Influence of mono-lithium based initiators with different steric volumes on 1,4 unit content of hydroxyl terminated polybutadiene using anionic polymerization

Abstract

In order to study the influence of steric structural factors of mono-lithium based anionic initiators on the 1,4 unit content of hydroxyl terminated polybutadiene (HTPB), a series of precursors with different steric volumes were designed and synthesized. The precursors were allowed to react with lithium in cyclohexane at 60 °C to prepare mono-lithium based initiators. The degree of lithium reaction in initiators was characterized by an indirect method in which a certain amount of highly reactive methanol was added into the mono-lithium based initiator solution, and then the highly reactive initiator was replaced by a hydrocarbon molecule to monitor the reaction process using NMR. By using the initiators prepared, HTPB was synthesized via anionic polymerization of 1,3-butadiene in cyclohexane. The structure and 1,4 unit content were characterized by SEC-MALLS and ¹H NMR. It was found that the content of the 1,4 unit in HTPB was markedly increased with an increase in the initiator's steric volume. It was also found that much higher relative 1,4 unit content in HTPB was achieved when using an initiator with a higher steric volume, compared to the initiators with lower steric volume. In addition, both 1,4 unit content and molecular weight, as well as the molecular weight distribution of HTPB, can be coordinately controlled by the steric structure of the initiators during anionic polymerization.