Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, USA.
J Am Chem Soc. 2010 Mar 3;132(8):2695-709. doi: 10.1021/ja908818y.
This contribution reports a combined synthetic, kinetic, mechanistic, and theoretical/computational study of the recently discovered catalyst-site-controlled coordination polymerization of polar vinyl monomers [such as methyl methacrylate (MMA) and N,N-dimethylacrylamide (DMAA)] into highly syndiotactic polymers. Among the 12 C(s)-ligated ansa-cyclopentadienyl (Cp)-R(2)E(C,Si)-fluorenyl (Flu) group 4 metallocene catalyst systems examined-which varied in metal center, anion structure, bridging atom and substituents, and ligand substitution pattern-cationic ansa-metallocene ester enolate catalyst 6(+)B(C(6)F(5))(4), derived from the activation of the precatalyst [Ph(2)C(Cp)(2,7-(t)Bu(2)-Flu)]ZrOC(O(i)Pr)=CMe(2) with [Ph(3)C][B(C(6)F(5))(4)], stood out as the best catalyst in all aspects of the MMA polymerization at room temperature, including the highest activity (1554 h(-1) TOF), efficiency (98% I*), syndiotacticity (94% rr), and control (predicted number-average molecular weight and 1.14 molecular weight distribution). Kinetic and mechanistic results are consistent with a catalyst-site-controlled, monometallic coordination-addition mechanism, involving fast intramolecular addition within the catalyst-monomer complex leading to the resting eight-membered ester enolate chelate, followed by the rate-limiting ring-opening of the chelate to regenerate the active species. This work has also uncovered several unique features of this polymerization system that are in marked contrast to the propylene polymerization by analogous C(s)-ligated cationic alkyl catalysts: a constant syndiotacticity of PMMA produced over a wide polymerization temperature range (i.e., from 0 degrees C, 94% rr to 25 degrees C, 94% rr to 50 degrees C, 93% rr); insensitivity of its high activity, degree of control, and stereoselectivity to solvent polarity and structure of weakly coordinating anions; and deviation from a pure site-control mechanism at high [MMA]/[catalyst] ratios. Computational results provide theoretical support for the proposed monomer-assisted, catalyst-site epimerization, after an enantiofacial mistake, to a thermodynamically more stable resting state, which accounts for the observed higher than expected [mr] contents based on a pure site-controlled mechanism. DFT calculations rationalize why the Ph(2)C< bridged catalyst 6 exhibits higher stereoselectivity than other catalysts with the Me(2)C< or Me(2)Si< bridge: the bridge rigidity pushes the eta(3)-bound Flu ligand closer to the growing chain and the monomer, thereby increasing DeltaE(stereo) between the competing transition states for the addition of a monomer molecule to the opposite (correct and wrong) enantiofaces of the enolate growing chain. The relative polymerization activity of this catalyst series is shown to correlate with the relative energetics of the back-biting of the penultimate unit and ion-pair formation.
这项贡献报告了一种综合的、动力学的、机制的和理论/计算研究，研究了最近发现的极性乙烯基单体（如甲基丙烯酸甲酯（MMA）和 N,N-二甲基丙烯酰胺（DMAA））的催化剂位控制配位聚合，形成高度间同立构聚合物。在所研究的 12 个 C(s)-配位的薁（Cp）-R(2)E(C,Si)-芴基（Flu）基团 4 金属茂催化剂体系中，阳离子ansa-金属茂酯烯醇催化剂 6(+)B(C(6)F(5))(4)脱颖而出，在室温下的 MMA 聚合的各个方面都表现出最佳的催化性能，包括最高的活性（1554 h(-1) TOF）、效率（98% I*）、间同立构规整性（94% rr）和控制（预测的数均分子量和 1.14 分子量分布）。动力学和机理研究结果与催化剂位控制的单金属配位加成机理一致，涉及催化剂-单体配合物内的快速分子内加成，导致休眠的八元酯烯醇螯合物形成，然后是螯合物的限速开环，以再生活性物种。这项工作还揭示了这个聚合体系的几个独特特征，与类似的 C(s)-配位的阳离子烷基催化剂的丙烯聚合形成鲜明对比：在较宽的聚合温度范围内（即从 0°C，94% rr 到 25°C，94% rr 到 50°C，93% rr），产生的 PMMA 的间同立构规整度保持不变；其高活性、控制程度和立体选择性对溶剂极性和弱配位阴离子结构不敏感；以及在高[MMA]/[催化剂]比下偏离纯位控制机制。计算结果为所提出的单体辅助、催化剂位对映体异构化提供了理论支持，在发生对映面错误后，异构化为热力学上更稳定的休眠状态，这解释了基于纯位控制机制观察到的高于预期的[mr]含量。DFT 计算解释了为什么 Ph(2)C<桥连催化剂 6 比其他具有 Me(2)C<或 Me(2)Si<桥的催化剂具有更高的立体选择性：桥的刚性将 eta(3)-键合的 Flu 配体推近于增长的链和单体，从而增加了烯醇增长链的相反（正确和错误）面之间单体分子加成的竞争过渡态之间的 DeltaE(stereo)。该催化剂系列的相对聚合活性与后咬最后一个单元和离子对形成的相对能量相关。
