Polymer chain-end control

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. 

The Bemoullian model (also referred to as the zero-order Markov model) assumes that only the last monomer unit in the propagating chain end is important in determining polymer stereochemistry. Polymer stereochemistry is not affected by the penultimate unit or units 

Probahilities Pm and Pr, the transition or conditional probabilities of forming meso and racemic dyads, respectively, are defined by 

Probabilities Pm and P, are synonymous with the dyad tactic fractions (m) and (r) defined in Sec. 8-2b. Triad probabilities, synonymous with the triad fractions, follow as 

The probability of forming a particular triad is the product of the probabilities of forming the two dyads making up the triad. The coefficient of 2 for the heterotactic triad is required since the heterotactic triad is produced in two ways thus, mr is produced as mr and rm. Tetrad probabilities are given by 

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Fig. 8-13 Polymer chain end control model for syndioselective polymerization. After Boor and Youngman [1966] (by permission of Wiley-Interscience, New York).

The polymer chain end control model is supported by the observation that highly syndiotactic polypropene is obtained only at low temperatures (about —78°C). Syndiotacticity is significantly decreased by raising the temperature to —40°C [Boor, 1979]. The polymer is atactic when polymerization is carried out above 0°C. 13C NMR analysis of the stereoerrors and stereochemical sequence distributions (Table 8-3 and Sec. 8-16) also support the polymer chain end control model [Zambelli et al., 2001], Analysis of propene-ethylene copolymers of low ethylene content produced by vanadium initiators indicates that a syndiotactic block formed after an ethylene unit enters the polymer chain is just as likely to start with an S- placement as with an R-placement of the first propene unit in that block [Bovey et al., 1974 Zambelli et al., 1971, 1978, 1979]. Stereocontrol is not exerted by chiral sites as in isotactic placement, which favors only one type of placement (either S- or R-, depending on the chirality of the active site). Stereocontrol is exerted by the chain end. An ethylene terminal unit has no preference for either placement, since there are no differences in repulsive interactions. 

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

Czv-Symmetric Catalysts. Syndiotactic polymers have been formed using metallocene catalysts where the polymer chain end controls the syndiospecificity of olefin insertion. Resconi has shown that Cp 2MCl2 (M = Zr. Hf) derived catalysts produce predominantly syndiotactic poly(l-butene) with an approximate 2 kcal/mol preference for syndiotactic versus isotactic dyad formation." At —20 °C. Cp 2HfCl2/MAO produces poly(l-butene) with 77% rr triads. Pellecchia had reported that the diimine-ligated nickel complex 30 forms moderately syndiotactic polypropylene at —78 °C when activated with MAO ([rr] = 0.80)." " Olefin insertion was shown to proceed by a 1.2-addition mechanism." in contrast to the related iron-based systems which insert propylene with 2.1-regiochemistry. ... 

Syndiotactic polymers have also been synthesized using metallocene catalysts where the polymer chain end controls the syndiospecificity of olefin insertion. Resconi has shown that Cp 2MCl2 (M = Zr, Hf) derived catalysts produce predominantly syndiotactic... 

In a chain-growth polymerization reaction, one end of the polymer chain remains at the active metal center during monomer enchainment. Thus, the stereogenic center in the polymer chain from the last enchained monomer unit will have an influence on the stereochemistry of monomer enchainment. If this influence is significant, the mode of stereochemical regulation is referred to as polymer chain-end control (Scheme 2(a)). If the active site is chiral and overrides the influence of the polymer chain end, the mechanism of... 

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