Entanglement Formation Mechanism in the POSS Modified Heterogeneous Ziegler-Natta Catalysts
Entanglement Formation Mechanism in the POSS Modified Heterogeneous Ziegler-Natta Catalysts
复制标题
POSS 改性异相 Ziegler-Natta 催化剂中的缠结形成机制
DOI：
10.1021/acs.macromol.9b00610
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发表时间：
2019-10-22
期刊：
影响因子：
5.5
通讯作者：
Yang, Yongrong
中科院分区：
文献类型：
--
作者：
Chen, Yuming;Liang, Peng;Yang, Yongrong
链缠结对于调节新生超高分子量聚乙烯（UHMWPE）的加工性能和机械性能非常重要。迄今为止，多相催化剂进行乙烯聚合时缠结的形成机制仍然是一个谜。在本研究中，通过多面体低聚倍半硅氧烷/MgCl2纳米聚集体改性齐格勒-纳塔催化剂合成了一系列弱缠结的UHMWPE。通过X射线光电子能谱、密度泛函理论模拟和扫描探针显微镜实验评估了纳米聚集体的结构，并研究了MgCl2与POSS羟基的配位策略。这些纳米聚集体对乙烯聚合表现出极低的活性，并被证明可以作为分离活性位点和生长链的隔离剂。新生UHMWPE的缠结密度（由初始储能模量G((t=0))'的值反映）随着纳米聚集体数量增加的聚合活性呈指数衰减。重要的是，这种由 POSS/MgCl2 隔离器数量增加引起的指数衰减效应抵消了温度升高时纠缠的快速幂函数增量，这是即使在 85 ℃ 下也能成功合成弱纠缠 UHMWPE 的根本原因。最后，我们提出了 G((t=0))') 的依赖性和敏感性（即，表明新生纠缠的初始纠缠密度） 聚合物）对聚合活性的影响，能够通过 POSS 改性的非均相催化剂追踪聚合过程中缠结的形成。
Chain entanglement was very important for adjusting the processability and mechanical property of nascent ultrahigh molecular weight polyethylene (UHMWPE). So far, it is still a mystery to unravel the formation mechanism of entanglements when the ethylene polymerization is conducted by the heterogeneous catalysts. In this study, a series of weakly entangled UHMWPE was synthesized by the polyhedral oligomeric silsesquioxane/MgCl2 nanoaggregates modified Ziegler-Natta catalysts. The structure of nanoaggregates was evaluated by X-ray photoelectron spectroscopy, density functional theory simulations, and scanning probe microscope experiments, where the coordination strategy of MgCl2 and hydroxyl of POSS was investigated. These nanoaggregates presented extremely low activity on ethylene polymerization and were proved to serve as isolators for separating the active sites and growing chains. The entanglement density of nascent UHMWPE (reflected by the value of initial storage modulus G((t=0))') was exponentially decayed with the polymerization activity of increased numbers of nanoaggregates. Importantly, this exponentially decayed effect contributed by increased numbers of POSS/MgCl2 isolators offset the rapid power function increment of entanglements upon rising the temperature, which was the essential reason for the successful synthesis of weakly entangled UHMWPE even at 85 degrees C. Finally, we have proposed the dependence and sensitivity of G((t=0))') (i.e., indicating the initial entanglement density of nascent polymers) on the polymerization activity, which was able to trace the formation of entanglements during polymerization through the POSS modified heterogeneous catalyst.