Anionic polymerization coupling reactions

Anionic polymerizations are well suited for the synthesis of polymers fitted at chain end with reactive functions. Block copolymers can result from reactions between suitable functions carried by two different functional precursors. In some cases the carbanionic sites themselves are the reactive functions. In other cases, functional polymers (obtained anionically, or by other methods) can be reacted with low molecular weight coupling agents. Here are a few examples ... 

An interesting procedure has been proposed for the synthesis of amylose-b-PS block copolymers through the combination of anionic and enzymatic polymerization [131]. PS end-functionalized with primary amine or dimethylsilyl, -SiMe2H groups were prepared by anionic polymerization techniques, as shown in Scheme 56. The PS chains represented by the curved lines in Scheme 56 were further functionalized with maltoheptaose oligomer either through reductive amination (Scheme 57) or hydrosilyla-tion reactions (Scheme 58). In the first case sodium cyanoborohydride was used to couple the saccharide moiety with the PS primary amine group. 

Narrowly distributed Pl-ft-PS-i-PI triblock copolymer chains with both of their ends capped with bromobutyl groups were prepared by sequential addition of living anionic polymerization and terminated by excess of 1,4-dibromobutane (PS block Mw = 3.5 x 103 g/mol PI blocks Mw = 3.1 x 103 g/mol Mw/Mn = 1.12 The degree of end-functionalization was 92% characterized by HNMR). Figure 6 shows the SEC profile of such prepared triblock copolymer chains. The small but a detectable amount ( 5%) of Pl-i-PS-i-PI dimers, PI-Z>-PS-Z>-PI-c-PI-Z>-PS-Z>-PI, is presumably formed via the Wurtz-type coupling reaction. 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

Anionic polymerization techniques have contributed to a very large extent to the development of tailot made molecules of various types. The long life time of the active sites is a factor of decisive importance for such synthesis it enables one to choose at will the molecular weights of the polymers to be made, and it ensures narrow molecular weight distribution of the samples. It involves the possibility of functionalizations at one or more chain ends, and of coupling reactions with bifunc-... 

Various types of well-defined block copolymers containing polypropylene segments have been synthesized by Doi et al. on the basis of three methods (i) sequential coordination polymerization of propylene and ethylene 83-m>, (ii) transformation of living polypropylene ends to radical or cationic ones which initiate the polymerization of polar monomers 104, u2i, and (iii) coupling reaction between iodine-terminated monodisperse polypropylene and living polystyrene anion 84). In particular, the well-defined block copolymers consisting of polypropylene blocks and polar monomer unit blocks are expected to exhibit new characteristic properties owing to the effect of microphase separation. 

The combination of living cationic and living anionic polymerizations provides a unique approach to the synthesis of block copolymers not available by a single method. Coupling of living anionic and cationic polymers is conceptually simple, but few examples have been reported so far. This is most likely due to the different reaction conditions required for living cationic and anionic polymerizations. 

Living anionic polymerization has been used to place a central polystyrene chain between two dendrimers [124]. Prior to the coupling reaction at -78 °C the polystyrylpotassium reactivity is reduced by end-capping with diphenyleth-ylene (Scheme 14e). 

Several studies of the mechanism of the sodium coupling reaction have been carried out, and a mechanistic model has emerged which accounts for most of the experimental observations. The reaction chain appears to be anionic in nature. The first step is believed to be the slow reaction of the dichlorosilane with sodium to give an anion (equation 34). The chain-extending process is the reaction of polymer silyl anions with dichlorosilane molecules (equation 35), followed by rapid reduction of the chlorine-terminated polymer to the anion (equation 36). Reduction must take place at the surface of a sodium particle. As polymerization occurs, some molecules may break free from the sodium surface once in solution away from the metal, chain extension would cease. This portion may give rise to the low-molecular weight polymer. Other molecules probably become entangled at the sodium surface and so cannot break free. They therefore continue to react with fresh dichlorosilane, and eventually... 

Although some block copolymers can be made by other techniques, anionic polymerizations are particularly useful in this application. This is mainly because of the absence of an inherent termination step in some anionic systems and because anions with terminal monomer units of one type can be used to initiate the polymerization of other selected monomers. The different anionic reaction sequences that are employed include sequential monomer addition, coupling reactions, and termination with reactive groups. 

Coupling Reactions. In this technique, a living AB block copolymer is made by monofunctional initiation and is then terminated with a bifunctional coupling agent likeadihaloalkane. ABA copolymers can be madeby joining AB polymeric anions ... 

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