Propagation isotactic

What is significant about these reactions is that only two possibilities exist addition with the same configuration (D -> DD or L LL) or addition with th< opposite configuration (D DL or L LD). We shall designate these isotactic (subscript i) or syndiotactic (subscript s) additions, respectively, and shal define the rate constants for the two steps kj and k. Therefore the rates o isotactic and syndiotactic propagation become... 

The propagating polymer then terminates, producing an isotactic polypropylene. Linear polyethylene occurs whether the reaction takes place by insertion through this sequence or, as explained earlier, by ligand occupation of any available vacant site. This course, however, results in a syndiotactic polypropylene when propylene is the ligand. 

Polymerization of t-butyl methacrylate initiated by lithium compounds in toluene yields 100% isotactic polymers 64,65), and significantly, of a nearly uniform molecular-weight, while the isotactic polymethyl methacrylate formed under these conditions has a bimodal distribution. Significantly, the propagation of the lithium pairs of the t-Bu ester carbanion, is faster in toluene than in THF. In hydrocarbon solvents the monomers seem to interact strongly with the Li+ cations in the transition state of the addition, while the conventional direct monomer interaction with carbanions, that requires partial dissociation of ion-pair in the transition state of propagation, governs the addition in ethereal solvents. 

The absolute value of the propagation constant was determined89). The detailed mechanism of this complex reaction and the factors determining the isotacticity were discussed in a following paper90). 

By studying mixtures of L- and D-LA of varying composition, Spassky et al. have demonstrated that (256) yields PLA containing long isotactic sequences, with a ratio of homo cross propagation of 2.8.796 Hence, an 80/20 L D mixture when polymerized to 70% conversion displayed an optical purity of 87%. Even at the relatively low optical purity of 65/35 L D, isotactic block lengths of >12 repeat units were reported. Achiral (259) converts rac-LA into highly isotactic PLA a Tm of 192 °C indicates that the chains of P(L-LA) and P(D-LA) form a stereocomplex.792 797-799... 

Diastereoisomeric transition states calculated for propene primary insertion in a model of the Ewen achiral metallocenes are shown in Figure 1.20. The two possible diastereomeric transition states correspond to si (Figure 1.20a) and re (Figure 1.20b) insertions of the monomer for the case of a si chain (i.e., a growing chain in which the last monomeric unit has been obtained by a cis addition of a -coordinated monomer molecule) and are suitable for like (isotactic) and unlike (syndiotactic) propagations, respectively.142,143... 

The probabilistic aspect of error propagation in isotactic polypropylene was treated both as a second-order Markov chain (in terms of m and r dyads) (408) and, in terms of a model of enantiomorphic catalyst sites, as asymmetric Ber-... 

The driving force for isoselective propagation results from steric and electrostatic interactions between the substituent of the incoming monomer and the ligands of the transition metal. The chirality of the active site dictates that monomer coordinate to the transition metal vacancy primarily through one of the two enantiofaces. Actives sites XXI and XXII each yield isotactic polymer molecules through nearly exclusive coordination with the re and si monomer enantioface, respectively, or vice versa. That is, we may not know which enantio-face will coordinate with XXI and which enantioface with XXII, but it is clear that only one of the enantiofaces will coordinate with XXI while the opposite enantioface will coordinate with XXn. This is the catalyst (initiator) site control or enantiomorphic site control model for isoselective polymerization. 

C NMR of isotactic polypropene shows the main error is pairs of racemic dyads instead of isolated racemic dyads (Table 8-3) [Heatley et al., 1969 Resconi et al., 2000 Wolfsgruber et al., 1975]. A stereoerror in the addition of a monomer molecule is immediately corrected when stereocontrol is by the chiral active site. If stereocontrol was due to the propagating chain end, an error would continue to propagate in an isotactic manner to yield a polymer, referred to as an isotactic stereoblock, containing long isotactic all-R and all-5 stereoblocks on each side of the error. 

C NMR of ethylene-propene copolymers of low ethylene content produced by initiators that yield isotactic polypropene shows that the isotactic propene units on each side of an ethylene unit have the same configuration (i.e., all-R or all-5) [Zambelli et al., 1971, 1978, 1979]. For stereocontrol by the propagating chain end, the amount of polymer in which the polypropene blocks on either side of an ethylene unit have the same configuration would equal that in which the blocks have the opposite configuration. 

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