Santhosh Kumar, Gnanou Yves, Champouret Yohan, Daran Jean-Claude, Poli Rinaldo
Laboratoire de Chimie des Polymères Organiques, ENSCPB-CNRS-Université Bordeaux, 1, 16, Avenue Pey Berland, 33607 Pessac Cedex, France.
Chemistry. 2009;15(19):4874-85. doi: 10.1002/chem.200802388.
The complex [Co(II)(tmhd)(2)] (4; tmhd = 2,2,6,6-tetramethylhepta-3,5-dionato) has been investigated as a mediator for controlled radical polymerization of vinyl acetate (VAc) and compared with the analogue [Co(II)(acac)(2)] (1; acac = acetylacetonato). A relatively well controlled process occurs, after an induction time, with 2,2'-azobis(4-methoxyl-2,4-dimethylvaleronitrile) (V-70) as radical initiator at 30 degrees C. However, whereas the polymerization essentially stops after about six initiator half-lives in the presence of 1, it continues with a first-order rate law in the presence of 4. The successful simulation of the kinetic data shows that 4 operates simultaneously by associative (degenerative transfer, DT) and dissociative (organometallic radical polymerization, OMRP) mechanisms. The occurrence of OMRP was confirmed by an independent polymerization experiment starting from an isolated and purified Co(tmhd)(2) macroinitiator. The polymer molecular weight evolves linearly with conversion in accordance with the expected values for one chain per Co atom when DT is the predominant mechanism and also during the pure OMRP process; however, observation of stagnating molecular weights at long reaction times with concomitant breakdown of the first-order rate law for monomer consumption indicates a competitive chain-transfer process catalyzed by an increasing amount of Co(II). In the presence of external donors L (water, pyridine, triethylamine) the DT pathway is blocked and the OMRP pathway is accelerated, and polymerization with complex 4 is then about five times slower than with complex 1. The reversal of relative effective OMRP rate constants k(eff) (4>1 in the absence of external donors, 4<1 in their presence) is rationalized through competitive steric effects on Co(III)-C and Co(II)-L bond strengths. These propositions are supported by (1)H NMR studies and by DFT calculations.
已对配合物[Co(II)(tmhd)₂]（4；tmhd = 2,2,6,6 - 四甲基庚 - 3,5 - 二酮酸根）作为醋酸乙烯酯（VAc）可控自由基聚合的引发剂进行了研究，并与类似物[Co(II)(acac)₂]（1；acac = 乙酰丙酮根）进行了比较。在30℃下，以2,2'-偶氮二(4 - 甲氧基 - 2,4 - 二甲基戊腈)（V - 70）作为自由基引发剂，经过诱导期后会发生相对可控的聚合反应。然而，在1存在的情况下，聚合反应在大约六个引发剂半衰期后基本停止，而在4存在的情况下，聚合反应以一级速率定律继续进行。动力学数据的成功模拟表明，4通过缔合（退化转移，DT）和解离（有机金属自由基聚合，OMRP）机制同时起作用。通过从分离纯化的Co(tmhd)₂大分子引发剂开始的独立聚合实验证实了OMRP的发生。当DT是主要机制时以及在纯OMRP过程中，聚合物分子量随转化率呈线性增长，符合每个Co原子一条链的预期值；然而，在长反应时间观察到分子量停滞，同时单体消耗的一级速率定律被破坏，这表明存在由不断增加的Co(II)催化的竞争性链转移过程。在外部给体L（水、吡啶、三乙胺）存在的情况下，DT途径被阻断，OMRP途径加速，此时配合物4引发的聚合反应比配合物1引发的聚合反应慢约五倍。相对有效OMRP速率常数k(eff)的反转（在没有外部给体时4＞1，在有外部给体时4＜1）通过对Co(III)-C和Co(II)-L键强度的竞争性空间效应得到合理解释。这些观点得到了¹H NMR研究和DFT计算的支持。
