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Termination kinetics of free-radical methyl methacrylate-dodecyl acrylate and dodecyl methacrylate-methyl acrylate copolymerizations
No full textCopolymerization termination rate coefficients (k(t)) of the methyl methacrylate-dodecyl acrylate (MMA-DA) and dodecyl methacrylate-methyl acrylate (DMA-MA) systems at 40degreesC and 1000 bar have been measured using the single pulse (SP)-pulsed laser polymerization (PLP) technique. Plateau regions of k(t) are observed in the initial polymerization period. The region of constant k(t) increases with the size of the alkyl ester group, e.g. it extends up to at least 60% monomer conversion in DA and DMA homopolymerizations. The plateau k(t) values of MA and MMA are significantly above the corresponding DA and DMA values. The k(t) penultimate unit effect model, which uses the so-called geometric mean approximation, is well suited for representation of the dependence on monomer composition of the plateau k(t) values. The dependence of k(t) on monomer composition is quite different at low and at high degrees of monomer conversion. The reason behind this is seen in different types of diffusion control being operative at low and at high degrees of monomer conversion. The plateau-type behaviour is assigned to segmental diffusion control, whereas high-conversion k(t) is controlled by reaction diffusion
Propagation rate coefficients of acrylate-methacrylate free-radical bulk copolymerizations
No full textCopolymerization propagation rate coefficients, kp,copo, have been measured for the binary systems methyl acrylate (MA)-dodecyl methacrylate (DMA), butyl acrylate (BA)-methyl methacrylate (MMA), dodecyl acrylate (DA)-DMA, and DA-MMA at 40 °C and 1000 bar by the pulsed laser polymerization (PLP)-size-exclusion chromatography (SEC) technique. These acrylate-methacrylate systems are interesting because of the significant difference, by more than 1 order of magnitude, between the homopropagation rate coefficients of the two families. Reactivity ratios, ri, are determined from monomer feed compositions and the NMR spectroscopically measured copolymer compositions. The resulting ri values for the four acrylate-methacrylate copolymerizations agree within experimental accuracy. Moreover, these ri data are surprisingly close to reactivity ratio data estimated from individual addition rate coefficients to MA and MMA, respectively, of appropriate small (meth)acrylate-type free radicals. Such addition rate coefficients have been determined via EPR in liquid solution by the Hanns Fischer group. The terminal model allows for excellent individual fits of composition and of kp,copo for each of the four systems. The implicit penultimate unit effect (IPUE) model (and the explicit penultimate unit effect (EPUE) model) are capable of simultaneously fitting composition and rate data for the MMA-BA and DMA-MA systems whereas both models fail to provide a satisfactory representation of the two DA-containing systems. The data suggest that, with DA being one of the comonomers, individual propagation rate coefficients are not adequately described by consideration of only terminal and penultimate units at the free-radical terminus. On the other hand, ratioing individual propagation rate coefficients of free radicals with the same penultimate units seems to eliminate most of the impact of the penultimate units. For this reason the resulting and widely used "terminal model" reactivity ratios are reasonable and meaningful kinetic quantities although penultimate effects on the individual propagation rate coefficients undoubtedly operate
Chain-length dependence of termination rate coefficients in acrylate and methacrylate homopolymerizations investigated via the SP-PLP technique
No full textTermination rate coefficients, k(t), of alkyl acrylate and alkyl methaerylate homopolymerizations at 40 degreesC and pressures of 1000 and 2000 bar have been measured up to high degrees of monomer conversion using the time-resolved single-pulse-pulsed-laser polymerization (SP-PLP) technique. The chain-length dependence (CLD) of k(t) has been deduced from SP-PLP data by adopting the power-law model, k(t) = k(t)(o)i(-alpha), where i is the chain length. For methacrylates at low degrees of monomer conversion, t alpha is close to the theoretically predicted value of 0.16. At conversions above 20% the exponent a increases significantly with increasing conversion. This effect becomes particularly pronounced in the gel effect region, where alpha, e.g. for MMA, reaches values close to unity. In the case of acrylates with small alkyl ester side chain, such as methyl acrylate, alpha is also close to 0.16 at low conversions and increases toward higher conversions. In the case of acrylates with larger alkyl ester side chain, such as dodecyl acrylate and 2-ethylhexyl acrylate, however, alpha is close to 0.4 even at low degrees of monomer conversion. The latter effect is strongly indicative of intramolecular chain transfer, which generates significant amounts of midchain radicals in the system. The fact that such transfer processes take place is supported by SP-PLP data on alkyl acrylates polymerized in mixtures with supercritical carbon dioxide
Investigations into the termination kinetics of homo- and copolymerizations of acrylates and methacrylates using the SP-PLP technique.
No full textInvestigations into the termination kinetics of homo- and copolymerizations of acrylates and methacrylates using the SP-PLP technique.
No full textTermination kinetics of acrylate and methacrylate homo- and copolymerizations
No full textTermination kinetics of acrylate and methacrylate homo- and copolymerizations at 40 degreesC and 1000 bar have been measured up to high conversion via the SP-PLP technique. DMPA and alpha-methyl-4-(methylmercapto)-alpha-morpholino propiophenone have been used as photoinitiators. An initial plateau region of constant k(t) is seen which, with MMA, extends up to about 20 monomer conversion and is followed by a reduction in k(t) by about three orders of magnitude up to 50 % conversion. Dodecyl acrylate (DA) shows a distinctly different behavior in that constant (plateau) k(t) is observed up to 75 % conversion. Copolymerization k(t) in the plateau region is adequately described by a penultimate unit effect model. The chain-length dependence of k(t) is investigated for several homopolymerizations using the expression: k(t) = k(t)(0 .) i(-alpha), where i is chain length. With the exception of DA, where alpha is about 0.4 in the entire conversion range, alpha is close to 0.16 in the plateau region and increases once the gel effect region is reached
Termination kinetics of acrylate and methacrylate homo- and copolymerizations
No full textTermination kinetics of acrylate and methacrylate homo- and copolymerizations at 40 degreesC and 1000 bar have been measured up to high conversion via the SP-PLP technique. DMPA and alpha-methyl-4-(methylmercapto)-alpha-morpholino propiophenone have been used as photoinitiators. An initial plateau region of constant k(t) is seen which, with MMA, extends up to about 20 monomer conversion and is followed by a reduction in k(t) by about three orders of magnitude up to 50 % conversion. Dodecyl acrylate (DA) shows a distinctly different behavior in that constant (plateau) k(t) is observed up to 75 % conversion. Copolymerization k(t) in the plateau region is adequately described by a penultimate unit effect model. The chain-length dependence of k(t) is investigated for several homopolymerizations using the expression: k(t) = k(t)(0 .) i(-alpha), where i is chain length. With the exception of DA, where alpha is about 0.4 in the entire conversion range, alpha is close to 0.16 in the plateau region and increases once the gel effect region is reached
Facile access to chain length dependent termination rate coefficients via reversible addition-fragmentation chain transfer (RAFT) polymerization: Influence of the RAFT agent structure
No full textA recently developed methodology for determining chain length dependent termination rate coefficients, (k(t)(i,i)), via reversible addition-fragmentation chain transfer (RAFT) polymerizations has been extended and validated for 1-phenylethyl phenyldithioacetate (PEPDA) and 3-benzylsulfanylthiocarbonylsulfanylpropionic acid (BSPA) mediated styrene (bulk) free radical polymerizations at 80 degreesC. While the use of cumyl phenyldithioacetate (CPDA) enables a highly precise mapping of the chain length dependence of the termination rate coefficient, employment of PEPDA and BSPA leads to considerable information loss for short chain lengths (i < 10). Careful simulations demonstrate that such behavior is caused by a substantial decrease in the initial transfer effectiveness of the RAFT agents when going from CPDA to BSPA, leading to hybrid behavior between conventional and living free radical polymerization. The observed hybrid behavior is quantifiable via (overall) transfer rate coefficients for the individual RAFT agents in the preequilibrium step [CPDA (k(tr,R) = 5.0 x 10(5) L mol(-1) s(-1)), PEPDA (k(tr,R) = 2.0 x 10(5) L mol(-1) s(-1)), and BSPA (k(tr,R) = 1.0 x 10(4) L mol(-1) s(-1)) at 80 degreesC] The underlying structural cause is the change from a tertiary (CPDA), via a secondary (PEPDA), to a primary (BSPA) leaving group in the initial RAFT agent. Further, the presented simulations open an efficient pathway for approximating overall preequilibrium transfer rate coefficients for the employed RAFT agents
Free-radical propagation rate coefficients via n-pulse periodic polymerization
No full textAspects of applying n-pulse periodic initiation in pulsed laser polymerization/size exclusion chromatography (PLP/SEC) experiments are studied via simulation of molecular weight distributions (MWDs). IN n-pulse periodic PLP/SEC, sequences of n laser pulses at successive time intervals Deltat(1), up to Deltat(n) are periodically applied. With the dark time intervals being suitably chosen, n-modal MWDs with n well separated peaks occur. The n-pulse periodic PLP/SEC method has the potential for providing accurate propagation rate coefficients, k(p). Among several measures for k(p), the differences in molecular weights at the MWD peak positions yield the best estimate of k(p) under conditions for medium and high pulse laser-induced free-radical concentration. Deducting k(p) from n dark time intervals (corresponding to n regions of free-radical chain length) within one experiment at otherwise identical PLP/SEC conditions allows addressing in more detail a potential chain-length dependence of k(p). Simulations are compared with experimental data for 2-pulse periodic polymerization of methylmethacrylate
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