High-Temperature Cyclic Olefin Copolymers via Thiophene-Scandium Catalysis

Cyclic olefin copolymers (COCs) have excellent optical and mechanical properties which make them attractive to use in various high-performance applications such as optical lenses, medical devices, and packaging materials. COCs are primarily synthesized using copolymerization process of ethylene with cyclic olefin monomers such as norbornene, dicyclopentadiene (DCPD). These materials have been used by chemical and materials engineers because they are highly transparent with low birefringence, low water absorption as well as impressive heat resistance. However, despite all the efforts and advancements in COC synthesis, there are still significant challenges and desire to improve their thermal properties specifically enhance the glass transition temperature (Tg) which currently limits their use in application that require higher operating temperatures. One of the most promising approaches that has been tried before is to increase the Tg of COCs by the incorporation of bulkier cyclic olefin monomers which can increase the rigidity of the polymer chains. Tricyclopentadiene (TCPD) which is considered a bulky cyclic olefin is an attractive candidate for this purpose. TCPD possesses two types of vinylidene groups and has the advantage of being derived from the readily available and inexpensive DCPD and this make it excellent potential for selective copolymerization. However, the copolymerization of TCPD with ethylene is challenging because it lacks of suitable catalysts that can effectively mediate this reaction. Moreover, traditional catalysts used in polyolefin industries including cyclopentadienyl-based titanium complexes often lead to undesirable side reactions such as crosslinking which makes the produced polymers unsuitable for high-performance applications.

To this account, recent study published in Polymer Chemistry Journal and conducted by Lingyan Huang, Shunan Zhang, Chunji Wu, Pan He & led by Professor Baoli Wang from the State Key Laboratory of Polymer Physics and Chemistry at Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, the researchers developed new and innovative catalytic system of thiophene-fused-heterocyclic cyclopentadienyl scandium complexes as superior catalysts for the copolymerization of TCPD and ethylene. They initially hypothesized that by carefully tuning the catalyst structure and polymerization conditions, they could achieve controlled copolymerization, leading to TCPD/ethylene copolymers with improved Tg values and enhanced thermal stability. Furthermore, they tested the effect of the TCPD distribution along the polymer chain as well as the potential for post-polymerization modification such as hydrogenation to improve even more the material properties.

To begin, the team synthesized a variety of scandium-based catalysts, each with slightly different structural features and tested their ability to catalyze the copolymerization of TCPD and ethylene. The catalysts were highly effective in promoting the polymerization, with high activity under optimal conditions. This high level of activity enabled the researchers to achieve a wide range of TCPD incorporations from 13.0% to 43.8% by simply varying the catalyst structure and polymerization conditions. They found as expected, higher incorporations of TCPD led to significant increases in the Tg of the resulting copolymers. For example, the copolymer with 43.8% TCPD incorporation exhibited a Tg as high as 252 °C, while those with lower TCPD content had proportionally lower Tgs, some as low as 50 °C. We believe this excellent correlation between TCPD content and Tg was a key finding by the authors and highlights the ability of the bulky TCPD units to increase the rigidity of the polymer chain and consequently its thermal properties. Another important discovery in their experimental studies was the observation that the distribution of TCPD units along the polymer chain also played a significant role in determining the Tg of the copolymers. They analyzed the microstructure of the copolymers using techniques such as ¹H–¹³C HSQC NMR and found that copolymers with a higher proportion of alternating {–TCPD–E–} units, rather than isolated {–E–(TCPD)–E–} units, had significantly higher Tgs even at similar TCPD incorporation levels. We believe this is an important discovery by the authors because it highlighted the importance of controlling not only the quantity of TCPD incorporated but also its precise arrangement within the polymer chain to maximize the material’s thermal stability. In addition, the researchers investigated the effect of post-polymerization hydrogenation on the properties of the copolymers. To do this, they subjected the TCPD/ethylene copolymers to hydrogenation using nickel acetylacetonate [Ni(acac)₂] as a catalyst under mild conditions and noticed that the hydrogenation process successfully saturated the double bonds in the copolymers without causing degradation or crosslinking and that was evidenced by the unchanged molecular weight distributions before and after hydrogenation. These results in Polymer Chemistry Journal demonstrated that hydrogenation could be used to improve the chemical stability of the copolymers without significantly compromising their thermal properties. Moreover, the researchers examined the optical properties of the TCPD/ethylene copolymers and their hydrogenated counterparts. They produced thin films of the copolymers and measured their transmittance in the visible light range. The results were impressive: all the copolymers, including the hydrogenated versions, exhibited high transparency, with transmittance values exceeding 89% across the visible spectrum. This characteristic, combined with the high Tg, makes the materials highly suitable for optical applications where clarity and thermal stability are essential, such as in the production of lenses for cameras or medical devices.

In conclusion, Professor Baoli Wang and his colleagues successfully designed thiophene-fused-heterocyclic cyclopentadienyl scandium complexes as catalysts and overcome the long-standing challenge of copolymerizing bulky cyclic olefins like TCPD with ethylene. The resulting copolymers achieved significantly higher Tg, reaching up to 252 °C, and this is critical for expanding the applicability of these materials in industries that demand heat-resistant polymers, such as electronics, optics, and medical devices. Because many commercial polymers currently in use have limited Tg which restrict their functionality in environments that require high thermal stability. However, we believe the innovative approach reported by Professor Baoli Wang offers a practical solution to this limitation. The ability to precisely control the structure and distribution of TCPD units within the polymer chain further enhances the thermal properties without compromising the optical clarity or mechanical integrity of the material is another finding that will open the door for new research in this field. Additionally, the successful hydrogenation of the TCPD/ethylene copolymers without significant loss in Tg indicates the possibility of modifying these materials post-polymerization to improve chemical stability and resistance to environmental degradation. Such high transparency of these copolymers, coupled with their thermal properties, suggests potential applications in the manufacture of advanced optical devices, including lenses and medical equipment that require both clarity and heat resistance.