Propagation rate coefficients of acrylate-methacrylate free-radical bulk copolymerizations

Copolymerization 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

Facile access to chain length dependent termination rate coefficients via reversible addition-fragmentation chain transfer (RAFT) polymerization: Influence of the RAFT agent structure

A 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

An in-depth analytical approach to the mechanism of the RAFT process in acrylate free radical polymerizations via coupled size exclusion chromatography-electrospray ionization mass spectrometry (SEC-ESI-MS)

Coupled size exclusion chromatography (SEC)-electrospray ionization mass spectrometry (ESI-MS) was applied to carefully map the product spectrum of a series of acrylate free radical polymerizations mediated via the reversible addition fragmentation chain transfer (RAFT) process. The product stream of a significantly rate retarded RAFT system (i.e. n-butyl acrylate (BA)/cumyl dithiobenzoate (CDB)) was compared with the less rate retarded RAFT polymerizations of BA mediated by cumyl phenyl dithioacetate (CPDA) and methyl acrylate (MA)/CPDA. In each case excellent agreement between the theoretical and experimental masses, as well as the simulated isotopic peak distributions, of polymeric species in the product stream was observed. Although conventional disproportionation and combination bimolecular termination products were clearly identified within the product spectra, the presence of irreversibly terminated RAFT intermediates, i.e. 3-armed star polymers, was not observed. The mass spectroscopic results are compared to modeling estimations (carried out via the PREDICI® program package) of the concentration ratios of 3-armed stars vs. conventional termination products. It is demonstrated that the occurrence of conventional termination products should be accompanied by a significant product stream associated with 3-armed star polymer material if cross termination was operational - at least under the current reaction conditions. The absence of three armed star polymer products in the polymers stream suggests that irreversible cross termination reactions may be of minor importance in the present systems. © 2005 Elsevier Ltd. All rights reserved

Reversible addition fragmentation chain transfer copolymerization: Influence of the RAFT process on the copolymer composition

Reversible addition fragmentation chain transfer (RAFT) mediated and conventional copolymerizations at low monomer conversions have been carried out for the systems methyl methacrylate (MMA)-styrene, methyl acrylate (MA)-styrene and methyl methacrylate-butyl acrylate (BA). The polymer samples have been analyzed via 1H-NMR spectroscopy to obtain the copolymer composition and the terminal model reactivity ratios. In the RAFT mediated copolymerizations, the polymer mole fraction of the monomer with the larger reactivity ratio is increased compared to the conventional copolymerization. Simulations have been carried out using the program package PREDICI ® to examine possible explanations for the experimental findings. The simulations demonstrate that the RAFT process itself may alter the macroradical populations and the copolymer composition by offering additional reaction pathways. Further, the rate coefficients for the initiation reaction and the pre-equilibrium play an important role in determining the copolymer composition. The rate coefficients governing the main equilibrium of the RAFT process have only a minor impact on the copolymer composition. © 2004 Elsevier Ltd. All rights reserved

Initiator efficiency of 2,2′-azobis(isobutyronitrile) in bulk dodecyl acrylate free-radical polymerizations over a wide conversion and molecular weight range

The initiator efficiency, f, of 2,2′-azobis(isobutyronitrile) (AIBN) in dodecyl acrylate (DA) bulk free-radical polymerizations has been determined over a wide range of monomer conversion in high-molecular-weight regimes (M n cong; 106 g mol-1 [≅ 4160 units of DA)] with time-dependent conversion data obtained via online Fourier transform near infrared spectroscopy (FTNIR) at 60°C. In addition, the required initiator decomposition rate coefficient, kd, was determined via online UV spectrometry and was found to be 8.4 · 10-6 s-1 (±0.5 · 10-6 s-1) in dodecane, n-butyl acetate, and n-dodecyl acetate at 60°C. The initiator efficiency at low monomer conversions is relatively low (f = 0.13) and decreases with increasing monomer to polymer conversions. The evolution of f with monomer conversion (in high-molecular-weight regimes), x, at 60°C can be summarized by the following functionality: f60°c (x) = 0.13-0.22 · x + 0.25 · x2 (for x ≤ 0.45). The reported efficiency data are believed to have an error of >50%. The ratio of the initiator efficiency and the average termination rate coefficient, (kt±, (f/〈kt〉) has been determined at various molecular weights for the generated polydodecyl acrylate (Mn = 1900 g mol-1 (≅ 8 units of DA) up to Mn = 36,500 g mol-1 (≅ 152 units of DA). The (f/〈kt〉) data may be indicative of a chain length-dependent termination rate coefficient decreasing with (average) chain length. © 2004 Wiley Periodicals, Inc

Access to chain length dependent termination rate coefficients of methyl acrylate via reversible addition-fragmentation chain transfer polymerization

The reversible addition - fragmentation chain transfer - chain length dependent - termination (RAFT-CLD-T) method is employed to map out the chain length dependence of the termination rate coefficient in methyl acrylate (MA) bulk free radical polymerizations at 80°C. Methoxycarbonylethyl phenyldithioacetate (MCEPDA) - a novel RAFT agent carrying a methyl acryl leaving group - is identified as suitable for the RAFT-CLD-T method applied to methyl acrylate, as interfering inhibition and rate retardation effects are avoided. The chain length dependency of the termination rate coefficient was constructed in a stepwise fashion since the MA/MCEPDA system displays hybrid behavior (between conventional and living free radical polymerization), resulting in initial high molecular weight polymers formed at low RAFT agent concentrations. The chain length dependency of kt in the MA system for chain lengths, i, ranging from 5 to 800 at 80°C may be described by a value for a of 0.36 × 0.05 (where a is the slope of the associated log kt i,i vs log i plot). An alternative RAFT agent, dimethoxycarbonylethyl trithiocarbonate (DMCETC), may not be as ideally applicable to map CLD dependent kt i,i in MA polymerizations. Since the leaving group of both RAFT reagents is identical to the propagating methyl acrylate radical, the addition rate coefficient of the methyl acrylate propagating radicals to the initial and polymeric RAFT agent, kβ, was determined and found to be close to 1.4 × 10 6 L mol-1 s-1 for MCEPDA and 2.1 × 106 L mol-1 s-1 for DMCETC at 60°C. © 2005 American Chemical Society

Consistent experimental and theoretical evidence for long-lived intermediate radicals in living free radical polymerization

The cumyl dithiobenzoate (CDB)-mediated reversible addition fragmentation chain transfer (RAFT) polymerization of styrene at 30°C is studied via both kinetic experiments and high-level ab initio molecular orbital calculations. The kinetic data clearly indicate the delayed onset of steady-state behavior. Such an observation is consistent with the slow fragmentation model for the RAFT process, but cannot be reconciled with the cross-termination model. The comprehensive failure of the cross-termination model is quantitatively demonstrated in a detailed kinetic analysis, in which the independent influences of the pre-equilibria and main equilibria and the possible chain length dependence of cross-termination are fully taken into account. In contrast, the slow fragmentation model can describe the data, provided the main equilibrium has a large fragmentation constant of at least 8.9 × 106 L mol-1. Such a high equilibrium constant (for both equilibria) is consistent with high-level ab initio quantum chemical calculations (K = 7.3 × 106 L mol-1) and thus appears to be physically realistic. Given that the addition rate coefficient for macroradicals to (polymeric) RAFT agent is 4 × 106 L mol-1 s -1, this implies that the lifetime of the RAFT adduct radicals is close to 2.5 s. Since the radical is also kinetically stable to termination, it can thus function as a radical sink in its own right

Living free radical polymerization (RAFT) of dodecyl acrylate: Chain length dependent termination, mid-chain radicals and monomer reaction order

The reversible addition fragmentation chain transfer-chain length dependent-termination (RAFT-CLD-T) methodology was employed to map chain length dependent termination rate coefficient, kti,i, in dodecyl acrylate (DA) free radical polymerization at 60 and 80 °C. The chain length of the propagating DA radicals was controlled by the RAFT agents methoxycarbonylethyl phenyldithioacetate (MCEPDA) and dimethoxycarbonylethyl trithiocarbonate (DMCETC). In addition, the reaction order of the polymerization process with respect to the monomer concentration was determined at both temperatures and found to be close to 1.55 (60 °C) and 1.75 (80 °C), commensurate with the increased presence of mid-chain radicals. A modeling study demonstrates that the obtained data for the reaction order can be transferred to RAFT polymerization systems. The RAFT-CLD-T procedure was modified to account for the determined reaction orders. The obtained chain length dependence of k t in dodecyl acrylate polymerizations is in good agreement with the composite model for chain length dependent termination, showing two distinct regions: For the initial chain-length regime up to a degree of polymerization of 20, kt decreases rapidly with α (in the expression kti,i=kt0·i-α) being close to 1.15 at 80 °C. At chain lengths exceeding 20, the decrease is significantly less pronounced (α close to 0.22 at 80 °C). At 60 °C, the chain length dependence in both regions is somewhat more pronounced. The RAFT agent DMCETC may not be as suited to map out CLD kt values in the DA system, since it induces some limited rate retardation effects. © 2005 Elsevier Ltd. All rights reserved

Erratum: Initiator efficiency of 2,2′-azobis(isobutyronitrile) in bulk dodecyl acrylate free-radical polymerizations over a wide conversion and molecular weight range (Journal Polymer Science Part A : Polymer Chemistry (2004) 42 (5170-5179))

Erratum is free to read on publisher websit