model-based optimization of polystyrene properties by ... l. bentein-methusalem...jun 17, 2011 ·...

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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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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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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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Side reactions

Diels Alder reaction:

Monomer assistedhomolysis:

Formation of 1,2-diphenylcyclobutane:

Ene reaction:

DIMER

THERMAL INITIATION

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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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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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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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Classical synthesis approach: results (3)

OBTAINED average chain length (-)

CONVERSION = 0.85

TCL= 1000

Non-classical synthesis (fed-batch) approach ?

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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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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


WITH INITIATOR RADICAL

+Rj

+R0

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PROPAGATION CHAIN TRF TO DIMER

TERMINATION (RECOMB)DEACTIVATION

REPEATED TEMPORARY SUPPRESSION OF CHAIN TRF TO

DIMER


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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IMPROVEMENT


MULTIPLE ADDITION of predeterminedamount

CONVERSION = 0.85

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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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PROPAGATION CHAIN TRF TO DIMER

TERMINATION (RECOMB)DEACTIVATION

EFFECTIVE SUPPRESSION OF

CHAIN TRF TO DIMER AND TERMINATION


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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IMPROVEMENT


MULTIPLE ADDITION of

criterion basedamount

MULTIPLE ADDITION of predeterminedamount

CONVERSION = 0.85

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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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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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‘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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‘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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‘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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‘Ideal’ miniemulsion: modeling

droplets with i macroradicals, r initiator radicals, j nitroxide radicals

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‘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)


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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‘Ideal’ miniemulsion NMP: modeling

Generalized Smith-Ewart equations


“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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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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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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Reaction probabilities


WITH MACRORADICAL

REACTION PROBABILITY FOR MACRORADICALS & INITIATOR RADICALS

Ri

+M PROPAGATION



DEACTIVATION


WITH MACRORADICAL

+X

+M

+D



+Rj

+R0

R0

+M PROPAGATION



DEACTIVATION +X

+M

+D



+Rj

+R0

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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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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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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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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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Region II: acceleration (particle distribution)

TCL = 300region II

dp = 30 nm

higher: in agreement with

higher polymerization rate

0 macroradicals0 initiator radicals0 nitroxide radicals



inactive particles

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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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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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Transition region (2)


dp = 70 nm inactive particles:0 macroradicals

0 initiator radicals

more nitroxide radicals:

retardation → “bulk”

high

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Transition region (2)


dp = 70 nm inactive particles:0 macroradicals


more nitroxide radicals:

retardation → “bulk”

high

active particles:1 macroradical


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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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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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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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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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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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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

model-based optimization of polystyrene properties by ... l. bentein-methusalem...jun 17, 2011 ·...

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