Coordination polymerization Gibson

Gibson described the synthesis of four-coordinate cationic aluminium alkyls 1 which were reported to be well-defined aluminium polymerization catalysts [12]. However, the polydispersities of the products obtained were high (2.9-6.3), showing that there is not a single well-defined active species. The experiments were carried out in metal autoclaves, and Fe and Co complexes of pyridine-diimine ligands are extremely active in ethene polymerization [34], so a transition-metal impurity does not seem an unreasonable explanation. 

Gibson, V. C. O Reilly, R. K. Wass, D. R White, A. J. R Williams, D. J. Polymerization of methyl methacrylate using four-coordinate (a-diimine)iron catalysts Atom transfer radical polymerization vs. catalytic chain transfer. Macromolecules 2003, 36, 2591-2593. 

Gibson, V.C., et al. (2002). Four-coordinate iron complexes bearing a-diimine ligands efficient catalysts for Atom Transfer Radical Polymerization (ATRP). Chem. Commun. (Cambridge) 1850-1851. 

As stated by Gibson, the most striking feature in the structure of the Bis(imido) chromium(VI) precursor is the manner in which the two benzyl ligands are bonded to the chromium atom through and r coordination modes. Figure 3.30 shows the two cationic complexes prepared from the precursor complex to form ethylene polymerization catalysts. 

Once it was recognized that coordinative unsaturaiion could facilitate insertion, the widely accepted rule of thumb that only early metals could give efficient polymerization catalysts seemed shortsighted. Brookhart et and Gibson... 

Gibson and his co-workers have intensively studied the bidentate diimine ligands, which form four-coordinate iron complexes (Figure 7). An interesting feature of the diimine complexes was to mediate the polymerization of styrene possibly via either the ATRP or GCT process, depending on the ligand stmcture. 

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