Cyclopentadienyl -Amido Initiators

The anra-cyclopentadienyl-amido initiators (CpA), often referred to as constrained-geometry initiators, retain one cyclopentadienyl ring of the metallocenes, but replace the other ring with a nitrogen substituent that coordinates with the metal center, usually a group 4 metal 

The absence of a second cyclopentadienyl ring coupled with the short bridge gives a very open environment at the metal site. This allows easier access for bulky monomers, including 1-alkenes and norbomene, compared to polymerization with metallocenes. CpA initiators yield ethylene copolymers not easily available with metallocenes. Copolymers containing significant amounts of comonomers such as styrene, norbomene, and a-olefins from 1-hexene to 1-octadecene are easily obtained with CpA, but not with metallocene or traditional Ziegler-Natta initiators. 

Long-chain branching via the in situ formation of vinyl macromonomers is also accessible with CpA initiators. A key feature of CpA initiators is that polymerizations can be carried out at higher temperatures ( 100°C) to generate the vinyl macromonomer without loss of activity. 

The open nature of the metal site limits catalyst site control by CpA initiators. Polymerization of propene proceeds with weak chain end control at low temperatures. The highest stereoselectivity reported is (mmmm) — 0.77 using Me2Si(Flu)(N-t-Bu)ZrCl2. 

Nickel and palladium a-diimine chelates in the presence of MAO polymerize ethylene to polymers with weight-average molecular weights of 10 -10 . Palladium initiators yield higher molecular weights than do Ni initiators. Unlike Ni and Pd, platinum shows no activity. 

Both Ni and Pd initiators polymerize propene to give a combination of different microstructures, including 1,2- and 3,1-placements as well as methyl branches (via 2,3-placement) and long-chain branching. Room-temperature polymerizations with both Ni and Pd initiators yield atactic polypropene. Low temperature polymerizations proceed by chain end control to yield moderate syndioselectivity, (rr) as high as 0.8, but usually less [Busico and Cipullo, 2001 McCord et al, 2001 Pappalardo et al., 2000 Zambelli et al., 2001], 

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