Polvethylene-polysiloxane Graft Copolymers Prepared Via Coordination Polymerization:Structure And Properties

As an important commodity polymer with excellent properties,polyethylene(PE)has been found versatile applications varying from grocery bags to bulletproof vests.Despite the superior commercial importance and technological potential,the nonpolar character of PE results in its poor performance in dyeability,printability,and adhesion,consequently limiting its applications in many high-end fields.The functionalization of PE has attracted great interest in both academia and industry.On the other hand,due to the high backbone flexibility,low surface tension,good heat resistance and excellent biocompatibility,polysiloxanes are recognized as a promising material in applications such as aerospace materials and elastomers for automotive fuel systems.But the unsatisfactory mechanical properties and high cost remain to be improved.In principle,the combination of polyolefins with polysiloxanes is of great potential to create superior materials with advantages of both sides.This work designed and synthesized a series of novel polysiloxane macromonomers via hydrosilylation and anionic ring-opening polymerization.Catalyzed by vanadium catalyst,direct copolymerizations of ethylene and polysiloxane macromonomers were carried out to prepare well-defined polyethylene-g-polysiloxane copolymers,including polyethylene-g-polydimethylsiloxane copolymers(PE-g-PDMS),polyethylene-g-polymethyltrifluoropropylsiloxane copolymers(PE-g-PMTFPS)and polyethylene-g-polymethylphenylsiloxane copolymers(PE-g-PMPS).These groups of polysiloxane-functionalized polyethylene copolymers could be used to effectively improve the melt flowing property and surface property of HDPE.Then further studies were conducted to clarify the effects of reaction conditions on the copolymerization reactions and the resulting copolymer microstructures.Also,polysiloxanes were introduced into ethylene-norbornene copolymers,yielding a series of novel polyolefin-polysiloxane hybridized materials with mechanical properties varying from plastics to elastomers.Finally,new crosslinkable copolymers were synthesized via direct copolymerization of ethylene and alkoxylsilane-functionalized norbornene.The major achievements of this thesis include:(1)The anionic ring-opening polymerizations of various cyclotrisiloxanes were performed in high yields using synthesized chlorosilanes as the capping agent to prepare polysiloxane macromonomers.Excellent control of the molecular weights of synthesized macromonomers could be achieved by adjustment of the feed ratio between cyclosiloxanes and the initiator.The obtained polysiloxane macromonomers were characterized with narrow molecular weight distribution and well-defined microstructure.(2)A facile approach for the efficient and controllable synthesis of well-defined PE-g-polysiloxane copolymers that consist of a PE backbone and polysiloxane side chains was reported for the first time.The copolymerizations of polysiloxane macromonomers with ethylene showed excellent catalytic activities up to 1.78×107 g/(mol·h)and polysiloxane macromonomers could be efficiently introduced into obtained copolymers.All copolymers exhibited narrow molecular weight distribution and homogeneous chemical composition distribution.The chain structure and crystallization behavior of copolymers were characterized by DSC and SSA,and a clear linear relationship between macromonomer feed-macromonomer incorporation-melting temperature of copolymers was established.The incorporation of PMPS significantly enhanced the thermal stability of resultant copolymers.(3)Systematic researches were carried out to determine the relationships between the microstructures of resulting PE-g-PMTFPS copolymers and reaction conditions,paramount of which were the initial feed of PMTFPS macromonomers,the molecular weight of PMTFPS macromonomers and the copolymerization temperature.A comprehensive database was established for the tailor-made synthesis of PE-g-PMTFPS copolymers with tunable microstructures and properties.The incorporation of polysiloxane macromonomers and the consequent branch density of graft copolymers could be precisely adjusted by tuning macromonomer feed.The length of polysiloxane side chains was determined by the molecular weight of macromonomers.The variation of reaction temperature was an effective tool to regulate PE backbone length.(4)A new type of norbornene-terminated polysiloxane macromonomer was synthesized.Its copolymerizations with ethylene showed extraordinary catalytic activities(up to 2.90×107 g/(mol·h)),which realized the efficient synthesis of polysiloxane-functionalized ethylene-norbornene copolymers.With the increase in macromonomer feed,the macromonomer incorporation increased from 2.8 mol%to 8.6 mol%,the weight average molecular weight of resultant copolymers decreased from 59.6 kg/mol to 46.9 kg/mol,and the molecular weight distribution decreased from 2.3 to 2.2.The crystallinity of these novel PE-polysiloxane hybridized materials could be altered by changing the macromonomer incorporation,which consequently adjusted the mechanical properties of resultant copolymers from plastic to elastomer.The copolymer with macromonomer incorporation of 5.7 mol%and weight-average molecular weight of 314 kg/mol was characterized with good elastic recovery performance and elongation at break(748%).(5)Investigations on the modification effect of these PE-g-polysiloxane copolymers on HDPE at a dosage of 2 wt%showed a remarkable improvement on the melt flowing property.The elasticity modulus and tensile strength decreased slightly,while the elongation at break increased.The increasing water-contact angles indicated that the hydrophobicity of HDPE blends was enhanced,owing to the silicon enrichment on the surface.