| Popis: |
Nuclear magnetic resonance (NMR) analysis of the 13C-labeled chain ends of polystyrene, polyMMA, and styrene-MMA copolymers prepared by polymerizations initiated using 13C-labeled-phenacyl radicals were investigated. The phenacyl radicals were generated by anaerobic oxidation of acetophenone-methyl-13C using a Cu(II) octanoate-pyridine complex in the presence of triethylamine and triphenylphosphine. NMR analysis of the 13C-labeled chain ends of these polymers afforded insight into the initiation mechanism. In copolymerization experiments using 13C-labeled acetophenone initiator, the NMR spectra provided evidence that the phenacyl radical reacts 2.7 times faster with styrene than with MMA. The resonances of the labeled phenacyl carbons also showed that the sequence and stereosequence distributions of monomer units at the chain ends are nearly the same as those that prevail along the polymer chains. Styrene–styrene, styrene–MMA, and MMA–styrene enchainments at the chain ends are equally likely to have meso (erythro) or racemic(threo) configurations but the ratio of meso to racemic MMA-MMA enchainments is ∼ 3/7. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2347–2356, 2008 |
Plný text