Polymerization methods transfer

A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Only particles of linear or very slightly cross-linked <0.6%) polymers may be produced by dispersion polymerization. Obviously, dispersion polymerization may be used for the production of monosized seed particles, which, after transfer to aqueous conditions, are used for the production of different cross-linked and macroporous particles by the activated swelling and polymerization method. 

Tung et al21> have reported on the use of a polymeric thiol transfer agent for use in block copolymer production. Various methods have been used for the anion thiol conversion. Near quantitative yields of thiol arc reported to have been obtained by terminating anionic polymerization with ethylene sulfide and derivatives (Scheme 7.27). Transfer constants for the polymeric thiols are reported to be similar to those of analogous low molecular weight compounds.273... 

Cationic polymerization was considered for many years to be the less appropriate polymerization method for the synthesis of polymers with controlled molecular weights and narrow molecular weight distributions. This behavior was attributed to the inherent instability of the carbocations, which are susceptible to chain transfer, isomerization, and termination reactions [48— 52], The most frequent procedure is the elimination of the cation s /1-proton, which is acidic due to the vicinal positive charge. However, during the last twenty years novel initiation systems have been developed to promote the living cationic polymerization of a wide variety of monomers. 

In 1995, a new class of controlled/ living radical polymerization methods was reported by the groups of Matyjaszewski [34] and Sawamoto [35], This new process, named atom transfer radical polymerization (ATRP) [34], has had a tremendous... 

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. 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

The main limitation of these methods is the rather small number of monomers that undergo anionic polymerizations without transfer or termination steps. Nevertheless, research on the various applications of anionic polymerization as an efficient tool for the synthesis of tailor made polymers has been pursued very actively over the past 20 years, and it has yielded a great number of results of significance which has also opened a broad field of potential applications. 

Saidel and Katz (97) have used an extension of the method of moments to calculate MJMn for a batch polymerization with transfer to polymer, and with termination exclusively by combination, but with restriction to mono-radicals. As the equations for the second and higher moments contain terms involving still higher moments it is necessary to approximate them in terms of lower moments, but this requires assumptions about the form of the distribution that may not be justified. 

Controlled free-radical polymerization methods, like atom-transfer radical polymerization (ATRP), can yield polymer chains that have a very narrow molecular-weight distribution and allow the synthesis of block copolymers. In a collaboration between Matyjaszewski and DeSimone (Xia et al., 1999), ATRP was performed in C02 for the first time. PFOMA-/)-PMMA, PFOMA-fr-PDMAEMA [DMAEMA = 2-(dimethylamino)ethyl methacrylate], and PMMA-/)-PFOA-/)-PM M A copolymers were synthesized in C02 using Cu(0), CuCl, a functionalized bipyridine ligand, and an alkyl halide initiator. The ATRP method was also conducted as a dispersion polymerization of MMA in C02 with PFOA as the stabilizer, generating a kine-... 

Among the several controlled radical polymerization methods, atom transfer radical polymerization (ATRP) is one of the most attractive techniques. The proposed mechanism of ATRP is shown in Fig. 10. The radicals were formed by atom transfer of a halogen from a dormant species to a metal. This transfer is possible due to the capacity of the metal to change its oxidation state. This... 

The patterned organosilane monolayers introducing organolsilane molecule with a polymerization initiating unit are useful as template surfaces for site-specific polymerization. Atom transfer radical polymerization (ATRP) unit was immobilized as a monolayer component. Since ATRP is one of the most successful methods for polymerizing a variety of monomers in a controlled fashion [44, 45], tailor-made surface topography is possible. Several reports have described the formation of polymer thin film by the radical polymerization from the immobilized ATRP initiator [46, 47],... 

The cation radical intermediate and the process of electron (hole) transfer have recently been shown to constitute the basis for a fundamentally new addition to the repertoire of polymerization methods [85], Both cation radical chain cyclobutana-tion polymerization (Scheme 44) and Diels-Alder polymerization have been demonstrated under the typical aminium salt conditions. 

A disadvantage of traditional acrylamide polymerization reactions is the heterogeneity of the products that result. A radical polymerization method that produces polymers of similar structure but that are much more homogeneous is atom-transfer radical polymerization (ATRP) [155,156]. ATRP has been used to synthesize carbohydrate-substituted polymers with low polydispersities [157,158,159,160,161]. Materials that display sugar residues such as glu-cofuranose [160], glucopyranose [161], and A-acetyl-D-glucosamines [159]. 

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