Copolymers derived from 7-acryloyloxy-4-methylcoumarin and acrylates: 1. Copolymerizability and photocrosslinking behaviours

Photoreactive copolymers containing 4-methylcoumarin pendant groups were prepared by radical copolymerization of 7-acryloyloxy-4-methylcoumarin (M1) with acrylates [M2 = methyl methacrylate (MMA), methyl acrylate (MA), ethyl acrylate (EA) and butyl acrylate (BA)] in dimethyl formamide (DMF) at 65°C using azobisisobutyronitrile as initiator. The monomer reactivity ratios (MRRs) of M1 and M2 were estimated by the Fineman—Ross and Kelen—Tüdos methods, by analysing copolymer compositions which were determined by 1H nuclear magnetic resonance (n.m.r.) spectra. It was found that the MRRs are r1 = 0.45, r2 = 1.68 for M1 MMA; r1 = 2.16, r2 = 0.48 for M1-MA; r1 = 2.04, r2 = 0.42 for M1-EA; r1 = 1.82, r2 = 0.53 for M1-BA, respectively. The order of monomer reactivity estimated from 1/r1 values is MMA > M1 > BA > EA > MA. The higher reactivity of MMA can be attributed to its extra hyperconjugating stabilization by methyl groups in addition to CO conjugation. Furthermore, from 13C n.m.r. spectra, it was found that π-electron density of the β-carbon is in the order of MA > EA > BA > M1. The lower electron density of β-carbons in a monomer leads to easier electron transfer from propagating radical to the monomer and consequently a higher reactivity. Photocrosslinking (300 nm) of the copolymers in the film state were investigated by their characteristic curves (WW0 versus time plot).