Syndioselective polymerization

Stereoselective polymerizations yielding isotactic and syndiotactic polymers are termed isoselective and syndioselective polymerizations, respectively. The polymer structures are termed stereoregular polymers. The terms isotactic and syndiotactic are placed before the name of a polymer to indicate the respective tactic structures, such as isotactic polypro-pene and syndiotactic polypropene. The absence of these terms denotes the atactic structure polypropene means atactic polypropene. The prefixes it- and st- together with the formula of the polymer, have been suggested for the same purpose it-[CH2CH(CH3)] and st-[CH2 CH(CH3)] [IUPAC, 1966],... 

While the properties and applications of isotactic polymers have been extensively studied, those of syndiotactic polymers received less attention until relatively recently. The reason is the relative ease of forming isotactic polymers. Syndioselective polymerizations were less frequently encountered or proceeded with less efficiency compared to isoselective polymerizations. But the situation is changing fast as initiators and reaction conditions have been developed for syndioselective polymerizations. In the case of polypropene, the properties of the syndiotactic polymer have been examined [Youngman and Boor, 1967]. Syndiotactic polypropene, like its isotactic counterpart, is easily crystallized, but it has a lower Tm by about 20°C and is more soluble in ether and hydrocarbon solvents. 

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. 

Syndioselective polymerizations of propene are somewhat less regioselective than the isoselective reactions, with the typical highly syndiotactic polymer showing a few percent of the monomer units in head-to-head placement [Doi, 1979a,b Doi et al., 1984a,b, 1985 Zambelli et al., 1974, 1987]. The mode of insertion is secondary, contrary to what is expected for a carbanion propagating center. Apparently, steric requirements imposed by the counterion derived from the initiator force propagation to proceed by secondary insertion. 

The initiator formed from VCLt and A1(C2H5)2C1 is one of the most efficient means for syndioselective polymerization of propene, especially in the presence of a Lewis base such as anisole (methoxybenzene) [Doi, 1979a,b Natta et al., 1962 Zambelli et al., 1978, 1980], Other vanadium compounds such as vanadium acetylacetonate and various vanadates [VO(OR)xClp x), where x — 1,2,3] can be used in place of VCI4 but are more limited in their stereoselectivity [Doi et al., 1979]. Trialkylaluminum can also be used as a coinitiator, but only for VCI4. Syndiotacticity increases with decreasing temperature most of these syndioselective polymerizations are carried out below —40°C and usually at —78°C. The initiators must be prepared and used at low temperatures since most of them undergo decomposition at ambient and higher temperatures. There is considerable reduction of V(III) to V(II) with precipitation of ill-defined products that are low in activity and do not produce syndiotactic polymer, when the initiators are prepared at or warmed to temperatures above ambient. 

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

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

The effect of the transition metal is similar to that in the C2 metallocenes. However, the differences between different metals are greater for the Cs metallocenes for example, titanium metallocenes do no yield even modestly syndioselective polymerization under any conditions of temperature and monomer concentration. 

The stereoselectivity of polymerization depends on the transition metal and the structure of the initiator. Syndioselective polymerization is more common than isoselective polymerization. Some titanium phenoxy-imine initiators yield highly syndioselective polymerization by chain end control. For example the initiator with R2 = R3 = t-butyl yields polypropene with (rr) = 0.92 [Tian and Coates, 2000]. The initiator with R2 = t-butyl and R1 = C6Fs yields polypropene with (rr) = 0.98 [Saito et al., 2001 Tian et al., 2001], Moderately isoselective polymerization is obtained with some zirconium and hafnium phenoxy-imine initiators [Saito et al., 2002]. 

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... 

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. 

SCHEME 2.2 Site epimerization and enantiofacial misinsertion are two principal sources of stereoerrors commonly observed in syndioselective polymerizations. The bridge substituents have been omitted for clarity P represents the growing polymer chain and M = cationic Zr+ or Hf ", generally. 

Transition Metal Complexes for the Syndioselective Polymerization of Styrene 365... 

Apart from Ti(IV), titanium complexes of other oxidation states (Ti(III) and Ti(II)), such as Ti(acac)3 (acac = acetylacetonate) and TiPh2, also show syndioselective polymerization activity and produce syndiotactic polystyrene. As for the Ti(0) complex, Ti(bipy)3 (bipy = 2,2 -bipyridine), only atactic polymer is produced (Table 14.1, entries 10-12). ... 

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