活性正离子聚合制备聚(异丁烯-b-α-甲基苯乙烯)嵌段共聚物

以2-氯-2,4,4-三甲基戊烷(TMPCl)/TiCl4/质子捕捉剂(DtBP)为引发剂体系,引发异丁烯聚合,随后加入1,1-二(4-甲基苯基)乙烯作为封端剂稳定末端碳正离子,再引入四异丙醇钛(Ti(OiPr)4),降低Lewis酸性,继续引发α-甲基苯乙烯聚合,实现活性正离子聚合制备聚(异丁烯-b-α-甲基苯乙烯)嵌段共聚物.考察了α-甲基苯乙烯聚合时间对单体转化率、产物的dn/dc值、分子量及其分布的影响以及四异丙醇钛对聚合速率的影响.并通过体积排斥色谱法/紫外检测器/示差折光指数/多角激光光散射、1H-NMR以及DSC以对产物进行表征.实验结果表明,嵌段共聚物分子量分布窄(MWD≤1.2),单体转化率与分子量呈线性关系,聚合速率对单体浓度呈一级动力学关系,具有活性聚合的特征.Ti(OiPr)4能有效稳定活性中心,降低聚合速率.聚(异丁烯-b-α-甲基苯乙烯)嵌段共聚物的DSC测试发现明显的两个Tg,表明存在微相分离结构.

Synthesis of Poly(isobutylene-b-a-methylstyrene)Copolymers by Living Cationic Polymerization

The efficient synthesis of poly(isobutylene-b-α-methylstyrene) (PIB-b-PαMeSt) copolymer has been accomplished by living cationic polymerization using sequential monomer additions.Living PIB was first prepared by the 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl_4/DtBP/C_6H_ 12 ∶MeCl=50∶50 V/V/-80℃ polymerization system.Subsequently,the living PIB chain end was converted to the corresponding diphenylalkyl end by capping with 1,1-di-p-tolylethylene (DTE).Ti(OiPr)_4 was added to reduce the Lewis acidity,followed by the addition of αMeSt.When Ti(OiPr)_4 was introduced into the polymerization system,the complexes of Ti_2Cl_ 8-x (OiPr)_x were formed rapidly.By changing the stoichiometry,the Lewis acidity of TiCl_4 can be moderated in a wide range.This was corroborated by our visual observation that upon addition of Ti(OiPr)_4 the orange color of the diphenyl carbenium ion turned into the yellow-green color.This indicated that highly reactive species convert into less reactive species.The PIB-b-PαMeSt copolymers were characterized by ~1H-NMR spectroscopy and triple detection SEC: refractive index (RI),multi-angle laser light scattering (MALLS) and ultraviolet (UV) detector.The effects of the polymerization time for αMeSt (t_2) on conversion of monomer,dn/dc value,molecular weight and molecular weight distribution as well as the effect of Ti(OiPr)_4 concentrations on the polymerization rate were investigated.The results showed that the dn/dc values of block copolymers increased with increasing polyαMeSt compositions.And molecular weight distributions of the resulting polymers stayed relatively narrow,less than 1.2 (MWD≤1.2).LS,RI and UV traces of the resulting PIB-b-PαMeSt copolymer had the same eluting time without any tailing which demonstrated the blocking efficiency is close to 100%.The kinetics of the cationic polymerization of αMeSt in the presence of Ti(OiPr)_4 were determined by the gravimetric monitoring of the polymer yield as a function of the reaction time.The monomer conversion gradually increased with increasing polymerization time,and the conversion decreased with increasing Ti(OiPr)_4]/TiCl_4] ratio.Interestingly,there was a precipitous drop in conversion at Ti(OiPr)_4]/TiCl_4] ratio >1,with the 1.1 Ti(OiPr)_4]/TiCl_4] ratio ca. 5% conversion was obtained in 1 h.On the basis of these observations,it was concluded that Lewis acidity of TiCl_4 decreased with increasing the Ti(OiPr)_4] to TiCl_4] ratio.When the Ti(OiPr)_4] to TiCl_4] ratio was more than 1.1,a slight excess of Ti(OiPr)_4 deactivated TiCl_4,and the active species lost.Linear plots of ln(M]_0/M]) versus time passed through the origin demonstrated the expected first-order dependence on the monomer concentration,which proved that the concentration of growing centers remained constant.The result combined with the linear M_n-conversion plots confirmed that the polymerization was living.The k~ app _P values obtained from the slope of the in(M]_0/M]) versus time linear plots also showed that the rate of the polymerization decreased with increasing Ti(OiPr)_4]/TiCl_4] ratio.The complexes of Ti_2Cl_ 8-x (OiPr)_x interacted with the growing chain end to reduce its"cationicity" and thus to decrease the polymerization rate,suppress chain transfer and termination reactions and narrow MWD.Differential scanning calorimetry (DSC) of the diblock copolymers showed two glass transitions demonstrating microphases separation.