Synthesis of hydroxyl-terminated polybutadiene bearing pendant carboxyl groups by combination of anionic polymerization and blue light photocatalytic thiol-ene reaction and its pH-triggered self-assemble behavior

Abstract

Polybutadiene with the simultaneous presence of hydroxyl end-groups and pendant carboxyl side groups was successfully synthesized by combination of living anionic polymerization and blue light photocatalytic thiol-ene “click” reaction. Firstly, hydroxyl-terminated polybutadiene (HTPB) with high content of 1,4-butadiene unit was synthesized by living anionic polymerization using hydroxy-protected initiator, and followed by de-protection. Then, double bonds in backbone of HTPB were modified into pendant carboxyl side groups by blue light photocatalytic thiol-ene “click” reaction using Ru(bpy)3Cl2 and p-toluidine as photoredox catalysts. The blue light photocatalytic thiol-ene reaction exhibit high-efficiency, and all the unsaturated bonds were modified after reacting for 5 h, in spite of that in-chain double bonds in 1,4-butadiene units are much less reactive than vinyl groups in 1,2-butadiene units. When all the double bonds were modified, the structure of resulting polymer can be deemed as carboxyl functionalized hydroxyl-terminated polyethylene. Furthermore, the solution properties of resulting polymer have been investigated by various technologies. The results showed carboxyl functionalized hydroxyl-terminated polyethylene experience a dissolution-assembly-aggregation transition process with pH value of solution decreased from 12.0 to 2.0. The mechanism of this pH-triggered phase transition behavior was supposed to depend on the ionization of carboxyl side groups at varying pH values.