Tetrahydrofuran-propylene oxide copolymers: 1. End-group analysis by 13C n.m.r. and the mechanistic inferences

Abstract

The boron trifluoride catalysed copolymerization of tetrahydrofuran and propylene oxide in the presence of 1,4-butanediol leads to linear, difunctional copolymers. The 13C n.m.r. spectra of these copolymers are exceptionally informative in that the compositions, the molecular weights and the types of end-groups can all be obtained independently from both regions of the spectrum. The types of end-groups were identified by an off-resonance and a gated decoupling technique. No tetrahydrofuran end-groups were found. Only propylene oxide end-groups with a 2:1 ratio of secondary alcohol to primary alcohol were observed, regardless of the composition or the molecular weight of the polymer. The propylene oxide end-groups of the copolymer are considered to be formed by initial propagation reactions. These reactions are the attack of either monomer on the protonated propylene oxide to open the oxirane ring and form a cyclic oxonium ion. Attack on protonated tetrahydrofuran is thought not to lead to initiation. 1,4-Butanediol serves as a chain transfer agent and thereby regulates the molecular weight of the copolymer. Polymerization stops when all propylene oxide has been consumed because no new chains can be initiated by the remaining tetrahydrofuran.