Density functional theory studies have been conducted for isoprene polymerization catalyzed by the cationic half-sandwich scandium alkyl species containing a methoxy side arm [(C₅Me₄C₆H₄OMe-o)Sc(CH₂SiMe₃)]⁺ (1) and that containing a phosphine oxide side arm [{C₅Me₄SiMe₂CH₂P(O)Ph₂}Sc(CH₂SiMe₃)]⁺ (2). It has been found that trans-1,4-polymerization of isoprene by species 1 prefers an insertion–isomerization mechanism: (i) an insertion of cis-isoprene into the metal–alkyl bond to give η³-π-anti-form, (ii) anti-syn isomerization of the resulting 1,2-disubstituted allyl complex to yield a syn-allyl form, (iii) repetitive insertion of cis-isoprene into the metal–syn-allyl bond and subsequent anti–syn isomerization. The resulting η³-π-syn-allyl species is suitable for more kinetically favorable cis-monomer insertion. The stability of the key transition state involved in the most feasible pathway could be ascribed to the smaller deformation of cis-isoprene and stronger interaction between the cis-isoprene moiety and the remaining metal complex. The origin of experimentally observed inertness of 2 toward isoprene polymerization is that the steric hindrance derived from the crowding of η³-π-syn-allyl species hampers the insertion of the incoming isoprene monomer. The modeling of 2-mediated chain propagation also has a high energy barrier and is endergonic. To corroborate the steric effect on the kinetic and thermodynamic aspects, various analogue complexes with smaller hindrance have been computationally modeled on the basis of 2. Expectedly, lower energy barrier and favorable thermodynamics are found for the monomer insertion mediated by these complexes with less steric hindrance around the metal center.

中文翻译：

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带有配位侧臂的阳离子半三明治Scan络合物催化的异戊二烯聚合反应的理论研究

已经对含有甲氧基侧臂[（C ₅ Me ₄ C ₆ H ₄ OMe- o）Sc（CH ₂ SiMe ₃）] ⁺（1的阳离子半三明治scan烷基物质催化的异戊二烯聚合反应进行了密度泛函理论研究。）和含有氧化膦侧臂的化合物[{C ₅ Me ₄ SiMe ₂ CH ₂ P（O）Ph ₂ } Sc（CH ₂ SiMe ₃）] ⁺（2）。已经发现反式通过物种异戊二烯-1,4-聚合1喜欢的插入-异构化机制：（ⅰ）的插入顺式-异戊二烯进入金属-烷基键，得到η ³ -π-抗-型，（ⅱ）抗-顺进行1,2-二取代的烯丙基络合物的异构化，生成顺式-烯丙基形式；（iii）顺式-异戊二烯重复插入金属-顺式-烯丙基键中，然后进行反式-顺式异构化。将所得的η ³ -π-顺式-烯丙基物种适合于更有利动力学顺-单体插入。最可行途径中涉及的关键过渡态的稳定性可以归因于顺式异戊二烯的较小变形以及顺式异戊二烯部分与其余金属配合物之间的较强相互作用。的实验观察到惰性的原点2朝向异戊二烯聚合的是，位阻从η的拥挤衍生³ -π-顺式-烯丙基物种篮传入异戊二烯单体的插入。2的造型-介导的链增长也具有高的能垒并且是带负电荷的。为了证实在动力学和热力学方面的空间效应，已经在2的基础上对各种具有较小障碍的类似物进行了建模。预期发现，由这些配合物介导的单体插入具有较低的能垒和有利的热力学，这些配合物在金属中心周围具有较小的空间位阻。