functional epoxide since its precursor, 1,4-cyclohexadiene, is a waste by-product generated during the self-

metathesis of polyunsaturated fatty acid methyl esters.44

Honda et al. described the first

synthesis of poly(cyclohexadiene carbonate) (PCHDC) through the alternating

copolymerization of CO2 and CHDO with tetraphenylporphyrinatocobalt(III)

chloride/dimethylaminopyridine as the catalyst system. The subsequent bromine

addition to the double bonds afforded brominated PCHC, regarded as a promising

material for optics, owing to its high transparency and expected high refractive index

(Scheme 1.8, Route a).45

Darensbourg et al. also investigated the CHDO/CO2

copolymerization and the post-modification of PCHDC.46

They employed a

(salen)CoDNP/PPNDNP (DNP = 2,4-dinitrophenolate) catalyst system to produce high

molecular weight PCHDC with 100% selectivity. PCHDC was then modified by

quantitative thiol-ene addition of thiolglycolic acid to afford an amphiphilic copolymer,

which was converted further into a water-soluble polymer upon deprotonation with

ammonium hydroxide (Scheme 1.8, Route b). A similar modification has also been

applied to poly(1,3-cyclohexadiene carbonate). Winkler et al. likewise reported the

preparation of PCHDC using di-zinc/magnesium or chromium (III)/cobalt(III) salen

complexes.47

The ring-opening copolymerizations of CHDO/phthalic anhydride were

also studied, affording renewable, unsaturated polyesters with Tgs up to 128 oC.

Limonene 1,2-monoepoxide (LO), derived mainly from the (R)-limonene isomer

present in orange oils, is used as a flavor agent in the fragrance industry and as a green

solvent. Since the first report in 2002 on the alternating copolymerization of LO and

CO2 catalyzed by a -diiminate (BDI) zinc acetate complex,48

efforts have been made

to study the specific properties and potential applications of limonene-based

polycarbonates. Hauenstein et al. improved the monomer quality by efficient removal

of protic impurities to achieve a high MW poly(limonene carbonate) (PLC, Mn >100

kDa) and almost quantitative conversion of LO. The polymer showed also a high Tg

(130 oC), excellent thermal resistance, hardness and transparency. The further multiple

modifications significantly tuned the properties of the PLC in many directions.49

The

thiol-ene reaction with butyl-3-mercaptopropionate transformed PLC, a potential

engineering thermoplastic, into a rubber. Moreover, the modification with 2-

(diethylamino)ethanethiol and the subsequent quaternization of the amine with an aryl

halide afforded materials with antibacterial activity. The hydrophilicity and water

solubility of the PLC were enhanced by reacting the pendant double bonds with

mercaptoethanol and mercaptoacetic acid, respectively (Scheme 1.9, Route a). The

acid-catalyzed electrophilic addition of monohydroxylated poly(ethylene glycol) (PEG)

to PLC led to a PLC exhibiting a remarkably enhanced hydrophilicity (Scheme 1.9,

Route b). A completely saturated counterpart of PLC exhibited improved heat

processability.

13

Scheme 1.9. Synthesis and post-modification of PLCs.48-50

Terpolymerizations of LO, CHO and CO2 were carried out using an Al(III)

aminotriphenolate complex/bis(triphenylphosphoranylidene)ammonium chloride

(PPNCl) catalyst system.50

The thermal properties of the terpolymers were improved by

thiol-ene crosslinking reactions with 1,2-ethanedithiol, affording interconnected

networks with decomposition temperatures, Tds, in the range of 250-280 oC and Tgs of

up to 150 oC.

The alkynyl group is also a very interesting functionality utilized in many organic

reactions, in particular in the 1,3-dipolar cycloaddition or click reaction. Frey et al.

synthesized propargyl-functional polycarbonates with different contents of functional

units via the terpolymerization of glycidyl propargyl ether, glycidyl methyl ether and

CO2.51

The reaction between the pendant propargyl groups and benzyl azide was highly

efficient, undergoing the copper-catalyzed Huisgen-1,3-dipolar addition (Scheme 1.10).

A wide range of functional APCs can be produced via this strategy.

Scheme 1.10. Synthesis and post-modification of propargyl-functional APCs.51

14

Furfuryl-functionalized epoxides are potentially available from renewable resources. The furfuryl groups are used for crosslinking and network formation via thermally

reversible Diels-Alder (DA) reactions. Thus, Hu and coworker prepared the first

furfuryl glycidyl ether (FGE)/CO2 copolymer (PFGEC) using a rare earth ternary

catalyst (Scheme 1.11, Route a).52

The new polycarbonate became yellowish at ambient

atmosphere due to post-polymerization via crosslinking of the highly reactive furan

rings. The effective DA reaction of PFGEC with N-phenylmaleimide was able to

stabilize the polymer by reducing the number of furan rings and introducing bulky

groups. Similarly, Hilf et al. synthesized well-defined FGE/glycidyl methyl ether

(GME)/CO2 terpolymers (PDI~1.2-1.4) of various compositions under solvent-free

conditions.51

The post-modification with a monofunctional maleimide-containing

catechol resulted in polycarbonates that may be useful for a wide range of applications

such as adhesives, coatings, sensors and smart hydrogels. The sol-gel transformation

via the DA reaction with 1,1-(methylenedi-4,1-phenylene)bismaleimide was fully

thermally reversible (Scheme 1.11, Route b), making this polycarbonate a possible

candidate for application as self-healing material.

Scheme 1.11. Synthesis and post-modification of furfuryl-functional APCs.51, 52

1.3.2.2 APCs with pendant ester/carbonate groups

Ester/carbonate-functionalized epoxides are seldomly used, particularly in the

copolymerization with CO2, for several reasons. First, many catalysts are often

15

promoting transesterifications and transcarbonations. As a consequence, branched and cyclic polymers are formed, which retards the polymer chain growth and causes a

broad molecular weight distribution. Duchateau and coworkers proved such behavior

by MALDI-ToF-MS studies on ester-functionalized polycarbonates obtained by

copolymerization of 3,4-cyclohexene-oxide-1-carboxylic acid methyl ester or 3,4-

cyclohexene-oxide-1-carboxylic acid phenyl ester and CO2.53

Moreover, due to the

similarity between the main chain carbonate linkages and the pendant ester/carbonate

groups, the post-modification via transesterification or hydrolysis can lead to the

breakdown of the polymer. Takanashi reported the first example of carbonate-

functionalized epoxide/CO2 copolymerization using a ZnEt2/water system as catalyst.54

The structures of the monomers are shown in Scheme 1.12 (Route a). The hydrolysis of

the resulting polycarbonates was investigated under basic or acidic conditions, during

which the compound attached to the pendant group was released. Those materials are

thus potential drug carriers for controlled drugs release.

A fatty acid has been modified to make an epoxide-carrying ester functionality. The

fatty acid-based epoxide, epoxy methyl-10-undecenoate, was copolymerized with CO2

using a zinccobalt(III) double metal cyanide complex [ZnCo(III) DMCC],

generating a dihydroxyl-terminated-polycarbonate with Tgs of -38 to -44 oC.

55 The fully

biobased polycarbonate was then used to initiate ring-opening polymerization of L-

lactide, affording a biodegradable triblock copolymer (Scheme 1.12, Route b). The

pendant ester functionalities remained untouched during the process.

Scheme 1.12. Synthesis and post-modification of APCs containing pendant

ester/carbonate groups.53-55

16

1.3.2.3 APCs with pendant hydroxyl and carboxylic acid groups The preparation of APCs bearing pendant hydroxyl and carboxylic acid groups with

protic functionalities, introduced directly via the copolymerization of CO2 and epoxides,

is rather challenging, since the protic species may deactivate the catalyst or act as chain

transfer agents, causing branching, crosslinking or limiting the growth of the polymer

chains. Such epoxides must then be modified by appropriate hydroxyl- or carboxyl

protective groups, before copolymerization with CO2, and subsequent deprotection is

required. The latter step should proceed quantitatively without degradation of the

polycarbonate backbone.

Protected monohydroxy-functional glycidyl ether

Scheme 1.13. Synthesis and degradation of of poly(1,2-glycerol carbonate).56-60

Glycidol/CO2 copolymerization attempts by Inoue using the ZnEt2/H2O system as

catalyst led exclusively to the cyclic glycerol carbonate.56

The hydroxyl group was then

protected by a trimethylsilyl group. The copolymerization of the new epoxide with CO2

afforded alternating copolymers with high molecular weight. The hydrolysis of the

pendant trimethylsilyl ethers under acidic conditions gave poly(1,2-glycerol carbonate)

(1,2-PGC), which readily degraded into cyclic glycerol carbonate while exposed to

water (Scheme 1.13, Route a). Geschwind and Frey later described the synthesis of 1,2-

PGC via two different approaches based on the use of ethoxy ethyl glycidyl ether

(EEGE) and benzyl glycidyl ether (BGE).57

The protecting groups were cleaved either

by acidic treatment (PEEGEC) or by hydrogenation (PBGEC) (Scheme 1.13, Route b).

The former method caused the degradation of the polymer backbone while the latter

did not. Isotactic linear 1,2-PBGECs was also prepared by Zhang and Grinstaff using

the same method based on butyl glycidyl ether.58

Poly(butyl ether 1,2-glycerol

carbonate) was investigated as a potential (thermally) stable solid polymer electrolyte.

17

It exhibited temperature-dependent conductivity with values comparable to those of optimized PEO-based electrolytes.

59 Wu and coworkers proposed a simple way for the

preparation of PPC-co-1,2-PGC terpolymers, using 2-[[(2-nitrophenyl)methoxy]-

methyl]oxirane (Scheme 1.13, Route c).60

The o-nitrobenzyl (ONB) protecting groups

were cleaved efficiently under UV light within minutes, affording hydroxyl-

functionalized PPCs with strong hydrophilicity and high Tgs.

Protected bishydroxy-functional epoxide

Geschwind and Frey also synthesized the glycerol-derived monomer 1,2-

isopropylidene glyceryl glycidyl ether (IGG, Scheme 1.14, Route a). IGG was

terpolymerized with GME and CO2, giving random terpolymers with different contents

of IGG units.61

The removal of the protecting acetal groups under acidic hydrolysis

resulted in a new type of hydroxyl-functional polycarbonates with diol side groups.

These polymers are rather hydrophilic materials with good hydrolytic stability.

Moreover, the pendant two vicinal hydroxyl groups allows their use as potential

substrates for the attachment of molecules with aldehyde or ketone functionalities via

the formation of cyclic acetal- or ketal-groups. The facile release of binding molecules

makes this class of compounds attractive for applications such as controlled drug

release.

Scheme 1.14. Synthesis of APCs with bishydroxyl functionalities.61-62

Liu and coworkers reported the synthesis of highly isotactic PCs via copolymerization

of meso-3,5-dioxaepoxides and CO2 using a chiral dinuclear Co(III) catalyst (Scheme

1.14, Route b).62

These isotactic PCs were semicrystalline with melting points (Tm)

between 179 and 257 oC. The polymer derived from 4,4-dimethyl-3,5,8-

trioxabicyclo[5.1.0]octane (CXO) was hydrolyzed into a stereoregular poly(1,2-

bis(hydroxymethyl)ethylene carbonate) with two primary hydroxyl groups per

monomer unit upon acid treatment. This hydroxyl-functional polymer was further used

as macro-initiator for grafting polymerization of lactide, affording fully degradable

brush polymers, which may be used in biomedical and pharmaceutical applications. In

18

addition, the optically active terpolymers of CHO, CXO and CO2 could be potential optical materials for applications like fiber optics.

Protected carboxyl-functional glycidyl ether

Tsai et al. described the synthesis of an interesting polycarbonate with pendant

carboxylic groups.63

In this report, tert-butyl 3,4-epoxybutanoate, derived from 3,4-

dihydroxybutyric acid, a normal human metabolite, was copolymerized with CO2 using

bifunctional cobalt (III) salen catalysts, followed by the removal of the tert-butyl

protecting group (Scheme 1.15). The resulting poly(3,4-dihydroxybutyric acid

carbonate) was further modified for evaluation as carrier for platinum-based anticancer

drugs. These results suggest that poly(3,4-dihydroxybutyric acid carbonate) and the

related derivatives have the potential to serve as platinum drug delivery carriers for

future anticancer pharmaceuticals.

Scheme 1.15. Synthesis and post-modification of APCs carrying carboxylic acids.63

These functional polymers possess the following advantages in the biomedical field: (i)

free functional groups for the binding of chemotherapeutic agents, antibacterial

compounds, anti-inflammatory agents, fluorescent tags, or material property modifiers;

(ii) a defined biodegradation route to afford nontoxic and nonacidic byproducts, e.g.,

glycerol and carbon dioxide; (iii) physical properties ranging from semicrystalline or

amorphous materials based on the polymer or copolymer composition; (iv) potential

processability for manufacturing methods such as electrospinning.

1.3.2.4 APCs with pendant hydrophilic/hydrophobic groups

CO2-based polycarbonates with rapid and reversible thermal response at body

temperature were synthesized by alternating copolymerization of CO2 and epoxides

19

with pendant hydrophilic (methoxyethoxy)ethoxy or methoxyethoxy chains (see Scheme 1.16).

64 These novel polymers possess interesting properties such as

biodegradability, well-defined thermo-sensitivity, nontoxicity and anti-immunogenicity,

making them promising materials for various biomedical applications.

Scheme 1.16. Synthesis of hydrophilic groups-containing APCs.64

Kim and Coates reported a method for the synthesis of multi-segmented polycarbonate

graft copolymers (Scheme 1.17).65

First, norbornenyl-terminated macromonomers with

variable block sequences were generated via living copolymerization of functionalized

CHOs and CO2, using a -diiminate zinc catalyst with a norbornene carboxylate

initiator. The segmented graft copolymers were then produced by the subsequent ring-

opening metathesis polymerization of these macromonomers. This method provides a

highly controlled way for the synthesis of core-shell molecular brushes, which could be

useful in fields of phase separation, self-assembly and nanostructure formation.

Scheme 1.17. Synthesis of norbornenyl-terminated PCHC from functional CHO

carrying a hydrophilic or hydrophobic group and subsequent ROMP.65

1.3.2.5 APCs with pendant epoxy/cyclic carbonate

Aliphatic polycarbonates with dual cyclic carbonate and epoxide functionalities were

synthesized via an ethylene-bridged imine-thiobis(phenolate) chromium complex

[(ONSO)CrCl]-mediated copolymerization of 4-vinyl-1-cyclohexene diepoxide with

carbon dioxide in one pot (see Scheme 1.18).66

However, the broad molecular weight

distributions of these polycarbonates, even for relatively low monomer conversions,

indicate a limited chemo-selectivity. Such dual functional APCs have Tgs up to 154 C.

The content of pendant cyclic carbonate and epoxy groups could be conveniently tuned

by varying the molar ratio of cocatalyst to metal complex and the reaction temperature.

20

Furthermore, these polycarbonates can undergo multiple post-modifications, providing an extensive library of functionalized polycarbonate derivatives with hydroxyl- or

azidoalcohol-, and hydroxyurethane-functional groups.

Scheme 1.18. Synthesis and post-modification of APCs carrying pendant cyclic

carbonate and epoxy groups.66

1.3.2.6 Side-chain liquid crystalline (SCLC) APCs

Side-chain liquid crystalline (SCLC) polycarbonates were prepared by

copolymerization of carbon dioxide with epoxides carrying a nitrostilbene mesogenic

group and spacers of different lengths (Figure 1.20, Route a).67

The corresponding five-

membered cyclic carbonates was also formed as side products. No clear relation

between phase transition temperatures and spacer length was found, probably due to

the quite broad molecular weight distribution (5.9-80). Similar observations were

reported for the SCLC polycarbonates with alkoxyphenylbenzoate side groups (Scheme

1.19, Route b).68, 69

The high Tg of these polymers (110 oC) may be interesting for

nonlinear optics applications or for optical data storage, provided that the mesophase is

nematic and can easily be aligned.

Scheme 1.19. Synthesis of APCs containing LC side groups.67-69

21

1.4 Aim of this study Terpenes form a large and diverse class of organic compounds, mainly existing as the

primary constituents of the essential oils of many types of plants and flowers. The use

of abundant, naturally occurring compounds for chemical synthesis is an important

strategy for reducing our dependence on petroleum-derived raw materials and for

enhancing the sustainability of chemical products. The aim of this project is to develop

new materials from copolymers based on monomers derived from natural terpenes,

particularly epoxides, and CO2. Among the available epoxides, limonene oxide (LO)

and limonene dioxide (LDO), derived from the natural cyclic monoterpene limonene,

are very promising monomers. Despite the enormous efforts to develop new catalysts

and monomers for the copolymerization of epoxides and CO2, the applications of the

traditional APCs like PPC and PCHC are still limited. The lack of functionalities in

these APCs makes it impossible to improve their properties via chemical modifications.

In comparison, the limonene-based polycarbonates can be fully biobased functional

polycarbonates, serving as the platform for multiple modifications or crosslinking

reactions, thereby making them suitable for various applications.

To address these issues, the following goals were defined:

1) Synthesis and characterization of polycarbonates from LO and LDO with moderate

molecular weight and proper functionalities.

2) Post-modifications of these limonene-based polycarbonates and exploration of the

structure-properties relationships.

3) Evaluation of some typical functional polycarbonates in (powder) coating

applications.

4) Studies on the degradation behavior during depolymerization of these polymers or

crosslinked networks with the aim of exploring their recyclability.

1.5 Outline of this thesis

APCs have received increasing attention due to their potential recyclability and

biodegradability. An attractive method to prepare APCs lies in the catalyzed alternating

copolymerization of epoxides with carbon dioxide. Among the available epoxides, LO

and LDO, derived from the naturally occurring cyclic monoterpene-limonene, are very

promising monomers. The rigid structure of the monomer units result in high Tg

polymers and their natural origin provides the advantage of renewability. Moreover, the

extra functionalities, viz. alkenyl and methyloxiranyl groups, in the resulting

amorphous, transparent and hard polymers allow the use of multiple curing and

22

modification chemistries and methods. This thesis describes the synthesis, post-modifications and the evaluation of limonene-based APCs as novel coating resins.

This chapter (Chapter 1) presents an introduction to the development of APCs by

epoxide/CO2 copolymerization and summarizes the literature describing the

functionalization of APCs. Also the aim and the outline of the thesis are stated.

In Chapter 2, the synthesis and the coating evaluation of hydroxyl-functionalized

PLCs will be discussed. First, hydroxyl-terminated poly(limonene carbonate)s (PLCs)

with desired molecular weight were prepared from LO and CO2 by two different

methods, including 1) the breaking down of high molecular weight polycarbonates via

transcarbonation using polyols and by 2) applying multifunctional chain transfer agents

(CTAs) during the LO / CO2 copolymerization. The curing of dihydroxyl PLCs via

urethane chemistry gave poorly performing coatings with moderate properties, which

could be explained by the limited reactivity of the secondary and tertiary OH end

groups with polyisocyanates. In order to increase the number of isocyanate-reactive

OH groups, post-modifications of PLCs with different mercapto-alcohols via thiol-ene

click reactions were performed. PLCs with tunable properties were obtained by

controlling the stoichiometry of these reactions. Some typical PCs were evaluated as

coating resins by solvent casting and subsequent curing with polyisocyanates. The

resulting coatings showed good acetone resistance and high hardness.

In Chapter 3, the direct curing of PLCs with a multifunctional thiol compound via

thiol-ene chemistry will be discussed. In-depth kinetic studies determined the optimal

conditions for thiol-ene curing after solvent-casting. Both thermal and UV curing were

employed. The crosslink density was controlled by using different amounts of curing

agent, and thereby, varying the thiol/ene ratio. All the cured coatings showed good

acetone resistance and high hardness. UV curing appeared to be more efficient than

thermal curing, as indicated by Knig hardness measurements. The high Tg of PLC

makes it a suitable candidate for application as a powder coating resin.

In Chapter 4, the synthesis, the post-modification and an application example of

poly(limonene-8,9-oxide carbonate) (PLOC) will be presented. The chemo-selective

copolymerizations of limonene dioxide with CO2 using an Et-BDI zinc amido complex

as catalyst generated PLOC, which exhibits Tg values up to 135 oC. Besides, PLOC

bears one pendant epoxide group in every repeat unit, which readily underwent

chemical modification by catalyzed epoxide ring-opening reactions with thiols and

carboxylic acids or by catalyzed CO2 insertion, without affecting the polycarbonate

main chain. Finally, a fatty acid-modified PLOC was evaluated as comb-like alkyd

resin.

23

In Chapter 5, the high Tg thiol-epoxy network formation based on PLOC and the corresponding coating evaluation will be described. PLOCs with different molecular

weights were cured by two multifunctional thiols. The optimal curing conditions were

determined by a kinetic study of the curing at different temperatures and with different

catalyst loadings. The stoichiometry of the curing reaction determined the crosslink

density of the networks as indicated by DMTA results obtained from the corresponding

free-standing films. The resulting coatings showed good acetone resistance and

variable hardness and Tgs, depending on the structure of the curing agents.

Chapter 6 will present the powder coating evaluations based on several formulations

containing either PLC or PLOC. The UV-cured powder coatings prepared from PLCs

and PLOCs with Mn values ranging from ca. 3 to 6 kDa showed promising properties,

like good metal adhesion, acetone resistance and high hardness. The results of the

coating tests indicated that these two PCs are promising functional thermoset coating

resins.

In Chapter 7, the recyclability and degradability of PLC and PLOC will be discussed.

The depolymerization of PLCs and PLOCs was investigated, employing various

catalysts, including strong organic or inorganic bases and several metal complexes. The

(metal-assisted) base-initiated depolymerization of PLC led to a quantitative

conversion of the polymer into LO. However, the full depolymerization of PLOC

resulted in a mixture of LDO and 1,2-epoxy cyclic limonene 8,9-carbonate, which can

be used for the synthesis of a multifunctional cyclic carbonate as intermediate for non-

isocyanate based polyurethanes.

In Chapter 8 a technology assessment of the possible industrial use of the APCs

described in this thesis will be provided, together with some comments and suggestions

for future research in this field.

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27

2

Hydroxy-functional poly(limonene carbonate)s

Abstract

,-Dihydroxyl poly(limonene carbonate)s (PLCs) have been prepared by

copolymerization of limonene oxide with CO2, using a -diiminate zinc-

bis(trimethylsilyl)amido complex as the catalyst, and subsequent transcarbonation

reactions with various (metallo)organic catalyst/polyol systems, viz. stannous octoate,

(salen)AlEt (salen=2,2'-ethylenebis(nitrilomethylidene)diphenol) and 1,5,7-

triazabicyclo-[4.4.0]dec-5-ene (TBD) as the transcarbonation catalysts, combined with

1,3-propanediol, 1,10-decanediol, isosorbide, 2-(hydroxymethyl)-2-ethylpropane-1,3-

diol and pentaerythritol as the transcarbonation agents. The structure and end groups

of the polymeric species of the PLCs were identified by MALDI-ToF-MS. For the first

time, the ,-dihydroxyl PLCs were used in solvent casting and curing. Nevertheless,

only moderate properties were achieved due to incomplete curing. A partial post-

modification of these polycarbonates was fulfilled via thiol-ene chemistry using two

mercaptoalcohols with different chain lengths, viz. 2-mercaptoethanol and 6-

mercaptohexanol. The thermal properties and hydroxyl values (OHVs) of the resulting

hydroxyl PLCs were modulated by controlling the type and the amount of incorporated

thioether species. The curing kinetics of the reactions between these PLCs and

blocked/non-blocked multifunctional isocyanates was studied by ATR-FTIR, followed

by solvent casting and curing under optimized conditions. The good acetone resistance

and high transparency and hardness of the coatings demonstrated that the fully

biobased PLCs with adequate molecular weights and OHVs are promising resins for

coating applications.

This work has been published as: 1) C. Li, R.J. Sablong, C.E. Koning, Eur. Polym. J., 2015, 67,

449-458; 2) C. Li, S. van Berkel, R.J. Sablong and C.E. Koning, Eur. Polym. J., 2016, 85, 466

477.

28

2.1 Introduction

The manufacturing of CO2-derived polycarbonates is one potential methodology for

utilizing CO2 as C1 feedstock for developing novel polymers and materials.1, 2

Since

the first report of the CO2 copolymerization reaction with propylene oxide by Inoue

and coworkers, such polycarbonates have attracted great attention.3 New epoxides and

different catalyst systems have been investigated in order to achieve various polymer

structures and properties.4, 5

The most commonly used epoxides are propylene oxide

(PO), cyclohexene oxide (CHO) and styrene oxide (SO). Many other petroleum-

derived epoxides carrying functional groups, like 4-vinyl-cyclohexene oxide (VCHO),6,

7 butadiene monoxide,

8 and the renewable 1,4-cyclohexadiene oxide (CHDO)

9, 10 and

limonene oxide (LO)11

have also been copolymerized with CO2, after which the

remaining functional groups were used for further modification or crosslinking

chemistry.

One important challenge is to find promising applications for the different

polycarbonates in line with their specific properties and functionalities. So far,

extensive studies have been carried out on the modification of CO2-derived PCs

bearing pendant functionalities. Coates and his coworkers successfully transformed

linear vinyl-containing polycarbonates into nanoparticles of controlled dimensions

using a second-generation Grubbs catalyst, which effectively cross-metathesized the

vinyl groups along the polymer chain. The resulting polymeric nanoparticles may be

applied in electronics as nanoporous insulators.12

Koning et al. reported a new

generation of polycarbonate resins, synthesized from CHO and VCHO, which were

further evaluated as powder coating resins, using thiol-ene chemistry for the

crosslinking reactions involving the pendant vinyl groups. The resulting coatings

showed promising properties, like processability, high pencil hardness and good

acetone resistance.6 Recently, Darensbourg and coworkers described a practical method

to prepare amphiphilic/water-soluble polycarbonates by post-polymerization

functionalization of PCs made from CO2 and 2-vinyloxirane. Poly(2-vinyloxirane

carbonate) (PVIC) was first functionalized by thiol-ene coupling with hydroxyl and

carboxylic groups and accordingly, became amphiphilic. The amphiphilic

polycarbonates were further modified by ring-opening and/or deprotonation.8 These

new PCs are expected to provide a powerful platform for bio-conjugation. All these

reports described potential applications for each kind of polycarbonate, but the great

dependence on the petroleum sources might limit their use in the future when the fossil

feedstock will dry up.13

Thus, the development of new PCs based on bio-renewable

resources is not only promising but even necessary.

Poly(limonene carbonate) (PLC) prepared from LO, a derivative of orange peel oil or

turpentine oils, is completely biobased and shows attractive thermal and optical

29

properties.14

Since the first report by Coates on the alternating copolymerization of

limonene oxide with CO2, to our knowledge, limited work related to the applications of

this new, fully renewable polycarbonate has been reported.15

As an amorphous and

highly transparent polymer, it is a promising thermoset resin for coating applications.

The presence of pendant isopropenyl groups in PLCs, like the previously mentioned

pendant vinyl groups in PVCHCs, allows relatively easy modifications that will alter

the properties of the polymers, and thus extend their potential applications.16-21

Several

studies have already demonstrated that those isopropenyl groups could be modified by

the earlier mentioned thiol-ene click reaction, a powerful and very versatile tool for

polymer functionalization. 15

For coating applications, the polymer resins should contain at least two functional

groups (hydroxyl, carboxylic acid or epoxy) for the crosslinking reactions with the

multifunctional curing agents. When the curing reactions are restricted to the functional

end groups, moderate molecular weights (MWs) (2-4k) are necessary to exhibit low

solution or melt viscosities and to obtain adequate crosslink densities. Two approaches

can be used to synthesize the relatively low MW, hydroxy-terminated polycarbonates,

either by applying multifunctional chain transfer agents (CTAs) in epoxide/CO2

copolymerizations or by breaking down pre-made high Mw polycarbonates via

transcarbonation reactions with polyols. Water and other protic species have been

proven to act as chain transfer agents (CTA)s for some catalyst systems, resulting in

relatively low MW polycarbonates. 22-24

Thus, in principle hydroxyl-terminated

polycarbonates could be produced by using dihydroxyl chain transfer agents during the

polymerization.

In this work, we describe a promising approach to make fully renewable ,-

dihydroxyl polycarbonate resins from LO and CO2 for coating applications (Scheme

2.1). A new generation of end-functionalized PLC resins was synthesized by

alternating polymerization of LO and CO2 and subsequent transcarbonation reactions

using various (metallo)organic catalyst/(renewable) polyol systems, as shown in

Scheme 2.1. Representative PLCs were evaluated as potential coating resins by solvent

casting and subsequent curing. Some properties of the resulting coatings were then

evaluated. We believe this is the first time that such fully biobased polycarbonates have

been evaluated for practical applications. Furthermore, the post-modification of high

MW PLCs with different mercaptoalcohols carrying primary OH functionalities was

also investigated. PLCs with tunable properties were obtained by controlling the

stoichiometry of the reactions and the nature of the mercaptoalcohols. Some typical

mercaptoalcohol-modified PLCs were evaluated as coating resins by solvent casting

and subsequent curing with polyisocyanates. As we will show in this chapter, the PLCs

carrying reactive primary OH groups can form fully crosslinked polycarbonate-

urethane network structures.

30

Scheme 2.1. Synthesis and post-modification of dihydroxyl-terminated PLCs.

2.2 Experimental section

2.2.1 Materials and general considerations

All reactions involving air- or water-sensitive compounds were carried out under dry

nitrogen using MBraun glove boxes or standard Schlenk line techniques. The

copolymerizations were performed in either a 10 mL high-pressure reactor (homemade)

or a 200 mL stainless steel reactor (Bchi). Toluene, tetrahydrofuran (THF),

dichloromethane (CH2Cl2) and diethyl ether were purchased from Biosolve and

purified using an activated alumina purification system. cis/trans (45/55)-R-Limonene

oxide (98% purity) and cis/trans-S-limonene oxide (99% purity) were purchased from

Aldrich, distilled from calcium hydride and stored under nitrogen. Carbon dioxide

(99.999% purity) from Linde Gas was used without further purification.

Hexamethylene diisocyanate-based polyisocyanate (trade name: Desmodur N3600) and

its corresponding caprolactam-blocked polyisocyanate (trade name: Desmodur BL3272)

were gifts from Bayer AG. Other chemicals were also obtained from Aldrich and used

as received. Zinc bis[bis(trimethylsilyl)amide] was synthesized as described in the

literature.25

(Et-BDI)Zn[N(SiMe3)2][Et-BDI = 2-((2,6-diethylphenyl)amido)-4-((2,6-

diethylphenyl)imino)-2-pentene)] and (salen)AlEt (salen = 2,2'-

ethylenebis(nitrilomethylidene)diphenol) have been synthesized according to published

procedures.26, 27

31

2.2.2 Characterization

NMR spectra were recorded on a Varian Mercury Vx (400 MHz) spectrometer at 25 C

in chloroform-d1 and referenced versus residual solvent shifts. Gel permeation

chromatography (GPC) analyses were carried out using a Waters Alliance system

equipped with a Waters 2695 separation module, a Waters 2414 refractive index

detector, a Waters 2487 dual absorbance detector, and a PSS SDV 5 m guard column

followed by two PSS SDV linearXL columns in series of 5 m (8 300) at 40 C. THF

with 1% v/v acetic acid was used as eluent at a flow rate of 1.0 mL min-1

. The columns

were calibrated using a series of polystyrene standards (Polymer Laboratories, Mp =

580 Da up to 7.1 106 Da). Before the analyses, the samples were filtered through a

0.2 m PTFE filter (13 mm, PP housing, Alltech). MALDI-ToF-MS analyses were

performed on a Voyager DE-STR from Applied Biosystems equipped with a 337 nm

nitrogen laser. An accelerating voltage of 25 kV was applied. Mass spectra of 1000

shots were accumulated. The polymer samples were dissolved in THF at a

concentration of 1 mg mL-1

. The cationization agent used was potassium

trifluoroacetate (Fluka, >99%) dissolved in THF at a concentration of 5 mg mL-1

. The

matrix trans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB)

(Fluka) was dissolved in THF at a concentration of 40 mgmL-1

. Solutions of matrix,

salt, and polymer were mixed in a volume ratio of 4:1:4, respectively. The mixed

solution was hand-spotted on a stainless steel MALDI target and left to dry. The

spectra were recorded in the reflection mode. All MALDI-ToF-MS spectra were

recorded from the crude products. An in-house developed software was used to

characterize the polymers in detail and allowed us to elucidate the individual chain

structures and the end groups of the polymer chains. Differential Scanning Calorimetry

(DSC) analyses of polymer samples were performed on a DSC Q100 from TA

Instruments. Between 4 and 6 mg of polymer were placed in an aluminum pan. The

samples were generally heated from room temperature to 160C at 10C min1

under

nitrogen. The reported DSC data from the second run were used to determine the glass

transition temperatures of the polymers. End group titrations: the acid values (AV, mg

KOH/g of polymer) and the hydroxyl values (OHV, mg KOH/g of polymer) were

measured titrimetrically according to ISO 2114-2000 and ISO 4629-1978, respectively.

The efficiency of the crosslinking reaction and the coating performance of the cured

coatings were evaluated at room temperature via the acetone rub and rapid deformation

tests (reverse impact test, ASTM D 2794), see below.

To examine the physical properties of the cured free-standing films, dynamic

mechanical thermal analysis (DMTA) was performed on a Q800 DMTA (TA

instruments), equipped with a film fixture for tensile testing. Film tension DMTA

measurements were performed on rectangular dry film samples between 20 and 180 oC,

with a heating rate of 3 oCmin

-1. The tests were performed in the controlled strain

32

mode with a frequency of 1 Hz, an oscillating amplitude of 0.12 m, and a force track

of 125%.

2.2.3. Copolymerizations of limonene oxide with CO2

The copolymerizations were performed at 25 C in bulk with different epoxide/catalyst

ratios. The catalyst was dissolved in LO and the solution was transferred into a 5 mL

glass insert placed in the high pressure reactor. The reactor was sealed, pressurized

with CO2 and allowed to react for the desired time. The LO conversion was determined

by 1HNMR. The reaction mixture was dissolved in a small amount of dichloromethane

and precipitated with a large excess of methanol. The polymer was washed with

methanol to remove the catalyst and the unreacted epoxide and dried in vacuo.

2.2.4. Transcarbonation reactions of PLCs with polyols

The PLCs and polyols (with monomer units/polyol = 10/1 mol/mol.) were dissolved in

toluene under nitrogen at the desired temperature. After a homogeneous mixture was

formed, the catalyst (1 mol % with respect to monomer units of the PLCs) was added.

The solution was allowed to react until the target MW was reached. The development

of molecular weight and its distribution was monitored by GPC. The reaction mixture

was dissolved in a small amount of CH2Cl2 and precipitated with methanol. The

resulting solid was redissolved in CH2Cl2 and reprecipitated in methanol two times.

The final product was dried in a vacuum oven to give a white powder.

2.2.5 Thiol-ene modification

A typical procedure for the synthesis of primary OH-functionalized PLC polymer

(PLC-OH) was as follows. The modification reactions were carried out with molar

ratios of reagents [C=C]0/[mercaptoalcohol]0/[AIBN]0 = 1/x/0.3. Thiol-ene click

reactions between PLC (2 g, 10.2 mmol of pendant isopropenyl C=C groups) and

mercaptoalcohol were conducted in a 100 mL Schlenk flask under nitrogen atmosphere

with 10 mL 1,4-dioxane as solvent and AIBN (0.557 g, 3.4 mmol) as initiator. The

reaction mixture was allowed to stir overnight at 80 oC. After filtration, the solvent was

removed by rotary evaporation. The residual solid was dissolved in THF and

precipitated in methanol/H2O (9/1). The product was re-precipitated twice and PLC-

OH was obtained after drying in vacuum oven for 2-3 days at 60 oC.

2.2.6. Solvent casting and curing of hydroxyl-functional polycarbonates

The PLC-OHs were cured using either a conventional polyisocyanate curing agent, viz.

a trimer of hexamethylene diisocyanate (curing agent I, Desmodur N3600, NCO

equivalent weight = 183 g/mol) or its derivative blocked with -caprolactam (curing

33

agent II, Desmodur BL3272) (Scheme 2.2). PLC-OHs with high OHVs were always

cured using the blocked polyisocyanate in order to prevent fast gelation at room

temperature before curing in the oven. The other curing experiments were carried out

using Desmodur N3600.

A solution of polycarbonate (0.3 g) and 0.5 wt% (calculated on polymer mass) of

dibutyltindilaurate (DBTDL) in anhydrous NMP (0.6 mL) was prepared along with a

separate solution of 1.05 molar equivalent of crosslinker (calculated according to the

titration results) in NMP (0.3 mL). The two solutions were mixed and a wet film of 220

m thickness was subsequently applied onto an aluminum panel, using a doctor blade.

The film was left to dry at room temperature and then cured at 180 C for 30 minutes

under nitrogen.

Scheme 2.2. Approximate structures of the curing agents.

2.2.7. Evaluation of the cured coatings by the acetone double rub test and

the reverse impact test

The coating performances were evaluated at room temperature using several tests. The

solvent resistance was evaluated using the acetone rub test. Hereto, the sample was

rubbed back and forth with a cloth drenched in acetone. If no damage was visible after

more than 150 rubs, i.e. 75 double rubs (DRs), the acetone resistance of the coating

was considered as good. The so-called reverse impact test is a rapid deformation test,

performed by dropping a 1 kg ball on the backside of a coated pan