Catalyst site control

Some chiral initiators have structures such that alternate monomer placements occur with opposite faces of the monomer to yield the syndiotactic polymer. This is syndioselective polymerization proceeding with catalyst site control and is usually observed only with some homogeneous initiators, both traditional Ziegler-Natta and metallocene. 

Fig. 8-11 Mechanism for catalyst site control model of isoselective polymerization. After Cossee [1964] (by permission of Academic Press, New York and Elsevier, Oxford).

Not all syndioselective polymerizations proceed with polymer chain end control. Some metallocene initiators yield syndioselective polymerization through catalyst site control (Sec. 8-5). 

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. 

The polymer stereosequence distributions obtained by NMR analysis are often analyzed by statistical propagation models to gain insight into the propagation mechanism [Bovey, 1972, 1982 Doi, 1979a,b, 1982 Ewen, 1984 Farina, 1987 Inoue et al., 1984 Le Borgne et al., 1988 Randall, 1977 Resconi et al., 2000 Shelden et al., 1965, 1969]. Propagation models exist for both catalyst (initiator) site control (also referred to as enantiomorphic site control) and polymer chain end control. The Bemoullian and Markov models describe polymerizations where stereochemistry is determined by polymer chain end control. The catalyst site control model describes polymerizations where stereochemistry is determined by the initiator. 

Syndioselective polymerization by a Cs metallocene such as Me2C(Cp)(Flu)ZrCl2 proceeds by catalyst site control. A statistical model for syndioselective catalyst site control has been described in terms of the parameter p [Resconi et al., 2000]. Parameter p is the probability of a monomer with a given enantioface inserting at one site of the initiator p is also the probability of the monomer with the opposite enantioface inserting at the other site of the initiator. The pentad fractions are given by... 

The term on the left is extremely sensitive, and this criterion should be used only with sufficiently accurate triad data. This is especially important if the polymer is very highly isotactic or syndiotactic, that is, with very small value of either (rr) or (mm). The term 4(mm)(rr)/ (mr)2 is considerably larger than one for the Markov and catalyst site control models. 

Having established that a particular polymerization follows Bemoullian or first-order Markov or catalyst site control behavior tells us about the mechanism by which polymer stereochemistry is determined. The Bemoullian model describes those polymerizations in which the chain end determines stereochemistry, due to interactions between either the last two units in the chain or the last unit in the chain and the entering monomer. This corresponds to the generally accepted mechanism for polymerizations proceeding in a nonco-ordinated manner to give mostly atactic polymer—ionic polymerizations in polar solvents and free-radical polymerizations. Highly isoselective and syndioselective polymerizations follow the catalyst site control model as expected. Some syndioselective polymerizations follow Markov behavior, which is indicative of a more complex form of chain end control. 

The enantiomorphic catalyst sites control mechanism was found to operate in the stereospecific polymerisation of tiiranes. Sigwalt et al. [79,153] found that... 

The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond of the monomer together with an electrophilic attack by the cationic counterion (G) on the alkene tt-electrons. The catalyst fragment acts essentially as a template or mold for the orientation and isotactic placement of incoming successive monomer units. Isotactic placement occurs because the Initiator fragment forces each monomer unit to approach the propagating center with the same face. This mechanism is referred to as catalyst site control or enantiomorphic site control. 

Isoselective Polymerization Catalyst site control Error 1 MM M M 1 1 1 1 Chain end control Error 1 1 ... 

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