Study On Cationic Ring-Opening Polymerization Of Tetrahvdrofuran Initiated By Phosphotungstic Acid

The polytetrahydrofuran (PTHF) is an important chemical material of elastomeric polyurethanes and spandex fibre, which is widely used in automotive industry, medical apparatus, textile industry and so on. Aiming at solving the problems of waste liquid discharge, equipment corrosion and difficult recycling of initiators, solid acid becomes a research hot spot at the present time. In this article, the THF polymerization was initiated by phosphotungstic heteropolyacid (HPW) using epichlorohydrin (ECH) as promoter. In-situ FT-IR was first applied for monitoring of the THF polymerization process, which provided full insight into the reaction kinetics. And effects of different factors on THF polymerization were systematically investigated. The influence of ethylene glycol as chain transfer agent on THF polymerization and the molecular weight (Mn) of PTHF product were emphatically studied. Furthermore, HPW was supported on the mesoporous SBA-15for heterogeneous catalysis in THF polymerization. It contains following parts:(1) The polymerization of THF carried out in CH2Cl2was monitored by in-situ mid-infrared spectroscopy system (ReactIR). The changes in absorbance intensity of the two characteristic peaks at1068and1109cm-1were used for monitoring the monomer and polymer, respectively. The experimental results demonstrated that the increase of apparent first-order rate constant kapp (kapp=kp×[P*]) for propagation is disproportional to the increase of HPW concentration and the THF polymerization reaction proved to be typically first-order reversible reaction. Thermodynamic parameters are determined to be△Hp=-3.9kcal·moF-1,△SP=-15.3cal·mol-1·K-1, Tc=155K from the temperature dependence of the monomer equilibrium concentration [M]e. And the ceiling temperature at standard state is determined to be155K. Also the apparent activation energy is determined to be Ea=44.53kJ·mol-1from the temperature dependence of kapp. Besides, it is confirmed that more chain-transfer occurred at higher temperatures (15or25℃), leading to a deviation from first-order propagation at the later stage of polymerization.(2) Considering that various reaction conditions have great influence on THF polymerization reaction and the Mn of PTHF, effects of various factors (polymerization temperature and time, content of ECH, HPW and EG) on the yield and Mn of PTHF were systematically investigated based on a L16(45) orthogonal experimental design using ethylene glycol (EG) as chain transfer agent. It was found that the initiator and promoter have synergistic effects on THF polymerization. The addition of EG as chain transfer agent in THF polymerization can effectively control the chain propagation and depress the back-biting reaction. In the presence of EG, cyclic fraction obviously diminishes in the resulted PTHF and its Mn decreases from12000～23000to1100～1700. Also PTHF’s MWD is significantly narrowed changing from unimodal distribution into single peak distribution. Moreover PTHF’s yield increases from50%to80%because of its higher mass transfer efficiency. It was also found that a trace amount of EG (mEG/mTHF=1.25×10-2～2.51×10-2) will slightly promote polymerization, while a large amount of EG (wEG//wTHF=3.76×10-2～7.52×10-2) will restrain polymerization, resulting in higher [M]e of THF.(3) In order to support HPW on the porous materials with larger pore size for THF polymerization, mesoporous silica SBA-15with tunable pore size (6-30nm) was synthesized through simple control of synthetic variables (/wPX/mP123, hydrothermal crystallization temperature and the addition of NH4F) using p-xylene (PX) as micelle expander and triblock copolymer as template agent. The results demonstrated that the increase of/mPX/wP123will expand the pore size and reduce the order degree of SBA-15. It suggested that the higher hydrothermal crystallization temperature and the addition of NH4F will lead to more efficient swelling of PX and subsequent pore expansion.(4) The initiator prepared by supporting HPW on mesoporous SBA-15was applied in THF polymerization. Effects of mass fraction of HPW (wHpw), mass ratio of APTES to SBA-15(/mAPTES/mSBA-15), the pore size and specific surface area of SBA-15on the performance of initiators and the conversion of THF were investigated. It was found that the supported initiator HPW/SBA-15prepared by impregnation with40%mass fraction of HPW exhibits highest activity. The amino-functionalized SBA-15can effectively improve the performance of HPW/SBA-15-NH2for repeated use in THF polymerization. And the initiator HPW/SBA-15-NH2has highest activity when the mass ratio of mAPTES/mSBA-15is0.0712. The initiator shows good performance in THF polymerization. PTHF product has higher molecular weight when SBA-15has higher specific surface area (853m2·g-1) or larger pore size (lower specific surface area,498～585m2·g-1). The performance of initiator and the Mn of PTHF increase with the pore size of SBA-15.