Placement isotactic, syndiotactic

For disubstituted ethylenes, the presence and type of tacticity depends on the positions of substitution and the identity of the substituents. In the polymerization of a 1,1-disubstituted ethylene, CH2=CRR, stereoisomerism does not exist if the R and R groups are the same (e.g., isobutylene and vinylidene chloride). When R and R are different (e.g., —CH3 and —COOCH3 in methyl methacrylate), stereoisomerism occurs exactly as in the case of a monosubstituted ethylene. The methyl groups can be located all above or all below the plane of the polymer chain (isotactic), alternately above and below (syndiotactic), or randomly (atactic). The presence of the second substituent has no effect on the situation since steric placement of the first substituent automatically fixes that of the second. The second substituent is isotactic if the first is isotactic, syndiotactic if the first substituent is syndiotactic, and atactic if the first is atactic. 

Fig. 14. The probabilities Pi, and of formation of isotactic, syndiotactic, and heterotactic triads, respectively, as a function, of o, the probability of isotactic placement of monomer units during chain propagation. Experimental points at the left are for methyl methacrylate polymers prepared with free radical initiators . those at the right for polymers prepared with anionic initiators ( O) t peaks ...

Wicke and Elgert studied the mode of a-methylstyrene placement in chains polymerized with butyllithium in tetrahydrofuran. They called attention to the possibility of various addition and depropagation rates for isotactic and syndiotactic monomer placement. Isotactic attachment is harder to form, and easier to decompose. For reversible processes this circumstance must be manifested at higher temperatures by thermodynamic, and at lower temperatures by kinetic, control of product stereoregularity [363],... 

Metallocenes offer a unique opportunity, unmatched by traditional catalysts as well as by other classes of SSCs, to control the placement of propylene. Sequences achievable range from atactic to isotactic. Syndiotactic sequences are not observed in metallocene-based EP copolymers, because the syndioselective metallocenes are characterized by low values of rir2 and thus do not allow the formation of propylene sequences long enough to discriminate between atactic and syndiotactic placement. Only a minor amount of regioirregular propylene placements are detectable in EP copolymers prepared with isoselective metallocenes. It should be noted that presently available aselective metallocenes are not able to prepare EP copolymers with sequences that are simultaneously stereo-and regioirregular. 

The term tactidty refers to the configuration of polymer chains when their constituent monomer residues contain a steric center. Figure 1.8 illustrates the three principal classes of tacticity as exemplified by polypropylene. In isotactic polypropylene, the methyl groups are all positioned on the same side of the chain, as shown in Fig. 1.8 a). In syndiotactic polypropylene, the methyl groups alternate from one side to the other, as shown in Fig. 1.8 b). Random placement of the methyl groups results in atactic polypropylene, which is shown in Fig. 1.8 c). We can readily observe the effects of tacticity on the properties of polypropylene isotactic polypropylene is hard and stiff at room temperature, syndiotactic polypropylene is soft and flexible, and atactic polypropylene is soft and rubbery. 

When adjacent monomers in a backbone share the same stereoconfiguration, the placement is known as a meso diad. When adjacent monomers have opposing stereoconfigurations, the placement is known as a racemic diad. Thus, a purely isotactic polymer comprises all meso placements, and a syndiotactic polymer consists of all racemic placements. 

The Fischer projections show that isotactic placement corresponds to meso or m-place-ment for a pair of consecutive stereocenters. Syndiotactic placement corresponds to racemo (for racemic) or r-placement for a pair of consecutive stereocenters. The configurational... 

It should be noted that other polymer structures can be postulated—those where one substituent is atactic while the other is either isotactic or syndiotactic or those where one substituent is isotactic while the other is syndiotactic. However, these possibilities are rarely observed since the factors that lead to ordering or disordering of one substituent during polymerization generally have the same effect on the other substituent. An exception is the formation of hemiisotactic polypropene where isotactic placements alternate with atactic placements [Coates, 2000]. 

Consider the description of the sequence distribution of isotactic and syndiotactic placements in the polymerization of a monosubstituted ethylene. The approach is general and can be applied with appropriate modification to the 1,4-polymerization of a 1,3-diene. Dyad tac-ticity is defined as the fractions of pairs of adjacent repeating units that are isotactic or syndiotactic to one another. The isotactic and syndiotactic dyads (XV) are usually referred to as meso and racemic dyads. The horizontal line in XV represents a segment of the polymer... 

The value of k,/km is determined by the difference AAG in the free energies of activation between the syndiotactic AG and isotactic AG placements... 

In summary, syndioselective initiators exaggerate the inherent tendency toward syndiotactic placement by accentuating the methyl-methyl repulsive interactions between the propagating chain end and incoming monomer. Isotactic placement occurs against this inherent tendency when chiral active sites force monomer to coordinate with the same enantioface at each propagation step. 

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. 

Both bridged and unbridged C2v-symmetric metallocenes, mostly the unsubstituted biscyclopentadienyl initiators, but also others such as (CH3)2SiFlu2ZrCl2, have been studied. These initiators are achiral, and their two coordination (active) sites are both achiral and homotopic. The result is that atactic polymer is formed via chain end control. Modest tendencies toward slight isotactic or syndiotactic placement are observed for some initiators, depending on the temperature and other reaction conditions. 

What are the mechanisms for syndiotactic and isotactic placements in propene polymerization Describe the reaction conditions that favor each type of stereoselective placement. 

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