Synthesis and characterization of linear and star-branched butadiene-isoprene block copolymers and their hydrogenated derivatives
Wood, Paul A. III
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The principal purpose of this investigation was to synthesize and hydrogenate well-characterized linear and star-branched block copolymers based on butadiene and isoprene. Sequential anionic addition techniques initiated by homogeneous organolithium species in hydrocarbon solvents were employed to prepare several series of butadiene-isoprene copolymers varying in block size and architecture. Linear Aâ Bâ A poly(butadieneâ isopreneâ butadiene) triblock copolymers were synthesized by two different living addition techniques, e.g., three-stage process using a monofunctional anionic initiator and a two-stage process using a difunctional anionic initiator. Alternatively, the synthesis of star block copolymers involved the sequential polymerization of poly(butadiene-isoprene) diblock arms which were then linked into stars via divinylbenzene. Hydrogenation of unsaturated polymers has widely attracted attention since this provides an alternate method for improving and optimizing the mechanical, thermal, oxidative and chemical resistance properties of these technological important materials. Homogeneous catalytic hydrogenation was employed to chemically modify these linear and star-branched copolymer into thermoplastic elastomers. Hydrogenation successfully converted the soft polybutadiene blocks into hard semicrystalline polyethylene segments, while the central polyisoprene blocks resulted in the formation of amorphous alternating rubbery copolymers of propylene-ethylene. The hard semicrystalline blocks form morphological domains that serve as physical crosslinking and reinforcement sites. The presence of semicrystalline segments in both the linear and starâ branched copolymers has important significance for processing. Above the endothermic melting temperature of the semicrystalline end. blocks, the now amorphous system can approach the melt behavior of a singlephase melt, that is, displaying negligible "physical" network structure in the melt. Overall, these systems display a valuable combination of good melt processability together with physical properties characteristic of A-Bâ A architectures.
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