Stereoregular Precursors to Poly(p-phenylene) via Transition-Metal-Catalyzed Polymerization. 2. The Effects of Polymer Stereochemistry and Acid Catalysts on Precursor Aromatization: A Characterization Study
Stereoregular Precursors to Poly(p-phenylene) via Transition-Metal-Catalyzed Polymerization. 2. The Effects of Polymer Stereochemistry and Acid Catalysts on Precursor Aromatization: A Characterization Study
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DOI：
10.1021/ja00103a008
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发表时间：
1994-11
影响因子：
15
通讯作者：
D. Gin;V. Conticello;R. Grubbs
中科院分区：
文献类型：
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作者：
D. Gin;V. Conticello;R. Grubbs
由基于5,6-二羟基-1,3-环己二烯的乙酰基衍生物的两种前体聚合物热转化产生的聚亚苯基的分子量和结构规整性取决于前体的立体化学和芳构化催化剂的存在。这些前体的本体热转化过程中会发生两个竞争反应：(1) 聚合物的热链断裂和 (2) 热诱导的酸消除（芳构化），导致聚亚苯基形成。这两个过程的相对速率最终决定最终产品的分子量，并在很大程度上取决于聚合物主链的立体化学。对于通过镍催化聚合制备的1, 4-连接的有规立构前体聚合物，链降解的开始发生在加热过程中芳构化开始之前。因此，尽管该前体具有适合轻松顺式热解酸消除的规则立体化学，但仅提供低质量的聚亚苯基低聚物。另一方面，对于含有 10% 1, 2-键的自由基聚合类似物，则存在相反的关系。尽管在这种无规立构前体的热解过程中仍然发生链降解，但主链断裂的相对量小于芳构化的相对量。与前体聚合物的有限热稳定性相关的问题可以通过在本体热解过程中使用布朗斯特德和路易斯酸催化剂来克服。通过选择性催化两种前体聚合物中的酸消除反应，酸将芳构化的起始温度降低到远低于热断链发生的温度。然而，由两种聚合物制成的所得聚苯撑的表征表明，通过酸催化芳构化过程产生的聚苯撑的结构规则性完全取决于初始前体的区域化学。高分子量、结构规则的聚（对亚苯基）仅通过 1, 4-连接的有规立构聚合物的酸催化本体芳构化来生产。自由基聚合类似物的酸催化本体热解仅提供含有大量1, 2-键的聚亚苯基。
The molecular weight and structural regularity of the polyphenylene produced from thermal conversion of two precursor polymers based on the acetyl derivative of 5, 6-dihydroxy-l, 3-cyclohexadiene depend on the stereochemistry of the precursor and the presence of aromatization catalysts. Two competing reactions occur during the bulk thermal conversion of these precursors:(1) thermal chain fracturing of the polymer and (2) thermally-induced acid elimination (aromatization) resulting in polyphenylene formation. The relative rates of these two processes ultimately determine the molecular weight of the final product and depend heavily uponthe stereochemistry of the polymer backbone. For a 1, 4-linked stereoregular precursor polymer made by nickel-catalyzed polymerization, the onset of chain degradation occurs before theonset of aromatization during heating. Consequently, this precursor only affords low-quality polyphenylene oligomers, despite having a regular stereochemistry that is ideal for facile cis pyrolytic acid elimination. Onthe other hand, the reverse relationships are true for its radically polymerized analog containing 10% 1, 2-linkages. Although chain degradation still occurs during the pyrolysis of this atactic precursor, the relative amount of backbone fracturing is less than that of aromatization. The problems associated with the limited thermal stability of the precursor polymers can be overcome through the use of Brpnsted and Lewis acid catalysts duringbulk pyrolysis. Acids lower the onset temperature of aromatization to a regime well below that at which thermal chain scission can occur by selectively catalyzing the acid elimination reaction in both precursor polymers. However, characterization of the resulting polyphenylenes made from both polymers indicates that the structural regularity of the polyphenylene produced by the acid-catalyzed aromatization process depends entirely on the regiochemistry of the initial precursor. High molecular weight, structurally regular poly (p-phenylene) is produced only bythe acid-catalyzed bulk aromatization of the 1, 4-linked stereoregular polymer. Acid-catalyzed bulk pyrolysis of the radically polymerized analog only affords polyphenylene containing substantial amounts of 1, 2-linkages.