Block copolymer ionic chain polymerization

Figure 14.13 Oil-in-water emulsions may be stabilized by (A) non-ionic surfactants, [B) poloxamer block copolymers or [C) polymeric materials. The hydrophilic chains produce repulsion by mixing interaction [osmotic) or volume restriction [entropic).

Several methods can be used to synthesize block copolymers. Using living polymerization, monomer A is homopolymerized to form a block of A then monomer B is added and reacts with the active chain end of segment A to form a block of B. With careful control of the reaction conditions, this technique can produce a variety of well-defined block copolymers. This ionic technique is discussed in more detail in a later section. Mechanicochemical degradation provides a very useful and simple way to produce polymeric free radicals. When a rubber is mechanically sheared (Ceresa, 1965), as during mastication, a reduction in molecular weight occurs as a result of the physical pulling apart of macromolecules. This chain rupture forms radicals of A and B, which then recombine to form a block copolymer. This is not a preferred method because it usually leads to a mixture of poorly defined block copolymers. 

Ionic polymerizations are almost exclusively solution processes. To produce monodisperse polymers or block copolymers, they must be mn batchwise, so that all chains grow for the same length of time under identical conditions. 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

The same conditions of initiator efficiency are to be observed when the living carban-ionic end of a polymer chain is used to initiate the polymerization of a second monomer 6,7,26.27) for tjjg synthesis of a block copolymer, the monomers are to be added... 

The broad applicability of ATRP in the ionic liquid [BMIMjPFg was shown by extending it to the polymerization of N-substituted maleimides with styrene. The ATRP was initiated with dendritic polyaryl ether 2-bromoisobutyrate as the initiator at room temperature. The dendritic-linear block copolymers formed in the ionic liquid were characterized by low polydispersity (1.05chain-extension polymerization, suggesting the living nature of the polymerization 247). 

This chapter is concerned primarily with the simultaneous polymerization of two monomers to produce statistical and alternating copolymers. The different monomers compete with each other to add to propagating centers, which can be radical or ionic. Graft and block copolymers are not synthesized by the simultaneous and competititive polymerization of two monomers. Each monomer undergoes polymerization alone. A sequence of separate, noncompetitive polymerizations is used to incorporate the different monomers into one polymer chain. The synthesis of block and graft copolymers and variations thereof (e.g., star, comb) are described in Secs. 3-15b-4, 3-15b-5, 5-4, and 9-9. 

For the synthesis of block copolymers chain addition polymerization (ionic or radical) as well as condensation polymerization and stepwise addition polymerization can be used. 

A special kind of termination in ionic polymerizations is the mutual combination of anionic and cationic living chains (see Chap. 5, Sect. 5.8). When the two polymers consist of different monomers, block copolymers are formed. The two macroions can also consist of the same monomer. 

Copolymers can be made not just from two different monomers but from three, four, or even more. They can be made not only by free-radical chain reactions, but by any of the polymerization methods we shall take up ionic, coordination, or step-reaction. The monomer units may be distributed in various ways, depending on the technique used. As we have seen, they may alternate along a chain, either randomly or with varying degrees of regularity. In block copolymers sections made up of one monomer alternate with sections of another ... 

In the case of ordered mesoporous oxides, the templating relies on supramolecular arrays micellar systems formed by surfactants or block copolymers. Surfactants consist of a hydrophihc part, for example, ionic, nonionic, zwitterionic or polymeric groups, often called the head, and a hydrophobic part, the tail, for example, alkyl or polymeric chains. This amphiphiUc character enables surfactant molecules to associate in supramolecular micellar arrays. Single amphiphile molecules tend to associate into aggregates in aqueous solution due to hydrophobic effects. Above a given critical concentration of amphiphiles, called the critical micelle concentration (CMC), formation of an assembly, such as a spherical micelle, is favored. These micellar nanometric aggregates may be structured with different shapes (spherical or cylindrical micelles, layered structures, etc. Fig. 9.8 Reference 70). The formation of micelles. 

Cationic surfactants, in contrast to anionic surfactants, usually reduce both the number of particles involved in the polymerization and the rate of polymerization. The nature of the stabilizing emulsifier has a marked effect on the polymerization kinetics. For example, addition of a non-ionic stabilizer [e.g., poly(vinyl alcohol), a block copolymer of carbowax 6000 and vinyl acetate, or ethylene oxide-alkyl phenol condensates] to a seed polymer stabilized by an anionic surfactant decreased the rate of polymerization to 25% of the original rate. The effect was as if the nonionic stabilizer (or protective colloid) acted as a barrier around the seed particles to alter the over-all kinetics. It may be that the viscosity of the medium in the neighborhood of the nonionic surfactant coating of the polymer particle is sufficiently different from that of an anionic layer to interfere with the diffusion of monomer or free radicals. There may also be a change in the chain-transfer characteristics of the system [156]. 

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