We present a comprehensive DFT investigation on the cationic ring-opening polymerisation (CROP) of ε-caprolactone (CL) using zirconocene/borate catalyst systems. All possible pathways of the interaction between cationic species [Cp₂ZrMe⁺] and counteranions, [A⁻] = [MeB(C₆F₅)₃]⁻ and [B(C₆F₅)₄]⁻, were examined during chain initiation, propagation, and termination steps. The calculations reveal an active chain-end mechanism with O-alkyl bond cleavage of the polymerisation. The catalytic performance of the two counteranions is found to be identical, and they influence the initial process through stabilisation of the cationic species via non-covalent interactions (NCI), with the [MeB(C₆F₅)₃]⁻ anion stabilising the catalyst–monomer complex more effectively than the [B(C₆F₅)₄]⁻ anion by 24.3 kJ mol⁻¹. The first two propagations are likely the rate-determining step, with calculated free-energy barriers of 61.4–71.2 and 73.9–80.6 kJ mol⁻¹ with and without the anions (A⁻), respectively. The presence of the counteranion significantly affects the third propagation rate, lowering the barriers up to 20 kJ mol⁻¹. Comparison of the first termination and the third propagation shows that they are not competitive, with the termination being less facile. We also studied the initiation and propagation steps for the hafnocene catalyst and found that the Hf catalyst slightly favours the CL CROP in comparison to the Zr catalyst. Analysis of solvent and dispersion interaction demonstrates that both factors play an important role in the process. NCI analysis reveals weak (van der Waals) interactions at the contacts between the cationic species and the counteranions during the reaction course. Overall, our results offer insights into the structures and interactions involved in the polymerisation.

中文翻译：

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使用茂金属/硼酸盐催化体系进行 ε-己内酯阳离子开环聚合的机理：链引发、增长和终止的 DFT 和 NCI 研究

我们对使用锆茂/硼酸盐催化剂体系的 ε-己内酯 (CL) 的阳离子开环聚合 (CROP) 进行了全面的 DFT 研究。阳离子物质 [Cp ₂ ZrMe ⁺ ] 和抗衡阴离子 [A ⁻ ] = [MeB(C ₆ F ₅ ) ₃ ] ⁻和 [B(C ₆ F ₅ ) ₄ ] ⁻之间相互作用的所有可能途径均在链的启动、传播和终止步骤。计算揭示了聚合反应中O-烷基键断裂的活性链端机制。发现两种抗衡阴离子的催化性能是相同的，它们通过非共价相互作用 (NCI) 稳定阳离子物质，从而影响初始过程，而 [MeB(C ₆ F ₅ ) ₃ ] ^-阴离子则稳定阳离子物质。催化剂-单体复合物比[B(C ₆ F ₅ ) ₄ ] ^-阴离子更有效24.3 kJ mol ^-1。前两次传播可能是速率决定步骤，计算出有和没有阴离子 (A ⁻ ) 时的自由能垒分别为 61.4–71.2 和 73.9–80.6 kJ mol ^-1 。抗衡阴离子的存在显着影响第三次传播速率，将势垒降低至20 kJ mol ^-1。第一次终止和第三次传播的比较表明它们不具有竞争性，并且终止不太容易。我们还研究了铪茂催化剂的引发和增长步骤，发现与 Zr 催化剂相比，Hf 催化剂稍微有利于 CL CROP。溶剂和分散体相互作用的分析表明，这两个因素在此过程中都发挥着重要作用。 NCI 分析揭示了反应过程中阳离子物质和抗衡阴离子之间接触处的弱（范德华）相互作用。总的来说，我们的结果提供了对聚合中涉及的结构和相互作用的见解。