Living radical polymerization transformation reactions

Many block and graft copolymer syntheses involving transformation reactions have been described. These involve preparation of polymeric species by a mechanism that leaves a terminal functionality that allows polymerization to be continued by another mechanism. Such processes are discussed in Section 7.6.2 for cases where one of the steps involves conventional radical polymerization. In this section, we consider cases where at least one of the steps involves living radical polymerization. Numerous examples of converting a preformed end-functional polymer to a macroinitiator for NMP or ATRP or a macro-RAFT agent have been reported.554 The overall process, when it involves RAFT polymerization, is shown in Scheme 9.60. 

Ruthenium catalysts found many applications in C-C bond formation reactions (selected reviews [157-161]). Ruthenium occurs mostly in oxidation states +2 and +3, but lower as well as higher oxidation states can easily be reached. Thus ruthenium compounds are frequently used in oxidative transformations proceeding by either single or two electron transfer pathways (selected reviews [162-164]). It has long been known that ruthenium complexes can be used for the photoactivation of organic molecules (selected reviews [165, 166]). Ruthenium complexes are applied as catalysts in controlled or living radical polymerizations [167-169]. 

Most of the methods for synthesizing block copolymers were described previously. Block copolymers are obtained by step copolymerization of polymers with functional end groups capable of reacting with each other (Sec. 2-13c-2). Sequential polymerization methods by living radical, anionic, cationic, and group transfer propagation were described in Secs. 3-15b-4, 5-4a, and 7-12e. The use of telechelic polymers, coupling and transformations reactions were described in Secs. 5-4b, 5-4c, and 5-4d. A few methods not previously described are considered here. 

Living polymerizations are limited to the realm of chain-growth polymerizations, in which a monomer is transformed to a polymer by a reactive species (an initiator, I) via a kinetic chain reaction (Scheme 15.1). An intrinsic limitation of a typical chain-growth process, such as free-radical polymerization, is the occurrence of termination reactions that lead to the formation of dead chains, chains that are incapable of further growth. 

The transformation reaction may be also possible by using an opposite strategy, from a controlled radical polymerization to a living anionic polymerization. The most widely applied controlled radical polymerization for this particular transformation is ATRP, due mainly to the fact that hydroxyl and amino groups, which are potential initiating sites for the AROP of certain monomers, are compatible with the ATRP of vinyl monomers. Examples of such transformations have been summarized recently [62, 63], while the general concept is shown in Scheme 11.15, based on an example of the combination ATRP of vinyl monomers with the AROP of lactides [71]. 

Figure 8 Main mechanistic transformation reactions in living and/or controlled polymerization methods. ATRP, atom transfer radical polymerization RAFT, reversible addition-fragmentation chain transfer NMRP, nitroxide-mediated free radical polymerization CROP, cationic ring-opening polymerization AROP, anionic ring-opening poiymerization.

Although many examples have been developed based on GRIM polymerization in conjunction with the living/controlled radical polymerization system, tedious site-transformation reactions as well as the limited control nature of radical polymerization frequently cause broad molecular-weight distributions (> 1.3). Furthermore, a thorough purification process to remove the residual copper catalyst from the products is sometimes essential for optoelectronic application. 

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. 

Many polymerization techniques have been combined with CRP through site transformation of the active species. These include non-living techniques like condensation (or step) and conventional free radical processes or living methods like anionic, cationic, and ring-opening polymerizations, as well as others. Early examples were undertaken perhaps just to show that two different techniques could be combined, while later examples show how elegant the combinations have become and provide a foundation for controlled synthesis of materials from any type of monomers. These types of reactions are detailed below. 

A straightforward method is the sequential monomer addition, which in most cases is performed in a one-pot polymerization reaction (Figure 1(a)). Provided that termination and/or transfer reactions are negligible, the consumption of the first monomer is followed by the ability of the macromolecular active sites formed to initiate polymerization of the new incoming chemically different second monomer. This aossover reaction must proceed fast and quantitatively in order to prevent unwanted side reactions, leading to the necessity of specific order in monomer addition depending always on the chosen polymerization technique, that is, anionic or radical, and on the polymerization conditions, such as type of initiator, solvent used, reaction temperature and/or duration of the reaction, and transformation of the living ends. 

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