Controllable Cationic Polymerization Of Olefin Monomers In Nano-confined Space

Cationic polymerization is characterized by rapid initiation,rapid growth,easy transfer and difficult termination.Lewis acids are commonly used as catalysts,such as SnCl₄,AlCl₃and TiCl₄.Because the active center of cationic polymerization reaction is carbocation,it is very active and easy to transfer to monomers and solvents.Therefore,cationic polymerization is generally carried out at the low temperature（-100℃）to inhibit the chain transfer reaction and increase the molecular weight of the polymer,which also increases the economic cost of industrial production and the complexity of the production process.How to obtain polymer with high molecular weight by cationic polymerization at the room temperature is a difficult problem in the field of polymer synthesis.Silicon-based mesoporous materials have the characteristics of high porosity,narrow pore size distribution and orderly arrangement.In this work,we developed a cationic polymerization method in nano-confined space,by using the confinement effect of silicon-based nano-pores to stabilize carbocation and inhibit the chain transfer reaction of cationic polymerization.A series of supported cationic polymerization catalysts were prepared with silica-based mesoporous SBA-15,MCM-41 and SBA-16 as carrier materials.These catalysts were obtained by the reaction of silanol（Si-OH）groups on the inner surface of nano-pores with Lewis acids such as SnCl₄,AlCl₃ and TiCl₄.We studied the polymerization behavior of these Lewis acids loaded in nano-channels on various vinyl ether monomers.The main results are as follows:（1）Supported catalyst SnCl₄@SBA-15 was prepared by modifying silicon-based mesoporous materials with SnCl₄.It was confirmed that SnCl₄ had been successfully loaded into the pores by nitrogen adsorption-desorption test（BET）,inductively coupled plasma test（ICP）,scanning electron microscope（SEM）and other characterization means.At the room temperature,SnCl₄@SBA-15 can catalyze the polymerization of monomers such as n-propyl vinyl ether and ethyl vinyl ether in high activity.The catalytic system has good controllability for cationic polymerization.The molecular weight of the polymer increases gradually with the increase of monomer/catalyst ratio,up to 126.5 kg/mol.More interestingly,SnCl₄@SBA-15can be recycled as a heterogeneous catalyst when catalyzing the polymerization of vinyl ether.（2）AlCl₃ functionalized silica-based mesoporous materials AlCl₃@SBA-15 and AlCl₃@MCM-41 were prepared.BET,ICP,SEM and other tests confirmed that AlCl₃ was successfully loaded into the pores of two kinds of silicon-based mesoporous materials.The basic morphology and mesoporous configuration of the original silicon-based mesoporous materials were remaining.The supported catalysts AlCl₃@SBA-15 and AlCl₃@MCM-41 can successfully catalyze the polymerization of isobutyl vinyl ether,ethyl vinyl ether,benzyl vinyl ether and other monomers.The polymerization reaction has good controllability.The highest molecular weight of the obtained polymer can reach to 86.0 kg/mol.（3）TiCl₄ functionalized silicon-based mesoporous materials TiCl₄@SBA-15,TiCl₄@MCM-41 and TiCl₄@SBA-16 were successfully prepared.All three supported catalysts can catalyze the polymerization of isobutyl vinyl ether.TiCl₄@SBA-15 catalytic system has the highest polymerization activity and the largest molecular weight of the polymer prepared.More importantly,the molecular weight of poly（isobutyl vinyl ether）obtained by the supported type TiCl₄@SBA-15 is 5 times that of the unsupported TiCl₄catalyst under the same conditions.It shows that the nano-confined space provided by the designed and synthesized supported catalyst can well inhibit the chain transfer reaction of cationic polymerization,and then improve the molecular weight of the polymer.This provides a new idea for controllable cationic polymerization at the room temperature.