De Stefano, Fabio (2023) From polyolefins to functional polymers via innovative coordination catalysts. [Tesi di dottorato]

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Abstract

The introduction of polar or non-polar functional groups into widely used polyolefins can enhance polymer properties and expand the use of polyolefins for advanced, value-added applications. However, efficient direct copolymerization of α-olefin with functional monomers to give functional copolymers with both high molecular weight and tacticity remains a big challenge. The developed research activity was aimed at synthesizing these new classes of polymers (functional and functionalizable polyolefins) with potentially new microstructures and enhanced properties by using innovative catalysts based on different transition metals. All the obtained polymers were carefully characterized with different techniques, in order to study the structure-property relationships. Ethylene, propylene and 5-ethylidene-2-norbornene terpolymers with high ethylene and diene contents were obtained from catalytic terpolymerization promoted by different vanadium based catalysts. The synthesized terpolymers behave as thermoplastic elastomers with enhanced mechanical properties, excellent elastic properties and remelting processability with no failure in properties for recycle use. Chromium complexes were investigated as precatalysts for the polymerization of ethylene using different aluminum cocatalysts and Lewis base NEt3 as additive. We demonstrated that with a simple and cheap additive, NEt3, is possible to produce ultra-high-molecular-weight polyethylene (UHMWPE) that is an engineering and smart polymer used in different application fields. In addition, the synthesis of some poly(cyclic olefin)s and polyesters, obtained with novel chromium complexes, were also explored. An extensive part of the project was devoted to the synthesis and characterization of new stereoregular crystalline polyolefins incorporating functional or functionalizable groups, produced by using an Hf-based catalytic system. In particular, it has been demonstrated that through the incorporation of allyltrimethylsilane and methylene-1,3-cyclopentane units into isotactic polypropylene and isotactic poly(4-methyl-1-pentene), respectively, it is possible to achieve improved mechanical properties of the original homopolymers and to induce the formation of new crystalline modifications under specific conditions, which have never been described in the literature. Furthermore, the effect of the presence of two α,ω-non conjugated dienes (1,5-hexadiene and 1,7-octadiene) on the polymorphic behaviour and on all the other properties of isotactic polybutene (iPB) was investigated. The study revealed that in butene/1,5-hexadiene copolymers the presence of methylene-1,3-cyclopentane units into iPB main chain accelerates the transformation from the metastable form II to the thermodynamically stable form I crystals, whereas in butene/1,7-octadiene copolymers the co-presence of both cyclic and linear co-units significantly stabilize the form II, slowing down the II – I phase transition. Several catalysts based on Nd, Co, Cu and Fe complexes have been used to synthesize poly(isoprene)s with different molecular structures and configurations. The study showed that the mechanical and the physical properties strictly depend on the concentration of different constitutional units. In particular poly(isoprene)s with almost regular cis-1,4 and alternating cis-1,4-alt-3,4 structures are amorphous with low glass transition temperatures (Tg) and mechanical properties characterized by viscous flow. By contrast, poly(isoprene)s with prevalent 3,4 syndiotactic structure are crystalline materials with higher Tgs, better mechanical properties and remarkable elastic behavior. Novel ethylene, CO and norbornene terpolymers were synthesized with two advanced Ni and Pd complexes, obtaining linear functional polyethylenes characterized by physical and mechanical properties that depend on crystallinity, which in turn depends on the concentration of the two comonomers. Finally, we also synthesized polyethylenes with combined in-chain and side-chain functional groups formed by catalytic terpolymerization of ethylene, CO, and methyl-5-norbornene-2-carboxylate achieving interesting polymers characterized by the presence of small amounts of polar co-units that impart photodegradability to the material and do not alter the desirable properties of polyethylene. This study is an example of how different classes of polymers with well controlled molecular structure and completely different properties can be obtained through the rational choice of catalysts and the optimization of polymerization conditions.

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