A New Type of Silicon Super Lewis Acids for Polymerization of Silyl Vinyl Ethers
A New Type of Silicon Super Lewis Acids for Polymerization of Silyl Vinyl Ethers
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一种用于硅乙烯基醚聚合的新型硅超级路易斯酸
DOI：
10.1021/ma002201t
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发表时间：
2001
期刊：
影响因子：
5.5
通讯作者：
Hisashi Yamamoto
中科院分区：
文献类型：
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作者：
M. Oishi;Hisashi Yamamoto
除了乙酸乙烯酯和烷基乙烯基醚的聚合之外，甲硅烷基乙烯基醚的1，2阳离子和羟醛基团转移聚合已被报道为可用于制备聚（乙烯醇）（PVA）的替代合成方法，因为乙醛烯醇化物单体的不可及性，区域特异性增长，并且容易转化为PVA。3-5此外，聚合物侧链上的甲硅烷基基团的变化以及主链结构的变化将扩大有机硅基大分子结构的范围，用于表面活性剂、酶和蛋白质的固定化、液体橡胶等。7据我们所知，羟醛基团转移聚合的现有限制包括甲硅烷基乙烯基醚的亲核性相对较弱，以及能够促进这些刘易斯碱性单体的链增长而没有显著终止的少量刘易斯酸。8-10我们最近发现甲硅烷基磺酸酯和空间位阻铝化合物，甲基铝双-（2，6-二叔丁基-4-甲基苯酚）（MAD）和甲基铝双（4-溴-2，6-二叔丁基苯酚）-（MABR）对Aldol反应的反应性具有协同增强作用。11本文中，我们报道了由硅超路易斯酸介导的甲硅烷基乙烯基单体1-4的有效均聚和共聚，所述硅超路易斯酸由甲硅烷基三氟甲磺酸酯和甲基铝双（芳氧基）组成（参见图1）。根据Sogah等人5报道的方法，在上述超级刘易斯酸存在下，使用1a进行初始聚合实验。通过在MeOH中沉淀分离结晶聚合物，并通过GPC表征Mn和Mw/Mn。表1中总结的聚合数据清楚地表明，Me 3SiOTf-MAD体系，无论温度、浓度和单体/引发剂比如何，都是高度活性的，尽管聚合数据没有显示出明显的活性。12该系统在较高温度下快速产生具有Mn（6.7-15.5）× 103范围和相对窄的Mw/Mn的所需甲硅烷基化PVA（条目10）。Mn和Mw/Mn的更好控制（条目1相对于条目6和7）可归因于Me 3Si基团的更高引发效率。不幸的是，在每种情况下，使用含水HF裂解甲硅烷基醚后获得的PVA的1H NMR分析表明聚合物是无规立构的。14关于内烯烃如异戊烯、巴豆酸酯、富马酸酯等的聚合的报道，非常有限。15-18我们进一步研究了1-丙烯基甲硅烷基醚（1b）在类似聚合条件下的聚合（参见方案1）。早期的工作表明，几种三烷基甲硅烷基乙烯基醚在常规或醛醇-GTP程序中进行聚合。19，20 M1 B，然而，没有。例如，ZnBr 2仅产生痕量的相应聚合物，而我们的系统成功地以较高的产率和中等的Mn值（Mcalcd）7.00× 103 g mol-1）制备了均聚物。13 C NMR谱中72-78、43-47和10-15 ppm处的宽共振可分别归属于CH-O、CH-CH 3和CH 3-CH的主链碳。在每个立体化学纯1b的聚合的初始阶段，检测到单体立体异构体的混合物。这表明
Besides polymerization of vinyl acetates and alkyl vinyl ethers, 1, 2 cationic and aldol group transfer polymerizations of silyl vinyl ethers have been reported as alternative synthetic methods that can be employed in the preparation of poly (vinyl alcohol)(PVA) because of the nonaccessibility of the acetaldehyde enolate monomer, regiospecific propagation, and easy conversion to PVA. 3-5 In addition, variation of the silyl groups on the polymer side chain as well as variations in the main chain structure would expand the scope of organosiliconbased macromolecular architecture for surfactants, immobilization of enzymes and proteins, liquid rubbers, etc. 6 The above polymerization technique has been far less studied than the original GTP. 7 To our knowledge, current limitations in aldol group transfer polymerizations include the relatively weak nucleophilicity of silyl vinyl ethers and the small number of Lewis acids that are capable of promoting chain propagation of these Lewis basic monomers without significant termination. 8-10 We have recently discovered a synergistic enhancement of silyl sulfonate esters and sterically encumbered aluminum compounds, methylaluminum bis-(2, 6-di-tert-butyl-4-methylphenoxide)(MAD), and methylaluminum bis (4-bromo-2, 6-di-tert-butylphenoxide)-(MABR) on reactivity of the aldol reaction. 11 Herein we report efficient homo-and copolymerization of silyl vinyl monomers 1-4 mediated by silicon super Lewis acids consisting of silyl triflates and methylaluminum bis-(aryloxide) s (see Chart 1). An initial polymerization experiment using 1a was performed in the presence of the above super Lewis acids according to the method reported by Sogah et al. 5 The crystalline polymer was isolated by precipitation in MeOH, and the Mn and Mw/Mn were characterized by GPC. The polymerization data summarized in Table 1 clearly demonstrate that the Me3SiOTf-MAD system, irrespective of temperature, concentration, and monomer/initiator ratio, is highly active although the polymerization data do not show the clear livingness. 12 The system rapidly produces the desired silylated PVA with a range of Mn’s (6.7-15.5)× 103 and relatively narrow Mw/Mn at higher temperature (entry 10). Better control of Mn and Mw/Mn (entry 1 vs entries 6 and 7) may be ascribed to the higher initiation efficiency of the Me3Si group. 13 Unfortunately, in each case 1H NMR analysis of PVA obtained after cleavage of the silyl ethers using aqueous HF indicated that the polymers are atactic. 14 The reports on polymerizations of internal olefins such as norbornene, crotonate, fumarate esters, etc., are very limited. 15-18 We further examined polymerization of 1-propenyl silyl ether (1b) under similar polymerization conditions (see Scheme 1). Earlier works showed that several trialkylsilyl vinyl ethers undergo polymerization in the conventional or aldol-GTP procedure. 19, 20 Monomer 1b, however, does not. For instance, ZnBr2 gave only a trace amount of the corresponding polymer, while our system successfully produced the homopolymer in higher yields and with moderate Mn values (Mcalcd) 7.00× 103 g mol-1). The broad resonances at 72-78, 43-47, and 10-15 ppm in 13C NMR spectra can be assigned to main-chain carbons of CH-O, CH-CH3, and CH3-CH, respectively. At the initial stage of polymerization of each stereochemically pure 1b, a mixture of monomer stereoisomers was detected. This indicates