Block copolymerization bonds

Block copolymerization was carried out in the bulk polymerization of St using 18 as the polymeric iniferter. The block copolymer was isolated with 63-72 % yield by solvent extraction. In contrast with the polymerization of MMA with 6, the St polymerization with 18 as the polymeric iniferter does not proceed via the livingradical polymerization mechanism,because the co-chain end of the block copolymer 19 in Eq. (22) has the penta-substituted ethane structure, of which the C-C bond will dissociate less frequently than the C-C bond of hexa-substituted ethanes, e.g., the co-chain end of 18. This result agrees with the fact that the polymerization of St with 6 does not proceed through a living radical polymerization mechanism. Therefore, 18 is suitably used for the block copolymerization of 1,1-diubstituted ethylenes such as methacrylonitrile and alkyl methacrylates [83]. 

Star polymers having several PS branches and only one poly(2-vinyl naphthalene), PVN branch were prepared by Takano et al. using anionic polymerization techniques [31]. Sequential anionic block copolymerization of (4-vinyl-phenyl) dimethylvinylsilane (VS) and VN was employed. The double bonds attached to silicon have to remain unaffected during the polymerization of VS. This was ac-... 

A handicap of Grignard reagents in the field of anionic polymerization is certainly their low reactivity toward nonpolar double bonds. Unlike organolithium compounds, organomagnesium compounds are nornally inert toward monomers sueh as styrene or butadiene. Thus, their scope in the field of anionic block-copolymerization is quite limited. 

Poly(l,4-butadiene) segments prepared by the ruthenium-mediated ROMP of 1,5-cyclooctadiene can be incorporated into the ABA-type block copolymers with styrene (B-106) and MMA (B-107).397 The synthetic method is based on the copper-catalyzed radical polymerizations of styrene and MMA from the telechelic poly(butadiene) obtained by a bifunctional chain-transfer agent such as bis(allyl chloride) or bis-(2-bromopropionate) during the ROMP process. A more direct route to similar block copolymers is based on the use of a ruthenium carbene complex with a C—Br bond such as Ru-13 as described above.67 The complex induced simultaneous or tandem block copolymerizations of MMA and 1,5-cyclooctadiene to give B-108, which can be hydrogenated into B-109, in one pot, catalyzed by the ruthenium residue from Ru-13. 

The improvement in the first two areas has been possible because of the progress made in understanding the mechanism and kinetics of radical polymerization and the stracture of radical intermediates. Further advancement requires detailed structure-reactivity correlation for radicals and also for dormant species. Both experimental measurements of rate and equilibrium constants as well as computational evaluation of thermodynamic (bond dissociation, redox properties) and kinetic properties of the involved species is needed. This will help to establish order of reactivities for various species and will assist selection of the efficient initiators or sequence of monomer addition for block copolymerization. 

When a polymer chain is ruptured mechanically, terminal-free radicals can be generated, and these can be utilized to initiate block copolymerization. Under an inert atmosphere, block copolymers can be produced by cold milling, or mastication of two different polymers or of a polymer in the presence of a second monomer. This generally results in the formation of graft copolymers in addition to the block copolymers since radicals can be located in nonterminal positions by chain transfer. However, predominant yield of block copolymers is obtained by milling monomer-swollen polymers. The success of this technique depends on the physical state of the polymer. Generation of radical is favored if the polymer exists at or near the glassy state otherwise, polymer flow rather than bond rapture will occur. Table 5.5 shows some block copolymers prepared by this technique. 

Furthermore, LCEs have been prepared by block copolymerization and hydrogen bonds (Cui et al., 2004 Li et al., 2004). Li et al. (2004) proposed a musclelike material with a lamellar structure based on a nematic triblock copolymer (Components 8a-c, Fig. 3.10). The material consists of a repeated series of nematic (N) polymer blocks and conventional rubber (R) blocks. The synthesis of block copolymers with well-defined structures and narrow molecular-weight distributions is a crucial step in the production of artificial muscle based on triblock elastomers. Talroze and coworkers studied the structure and the alignment behavior of LC networks stabilized by hydrogen bonds under mechanical stress (Shandryuk et al., 2003). They synthesized poly[4-(6-acryloyloxyhexyloxy)benzoic acid], which... 

Since destruction of polymer materials is very important for practical purposes, a large number of investigations on fracture phenomena in polymers have been carried out from both the experimental and theoretical points of view. Several reports provide indirect evidence for main chain scissions, for example decreases in molecular weight or initiations of the graft or block copolymerization after mastication. Direct evidence for chemical bond scission can be obtained from ESR measurements on fractured polymer materials [21]. The high reactivity and high mobility of free radicals produced by mechanical fracture (mechano-radicals) can also be followed. The ESR application to mechanical destruction of polymer materials is presented below. Temperature-dependent ESR spectra of polymer radicals produced... 

The synthesis of block, as well as random copolymers of 3-pinene with styrene and / -methylstyrene (pMeSt), was studied by living cationic polymerization, using both the styrene and vinyl ether adducts as initiators in the presence of Ti(OiPr)Cl3 in methylene chloride at —40°C [44,47]. For styrene (A) and 3-pinene (B), both AB and BA block copol3mers were obtained, as shown in Fig. 2.13, with Mn values of 4 000 and 3 600, respectively, and narrow Mw/Mn ratios (1.26 and 1.38, respectively). The efficiency of these block copolymerizations was attributed to the similar reactivity of the C—Cl bond derived from the two monomers [44]. 

Styrenic block copolymer (SBS) TPEs are multiphase compositions in which the phases are chemically bonded by block copolymerization (see chapter Introduction to Plastics and Polymers). At least one of the phases is a hard styrenic polymer. This styrenic phase may become fluid when the TPE composition is heated. Another phase is a softer elastomeric material that is rubber-like at room temperature. The polystyrene blocks act as cross-links, tying the elastomeric chains together in a three-dimensional network. SBS TPEs have no commercial applications when the product is just a pure polymer. They must be compounded with other polymers, oils, fillers, and additives to have any commercial value. 

Controlled radical polymerization of MMA followed by n-BuMA produces linear AB diblock copolymer LB-1 with a narrow molecular weight distribution (MWD, M /Mn = 1.2), which can be extended further into ABA triblock copolymer LB-2 with a similarly narrow MWD (M ,/Mn = 1.2) [21]. The block copolymers of MMA and MA, LB-3 and LB-4, were prepared using catalysts based on nickel, copper or iron complexes. Due to the higher activity of the carbon halogen terminal bonds in poly(methacrylate)s than in poly(acrylate)s, the block copolymerization of MA using a PMMA macroini-... 

Adventitious routes to partially blocky copolyamides have been mentioned in an earlier section, and block copolymer syntheses by conventional random block copolymerization and by oligomer combination reactions are summarized in Tables 3 and 4. It should be noted that the high melting points and restricted solubilities which are the source of useful properties in intermolecularly hydrogen-bonded polyamides and analogous polymers are also a frequent source of practical difficulties in the preparation of their block copolymers. 

Besides condensation reactions, the bifunctional ACPC may also be used for cationic polymerization [52-55] (e.g., of tetrahydrofuran). The polymer obtained by the method depicted in reaction (12) contains exactly one central azo bond [56] and is a suitable macroinitiator for the thermally induced block copolymerization of vinyl monomers. Initiators like ACPC are referred to as transformation agents becaiae they are able to initiate... 

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