Polymer conformation and NMR chemical shifts, 7. Conformational analysis of random copolymers

A method is developed for the conformational analysis of copolymers with randomly distributed stereo- and comonomer-sequences. The conformational probabilities of dyad sequences constituting a random copolymer chain are calculated as a function of the copolymer composition which is derived from the monomer reactivity ratios and the comonomer feed ratio. The applicability of this method is verified in the conformational analyses of poly(methyl acrylate-co-styrene) and poly(methyl methacrylate-co-styrene). The calculation indicates that the conformational stabilities of dyad sequences are scarcely dependent on the copolymer composition in the former copolymer, whereas they are susceptible to the composition in the latter copolymer. This conformational behavior apparently reflects the geometrical difference between the copolymers: the former is composed of two mono-substituted monomers and the latter of sterically nonequivalent monomers, viz. mono- and di-substituted vinyl compounds. The composition-dependences of ¹³C NMR chemical shifts of these copolymers are interpreted, based on the conformational calculation by this method.