Jeong Wonhee, Shin Eun Ji, Culkin Darcy A, Hedrick James L, Waymouth Robert M
Department of Chemistry, Stanford University, Stanford, California 94305, USA.
J Am Chem Soc. 2009 Apr 8;131(13):4884-91. doi: 10.1021/ja809617v.
The zwitterionic ring-opening polymerization of lactide initiated by N-heterocyclic carbenes generates cyclic polylactides with well-defined molecular weights between M(n) = 5000 and 30,000 g/mol with narrow polydispersities (M(w)/M(n) < or = 1.31). These zwitterionic polymerizations are extremely rapid (k(p) = 48.7 M(-1) s(-1)), but also exhibit exceptional control of molecular weight and molecular weight distribution. The unusual kinetic features of these zwitterionic polymerizations are illuminated with kinetic and mechanistic investigations, which implicate a mechanism that involves a slow initiation step (second order in [M]), a propagation step (first order in [M]) that is much faster than initiation (k(i) = 0.274 M(-2) s(-1)), cyclization (k(c) = 0.0575 s(-1)), and depropagation (k(d) = 0.208 s(-1)). Numerical and stochastic simulations of the kinetic data provide a kinetic rationale for the evolution of molecular weight with monomer conversion: the molecular weights increase with increasing monomer conversion, exhibit a nonzero intercept near 0% monomer conversion, and are relatively insensitive to the initial monomer-to-initiator ratio. The observed narrow molecular weight distributions are due to a high rate of propagation relative to cyclization and chain transfer. Kinetic simulations define the kinetic criteria under which the active zwitterions remain in solution; these simulations were substantiated by chain-extension experiments, which provide experimental evidence for chain extension of the zwitterions and reinitiation by the N-heterocyclic carbenes liberated upon macrocyclization. The kinetic model rationalizes some of the unique features of zwitterionic ring-opening polymerization and provides a useful mechanistic framework to optimize these polymerizations as a strategy to generate well-defined cyclic polyesters.
由氮杂环卡宾引发的丙交酯两性离子开环聚合反应生成了分子量明确的环状聚丙交酯，其数均分子量（(M_n)）在5000至30,000 g/mol之间，且多分散性较窄（(M_w/M_n \leq 1.31)）。这些两性离子聚合反应极其迅速（(k_p = 48.7 M^{-1} s^{-1})），同时对分子量和分子量分布表现出出色的控制能力。通过动力学和机理研究揭示了这些两性离子聚合反应不同寻常的动力学特征，其涉及的机理包括一个缓慢的引发步骤（对([M])为二级反应）、一个比引发快得多的增长步骤（对([M])为一级反应，(k_i = 0.274 M^{-2} s^{-1})）、环化（(k_c = 0.0575 s^{-1})）和解聚（(k_d = 0.208 s^{-1})）。动力学数据的数值模拟和随机模拟为分子量随单体转化率的变化提供了动力学依据：分子量随单体转化率的增加而增大，在单体转化率接近0%时呈现非零截距，并且相对不依赖于初始单体与引发剂的比例。观察到的窄分子量分布归因于相对于环化和链转移而言较高的增长速率。动力学模拟确定了活性两性离子保留在溶液中的动力学条件；这些模拟通过扩链实验得到了证实，扩链实验为两性离子的扩链以及大环化时释放的氮杂环卡宾的再引发提供了实验证据。该动力学模型解释了两性离子开环聚合反应的一些独特特征，并提供了一个有用的机理框架，以优化这些聚合反应，作为生成分子量明确的环状聚酯的一种策略。
