| Carbocationic polymerizations of isobutylene (IB) or/and styrene (St) with initiating system of protonic initiator H2O in conjunction with coinitiator TiCl4were investigated in the presence of nucleophiles (Nu). The controlled carbocationic polymerization of IB or St was achieved under proper conditions by investigating the effects of the concentrations of H2O, TiCl4, Nu and monomer, solvent polarity and temperature on the polymerization of IB or St. The block copolymerizations of IB and St involving the crossover of IB from PS chains and the crossover of St from PIB chains were investigated based on the homopolymerization of IB or St and diblock copolymer of poly(styrene-b-isobutylene)(PS-PIB) or poly(isobutylene-b-styrene)(PIB-PS) were prepared. Then, triblock copolymers of poly(styrene-b-isobutylene-b-styrene)(PS-PIB-PS) with different molecular weight were synthesized via three-step sequential carbocationic block copolymerization and characterized by GPC, UV,’H-NMR,13C-NMR, DSC, XPS and TEM respectively.The experimental results show that the fast propagation through free ions with H2O/TiCl4initiating system which led to PIB with high molecular weight and broad molecular weight distribution was suppressed whereas slow propagation through carbenium ion pairs was promoted in the presence of some nucleophiles, such as acetone (MMK), N,N-dimethylacetamide (DMA), pyridine (Py) and triethylamine (TEA). In the presence of sufficient nucleophile propagation proceeded mostly through monomer addition to the active carbenium ion pairs and the active species became dormant through reversible termination which based on the rapid equilibration between small amounts of active carbenium ion pairs and large amounts of dormant covalent species. The polymerization therefore decelerated and molecular weight of polymer increased with conversion together with relatively narrow molecular weight distribution. TiCl4concentration strongly influenced the equilibration between active and dormant species. With the increase in TiCl4concentration the exchange between dormant and active species became rapid and therefore the propagation through ion-paired species accelerated. Simultaneously increasing the concentrations of TiCl4and nucleophile was benefit for suppressing the propagation through free ions and chain transfer reaction on the one hand and favorite for accelerating the propagation through carbenium ion pairs on the other hand.With the increase in solvent polarity the ion-paired species were solvated greatly, which led to rapid propagation and side reaction of chain transfer. The propagation through ion-paired species decreased due to slow the exchange between dormant and active species and chain transfer reaction was induced due to the less stability of active carbenium ion pairs at relatively high temperature, which resulted in increasing content of olefin PIB in products. The chain transfer reaction became serious when monomer concentration was higher than3.0mol/L, which was evidenced from the apparent low molecular weight tail in GPC trace. The constants of chain transfer to monomer in the presence of MMK, DMA, Py and TEA were determined to be7.7×10-4、5.5×10-4、5.7×10-4and6.6×10-4respectively in monomer concentration range of1.0～2.5mol/L under conditions where [TiCl4]0=0.08mol/L and [Nu]0=0.0064mol/L at-60℃in CH2Cl2/n-Hex(50/50,v/v) solvent mixture. PIB chains completely terminated with tertiary chloride groups could be obtained under identical conditions and the tertiary chloride end groups were found to be fairly stable even after complete IB conversion and the polymerization continued upon fresh monomer addition.Kinetic investigation of IB polymerization by H2O/TiCl4/DMA initiating system in CH2Cl2/n-Hex(50/50,v/v) solvent mixture at-60℃shows that the polymerization rate was in first-order dependency on monomer concentration, second order dependency on TiCl4concentration in the concentration range from0.04mol/L to0.13mol/L and closer to first order dependency on H2O concentration in the concentration range from1.40mmol/L to4.97mmol/L. The polymerization rate had little dependence on DMA in the concentration range from0.001mol/L to0.005mol/L under conditions when [TiCl4]0=0.10mol/LKinetic investigation of St polymerization initiated by H2O/TiCl4/DMA system in CH2Cl2/n-Hex (50/50, v/v) solvent mixture at-60℃reveals that the polymerization rate was in first-order dependency on monomer concentration and a second-order kinetic dependency on TiCl4concentration. H2O concentration had little effect on the polymerization rate in the concentration range from0.57mmol/L to2.44mmol/L due to rapid propagation compared to initiation. The effect of DMA on polymerization rate was complex and depended on its concentration range. DMA concentration slightly affected the polymerization rate when it was in the range from0.002mol/L to0.011mol/L. However, the polymerization rate decreased apparently with increasing DMA concentration from0.011mol/L to0.029mol/L, and the reaction order with respect to DMA concentration was determined to be-1.84.The molecular weight of PS increased linearly with conversion even to complete monomer consumption and nearly doubled at similar conversion when St concentration was augmented from0.2mol/L to0.4mol/L.The constant of chain transfer to monomer (CM) was determined to be around3×10-3in the monomer concentration range from0.2mol/L to0.6mol/L. It was shown from lH-NMR that the chloride end groups of PS chains was less stable and decomposed rapidly with the depletion of monomer. Thus it was advised to add IB at around90%of St conversion in the block copolymerization in order to obtain higher crossover efficiency.It was feasible to synthesize diblock copolymers of PS-PIB, PIB-PS and triblock copolymers of PS-PIB-PS with H2O/TiCl4initiating system in the presence of MMK, DMA or MMK/DMA via two or three-step sequential cationic block copolymerization in CH2Cl2/n-Hex (50/50, v/v) solvent mixture at-60℃. The crossover efficiency of IB from PS chains was strongly influenced by St conversion or the timing of IB addition and decreased dramatically with time due to rapid decomposition of chloro-terminals of PS chains. However, the crossover of St from PIB chains was efficient and diblock copolymers of PIB-PS were prepared with two glass transition temperatures and microphase separation morphologies. A series of triblock copolymers SIBS with number average molecular weight (Mn) ranging from74,500g/mol to102,400g/mol and narrow molecular weight distributions (MW/Mn=1.26-1.42) were obtained. The styrene content in SIBS was in the range from26.7wt%to54.2wt%. The formation of blocking was directly evidenced by examining the crossover sites between PS and PIB segments from13C-NMR spectra. SIBS with styrene content of34.6-54.2wt%exhibited phase separation morphologies of bicontinuous alternating lamellae by Transmission Electron Microscopy. |
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