Synthesis of novel gradient copolymers via atom transfer radical polymerization

Synthesis of novel copolymers with gradient of composition along the chain, gradient copolymers, is reported. Two methods allowing for preparation of gradient copolymers were developed: spontaneous gradient formation via simultaneous copolymerization and forced gradient formation via copolymerization with addition of comonomer. Spontaneous gradient formation takes place during a simultaneous copolymerization of monomers with reactivity ratios such that {dollar}rm rsb1rsb2approx1 and rsb1>rsb2.{dollar} Copolymerization with addition of comonomer is applicable toward preparation of gradient copolymers of various monomers irrespective of their reactivity ratios.; Gradient copolymers can be prepared only under controlled free radical polymerization conditions. The controlled free-radical polymerizations investigated in this project for their suitability towards preparation of gradient copolymers include Atom Transfer Radical Polymerizations (ATRP), TEMPO-mediated polymerizations, polymerizations in the presence of alkyl phosphites and polymerization initiated by (i-Bu){dollar}sb3{dollar}Al/TEMPO/2,2{dollar}spprime{dollar}-bipyridine. Only ATRP provided well defined gradient copolymers with composition gradually changing with the chain length, predicted molecular weights and narrow molecular weight distributions. The following monomers were copolymerized to give gradient copolymers: methyl acrylate and methyl methacrylate, methyl acrylate and styrene, butyl acrylate and styrene, acrylonitrile and styrene. The gradient copolymers obtained by ATRP were well defined materials with molecular weights up to {dollar}rm Msb{lcub}n{rcub}=30,000{dollar} and molecular weight distributions {dollar}rm Msb{lcub}w{rcub}/Msb{lcub}n{rcub}<1.2{dollar} on average. The shape of the gradient of composition was determined from the evolution of instantaneous copolymer composition with chain length. Preliminary characterization of physical properties of these gradient copolymers by DSC, DMA and SAXS indicated broad glass transitions and solid-phase separation with transition temperatures and morphologies of the phase-separated system dependent on the shape of the gradient.