Chain‐Length‐Dependent Termination in Radical Polymerization of Acrylates

Abstract

SUMMARY:The technique of SP PLP EPR, which is single-pulse pulsed-laser polymerization (SP PLP) in conjunction with electron paramagnetic resonance (EPR) spectroscopy, is used to carry out a detailed investigation of secondary (chain-end) radical termination of acrylates. Measurements are performed on methyl acrylate, n-butyl acrylate and dodecyl acrylate in bulk and in toluene solution at -40 °C. The reason for the low temperature is to avoid formation of mid-chain radicals, a complicating factor that has imparted ambiguity to the results of previous studies of this nature. Consistent with these previous studies, composite-model behavior for chain-length-dependent termination rate, is found in this work. However, lower and more reasonable values of α s , the exponent for variation of k t i,i at short chain lengths, are found in the present study.

Chain‐Length‐Dependent Termination of Vinyl Acetate and Vinyl Pivalate Bulk Homopolymerizations Studied by SP‐PLP‐EPR

“…The exponent α l determined for −4 °C should however be valid also for other polymerization temperatures. [ 15,20,27 ] Our α l is in full agreement with the theoretically predicted α l = 0.16 for long chain-end (SPR) radicals. [ 22,23 ] For the chain-length regime between 18 and 1780, Theis et al deduced a lower power-law exponent of α l = (0.09 ± 0.05) via the RAFT-CLDT technique for low degrees of monomer conversion (10 %).…”

“…Above i c , termination is assigned to segmental diffusion of two entangled chains. Depending on the structure of the macroradicals (random coil vs rod-like coil), α s values are expected to be in the range 0.5 up to 1.0, [18][19][20][21] whereas theory of coil dynamics predicts α l of two large macroradicals with radical functionality at the chain end to be 0.16. [22][23][24] As the short-chain regime is correlated with center-of-mass diffusion, k t 1,1 may be discussed in terms of the Smoluchowski equation (Equation 4 ), which correlates k t 1,1 to the self-diffusion coeffi cient, D , the capture radius for monomeric radicals, R c , and to the spin factor, P spin .…”

“…The initiator MMMP has proven to be an almost perfect photoinitiator for polymerization of acrylates, methacrylates, and itaconates [ 10,11 ] and thus is used as the standard SP-PLP photoinitiator for polymerizations in organic phase. [ 20,27,[31][32][33] To our surprise, MMMPinduced PLP of VAc resulted in EPR spectra, which were not easily understood (see Figure 1 ).…”

“…As checked gravimetrically, monomer-to-polymer account. [ 13,14,17,20 ] The time required for a single propagation step, t p , is given by ( k p c M ) −1 . The densities required for c M determination were taken from literature.…”

“…The crossover chain length for VAc appears to be somewhat below the one for the structural isomer methyl acrylate, i c (MA) = (35 ± 10). [ 20 ] Both values are far below the crossover chain length determined for methyl methacrylate: i c (MMA) = (100 ± 10). [ 55 ] Large i c has been associated with poor chain fl exibility.…”