Dual-catalyst systems, reversible chain transfer

Reversible Chain Transfer in Dual-Catalyst Systems. 71... 

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

While CCTP character has been demonstrated in several single catalyst systems, examples of the use of main-group metals to exchange polymer chains between two different catalysts in a common reaction environment are much less common. Dual-catalyst systems claimed to demonstrate some level of reversible chain transfer (see Table 7) are summarized below for both ethylene- and propylene-based systems. Characterization data demonstrating that the primary product is BCP are described in greater detail in Section 3.22.9. 

As stated above, reversible chain transfer between two different catalysts, also known as chain shuttling, is a useful strategy for the catalytic synthesis of BCPs. BCPs can be envisioned from dual-catalyst chain shuttling processes through both homopolymerization and copolymerization systems. In either case, catalysts must be selected that give polymers with chemically... 

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See also in sourсe #XX -- [ ]

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