Popeney Chris S, Guan Zhibin
Department of Chemistry, University of California, 1102 Natural Sciences 2, Irvine, California 92697, USA.
J Am Chem Soc. 2009 Sep 2;131(34):12384-93. doi: 10.1021/ja904471v.
A detailed mechanistic investigation of the copolymerization of ethylene and methyl acrylate (MA) by a Pd(II) cyclophane-based alpha-diimine catalyst is reported. Our previous observations of unusually high incorporations of acrylates in copolymerization using this catalyst (J. Am. Chem. Soc. 2007, 129, 10062) prompted us to conduct a full mechanistic study on ethylene/MA copolymerization, which indicates a dramatic departure from normal Curtin-Hammett kinetic behavior as observed in copolymerization using the normal Brookhart type of Pd(II) alpha-diimine catalysts. Further investigation reveals that this contrasting behavior originates from the axial blocking effect of the cyclophane ligand hindering olefin substitution and equilibration. In equilibrium studies of ethylene with nitriles, the cyclophane catalyst was found to more strongly favor the linearly binding nitrile ligands as compared to the standard acyclic Pd(II) alpha-diimine catalysts. Ethylene exchange rates in the complexes (N--N)PdMe(C(2)H(4)) (N--N = diimine) were measured by 2D EXSY NMR spectroscopy and found to be over 100 times slower in the cyclophane case. Measurement of the slow equilibration of ethylene, methyl acrylate, and 4-methoxystyrene in cyclophane-based Pd(II) olefin complexes by (1)H NMR and fitting of the obtained kinetic plots allowed for the estimation of exchange rates and equilibrium constants of the olefins. After extrapolation to typical polymerization temperature, DeltaG(double dagger) = 20.6 and 16.4 kcal/mol for ethylene-methyl acrylate exchange in the forward (ethylene displacement by methyl acrylate) and reverse directions, respectively. These values are of similar magnitude to the previously determined migratory insertion barriers of ethylene (DeltaG(double dagger) = 18.9 kcal/mol) and methyl acrylate (DeltaG(double dagger) = 16.3 kcal/mol) under equivalent conditions, but contrast strongly to the rapid olefin exchange seen in the Brookhart acyclic catalyst. The large barrier to olefin exchange hinders olefin pre-equilibrium, decreasing the cyclophane catalyst's ability to preferentially incorporate one monomer (in this case ethylene) over the other, thus giving rise to high comonomer incorporations.
本文报道了基于环芳基钯（II）α-二亚胺催化剂对乙烯与丙烯酸甲酯（MA）共聚反应的详细机理研究。我们之前观察到使用该催化剂进行共聚反应时丙烯酸酯的异常高掺入量（《美国化学会志》，2007年，129卷，10062页）促使我们对乙烯/MA共聚反应进行全面的机理研究，结果表明其与使用常规布鲁克哈特型钯（II）α-二亚胺催化剂进行共聚反应时观察到的正常柯廷-哈米特动力学行为有显著差异。进一步研究表明，这种截然不同的行为源于环芳基配体的轴向位阻效应阻碍了烯烃取代和平衡。在乙烯与腈的平衡研究中发现，与标准的无环钯（II）α-二亚胺催化剂相比，环芳基催化剂更倾向于线性结合腈配体。通过二维交换光谱法（2D EXSY NMR）测量了配合物[(N–N)PdMe(C₂H₄)]⁺（N–N =二亚胺）中的乙烯交换速率，发现环芳基情况下的交换速率慢了100多倍。通过¹H NMR测量环芳基钯（II）烯烃配合物中乙烯、丙烯酸甲酯和4-甲氧基苯乙烯的缓慢平衡，并对所得动力学曲线进行拟合，从而估算烯烃的交换速率和平衡常数。外推至典型聚合温度后，乙烯-丙烯酸甲酯正向（丙烯酸甲酯取代乙烯）和反向交换的ΔG‡分别为20.6和16.4 kcal/mol。这些值与之前在等效条件下测定的乙烯迁移插入势垒（ΔG‡ = 18.9 kcal/mol）和丙烯酸甲酯迁移插入势垒（ΔG‡ = 16.3 kcal/mol）大小相似，但与布鲁克哈特无环催化剂中快速的烯烃交换形成强烈对比。烯烃交换的高势垒阻碍了烯烃预平衡，降低了环芳基催化剂优先掺入一种单体（在此为乙烯）而非另一种单体的能力，从而导致共聚单体的高掺入量。
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