来源：ACS Publications
The impact of the group 13 metals aluminum, gallium, and indium on isoprene polymerization was investigated for binary catalyst systems composed of the homoleptic tetramethyltrielate rare-earth-metal compounds Ln(EMe4)3 (Ln = La, Nd, Y; E = Al, Ga, In) and the alkylated metal chlorides R2ECl (E = Al, Ga, In; R = Me, Et) at varying nLn: nCl ratios. The formation of the active species is controlled by a favorable association of the precatalyst Ln(EMe4)3 with the cocatalyst R2ECl and concomitant halogenation of the rare-earth-metal center. Group 13 metal alkyl species also have a stabilizing/mediating effect on the active catalyst because a very weak association can lead to temperature-dependent decomposition reactions. A significant “indium effect” was detected for the small yttrium center. The ternary mixture of [YMe3]n/InMe3/Et2InCl (nY:nIn:nCl = 1:3:2) fabricated cis-1,4 polyisoprene (PI) selectively (99%), exceeding the selectivities of Y(AlMe4)3 (47% cis-1,4 PI) and Y(GaMe4)3 (85% cis-1,4 PI) by far. Note that Y(InMe4)3 is not isolable. The importance of balancing the association/dissociation of the active species with the group 13 metal was also revealed when the precatalyst Ln(EMe4)3 was combined with the borate cocatalysts [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4]. The binary catalyst system Nd(InMe4)3/B(C6F5)3 produced PI with a remarkably high cis-1,4 selectivity of 92%, compared to Nd(AlMe4)3/B(C6F5)3 (57% cis-1,4 PI). This study demonstrates that the group 13 metal alkyl has a significant stereoregulating effect, most likely driven by the degree of association with the active rare-earth-metal center.