Synthesis and Properties of Star-Shaped Hydroxyl-Terminated Polybutadiene via RAFT Polymerization: A Model for a Binder Prepolymer

Fundamental knowledge on how polymer architecture affects curing and material properties of solid rocket motors (SRMs) using hydroxyl-terminated poly-(butadiene) (HTPB) as a prepolymer has historically been based on studies employing impure samples. Herein, we present the synthesis of highly controlled HTPB via reversible addition-fragmentation chain-transfer (RAFT) polymerization and explore how the hydroxyl group content affects viscosity and pot-life. We further examine the kinetics of curing in order to gain a mechanistic insight. The synthesis of these polymers involved the design and preparation of chain-transfer agents, which allowed for star-shaped polymers via the Z-group approach. We demonstrate that increasing the number of hydroxyl groups serves to decrease the pot-life, despite the fact that network forming reactions (e.g., urethane formation) counterintuitively proceed more slowly, providing insight into the mobility of reactive chain ends during curing reactions relevant to SRM loading. Further, the architecturally pure materials presented here all have longer pot-lives than the commercially obtained HTPB, highlighting the benefit of using more controlled polymers for energetics applications. This represents the first examination of these processes using architecturally pure HTPB, a rare example of homopolymerization of butadiene using RAFT polymerization and a facile approach to more complex structures of poly-(butadiene) than have been reported previously.