Living initiation

Song, J., Bodis, J., and Fhiskas, J.E. Direct functionalization of poly isobutylene by living initiation with alpha-methylstyrene epoxide, J. Polym. Set, Polym. Chem., 40, 1005, 2002. 

Until quite recently, the best known answer to the challenge of combining different properties in an additive manner in a single polymeric product, has been the use of so-called "living" initiators to produce block copolymers, in a 2- or 3-steps process depending wether a di- or a tri-blockwas needed, (see scheme I). 

First, new "living" initiators have been discovered (although not always as efficient), which respond to other mechanisms, i.e. cationic (5) or even radical ones (6), and can accordingly accomodate other types of monomers. A recent typical example is the coordination polymerization of butadiene by bis (n3-allyl-trifluoro-acetato-nickel) to yield a "living" pure 1.4 cis-poly-butadienyl-nickel, able to initiate in turn the polymerization of monomers like isoprene or styrene (7). 

Due to the fact that the living initiator is almost quantitatively located at the surface of the microglobides, the efficiency of metal removal from the monolith after polymerization is high. Investigations revealed that the remaining ruthenium concentration after capping with ethyl vinyl ether is below 10 xg/g, corresponding to a metal removal of more than 99.8%. 

Polymerization of l-105 with 97 (R = ethyl), a living initiator for isocyanide polymerization, simultaneously produces a mixture of diastereomeric right- and left-handed helices with different molecular weights and narrow molecular weight distributions. The two helices can be completely separated by fractionation using acetone. Fractionated polymers exhibited smectic orderings. 

In the different solvents used the cis.trans product ratio varies and, although first-order kinetics operate over more than three half-lives, initial rates only were used. The first-order rate constants are fairly insensitive to the nature of the alcoholic solvent. Unlike the corresponding isomerization of [Pt( -Cl2C— CCl2)(PPhs)2], which apparently occurs by an 5n1 chloride loss and recombination, the chloroalkyne complex isomerizes by an intramolecular pathway more like that found for [Pt( -BrFO= CF2)(AsPh3)2]. These deductions are based on the observed solvent dependence of the rates. 

Chamberlain et al. [13] prepared a series of zinc alkoxide complexes that act as single-site living initiators for the polymerization of DLA or LLA to isotactic PLA, rac-LA to atactic PLA, and mcra-LA to syndiotactic PLA. The synthesis of chiral aluminum and yttrium alkoxides as stereospecific catalysts used for LA polymerization has also been reported by Ovitt and Coates [14]. 

Norbornene can be copolymerized with di-, tri-, and tetrasubstituted fluoroalkyl norbornenes in the presence of the above-mentioned living initiator Mo(CHt-Bu)(NAr)(Ot-Bu)2 to form products of narrow molecular weight distribution (87), having attractive physico-chemical properties [61]. 

Jansen, J.C., Mendichi, R., Sacchi.M.C., and Tritto, 1. (2003) Kinetic studies of the copolymerization of ethylene with norbornene by ansa-zirconocene/methylaluminoxane catalysts Evidence of a long-lasting quasi-living initial period. Macromolecular Chemistry and Physics, 204,522-530. 

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