Sequential copolymers, controlled initiation

Possibilities offered by controlled initiation have been exploited for the preparation of new sequential copolymers and mac-romers and are discussed below. 

A large number of sequential copolymers comprising many combinations of glassy and elastomeric segments have been prepared (1). The block and graft copol3nners prepared by controlled initiation are remarkably well defined and many are free of homopolymers. Sequential copol3nners obtained by controlled cationic initiation have been recently comprehensively surveyed (1,8). 

A series of bis(phenoxide) aluminum alkoxides have also been reported as lactone ROP initiators. Complexes (264)-(266) all initiate the well-controlled ROP of CL, NVL.806,807 and L-LA.808 Block copolymers have been prepared by sequential monomer addition, and resumption experiments (addition of a second aliquot of monomer to a living chain) support a living mechanism. The polymerizations are characterized by narrow polydispersities (1.20) and molecular weights close to calculated values. However, other researchers using closely related (267) have reported Mw/Mn values of 1.50 and proposed that an equilibrium between dimeric and monomeric initiator molecules was responsible for an efficiency of 0.36.809 In addition, the polymerization of LA using (268) only achieved a conversion of 15% after 5 days at 80 °C (Mn = 21,070, Mn calc 2,010, Mw/Mn = 1.46).810... 

More recently, Kennedy reported another initiating system that controls styrene polymerization with an added nucleophile 2,2,4-trimeth-ylpentyl chloride (TMP-Cl)/TiCl4 with N,N-dimethylacetamide (DMA) in CH3Cl/methylcyclohexane (4 6 v/v) mixture at -80° C [165]. The use of another additive, 2,6-di-ferf-butylpyr idine (proton trap), is described as beneficial. The molecular weight and MWD are controlled in this system, but the role of the added DMA is still ambiguous [166]. This system with the aliphatic (erf-chloride was designed to extend to the synthesis of isobutene-styrene block copolymers via sequential cationic polymerization (Chapter 5). 

The molecular weight can be controlled by the ratio of the initator to the monomer, the molecular weight distribution by the type of polymerization (discontinuous or continuous) and the modality by single or multiple initiation. Sequential addition of different monomers leads to block copolymers with sharp or tapered transitions. In the presence of Lewis bases, statistical or alternating copolymers can be obtained. 

Recent advances in polymer synthetic chemistry have allowed the development of elegant and more complicated architectural polymers. This has been driven predominantly by the development of various controlled polymerization methodologies, particularly in the area of free radical polymerization [45-49]. This has equipped the polymer chemist with a rich and abundant synthetic toolbox. In general, these architectural polymers are based on the principle of being able to sequentially add different polymeric blocks with defined molecular weight into a single polymer chain [50]. The synthesis of block copolymers is particularly suited to the combination of two different polymerization techniques. This can be quite easily achieved by the use of a bifunctional initiator and is an elegant synthetic... 

The preparation of well-defined sequential copolymers by an anionic mechanism has been explored and utilized commercially for some time now. Initially, the cationic methods received less attention until it was demonstrated by Kennedy that a large variety of block copolymers can be formed. The key to Kennedy s early work is tight control over the polymerization reaction. The initiation and propagation events must be fundamentally similar, although not identical ... 

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