Crystallization during Polymerization: A Route to Low-Entangled Solid-State Processable Ultra-High-Molecular-Weight trans -1,4-Polybutadiene

Abstract Trans-1,4-polybutadiene (TPBD) is a material of commercial interest whose properties strongly depend on its trans isomer content. The polymer exhibits distinctive features, including a highly linear backbone without pendant groups, excellent dynamic performance, low rolling resistance, low heat buildup, good strength, outstanding fatigue resistance, and high abrasion resistance. TPBD also shows a relatively high melting temperature (∼145 °C). Notably, it exhibits unique polymorphism, with a thermodynamically reversible monoclinic to high entropy hexagonal phase transition at atmospheric pressure. However, because of the unsaturated bonds along the main chain, the material faces challenges in melt processing including uniaxial chain-orientation needed for fiber development. To overcome the fundamental challenge of thermal cross-linking and entropy driven relaxation of the oriented chains in melt processing, here, we investigate the strong influence of polymerization conditions that allow solid-state processing of the nascent polymer without melting. The paper addresses the strong influence of polymerization medium, while using a single-site catalytic system, in the synthesized polymer. The findings indicate that in comparison to the conventionally used toluene, when heptane is used as a polymerization medium, the growing chains crystallize rapidly during the synthesis, leading to the formation of thinner crystals. Compared to toluene, the polymer synthesized in heptane shows higher trans content, higher molecular weight, higher melting temperature, and thermal stability. Fascinatingly, by reducing the catalyst amount and increasing the reaction time in heptane, TPBD having a molecular weight exceeding 2000 kg mol–1, with a distinct platelet-like morphology, is observed by SEM. Thus, the synthesized nascent polymer exhibits the properties of the low-entanglement state that can be sintered and drawn in the solid state to a draw ratio of λ ∼ 9, exceeding the previously reported drawability of λ < 2 in the melt-crystallized TPBD. The uniaxial drawn tapes exhibit a higher melting temperature of 151 °C, an average Young’s modulus of ∼9 ± 2.16 GPa, an average tensile strength of 353 ± 18.15 MPa, and an average engineering toughness of 8035 ± 2033.12 kJ m–3.