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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Model-based optimization of polystyrene
properties by Nitroxide Mediated
Polymerization (NMP) in homogeneous and
dispersed media
Lien Bentein
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
NMP: principle and objective
active speciesdormant species
Nitroxide mediated polymerization (NMP) principle:
Objective of NMP: synthesis of well-defined polymers, i.e., polymers having a high end-group
functionality and a low polydispersity index, in homogeneous and heterogeneousmedia
Synthesis challenge:controlled polymer properties for average chain lengths higher than ~500
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Bulk NMP: system & kinetic model
initiator(alkoxyamine)
monomer
Bulk NMP of styrene initiated by SG1-phenylethyl at 396 K
Classical synthesis approach:initial molar ratio of monomer to initiator
equal to targeted chain length at complete conversion
TARGETED chain length (TCL) = [styrene]0/[SG1-phenylethyl]0
Kinetic model: main reactions (activation, deactivation, propagation, termination)
side reactions (thermal initiation, (chain) transfer reactions)
diffusional limitations accounted for (mainly important on termination & deactivation)
Bentein et al. Macromol. Theory Simul. 2011, 20, 238
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Side reactions
Diels Alder reaction:
Monomer assistedhomolysis:
Formation of 1,2-diphenylcyclobutane:
Ene reaction:
DIMER
THERMAL INITIATION
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Side reactions
Chain transfer to monomer:
Chain transfer to dimer:
Transfer from nitroxide to dimer:
Transfer from nitroxide to monomer:
(CHAIN) TRANSFER REACTIONS
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Classical synthesis approach: results
TCL(-)
TCL(-)
Experimental data fromLutz et al., Macromol. Rapid
Commun., 2001, 189
TCL(-)
OBTAINED average chainlength = 557
end groupfunctionality = 0.57
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Classical synthesis approach: results (2)
NUMBER CLD MASS CLD
Chain transfer to dimer mainly responsible for loss of control over average chain length, PDIpol and polymer end-group functionality
TCL=960
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Classical synthesis approach: results (3)
OBTAINED average chain length (-)
CONVERSION = 0.85
TCL= 1000
Non-classical synthesis (fed-batch) approach ?
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Case I: predetermined amount of M added (1)
15 % improvement
OBTAINEDaverage CL
polymer end groupfunctionality
PDIpol
794 0.39 1.65
800 0.54 1.46
nstyrene = 8.74 10-2 mol
TCL 2000
initial TCL = 500nstyrene = 8.74 10-2 mol
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Case I: predetermined amount of M added (2)
REACTION PROBABILITY FOR MACRORADICALS (RPibulk)
Ri
+M PROPAGATION
CHAIN TRANSFER TO MONOMER
CHAIN TRANSFER TO DIMER
DEACTIVATION
TERMINATION BY RECOMBINATION
WITH MACRORADICAL
+X
+M
+D
TERMINATION BY RECOMBINATION
WITH INITIATOR RADICAL
+Rj
+R0
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Case I: predetermined amount of M added (3)
PROPAGATION CHAIN TRF TO DIMER
TERMINATION (RECOMB)DEACTIVATION
REPEATED TEMPORARY SUPPRESSION OF CHAIN TRF TO
DIMER
REACTION PROBABILITY FOR MACRORADICALS (RPibulk)
RP
i bu
lk,p
rop
agat
ion
(-)
RP
i bu
lk,c
hai
nTR
F to
D (-
)R
Pi b
ulk
,te
rmin
atio
nb
yre
com
b(-
)
RP
i bu
lk,d
eac
tiva
tio
n(-
)
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Case I: predetermined amount of M added (4)
IMPROVEMENT
OBTAINED average chain length (-)
MULTIPLE ADDITION of predeterminedamount
CONVERSION = 0.85
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Case II: criterion based amount of M added (1)
OBTAINEDaverage CL
polymer end groupfunctionality
PDIpol
1042 0.19 1.90
1594 0.62 1.37
>43 % improve-ment
initial TCL = 100
TCL 5000
after each addition:[styrene]/[alkoxyamine] =
100
no classical equivalent
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Case II: criterion based amount of M added (2)
PROPAGATION CHAIN TRF TO DIMER
TERMINATION (RECOMB)DEACTIVATION
EFFECTIVE SUPPRESSION OF
CHAIN TRF TO DIMER AND TERMINATION
REACTION PROBABILITY FOR MACRORADICALS (RPibulk)
RP
i bu
lk,p
rop
agat
ion
(-)
RP
i bu
lk,c
hai
nTR
F to
D (-
)R
Pi b
ulk
,te
rmin
atio
nb
yre
com
b(-
)
RP
i bu
lk,d
eac
tiva
tio
n(-
)
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Case II: criterion based amount of M added (3)
IMPROVEMENT
OBTAINED average chain length (-)
MULTIPLE ADDITION of
criterion basedamount
MULTIPLE ADDITION of predeterminedamount
CONVERSION = 0.85
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Fed-batch NMP of styrene
Theoretically, polymer properties can beimproved for average chain lengths higher than
500 by a fed-batch approach
But will the approach really work in practice?
…the experiments are currentlybeing performed in collaboration with the
Polymer Chemistry Research Group…
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
CRP in dispersed systems; miniemulsion?
General• industrially attractive: excellent heat transfer, ease of mixing and
handling/transporting of the final product
• water-borne systems: more environmentally friendly and economically interesting
• for CRP: emphasis on (mini)emulsion due to the expectation of similar/betterproperties than in bulk (inherent compartmentalization of radical species abilityto manipulate overall reaction rates and control over polymer properties by adaptingthe particle size)
CRP in miniemulsion• alter particle size by amount of added surfactant
• ideally polymerization reactions only inside the particles, in which controlling agent is present
styrene: radicals from thermal initiation captured by controlling agent
• encapsulation of additives (pigments)
• copolymerization of highly water-insoluble monomers
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
‘Ideal’ miniemulsion: concept
emulsifier
initiator(alkoxyamine)
EMULSIFICATION
BEFORE POLYMERIZATION
ASSUMPTIONS:- oil-soluble initiator
- uniform monomer droplet size- homogeneous initiator concentration
monomer
water
monomerdroplets
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
‘Ideal’ miniemulsion: concept
BEFORE POLYMERIZATION
ASSUMPTIONS:- oil-soluble initiator
- uniform monomer droplet size- homogeneous initiator concentration
POLYMERIZATION
ASSUMPTIONS:- polymerization only in oil phase
- no mass transfer to aqueous phase- constant particle size
monomerdroplets
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
‘Ideal’ miniemulsion: modeling approaches for NMP
• Generalized Smith-Ewart equations
• detailed reaction network (thermal initiation through Mayo mechanism, chaintransfer to monomer, to dimer and transfer from nitroxide to dimer)
• distinction between initiator radicals and macroradicals
• diffusional limitations included up to high conversion
• effect of particle size on overall polymerization rate as well as polymerproperties
In literature: mainly TEMPO/styrene
• Modified Smith-Ewart equations
• intrinsic kinetic model
• often limited to low conversion (Zetterlund: TEMPO/TIPNO)
• no thermal initiation, no compartmentalization of nitroxide, termination bydisproportionation (Charleux: SG1)
• Kinetic Monte Carlo (Tobita)
• intrinsic kinetic model
• focus on the effect of particle size on overall polymerization rate
Our approach: SG1/styrene
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
‘Ideal’ miniemulsion: modeling
droplets with i macroradicals, r initiator radicals, j nitroxide radicals
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
‘Ideal’ miniemulsion NMP: modeling
Generalized Smith-Ewart equations
= NA vp ka,app τ0 (Ni-1,rj-1 - Ni,r
j)dNi,r
j
dt+ NA
-1 vp-1 <kda,app ,0> ( (i+1)(j+1)Ni+1,r
j+1 – (i)(j)Ni,rj)
+ NA-1 vp
-1 <kda0,app ,0> ((r+1)(j+1)Ni+1,rj+1 – (r)(j)Ni,r
j)
+ NA vp kthi,app [M][D](Ni,r-2j – Ni,r
j)
+ NA-1 vp
-1 <ktc,app ,0> ((i+2)(i+1)Ni+2,rj – (i)(i-1)Ni,r
j)
+ NA-1 vp
-1 ktc00/2 ((r+2)(r+1)Ni,r+2j – (r)(r-1)Ni,r
j)
+ NA-1 vp
-1 <ktc0,app ,0> ((i+1)(r+1)Ni+1,r+1j – (i)(r)Ni,r
j)
+ <ktrM,app ,0>[M]((i+1)Ni+1,r-1j – (i)Ni,r
j)
+ <ktrD,app ,0> [D]((i+1)Ni+1,r-1j – (i)Ni,r
j)
+ kp0 [M]((r+1)Ni-1,r+1j – (r)Ni,r
j) + ktrXD [D]((j+1)Ni,r-1j+1 – (j)Ni,r
j)
droplets with i macroradicals, r initiator radicals, j nitroxide radicals
i-1 r j-1 i+1 r j+1i r-1 j-1 i r+1 j+1
+ NA vp ka0,app [R0X] (Ni,r-1j-1 - Ni,r
j)
i+1 r j+1 i-1 r j-1i r+1 j+1 i r-1 j-1i r-2 j i r+2 ji+2 r j i-2 r ji r+2 j i r-2 ji+1 r+1 j i-1 r-1 ji+1 r-1 j i-1 r+1 ji-1 r+1 j i+1 r-1 ji r-1 j+1 i r+1 j-1
number of dropletswith i, r, j
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
‘Ideal’ miniemulsion NMP: modeling
Generalized Smith-Ewart equations
droplets with i macroradicals, r initiator radicals, j nitroxide radicals
“Bulk” concentrations and conversion: continuity equations
Average properties: modified moment equations
e.g. total concentration
dormant macrospecies
dτ0
dt=
<kda,app,0>
(NAvp)2i,j,r
(i) (j) Ni,rj
- <ka,app ,0> τ0 Ni,rj
i,j,r
Avogadro constant
droplet volume
total number of droplets
DEACTIVATION ACTIVATION
Viscosity effects includedNumber of dropletswith (i,r,j)
Analogous as for normal bulk NMP: Bentein et al. Macromol. Theory Simul. 2011, 20, 238
Ni,rj
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Polymerization rate regions (dp)
TCL = 300
Miniemulsion NMP of styrene initiated by SG1-PhEt at 396 Kregion I
region II
acceleration
retardation
region III
bulk
MAXIMUM acceleration (conversion)
conversion
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Control over chain length & livingness (dp)
TCL = 300
Full line = miniemulsion
Dotted line = bulk
always betterworse
higher
MAX
MAX
MAX
MAXIMUM in region II
bulk
bulk
bulk
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Reaction probabilities
TERMINATION BY RECOMBINATION
WITH MACRORADICAL
REACTION PROBABILITY FOR MACRORADICALS & INITIATOR RADICALS
Ri
+M PROPAGATION
CHAIN TRANSFER TO MONOMER
CHAIN TRANSFER TO DIMER
DEACTIVATION
TERMINATION BY RECOMBINATION
WITH MACRORADICAL
+X
+M
+D
TERMINATION BY RECOMBINATION
WITH INITIATOR RADICAL
+Rj
+R0
R0
+M PROPAGATION
CHAIN TRANSFER TO MONOMER
CHAIN TRANSFER TO DIMER
DEACTIVATION +X
+M
+D
TERMINATION BY RECOMBINATION
WITH INITIATOR RADICAL
+Rj
+R0
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Region I: retardation (reaction probabilities)
TCL = 300region I
dp = 15 nm
fast decrease [R0X] with
conversion lower PDI
initiator radicals (exception):
macroradicals:
segregation of radicals
and similar overall
importance of chain transfer
to dimer:
higher livingness
confined space effect:
lower polymerization rate and
positive effect on control over chain
length and end-group functionality
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Region I: retardation (particle distribution)
region I
dp = 15 nm
inactive particle:0 macroradicals
0 initiator radicals0 nitroxide radicals
very low: confirming
lower polymerization rate
TCL = 300
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Region I: retardation (particle distribution)
region I
dp = 15 nm
inactive particle:0 macroradicals
0 initiator radicals0 nitroxide radicals
very low: confirming
lower polymerization rateactive particle:0 macroradicals1 initiator radical1 nitroxide radical
TCL = 300
1 nitroxide radical in very small volume
→ high concentration
(Tobita: Single Molecule Concentration Effect)
active particle:1 macroradical
0 initiator radicals1 nitroxide radical
„living‟
characteristics:
confirming good
control
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Region II: acceleration (reaction probabilities)
TCL = 300region II
dp = 30 nm
better overall suppression of
termination and chain transfer to
dimer reactions (compared to region I):
higher livingness
clearly propagation favored:
higher polymerization rate,
higher initial chain lengths
very slow decrease [R0X] with
conversion higher PDI
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Region II: acceleration (particle distribution)
TCL = 300region II
dp = 30 nm
higher: in agreement with
higher polymerization rate
0 macroradicals0 initiator radicals0 nitroxide radicals
0 macroradicals0 initiator radicals2 nitroxide radicals
0 macroradicals0 initiator radicals4 nitroxide radicals
inactive particles
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Region II: acceleration (particle distribution)
TCL = 300region II
dp = 30 nm
1 0 5
higher: in agreement with
higher polymerization rate
well-balanced amount of
nitroxide radicals: good livingness
1 macroradical0 initiator radicals1 nitroxide radical
1 macroradical0 initiator radicals3 nitroxide radicals
active particles
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Transition region II to region III
TCL = 300region II → III
dp = 70 nm similar rates on average:
indicative of transition
convergence to “bulk” properties:
diminished suppression of
termination and chain transfer to
dimer lower livingness
faster decrease [R0X] with
conversion lower PDI
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Transition region (2)
TCL = 300region II → III
dp = 70 nm inactive particles:0 macroradicals
0 initiator radicals
more nitroxide radicals:
retardation → “bulk”
high
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Transition region (2)
TCL = 300region II → III
dp = 70 nm inactive particles:0 macroradicals
0 initiator radicals
more nitroxide radicals:
retardation → “bulk”
high
active particles:1 macroradical
0 initiator radicals
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Effect of diffusional limitations (dp)
TCL = 300
most pronounced at higher dp (bulk limit)
Macroradicals
Nitroxide radicals
r i j
jri
p
R iNN
n ,,
1
r i j
jri
p
X jNN
n ,,
1
Full line = with diff. lim.
Dotted line = without diff. lim.
region II
dp = 30 nm
region II → III
dp = 70 nm
region I
dp = 15 nm
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Effect of diffusional limitations (dp)
TCL = 300
most pronounced at higher dp (bulk limit)
Macroradicals
Nitroxide radicals
r i j
jri
p
R iNN
n ,,
1
r i j
jri
p
X jNN
n ,,
1
Full line = with diff. lim.
Dotted line = without diff. lim.
main effect at high
conversion
region II
dp = 30 nm
region II → III
dp = 70 nm
region I
dp = 15 nm
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Interplay TCL and dp for miniemulsion characteristics
TCL = 300 TCL = 800 TCL = 2000
higher TCL: more
improvement at higher dp
higher TCL: maximal acceleration at higher dp
higher TCL: more effect at higher dp
higher TCL: limited increase PDI
conversion = 0.70
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Conclusions
bulk NMP of S (SG1-mediated; 396 K)
• chain transfer to dimer reactions are important for high TCL• fed-batch approach theoretically proven to improve polymer properties
miniemulsion NMP of S (SG1-mediated; 396 K)
• strong effect of droplet/particle size on polymerization rate and controlover polymer properties:
• polymer end-group functionality always higher than in bulk• maximal acceleration corresponding with maximal end-groupfunctionality
• improvement of all properties compared to bulk only for very smallparticles
• diffusional limitations are only important for high particle sizes at highconversion
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Acknowledgements
1. L. Bentein acknowledges financial support from a doctoral fellowshipfrom the Fund for Scientific Research Flanders (FWO).
2. This work was supported by the Interuniversity Attraction PolesProgramme - Belgian State - Belgian Science Policy and the LongTerm Structural Methusalem Funding by the Flemish Government.The research leading to these results has received funding from theEuropean Community’s Sixth framework Programme (contract nr011730).
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Methusalem Advisory Board meeting, Ghent, 17 June 2011
Glossary CRP: controlled radical polymerization
Livingness: polymer end-group functionality
NMP: nitroxide mediated polymerization
Targeted chain length (TCL): the chain length that would be obtained by an ideal, controlled polymerization at 100% conversion, i.e., the initial ratio of monomer/initiator
Reaction probability of a molecule: the ratio of the rate of a particular reaction to the rates of all other possible reactions that the molecule can undergo
Segregation effect of radicals: physical segregation of radicals in particles, allowing the suppression of bimolecular termination
Confined space effect: smaller particle/smaller volume leads to increasedconcentrations and increased rates (in this case: of deactivation)
Single molecule concentration effect: one molecule present in such a small volume that its concentration is higher than the concentration of this species in the equivalent bulk system
Mnpol: number average molar mass of the polymer
PDIpol: polydispersity index of the polymer