Copolymerization anionic synthesis

The arm-first synthesis of star microgels by initiating polymerization or copolymerization of a divinyl monomer such as diviny lbenzene or a bis-maleimide with a polystyryl alkoxyamine was pioneered by Solomon and coworkers.692 693 The general approach had previously been used in anionic polymerization. The method has now been exploited in conjunction with NMP,692 6 ATRP69 700 and RAFT.449 701 702 The product contains dormant functionality in the core. This can be used as a core for subsequent polymerization of a monoene monomer to yield a mikto-arm star (NMP ATRP704). 

Pioneering work in living anionic copolymerization of siloxanes was reported by Morton and co-workers 139 140, who synthesized isoprene-dimethylsiloxane block copolymers utilizing D4 as the siloxane monomer. The use of D3 in the synthesis of siloxane block copolymers with controlled structures was demonstrated by Bostick and others. Excellent reviews of these earlier studies and subsequent developments are available in the literature 22 137 13S). 

Another important consequence of the limitations concerning cross-addition is that anionic polymerization is not suited for the synthesis of random copolymers. If a mixture of two anionically polymerizable monomers is reacted with an initiator, the most electrophilic monomer will polymerize while the other is left almost untouched 30). In other words, a general feature of anionic binary copolymerization is that one of the reactivity ratios is extremely high while the other is close to zero. 

Recently it has been shown that anionic functionalization techniques can be applied to the synthesis of macromonomers — macromolecular monomers — i.e. linear polymers fitted at chain end with a polymerizable unsaturation, most commonly styrene or methacrylic ester 69 71). These species in turn provide easy access to graft copolymers upon radical copolymerization with vinylic or acrylic monomers. 

The role of reactive centers is performed here by free radicals or ions whose reaction with double bonds in monomer molecules leads to the growth of a polymer chain. The time of its formation may be either essentially less than that of monomer consumption or comparable with it. The first case takes place in the processes of free-radical polymerization whereas the second one is peculiar to the processes of living anionic polymerization. The distinction between these two cases is the most greatly pronounced under copolymerization of two and more monomers when the change in their concentrations over the course of the synthesis induces chemical inhomogeneity of the products formed not only for size but for composition as well. 

A radical initiator based on the oxidation adduct of an alkyl-9-BBN (47) has been utilized to produce poly(methylmethacrylate) (48) (Fig. 31) from methylmethacrylate monomer by a living anionic polymerization route that does not require the mediation of a metal catalyst. The relatively broad molecular weight distribution (PDI = (MJM ) 2.5) compared with those in living anionic polymerization cases was attributed to the slow initiation of the polymerization.69 A similar radical polymerization route aided by 47 was utilized in the synthesis of functionalized syndiotactic polystyrene (PS) polymers by the copolymerization of styrene.70 The borane groups in the functionalized syndiotactic polystyrenes were transformed into free-radical initiators for the in situ free-radical graft polymerization to prepare s-PS-g-PMMA graft copolymers. 

Applying these methodologies monomers such as isobutylene, vinyl ethers, styrene and styrenic derivatives, oxazolines, N-vinyl carbazole, etc. can be efficiently polymerized leading to well-defined structures. Compared to anionic polymerization cationic polymerization requires less demanding experimental conditions and can be applied at room temperature or higher in many cases, and a wide variety of monomers with pendant functional groups can be used. Despite the recent developments in cationic polymerization the method cannot be used with the same success for the synthesis of well-defined complex copolymeric architectures. 

An alternative route for the preparation of styrenic macromonomers is the reaction of living chains with 4-(chlorodimethylsilyl)styrene (CDMSS) [192]. The key parameter for the successful synthesis of the macromonomers is the faster reaction of the living anionic chain with the chlorosilane group rather than with the double bond of the CDMSS. Anionic in situ copolymerization of the above macromonomers (without isolation) with conventional monomers leads, under appropriate conditions, to well-defined comb-like chains with a variety of structures. 

The synthesis and properties of heat-resistant polyazomethines containing 2,5-disubstituted oxadiazole fragments, being insulators convertible into semiconductors by doping with iodine, have been described. The radical copolymerization of alkenes with the fluorescent co-monomer 2-/-butyl-5-(4 -vinyl-4-biphenylyl)-l,3,4-oxadiazole has resulted in useful macromolecular scintillators. Anionic polymerization of 2-phenyl-l,3,4-oxadiazolin-5-one has produced a nylon-type product <1996CHEC-II(4)268>. 

However, the practical, direct synthesis of functionalized linear polyolefins via coordination copolymerization olefins with polar monomers (CH2 = CHX) remains a challenging and industrially important goal. In the mid-1990s Brookhart et al. [25, 27] reported that cationic (a-diimine)palladium complexes with weakly coordinating anions catalyze the copolymerization of ethylene with alkylacrylates to afford hyperbranched copolymers with the acrylate functions located almost exclusively at the chain ends, via a chain-walking mechanism that has been meticulously studied and elucidated by Brookhart and his collaborators at DuPont [25, 27], Indeed, this seminal work demonstrated for the first time that the insertion of acrylate monomers into certain late transition metal alkyl species is a surprisingly facile process. It spawned almost a decade of intense research by several groups to understand and advance this new science and to attempt to exploit it commercially [30-33, 61]. 

A totally different route based on dehydrogenation of a saturated polymer precursor was introduced by Francois et al. [49] (Scheme 2.9). The method is based on anionic copolymerization of cyclohexadiene with styrene, followed by oxidation with chloranil. Due to possible coupling of two styrene (or two cyclohexadiene) molecules, a block copolymer, containing oligo(phenylene vinylene) units separated by oligo(phenylacetylene) and oligo(phenylene) blocks, is obtained. To the best of our knowledge, it was, so far, used only in the synthesis of phenyl-substituted PPV 10. 

While DADMAC is primarily employed in homo- and copolymerization as a monomer to produce cationic or amphoteric polymers or gels, it is also a source for the synthesis of sulfobetains or sulfobetains with additional anionic groups by sulfocyclization, sulfocyclosulfonation, or sulfocyclosulfination [36]. Figure 3 summarizes these reactions. 

The telechelica,(i -bis(2,6-dimethylphenol)-poly(2,6-dimethylphenyl-ene oxide) (PP0-20H) [174-182] is of interest as a precursor in the synthesis of block copolymers [175] and thermally reactive oligomers [179]. The synthesis has been accomplished by five methods. The first synthetic method was the reaction of a low molecular weight PPO with one phenol chain end with 3,3, 5,5 -tetramethyl-l,4-diphenoquinone. This reaction occurred by a radical mechanism [174]. The second method was the electrophilic condensation of the phenyl chain ends of two PPO-OH molecules with formaldehyde [177,178], The third method consists of the oxidative copolymerization of 2,6-dimethylphenol with 2,2 -di(4-hydroxy-3,5-di-methylphenyl)propane [176-178]. This reaction proceeds by a radical mechanism. A fourth method was the phase transfer-catalyzed polymerization of 4-bromo-2,6-dimethylphenol in the presence of 2,2-di(4-hy-droxy-3,5-dimethylphenyl)propane [181]. This reaction proceeded by a radical-anion mechanism. The fifth method developed was the oxidative coupling polymerization of 2,6-dimethylphenol (DMP) in the presence of tetramethyl bisphenol-A (TMBPA) [Eq. (57)] [182],... 

The polyoxymethylenes are presently widely used in different areas. Approximatively one-third of the market is represented by homopolymers and two-thirds by copolymers. Homopolymers are produced by anionic polymerization of formaldehyde using amines, alkoxides, and other types of anionic initiators. The details of these polymerizations will not be discussed in this paper, although some of their properties will be compared to those of copolymers which are obtained by cationic copolymerization of trioxane with cyclic ethers or cyclic esters. Comprehensive reviews on general aspects of synthesis and properties of acetal resins are available [158-162],... 

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