【摘要】 在自由基聚合中,生成间规立构的趋势稍优先于等规立构。通常认为,碳碳双键上取代基的体积效应和极性效应,是加成反应的立体定向力;而作者则认为,极性呈静电排斥效应是决定间规立构含量较优的主要因素。取代基的体积效应具有两个因素同时决定着立体异构的形成,其一是热力学因素,即体积的排斥效应使间规立构含量增加;另一是动力学因素,即在加成反应瞬间影响链末端碳碳单键内旋转的自由度。因此,取代基的存在不仅使得间规立构占主导,而且取代基体积越小,则自由基的碳碳单键内旋转越容易,形成间规立构的比例亦越高;反之则内旋转不自由,形成随机取向(即无规立构)的比例增加。更多还原

【Abstract】 The slight preference for syndiotactic placement over isotactic placement in methacrylate radical polymerization is normally considered as a consequence of steric and polarity effects between substituents at the carbon-carbon double bonds in the polymer chains. In the view of the present author, the polarity of the side groups were considered as electrostatic repulsion which was a dominant factor favouring the syndiotactic structure. It is proposed that steric hindrance due to the substituents has two effects on the microtacticity of polymer chains: one is thermodyna-mical effect which favours the formation of the syndiotactic structure because of its lower internal energy, the other is dynamical effect which affects the rotation of the last carbon-carbon single bond of the growing chain at new monomer propagation temporal state. The smaller the steric hindrance of the substituents is, the easier the rotation will be, tending in the formation of more syndiotatic structures. On the contrary, for the bulky groups the rotation is less easily and the polymer obtained will be more atactic.更多还原