Radical polymerization, block copolymers

The resulting polymers can further induce the radical polymerization of second monomers to give block copolymers. The polymerization with the alkoxyamines has developed to the recent living radical polymerization providing polymers with well-controlled molecular weight and molecular weight distribution, as will be described in Sect 6.1. 

Block Copolymers from Polymeric Azo Initiators 4.1 Radical Block Copolymerization... 

Fig. 15 Solid phase strategy towards peptide-containing block copolymers by polymerization from the supported peptide via atom transfer radical polymerization [66]...

The main techniques for synthesis of block copolymers in research laboratories around the world are presently anionic polymerization and living polymerization, cationic and radical. The older technique of anionic polymerization is still used widely in the industrial manufacture of block copolymers. Cationic polymerization may be used to polymerize monomers that cannot be polymerized anionically, although it is used for only a limited range of monomers. A summary of block copolymer synthesis techniques has been provided (5). 

The isolated polymers could further initiate the polymerization in the presence of second monomer to give block copolymers. Iniferter polymerization has three main characteristics over classical radical polymerization ... 

Polymerization of methacrylates is also possible via what is known as group-transfer polymerization. Although only limited commercial use has been made of this technique, it does provide a route to block copolymers that is not available from ordinary free-radical polymerizations. In a prototypical group-transfer polymerization the fluoride-ion-catalyzed reaction of a methacrylate (or acrylate) in the presence of a silyl ketene acetal gives a high molecular weight polymer (45—50). 

If (P ) is terminated by a chain transfer to a solvent or a monomer, a graft copolymer is formed, or, if the termination is from a combination, a crosslinked network polymer is formed. If the pre-existing polymer (B) contains an end group that itself is photosensitive (or can produce a radical by interacting with photoinitiator) and in the presence of a vinyl monomer (A), block copolymer of type AB can be produced if the photosensitive group is on one end of the polymeric chain. Type ABA block copolymer can be produced if the polymer chain (B) contains a photosensitive group on both ends. 

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,... 

A number of techniques for the preparation of block copolymers have been developed. Living polymerization is an elegant method for the controlled synthesis of block copolymers. However, this technique requires extraordinarily high purity and is limited to ionically polymerizable monomers. The synthesis of block copolymers by a radical reaction is less sensitive toward impurities present in the reaction mixture and is applicable to a great number of monomers. 

Low-molecular weight azo compounds have frequently been used in cationic polymerizations producing azo-containing polymers. Thus, the combination of ionically and radically polymerizable monomers into block copolymers has been achieved. Azo compounds were used in all steps of cationic polymerization without any loss of azo function as initiators, as monomers and, finally, as terminating agents. 

MAIs may also be formed free radically when all azo sites are identical and have, therefore, the same reactivity. In this case the reaction with monomer A will be interrupted prior to the complete decomposition of all azo groups. So, Dicke and Heitz [49] partially decomposed poly(azoester)s in the presence of acrylamide. The reaction time was adjusted to a 37% decomposition of the azo groups. Surface active MAIs (M, > 10 ) consisting of hydrophobic poly(azoester) and hydrophilic poly(acrylamide) blocks were obtained (see Scheme 22) These were used for emulsion polymerization of vinyl acetate—in the polymerization they act simultaneously as emulsifiers (surface activity) and initiators (azo groups). Thus, a ternary block copolymer was synthesized fairly elegantly. 

Hepuzer et al. [91] have used the photoinduced homolytical bond scission of ACPB to produce styrene-based MAIs. These compounds were in a second thermally induced polymerization transferred into styrene-methacrylate block copolymers. However, as Scheme 24 implies, benzoin radicals are formed upon photolysis. In the subsequent polymerization they will react with monomer yielding nonazofunctionalized polymer. The relatively high amount of homopolymer has to be separated from the block copolymer formed after the second, thermally induced polymerization step. 

Moreover, block copolymers with two radically polymerizable monomers can be synthesized with a combination of thermal and photochemical polymerizations. Regarding their utilization in block copolymer synthesis, azocompounds with photoactive benzoin [103,109-111] and azyloximester groups [112] have been described. Two low-molecular weight azo benzoin initiators of the general formula (Scheme 32) were synthe-... 

Generation of radicals by redox reactions has also been applied for synthesizing block copolymers. As was mentioned in Section II. D. (see Scheme 23), Ce(IV) is able to form radical sites in hydroxyl-terminated compounds. Thus, Erim et al. [116] produced a hydroxyl-terminated poly(acrylamid) by thermal polymerization using 4,4-azobis(4-cyano pentanol). The polymer formed was in a second step treated with ceric (IV) ammonium nitrate, hence generating oxygen centered radicals capable of starting a second free radical polymeriza-... 

Block copolymers have been synthesized on an industrial scale mainly by anionic or cationic polymerization, although monomers for block components are limited to ones capable of the process. Intensive academic and technological interest in radical block copolymerization using macroinitiators is growing. This process can be implemented in plants with easier handling of materials, milder conditions of operation, and a variety of materials to give various kinds of block copolymers to develop a wide application area [1-3]. 

Polystyrene homopolymer produced by free radical initiators is highly amorphous (Tg = 100°C). The general purpose rubber (SBR), a block copolymer with 75% butadiene, is produced by anionic polymerization. 

Currently, more SBR is produced by copolymerizing the two monomers with anionic or coordination catalysts. The formed copolymer has better mechanical properties and a narrower molecular weight distribution. A random copolymer with ordered sequence can also be made in solution using butyllithium, provided that the two monomers are charged slowly. Block copolymers of butadiene and styrene may be produced in solution using coordination or anionic catalysts. Butadiene polymerizes first until it is consumed, then styrene starts to polymerize. SBR produced by coordinaton catalysts has better tensile strength than that produced by free radical initiators. 

The C-S bond of the sulfide end groups can be relatively weak and susceptible to thermal and photo- or radical-induced homolysis. This means that certain disulfides [for example 7-9] may act as iniferters in living radical polymerization and they can be used as precursors to block copolymers (Sections 7.5.1 and 9.3.2). 

Disulfide derivatives and hexasubstituted ethanes2,15 may also be used in this context to make cnd-functional polymers and block copolymers. The use of dilhiuram disulfides as thermal initiators was explored by Clouet, Nair and coworkers.206 Chain ends are formed by primary radical termination and by transfer to the dilhiuram disulfide. The chain ends formed are thermally stable under normal polymerization conditions. The use of similar compounds as photoin iferters, when some living characteristics may be achieved, is described in Section 9.3.2.1.1. 

Many block and graft copolymer syntheses involve radical polymerization at some stage of the overall preparation. This section deals with direct syntheses of... 

The multifunctional initiators may be di- and tri-, azo- or peroxy-compounds of defined structure (c.g. 20256) or they may be polymeric azo- or peroxy-compounds where the radical generating functions may be present as side chains 57 or as part of the polymer backbone."58"261 Thus, amphiphilic block copolymers were synthesized using the polymeric initiator 21 formed from the reaction between an a,to-diol and AIBN (Scheme 7.22).26 Some further examples of multifunctional initiators were mentioned in Section 3.3.3.2. It is also possible to produce less well-defined multifunctional initiators containing peroxide functionality from a polymer substrate by autoxidalion or by ozonolysis.-0... 

The reactions of polymeric anions with appropriate azo-compounds or peroxides to form polymeric initiators provide other examples of anion-radical transformation (e.g. Scheme 7. 6). ""7i However, the polymeric azo and peroxy compounds have limited utility in block copolymer synthesis because of the poor efficiency of radical generation from the polymeric initiators (7.5.1). 

Further examples of micellar stabilization when micelles are composed of block copolymers formed by living radical polymerization are mentioned in Section 9.9.2. 

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