Stereoregulation in the Free-Ion Propagation of Optically Active Alkyl Vinyl Ethers
Stereoregulation in the Free-Ion Propagation of Optically Active Alkyl Vinyl Ethers
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光学活性烷基乙烯基醚的自由离子传播中的立体调控
DOI：
10.1021/ma60068a013
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发表时间：
1979
期刊：
影响因子：
5.5
通讯作者：
R. Solaro
中科院分区：
文献类型：
--
作者：
A. Ledwith;E. Chiellini;R. Solaro
以六氯锑酸托吡酯为阳离子引发剂，在自由碳正离子为重要增长中间体的条件下，合成了三甲基乙烯基氧基硅烷（TMVO）、（-）-薄荷基乙烯基醚（MVE）和（S）-1-甲基丙基乙烯基醚（BVE）。庞大的和nonoptical活性TMVO产生高度间同立构聚合物，根据目前的理论，立体调控传播的非关联物种，如自由基。与此完全相反，同样庞大但光学活性的MVE产生了高度全同立构的聚合物，表明在链生长过程中存在明显的手性诱导的立体调控。BVE得到的聚合物具有与文献中报道的最全同立构部分的全同立构规整度的大约一半一致的光学活性。本文讨论了这些结果对烷基乙烯基醚阳离子聚合机理的意义.在增长物种不受溶剂、反离子等阻碍的条件下，由简单乙烯基单体形成的聚合物链的立构规整性，分别由插入产生全同立构或间同立构二单元的单体单元的活化能差异控制。1自由基聚合是涉及“自由”增长物种的聚合中最为人所知的，现在已经确定，在自由基增长中，间同立构位置比全同立构位置稍有利，随着聚合温度的降低，这种差异变得越来越重要。1与自由基系统相反，通过离子
Trimethylvinyloxysilane (TMVO),(-)-menthyl vinyl ether (MVE), and (S)-l-methylpropyl vinyl ether (BVE) have been polymerized using tropylium hexachloroantimonate as cationic initiator under conditions in which free carbocations are the important propagating intermediates. The bulky and nonoptically active TMVO yielded highly syndiotactic polymers in accordance with current theories regarding stereoregulation in propagation by unassociated species such as free radicals. In complete contrast, the equallybulky but optically active MVE yielded highly isotacticpolymers indicative of a pronounced chirality induced stereoregulation during the chain growth process. BVE yielded polymers having optical activity consistent with a degree of isotacticity approximately one half of that forthe most isotactic fraction reported in the literature. Implications of these results for themechanisms of cationic polymerization of alkyl vinyl ethers are discussed.Stereoregularity of polymer chains formed from a simple vinyl monomer under conditions where the propagating species is thought to be free from hindrance by solvent, counterion, etc., is controlled by the difference in activation energies for the insertion of a monomer unit yielding an isotactic or syndiotactic diad, respectively. 1 Radical po-lymerizations are the best known of those involving “free” propagating species and it is now well established that in free-radical propagation, syndiotactic placements are slightly favored over isotactic placements with the dif-ference becoming progressively more important as the polymerization temperature is lowered. 1 In contrast to free-radical systems, propagation by ionic