Model-based optimization of polystyrene properties by Nitroxide Mediated Polymerization (NMP) in homogeneous and dispersed media

1 Model-based optimization of polystyrene properties by Nitroxide Mediated Polymerization (NMP) in homogeneous and dispersed media Lien Bentein

NMP: principle and objective Nitroxide mediated polymerization (NMP) principle: dormant species active species 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 heterogeneous media Synthesis challenge: controlled polymer properties for average chain lengths higher than ~500 2

Bulk NMP: system & kinetic model Bulk NMP of styrene initiated by SG1-phenylethyl at 396 K monomer initiator (alkoxyamine) 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 3

Side reactions 4 THERMAL INITIATION Diels Alder reaction: DIMER Monomer assisted homolysis: Formation of 1,2- diphenylcyclobutane: Ene reaction:

5 (CHAIN) TRANSFER REACTIONS Methusalem Advisory Board meeting, Ghent, 17 June 2011 Side reactions Chain transfer to monomer: Chain transfer to dimer: Transfer from nitroxide to monomer: Transfer from nitroxide to dimer:

Classical synthesis approach: results OBTAINED average chain length = 557 TCL(-) TCL(-) end group functionality = 0.57 Experimental data from Lutz et al., Macromol. Rapid Commun., 2001, 189 TCL(-) 6

Classical synthesis approach: results (2) NUMBER CLD TCL=960 MASS CLD Chain transfer to dimer mainly responsible for loss of control over average chain length, PDI pol and polymer end-group functionality 7

Classical synthesis approach: results (3) 8 CONVERSION = 0.85 TCL= 1000 OBTAINED average chain length (-) Non-classical synthesis (fed-batch) approach?

Case I: predetermined amount of M added (1) TCL 2000 n styrene = 8.74 10-2 mol 15 % improvement initial TCL = 500 n styrene = 8.74 10-2 mol OBTAINED average CL polymer end group functionality PDI pol 794 0.39 1.65 800 0.54 1.46 9

10 Methusalem Advisory Board meeting, Ghent, 17 June 2011 Case I: predetermined amount of M added (2) REACTION PROBABILITY FOR MACRORADICALS (RP i bulk) DEACTIVATION +X +M PROPAGATION TERMINATION BY RECOMBINATION WITH MACRORADICAL +R j +R 0 R i +M +D CHAIN TRANSFER TO MONOMER CHAIN TRANSFER TO DIMER TERMINATION BY RECOMBINATION WITH INITIATOR RADICAL

Case I: predetermined amount of M added (3) RP i bulk,deactivation (-) RP i bulk,propagation (-) RP i bulk,chain TRF to D (-) RP i bulk,termination by recomb (-) 11 REACTION PROBABILITY FOR MACRORADICALS (RP i bulk) PROPAGATION CHAIN TRF TO DIMER REPEATED TEMPORARY SUPPRESSION OF CHAIN TRF TO DIMER DEACTIVATION TERMINATION (RECOMB)

Case I: predetermined amount of M added (4) 12 IMPROVEMENT CONVERSION = 0.85 MULTIPLE ADDITION of predetermined amount OBTAINED average chain length (-)

Case II: criterion based amount of M added (1) 13 no classical equivalent TCL 5000 >43 % improvement initial TCL = 100 after each addition: [styrene]/[alkoxyamine] = 100 OBTAINED average CL polymer end group functionality PDI pol 1042 0.19 1.90 1594 0.62 1.37

Case II: criterion based amount of M added (2) RP i bulk,deactivation (-) RP i bulk,propagation (-) RP i bulk,chain TRF to D (-) RP i bulk,termination by recomb (-) 14 REACTION PROBABILITY FOR MACRORADICALS (RP i bulk) PROPAGATION CHAIN TRF TO DIMER EFFECTIVE SUPPRESSION OF CHAIN TRF TO DIMER AND TERMINATION DEACTIVATION TERMINATION (RECOMB)

Case II: criterion based amount of M added (3) 15 IMPROVEMENT CONVERSION = 0.85 MULTIPLE ADDITION of predetermined amount MULTIPLE ADDITION of criterion based amount OBTAINED average chain length (-)

Fed-batch NMP of styrene 16 Theoretically, polymer properties can be improved for average chain lengths higher than 500 by a fed-batch approach But will the approach really work in practice? the experiments are currently being performed in collaboration with the Polymer Chemistry Research Group

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/better properties than in bulk (inherent compartmentalization of radical species ability to manipulate overall reaction rates and control over polymer properties by adapting the 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 17

Ideal miniemulsion: concept monomer BEFORE POLYMERIZATION initiator (alkoxyamine) water EMULSIFICATION emulsifier monomer droplets ASSUMPTIONS: - oil-soluble initiator - uniform monomer droplet size - homogeneous initiator concentration 18

Ideal miniemulsion: concept POLYMERIZATION BEFORE POLYMERIZATION ASSUMPTIONS: - polymerization only in oil phase - no mass transfer to aqueous phase - constant particle size monomer droplets ASSUMPTIONS: - oil-soluble initiator - uniform monomer droplet size - homogeneous initiator concentration 19

Ideal miniemulsion: modeling approaches for NMP Kinetic Monte Carlo (Tobita) intrinsic kinetic model In literature: mainly TEMPO/styrene focus on the effect of particle size on overall polymerization rate Modified Smith-Ewart equations intrinsic kinetic model often limited to low conversion (Zetterlund: TEMPO/TIPNO) no thermal initiation, no compartmentalization of nitroxide, termination by disproportionation (Charleux: SG1) Our approach: SG1/styrene Generalized Smith-Ewart equations detailed reaction network (thermal initiation through Mayo mechanism, chain transfer 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 polymer properties 20

Ideal miniemulsion: modeling 21 droplets with i macroradicals, r initiator radicals, j nitroxide radicals

Ideal miniemulsion NMP: modeling i+1 i-1 ir+1 i+2 ir-1 r+2 r-2 r+1 r r-1 rj+1 j-1 j j droplets with i macroradicals, r initiator i-1 i+1 i i-2 r+1 r-1 r+2 r-2 r+1 r r-1 r j+1 j-1 j j radicals, j nitroxide radicals number of droplets with i, r, j dn i,r j dt Generalized Smith-Ewart equations = N A v p k a,app τ 0 (N i-1,r j-1 - N i,rj ) + N A -1 v p -1 <k da,app,0> ( (i+1)(j+1)n i+1,r j+1 (i)(j)n i,rj ) + N A -1 v p -1 <k da0,app,0> ((r+1)(j+1)n i+1,r j+1 (r)(j)n i,rj ) + N A v p k thi,app [M][D](N i,r-2j N i,rj ) + N A -1 v p -1 <k tc,app,0> ((i+2)(i+1)n i+2,rj (i)(i-1)n i,rj ) + N A -1 v p -1 k tc00 /2 ((r+2)(r+1)n i,r+2j (r)(r-1)n i,rj ) + N A -1 v p -1 <k tc0,app,0> ((i+1)(r+1)n i+1,r+1j (i)(r)n i,rj ) + <k trm,app,0>[m]((i+1)n i+1,r-1j (i)n i,rj ) + <k trd,app,0> [D]((i+1)N i+1,r-1j (i)n i,rj ) + N A v p k a0,app [R 0 X] (N i,r-1 j-1 - N i,rj ) + k p0 [M]((r+1)N i-1,r+1j (r)n i,rj ) + k trxd [D]((j+1)N i,r-1 j+1 (j)n i,rj ) 22

Ideal miniemulsion NMP: modeling droplets with i macroradicals, r initiator radicals, j nitroxide radicals Generalized Smith-Ewart equations N i,r j Number of droplets with (i,r,j) Viscosity effects included Bulk concentrations and conversion: continuity equations e.g. total concentration dormant macrospecies dτ 0 dt = i,j,r DEACTIVATION <k da,app,0> (i) (j) N j i,r (N A v p ) 2 ACTIVATION - <k a,app,0> τ 0 N j i,r i,j,r Avogadro constant droplet volume total number of droplets Average properties: modified moment equations Analogous as for normal bulk NMP: Bentein et al. Macromol. Theory Simul. 2011, 20, 238 23

Polymerization rate regions (d p ) MAXIMUM acceleration (conversion) 24 Miniemulsion NMP of styrene initiated by SG1-PhEt at 396 K region I retardation conversion TCL = 300 acceleration region II region III bulk

Control over chain length & livingness (d p ) MAX higher TCL = 300 Full line = miniemulsion Dotted line = bulk bulk MAX worse always better bulk MAX bulk MAXIMUM in region II 25

26 Methusalem Advisory Board meeting, Ghent, 17 June 2011 Reaction probabilities REACTION PROBABILITY FOR MACRORADICALS & INITIATOR RADICALS DEACTIVATION +X +M PROPAGATION TERMINATION BY RECOMBINATION WITH MACRORADICAL +R j +R 0 R i +M +D CHAIN TRANSFER TO MONOMER CHAIN TRANSFER TO DIMER TERMINATION BY RECOMBINATION WITH INITIATOR RADICAL TERMINATION BY RECOMBINATION WITH MACRORADICAL DEACTIVATION +X +R j +R 0 R 0 +M PROPAGATION +M +D CHAIN TRANSFER TO MONOMER CHAIN TRANSFER TO DIMER TERMINATION BY RECOMBINATION WITH INITIATOR RADICAL

Region I: retardation (reaction probabilities) 27 region I d p = 15 nm macroradicals: segregation of radicals and similar overall importance of chain transfer to dimer: higher livingness confined space effect: TCL = 300 lower polymerization rate and positive effect on control over chain length and end-group functionality initiator radicals (exception): fast decrease [R 0 X] with conversion lower PDI

Region I: retardation (particle distribution) 28 region I d p = 15 nm TCL = 300 inactive particle: 0 macroradicals 0 initiator radicals 0 nitroxide radicals very low: confirming lower polymerization rate

Region I: retardation (particle distribution) region I d p = 15 nm TCL = 300 inactive particle: 01 macroradicals 0 initiator radicals 01 nitroxide radicals active particle: 0 macroradicals 1 initiator radical 1 nitroxide radical living characteristics: confirming good control very low: confirming lower polymerization rate 1 nitroxide radical in very small volume high concentration (Tobita: Single Molecule Concentration Effect) 29

Region II: acceleration (reaction probabilities) 30 region II d p = 30 nm clearly propagation favored: higher polymerization rate, higher initial chain lengths TCL = 300 better overall suppression of termination and chain transfer to dimer reactions (compared to region I): higher livingness very slow decrease [R 0 X] with conversion higher PDI

Region II: acceleration (particle distribution) 31 region II d p = 30 nm TCL = 300 inactive particles 0 macroradicals 0 initiator radicals 0 nitroxide radicals higher: in agreement with higher polymerization rate 0 macroradicals 0 initiator radicals 2 nitroxide radicals 0 macroradicals 0 initiator radicals 4 nitroxide radicals

Region II: acceleration (particle distribution) region II d p = 30 nm active particles 1 macroradical 0 initiator radicals 1 nitroxide radical TCL = 300 higher: in agreement with higher polymerization rate 1 macroradical 0 initiator radicals 3 nitroxide radicals 1 0 5 well-balanced amount of nitroxide radicals: good livingness 32

Transition region II to region III 33 region II III d p = 70 nm similar rates on average: indicative of transition TCL = 300 convergence to bulk properties: diminished suppression of termination and chain transfer to dimer lower livingness faster decrease [R 0 X] with conversion lower PDI

Transition region (2) 34 region II III d p = 70 nm inactive particles: 0 macroradicals 0 initiator radicals TCL = 300 high more nitroxide radicals: retardation bulk

Transition region (2) 35 region II III d p = 70 nm inactive particles: 01 macroradicals 0 initiator radicals TCL = 300 high more nitroxide radicals: retardation bulk

Effect of diffusional limitations (d p ) TCL = 300 region I d p = 15 nm region II d p = 30 nm region II III d p = 70 nm Macroradicals 1 n R ini, r, j N p r i j Nitroxide radicals 1 n X jni, r, j N p r i j Full line = with diff. lim. Dotted line = without diff. lim. most pronounced at higher d p (bulk limit) 36

Effect of diffusional limitations (d p ) TCL = 300 region I d p = 15 nm region II d p = 30 nm region II III d p = 70 nm Macroradicals 1 n R ini, r, j N p r i j Nitroxide radicals 1 n X jni, r, j N p r i j Full line = with diff. lim. Dotted line = without diff. lim. main effect at high most pronounced at higher d p (bulk limit) conversion 37

38 Methusalem Advisory Board meeting, Ghent, 17 June 2011 Interplay TCL and dp for miniemulsion characteristics TCL = 300 TCL = 800 TCL = 2000 conversion = 0.70 higher TCL: maximal acceleration at higher d p higher TCL: more improvement at higher d p higher TCL: more effect at higher d p higher TCL: limited increase PDI

Conclusions 39 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 control over polymer properties: polymer end-group functionality always higher than in bulk maximal acceleration corresponding with maximal end-group functionality improvement of all properties compared to bulk only for very small particles diffusional limitations are only important for high particle sizes at high conversion

Acknowledgements 40 1. L. Bentein acknowledges financial support from a doctoral fellowship from the Fund for Scientific Research Flanders (FWO). 2. This work was supported by the Interuniversity Attraction Poles Programme - Belgian State - Belgian Science Policy and the Long Term Structural Methusalem Funding by the Flemish Government. The research leading to these results has received funding from the European Community s Sixth framework Programme (contract nr 011730).

41 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 increased concentrations 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 M n pol : number average molar mass of the polymer PDI pol : polydispersity index of the polymer