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

TL;DR

Scientists created a pure version of a polymer used in rocket fuel to better understand how its structure affects performance.

Contribution

First synthesis of architecturally pure HTPB using RAFT polymerization, enabling controlled study of its properties.

Findings

Increasing hydroxyl groups decreases pot-life despite slower network-forming reactions.

Architecturally pure HTPB has longer pot-life than commercial HTPB.

Star-shaped HTPB was synthesized via Z-group RAFT polymerization.

Abstract

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.

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