Abstract It is well known that ring-opening polymerization (ROP) of sterically unhindered cyclic ethylene phosphates, initiated by both organocatalysts and coordination catalysts, is accompanied by transesterification (TE) even at subzero temperatures. To clarify this phenomenon from the mechanistic point of view, we calculated (DFT, B3PW91/DGTZVP) the reaction profiles of the ROP for 2-methoxy-1,3,2-dioxaphospholane-2-oxide (methyl ethylene phosphate, MeOEP) and of the transesterification processes for poly(MeOEP) and trimethyl phosphate (TMP) in the presence of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) organocatalyst. We found that the free activation energy of TE is substantially higher (10–13 kcal/mol) than the ROP activation barrier. To verify the calculation results, we investigated the chemical behavior of low molecular weight trimethyl phosphate (TMP) in the TBD-catalyzed polymerization of MeOEP. We demonstrated that TMP was not affected by transesterification, which correlates with the calculation results. However, we found that the polymerization of MeOEP in the presence of TMP leads to the formation of linear poly(MeOEP) with given P n and narrow MWD even at >99% monomer conversion degree. A similar pattern was revealed for the synthesis of poly(MeOEP) with the coordination catalyst of aryloxy-alkoxy magnesium complex [(BHT)Mg(μ-OBn)(THF)] 2 (BHT = 2,6-di- tert -butyl-4-methylphenoxy). To explain the difference in the chemical behavior of TMP and polyphosphate, we assumed that poly(MeOEP) in the solution represents a dense globule, which leads to a decrease in the entropy of transesterification. To confirm this assumption, we studied the solution behavior of poly(MeOEP) in CHCl 3 and in TMP. Both CHCl 3 and TMP were found to be poor solvents for poly(MeOEP) in terms of KMH solution theory (α = 0.53 and 0.33, respectively). We assume that it is the conformation of the polymer in the solution that determines the tendency of the polyphosphate towards transesterification.

中文翻译：

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TBD催化磷酸亚乙酯开环聚合中酯交换反应机理研究

摘要 众所周知，由有机催化剂和配位催化剂引发的空间不受阻碍的环状磷酸乙烯酯的开环聚合 (ROP) 即使在零下温度也伴随着酯交换 (TE)。为了从机理的角度阐明这一现象，我们计算了 (DFT, B3PW91/DGTZVP) ROP 对 2-甲氧基-1,3,2-二氧杂磷烷-2-氧化物（甲基亚乙基磷酸酯，MeOEP）和聚（MeOEP）和磷酸三甲酯（TMP）在 1,5,7-三氮杂双环[4.4.0]dec-5-烯（TBD）有机催化剂存在下的酯交换过程。我们发现 TE 的自由活化能明显高于 ROP 活化能垒（10-13 kcal/mol）。为了验证计算结果，我们研究了低分子量磷酸三甲酯 (TMP) 在 TBD 催化的 MeOEP 聚合中的化学行为。我们证明了 TMP 不受酯交换的影响，这与计算结果相关。然而，我们发现在 TMP 存在下 MeOEP 的聚合导致形成具有给定 P n 和窄 MWD 的线性聚（MeOEP），即使单体转化率 >99%。在芳氧基-烷氧基镁配合物 [(BHT)Mg(μ-OBn)(THF)] 2 (BHT = 2,6-二叔丁基-4-甲基苯氧基）。为了解释 TMP 和聚磷酸盐的化学行为差异，我们假设溶液中的聚 (MeOEP) 代表一个致密的小球，这导致酯交换的熵降低。为了证实这一假设，我们研究了聚 (MeOEP) 在 CHCl 3 和 TMP 中的溶液行为。根据 KMH 溶液理论（分别为 α = 0.53 和 0.33），发现 CHCl 3 和 TMP 都是聚（MeOEP）的不良溶剂。我们假设溶液中聚合物的构象决定了多磷酸盐的酯交换趋势。