Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, USA.
J Am Chem Soc. 2010 Aug 25;132(33):11461-3. doi: 10.1021/ja105107y.
Stopped-flow NMR measurements suitable for determination of reaction kinetics on time scales of 100 ms or longer have been achieved by adaptation of a commercial NMR flow probe with a high-efficiency mixer and drive system. Studies of metallocene-catalyzed alkene polymerization at room temperature have been complicated by high rates, imprecise knowledge of the distribution of different catalyst species with time, and the high sensitivity of the catalysts to low concentrations of impurities. Application of the stopped-flow NMR method to the study of the kinetics of 1-hexene polymerization in the presence of (EBI)ZrMe[MeB(C(6)F(5))(3)] demonstrates that NMR spectroscopy provides an efficient method for direct and simultaneous measurement of substrate consumption and catalyst speciation as a function of time. Kinetic modeling of the catalyst and substrate concentration time courses reveal efficient determination of initiation, propagation, and termination rate constants. As first suggested by Collins and co-workers (Polyhedron 2005, 24, 1234-1249), a kinetic model in which Zr-HB(C(6)F(5))(3) forms rapidly upon beta-hydride elimination but reacts relatively slowly with alkene to reinitiate chain growth is supported by these data.
通过对具有高效混合器和驱动系统的商业 NMR 流探针进行改造，实现了适用于在 100ms 或更长时间尺度上测定反应动力学的停流 NMR 测量。室温下茂金属催化烯烃聚合的研究受到以下因素的困扰：反应速率高、不同催化剂物种随时间的分布知识不精确，以及催化剂对低浓度杂质的高灵敏度。将停流 NMR 方法应用于（EBI）ZrMe[MeB（C（6）F（5））（3）]存在下 1-己烯聚合动力学的研究表明，NMR 光谱法提供了一种直接、同时测定底物消耗和催化剂形态随时间变化的有效方法。对催化剂和底物浓度时间曲线的动力学建模揭示了引发、增长和终止速率常数的有效测定。正如 Collins 及其同事首次提出的（Polyhedron 2005，24，1234-1249），在该动力学模型中，Zr-HB（C（6）F（5））（3）在β-氢化物消除后迅速形成，但与烯烃反应相对缓慢，从而重新引发链增长，这些数据支持了这一模型。
