Isotactic placement

Fox and Schneckof carried out the free-radical polymerization of methyl methacrylate between -40 and 250 C. By analysis of the a-methyl peaks in the NMR spectra of the products, they determined the following values of a, the probability of an isotactic placement in the products prepared at the different temperatures ... 

Isobutene polymerized, 157 lsoelectronic system, 246 Isomerization energies, for butane, 75 for pentane, 75 Isoprene, 169 Isotactic placement, 172... 

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]. 

Fig. 8-10 Mechanism of stereoselective polymerization with isotactic placement.

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. 

When nonpolar solvents are employed, polymerization proceeds by an anionic coordination mechanism. The counterion directs isotactic placement of entering monomer units into the polymer chain. The extent of isotactic placement increases with the coordinating power of the counterion (Li > NaK. Cs). The small lithium ion has the greatest coordinating power and yields the most stereoselective polymerization. Increased reaction temperature decreases the isoselectivity. 

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

This indicates that a close contact of the carbanion with the counterion favours isotactic placements as well as short sequence length (corresponding to persistence ratios below 1). In the system Cs/THF the marked non-Bernoullian behaviour can be described by Markovian statistics rather than the Coleman-Fox model, i.e. the penultimate monomer unit influences the stereochemistry of the monomer addition (29). This effect can be interpreted by decreasing external solvation (III,IV) and increasing intramolecular solvation (I,II). 

The addition of the basic DME decreased the probability of isotactic placement by butyllithium. It also altered the nature of the already strongly anionic amylsodium and octylpotassium catalysts. This data shows that the more strongly anionic catalytic system looses the ability for isotactic steric control and produces instead the normal strongly anionic syndiotactic polymethylmethacrylate. 

The signal a of methyl protons consists of two doublet resonances. The weak doublet resonance (d, 8 1.98, JH H = 6.8 Hz) at lower field is related to the erythro (isotactic) placement of the last propylene unit (4-e), whereas the strong doublet resonance (d, 8 1.95, JH H = 6.8 Hz) at higher field is related to the threo (syndiotactic) placement of the same unit (4-t),... 

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 ...

Thus, the preference for syndiotactic placement is to be ascribed entirely to a small additional energy required for isotactic placement,... 

AFf — AFf, calculated from is approximately 0.9 kcal. at pH 2.0 and 1.3 kcal. at pH 10.0. It appears reasonable to interpret the additional 0.4 kcal. free energy requirement at high pH as arising from a greater coulombic repulsion in the activated state for isotactic placement than in the activated state for syndiotactic placement. The ionic strength of the polymerization medium is very high (of the order of 2) at high pH it would therefore appear that an even more marked effect of pH... 

With these catalysts, the cation complexes with the monomer so weakly that a solid surface and low polymerization temperatures are required to achieve sufficient orientation for stereospecificity. Braun, Herner and Kern (217) have shown that lower polymerization temperatures are required (in n-hexane diluent) to obtain isotactic polystyrene as the alkyl metal becomes more electropositive (RNa, —20° C. RK, —60° to —70° C. and RRb, —80° C.). They correlate isotacticity with the polymerization rate as a function of catalyst, temperature or solvent. However, with Alfin catalysts, stereospecific polymerization of styrene is unrelated to rate (226). A helical polymerization mechanism as proposed by Ham (229) and Szwarc (230) is also inadequate for explaining the temperature effects since the probability for adventitious formation of several successive isotactic placements should have been the same at constant temperature in the same solvent for all catalysts. 

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