Radical polymerization, block

The precipitated silica (J. Crosfield Sons) was heated in vacuo at 120° for 24h. before use. Two grades of surface areas 186 and 227 m g l (BET,N2), were used during this project. Random copolymers, poly(methyl methacrylates) and polystyrene PS I were prepared by radical polymerization block polymers and the other polystyrenes were made by anionic polymerization with either sodium naphthalene or sodium a methylstyrene tetramer as initiator. The polymer compositions and molecular weights are given in Table I. 

Controlled expansion alloys, 13 520-522 Controlled flavor release systems, 11 528, 543-553, 554-555 characteristics of, ll 544t demand for, 11 555 developments in, 11 558 elements of, 11 555-557 extrusion encapsulation for, 11 550 key aspects of, 11 556t morphologies of, 11 545 Controlled free-radical polymerization, block copolymers, 7 646 Controlled humidity drying, ceramics processing, 5 655-656 Controlled indexing, 18 241 Controlled initiation, 14 268-269 Controlled laboratory studies, in... 

Keywords Review, Copolymer, Controlled/living radical polymerization, Block, Graft, Gradient, Statistical... 

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). 

Polyethylene (PE) is a genetic name for a large family of semicrystalline polymers used mostiy as commodity plastics. PE resins are linear polymers with ethylene molecules as the main building block they are produced either in radical polymerization reactions at high pressures or in catalytic polymerization reactions. Most PE molecules contain branches in thek chains. In very general terms, PE stmcture can be represented by the following formula ... 

As discussed in Section 7.3, conventional free radical polymerization is a widely used technique that is relatively easy to employ. However, it does have its limitations. It is often difficult to obtain predetermined polymer architectures with precise and narrow molecular weight distributions. Transition metal-mediated living radical polymerization is a recently developed method that has been developed to overcome these limitations [53, 54]. It permits the synthesis of polymers with varied architectures (for example, blocks, stars, and combs) and with predetermined end groups (e.g., rotaxanes, biomolecules, and dyes). 

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,... 

There are some indications that the situation described above has been realized, at least partially, in the system styrene-methyl methacrylate polymerized by metallic lithium.29 29b It is known51 that in a 50-50 mixture of styrene and methyl methacrylate radical polymerization yields a product of approximately the same composition as the feed. On the other hand, a product containing only a few per cent of styrene is formed in a polymerization proceeding by an anionic mechanism. Since the polymer obtained in the 50-50 mixture of styrene and methyl methacrylate polymerized with metallic lithium had apparently an intermediate composition, it has been suggested that this is a block polymer obtained in a reaction discussed above. Further evidence favoring this mechanism is provided by the fact that under identical conditions only pure poly-methyl methacrylate is formed if the polymerization is initiated by butyl lithium and not by lithium dispersion. This proves that incorporation of styrene is due to a different initiation and not propagation. 

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). 

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

Bamford, Eastmond and coworkers have employed metal complexpolymeric halide redox systems to initiate block and graft copolymerization. The polymeric halides can be synthesized by a variety of techniques, including radical polymerization,281 anionic polymerization (Scheme 7.28),"so... 

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

Chung and coworkers have reported on the use of stable borinale or boroxyl radicals (e.g. 114) to mediate radical polymerization." Methacrylates (MMA) and acrylates (trifluoroelhyl acrylate) have been polymerized at ambient temperature to yield polymers with relatively narrow molecular weight distributions.231233 The method has been used to prepare block copolymers and polyolefin graft copolymers.2 4 37... 

Living polymerization processes immediately lend themselves to block copolymer synthesis and the advent of techniques for living radical polymerization has lead to a massive upsurge in the availability of block copolymers. Block copolymer synthesis forms a significant part of most reviews on living polymerization processes. This section focuses on NMP,106 A TRP,265,270 and RAFT.- 07 Each of these methods has been adapted to block copolymer synthesis and a substantial part of the literature on each technique relates to block synthesis. Four processes for block copolymer synthesis can be distinguished. 

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. 

The generic features of these approaches are known from experience in anionic polymerization. However, radical polymerization brings some issues and some advantages. Combinations of strategies (a-d) are also known. Following star formation and with appropriate experimental design to ensure dormant chain end functionality is retained, the arms may be chain extended to give star block copolymers (321). In other cases the dormant functionality can be retained in the core in a manner that allows synthesis of mikto-arm stars (324). 

The grafting through approach involves copolymerization of macromonomers. NMP, ATRP and RAFT have each been used in this context. The polymerizations are subject to the same constraints as conventional radical polymerizations that involve macromonomers (Section 7.6.5). However, living radical copolymerization offers greater product uniformity and the possibility of blocks, gradients and other architectures. 

The new knowledge and understanding of radical processes has resulted in new polymer structures and in new routes to established materials many with commercial significance. For example, radical polymerization is now used in the production of block copolymers, narrow polydispersity homopolymers, and other materials of controlled architecture that were previously available only by more demanding routes. These commercial developments have added to the resurgence of studies on radical polymerization. 

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