Ionic polymerization group-transfer

Examples of vinyl monomers for addition polymerization include acrylates, methacrylates, vinyl ethers and styrene derivatives. Radical, ionic, and group-transfer polymerizations are possible according to polymerizabil-ity of the monomers. Living polymerization is difficult because mesogenic monomers often contain bonds such as benzoate ester, which are easily attacked by growing ends. Cyclic and condensation monomers are less... 

The term acrylic apphes to a family of copolymers of monomers that are polymerized by a chain growth mechanism. Most often, the mechanism of polymerization is by free radical initiation. Other mechanisms of polymerization, such as ionic and group transfer polymerization, are possible but will not be discussed in this publication. For a description of other polymerization mechanisms, polymer textbooks are available (5,6). Technically, acrylic monomers are derivatives of acrylic or methacrylic acid. These derivatives are nonfunctional esters (methyl methacrylate, butyl acrylate, etc.), amides (acrylamide), nitrile (acrylonitrile), and esters that contain functional groups (hydroxyethyl acrylate, glycidyl methacrylate, dimethylaminoethyl acrylate). Other monomers that are not acryhc derivatives are often included as components of acryhc resins because they are readily copolymerized with the acryhc derivatives. Styrene is often used in significant quantities in acryhc copolymers. 

Although group transfer polymerization does not involve ionic reactions, it is reviewed in this chapter because it bears many practical similarities to anionic polymerizations and is an alternative route to (mcth-)acrylic polymers and block... 

In contrast to radical polymerizations in which there is only one type of propagating species, ionic polymerizations may involve several active species, each with different reactivities and/or lifetimes. As outlined in Scheme 2, ionic polymerizations may potentially involve equilibria between covalent dormant species, contact ion pairs, aggregates, solvent separated ion pairs, and free ions. Although ion pairs involving alkali metal countercations can not collapse to form covalent species, group transfer polymerization apparently operates by this mechanism. In anionic polymerizations, free ions are much more reactive than ion pairs although the dissociation constants are quite small = 10 ) [5]. 

TYPES OF POLYMERIZATION Free-radical or ionic polymerization of methacrylonitrile (2-cyanopropylene) in bulk, emulsion, or solution group-transfer polymerization also has been used. Ionic polymerization in inert solvents can produce either amorphous poly(methacrylonitrile) (by use of anionic catalysts such as n-butyllithium) or primarily isotactic poly(methacrylonitrile) (by use of coordination catalysts such as ethylberyllium or diethylmagnesium). 

The benzyl ester of malolactone was used as the monomer instead of malolactonic acid to eliminate potential problems in the ionic polymerization reactions. The carboxylic acid group, if present, could react with the initiators or cause either chain transfer or termination reactions during the polymerization. The polyester of this pendant ester was readily converted to poly-3-malic acid by hydrogenolysis without change in its molecular weight, according to the following reaction ... 

The existence of a dynamic equilibrium between dormant (covalent) and active (ionic) species in controlled carbocationic polymerizations had been debated for years. It has been argued that under certain conditions, polarized covalent species can directly react with monomer examples are the pseudocationic mechanism proposed for the polymerization of styrene initiated by perchloric acid (123,124) (Fig. 5) or the two-component group transfer polymerization proposed for the polymerization of isobutylene initiated by the dicumylacetate/BCls system (125) (Fig. 6). Recent results and theoretical considerations support the now generally accepted view that the true active species are ions, and the dormant species serve as a reservoir from which the propagating ion pairs are formed (126-131). The existence of a dynamic equilibrium between dormant and active species and the ability to suppress the formation of free ions made possible the synthesis of pol5miers with controlled molecular architecture via carbocationic polymerization. 

Living ionic polymerization is frequently used to synthesize amphiphilic block copolymers in the absence of irreversible chain transfer and chain termination, where aU polymer chains are instantaneously initiated and grow simultaneously [30]. The presence of counterions in the reaction medium associated witli the propagating chains preserves the electroneutrality of the entire system. However, these living polymerization methods are affected significantly by the nature of the solvent and the presence of water, carbon dioxide, and/or impurities. Moreover, they have limited apphcations for the synthesis of block copolymers with functional groups as side chains. 

---