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

Copolymerization propagation rate coefficients, k(p,copo), have been measur ed for the binary systems methyl acrylate (MA)-dodecyl methaerylate (DMA), butyl acrylate (BA)-methyl methaerylate (MMA), dodecyl acrylate (DA)-DMA, a nd DA-MMA at 40 degreesC and 1000 bar by the pulsed laser polymerization (P LP)-size-exclusion chromatography (SEC) technique. These acrylate-methaeryl ate 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, r(i), are determined from monomer fee d compositions and the NMR spectroscopically measured copolymer composition s. The resulting r(i) values for the four acrylate-methacrylate copolymeriz ations agree within experimental accuracy. Moreover, these r(i) data are su rprisingly close to reactivity ratio data estimated from individual additio n rate coefficients to MA and MMA, respectively, of appropriate small (meth )acrylate-type free radicals. Such addition rate coefficients have been det ermined via EPR in liquid solution by the Hanns Fischer group. The terminal model allows for excellent individual fits of composition and of k(p,copo) for each of the four systems. The implicit penultimate unit effect (IPUE) model (and the explicit penultimate unit effect (EPUE) model) are capable o f simultaneously fitting composition and rate data for the MMA-BA and DMA-M A 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 adequa tely described by consideration of only terminal and penultimate units at t he free-radical terminus. On the other hand, ratioing individual propagatio n rate coefficients of free radicals with the same penultimate units seems to eliminate most of the impact of the penultimate units. For this reason t he resulting and widely used "terminal model" reactivity ratios are reasona ble and meaningful kinetic quantities although penultimate effects on the i ndividual propagation rate coefficients undoubtedly operate.