Faster cyclopolymerisation of 4,4-disubstituted 1,7-octadiynes through an enhanced Thorpe–Ingold effect

Abstract

Conjugated polyenes containing cyclohexene moieties were synthesised from 4,4-disubstituted 1,7-octadiyne monomers via diyne cyclopolymerisation. Although 1,7-octadiynes are known to be more challenging monomers in cyclopolymerisation than 1,6-heptadiynes due to slower polymerisation rates, propagation rates could be accelerated by the introduction of dimethyl substitution at the α-position of the side chain as a result of an enhanced Thorpe–Ingold effect. This was confirmed by polymerisation kinetic studies using ¹H-NMR analysis, which showed much faster propagation rates for 1,7-octadiyne monomers containing dimethyl substitution compared to analogous monomers without dimethyl substitution. This new design was applied to polymerisation of various monomers and complete monomer consumption at room temperature was achieved in one hour compared to 24 h (previously reported). More significantly, monomers containing bulky silyl ether groups achieved controlled polymerisation at 5 °C in shorter reaction times, yielding polymers with predictable molecular weights and narrow PDIs (6 h vs. 24 h). Moreover, the introduction of dimethyl substitution to monomers containing an ester side chain increased the reactivity, giving the highest catalyst turn-over numbers (TON up to 240). Lastly, using the more reactive 1,7-octadiyne monomer, a diblock copolymer was successfully synthesised in a much shorter reaction time (2 h vs. 24 h).