SP–PLP–EPR Investigations into the Chain‐Length‐Dependent Termination of Methyl Methacrylate Bulk Polymerization

Abstract

Termination kinetics of methyl methacrylate (MMA) bulk polymerization has been studied via the single pulsed laser polymerization-electron paramagnetic resonance method. MMA-d(8) has been investigated to enhance the signal-to-noise quality of microsecond time-resolved measurement of radical concentration. Chain-length-dependent termination rate coefficients of radicals of identical size, k ti,i, are reported for 5-70 °C and up to i = 100. k ti,i decreases according to the power-law expression $k_{\rm t}^{i,i} = k_{\rm t}^{{\rm 1,1}} \cdot i^{ - \alpha }$. At 5 °C, k(t) for two MMA radicals of chain-length unity is k t1,1 = (5.8 ± 1.3) · 10(8) L · mol(-1) · s(-1) . The associated activation energy and power-law exponent are: E(A) (k t1,1) ≈ 9 ± 2 kJ · mol(-1) and α ≈ 0.63 ± 0.15, respectively.

Chain‐Length‐Dependent Termination in Radical Polymerization of Acrylates

“…[29,30] Finally, there are the additional facts that measured values of k t 1,1 are in accord with small-molecule measurements [27] and are in broad quantitative agreement with Equation (2). [29] All these findings give confidence that k t 1,1 can now be accurately measured and that for many systems these values can largely be understood through the simple framework of the Smoluchowski and Stokes-Einstein equations.…”

“…[30,45] This suggests that any of the peculiarities of RAFT polymerization -such as cross-termination of the RAFT intermediate or variation of k t i,i with conversion -appear to have no significant effect on determination of k t i,i…”

“…For methacrylate polymerizations, representative α s values are 0.63 [30] and 0.65 [45] for MMA (obtained via SP PLP EPR and RAFT CLDT respectively), 0.65 for BMA, [36] 0.50 for benzyl methacrylate [37] and 0.64 for dodecyl methacrylate (DMA). [37] All of these values are remarkably close to those quoted above from diffusion studies, noting that benzyl methacrylate, with its cyclic pendant group, should be like styrene.…”

“…In earlier times it was the same story with propagation rate coefficients, k p : progress in unraveling the cloudy picture at higher temperatures was only made once k p for SPRs was unambiguously measured at the same low temperatures as employed here. [6] The second aspect of the counter argument is that in our earlier study of BA termination in toluene using SP PLP EPR it was found that α s , α l and i c are relatively constant with temperature, [13,29,30,36] and thus the values found at low temperature serve as reasonable estimates for higher temperatures. It was further found that E a (k t 1,1 ) ≈ E a (η -1 ) for BA SPRs, [13] which means that values of k t 1,1 measured at low temperatures may easily be extrapolated to higher temperatures: all one needs to know is the temperature dependence of the fluidity of the reaction medium, which may easily be measured if it is not available in the literature.…”

Termination Rate Coefficients for Radical Homopolymerization of Methyl Methacrylate and Styrene at Low Conversion

“…[6] Briefly, for MMA it has been found that α S • 0.65, [17][18][19] this parameter giving the variation of k t i,i with i for small ('S') chains. The crossover ('c') to long-chain behavior occurs at chain length i c • 100.…”