In the synthesis of polyethylene using supported Ziegler–Natta catalyst, the nascent polyethylene adopts distinct structures influenced by factors such as active site distribution, temperature, and growth rate. While experiments can achieve atomic-level observations through scanning tunnel microscopy (STM), acquiring statistical data on chain conformation and aggregation structures throughout the growth period remains challenging. To address this, we developed an in situ polymerization molecular dynamics simulation model that concurrently characterizes the growth and mobility of nascent polyethylene chains on supported catalysts. We unveiled the competition between the diffusion of monomers and the growth of nascent polymer chains. The simulation results demonstrated that conformational variations of nascent chains affect the diffusion resistance of the polymer melt to monomers with chain conformations aligned along the Z-axis facilitating monomer diffusion. Furthermore, by introducing a sidewall to simulate the nanoparticles on supported catalysts, we derived the compromise in competition between the attractive energy arising from the sidewalls and the cohesive energy within the nascent chains, resulting in diverse spatial and chain length distributions of the nascent chains. This work provides a new approach to investigate the reaction dynamics and the underlying microscopic mechanisms of in situ ethylene polymerization with the potential of further extension to the study of in situ growth for other nascent polymers.

中文翻译：

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原位聚合的分子动力学模拟：链构象转变


在使用负载型齐格勒-纳塔催化剂合成聚乙烯时，新生聚乙烯采用受活性位点分布、温度和生长速率等因素影响的独特结构。虽然实验可以通过扫描隧道显微镜（STM）实现原子级观察，但获取整个生长期间链构象和聚集结构的统计数据仍然具有挑战性。为了解决这个问题，我们开发了一种原位聚合分子动力学模拟模型，该模型同时表征了负载型催化剂上新生聚乙烯链的生长和移动性。我们揭示了单体扩散和新生聚合物链生长之间的竞争。模拟结果表明，新生链的构象变化会影响聚合物熔体对单体的扩散阻力，其中链构象沿 Z 轴排列，有利于单体扩散。此外，通过引入侧壁来模拟负载型催化剂上的纳米粒子，我们得出了侧壁产生的吸引力与新生链内的内聚能之间的竞争折衷，从而导致新生链的不同空间和链长分布。这项工作提供了一种研究原位乙烯聚合的反应动力学和潜在微观机制的新方法，并有可能进一步扩展到其他新生聚合物的原位生长研究。