Anionic polymerization random copolymer

Commercially, anionic polymerization is limited to three monomers styrene, butadiene, and isoprene [78-79-5], therefore only two useful A—B—A block copolymers, S—B—S and S—I—S, can be produced direcdy. In both cases, the elastomer segments contain double bonds which are reactive and limit the stabhity of the product. To improve stabhity, the polybutadiene mid-segment can be polymerized as a random mixture of two stmctural forms, the 1,4 and 1,2 isomers, by addition of an inert polar material to the polymerization solvent ethers and amines have been suggested for this purpose (46). Upon hydrogenation, these isomers give a copolymer of ethylene and butylene. 

Radical copolymerization is used in the manufacturing of random copolymers of acrylamide with vinyl monomers. Anionic copolymers are obtained by copolymerization of acrylamide with acrylic, methacrylic, maleic, fu-maric, styrenesulfonic, 2-acrylamide-2-methylpro-panesulfonic acids and its salts, etc., as well as by hydrolysis and sulfomethylation of polyacrylamide Cationic copolymers are obtained by copolymerization of acrylamide with jV-dialkylaminoalkyl acrylates and methacrylates, l,2-dimethyl-5-vinylpyridinum sulfate, etc. or by postreactions of polyacrylamide (the Mannich reaction and Hofmann degradation). Nonionic copolymers are obtained by copolymerization of acrylamide with acrylates, methacrylates, styrene derivatives, acrylonitrile, etc. Copolymerization methods are the same as the polymerization of acrylamide. 

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. 

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. 

Polystyrene is unusual among commodity polymers in that we can prepare it in a variety of forms by a diversity of polymerization methods in several types of reaction vessel. j Polystyrene may be atactic, isotactic, or syndiotactic. Polymerization methods include free radical, cationic, anionic, and coordination catalysis. Manufacturing processes include bulk, solution, suspension, and emulsion polymerization. We manufacture random copolymers ... 

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. 

In addition to the triblock thermoplastic elastomers, other useful copolymers of styrene with a diene are produced commerically by living anionic polymerization. These include di-and multiblock copolymers, random copolymers, and tapered block copolymers. A tapered (gradient) copolymer has a variation in composition along the polymer chain. For example, S-S/D-D is a tapered block polymer that tapers from a polystyrene block to a styrene-diene random copolymer to polydiene block. (Tapered polymers need not have pure blocks at their ends. One can have a continuously tapered composition from styrene to diene by... 

Inaki (1992) synthesized a wide range of nucleobase-functionalized random and homopolymers. In addition, Inaki et al. (1980) synthesized block copolymers containing thymine and uracil groups in the main chain through ring-opening cationic and anionic polymerization of cychc derivatives of the nucleobases. 

The low temperature peroxyborane system is very effective for converting thiocarbonyl fluoride to homopolymer. The product is comparable to those formed by anionic polymerization. Since polymerization of thiocarbonyl fluoride is substantially slower than that of the chlorofiuoride, this monomer copolymerizes with exceptional ease with a large number of vinyl compounds to give products that appear to be random copolymers. 

Figure 15. Molecular weight distribution of anionically polymerized styrene-butadiene random copolymer and emulsion polymerized SBR.

Dynamic viscoelastic and stress-optical measurements are reported for blends of crosslinked random copolymers of butadiene and styrene prepared by anionic polymerization. Binary blends in which the components differ in composition by at least 20 percentage units give 2 resolvable loss maxima, indicative of a two-phase domain structure. Multiple transitions are also observed in multicomponent blends. AU blends display an elevation of the stress-optical coefficient relative to simple copolymers of equivalent over-all composition. This elevation is shown to be consistent with a multiphase structure in which the domains have different elastic moduli. The different moduli arise from increased reactivity of the peroxide crosslinking agent used toward components of higher butadiene content. 

By the hydrogenation of BD/IP-copolymer feedstocks fully and partially hydrogenated rubbers with low crystallinities and low glass transition temperatures are obtained. For the preparation of highly amorphous hydrogenated rubbers a random copolymer composition is essential. In this respect feedstock preparation by the use of the Nd-carboxylate-based system NdV/DIBAH/EASC yielded better results than BD/IP copolymer feedstocks prepared by anionic polymerization with BuLi [484,485]. 

From the literature developing, it does not appear that anionic propagation produces random copolymers, but rather in those cases where two monomers polymerize a block polymer is produced. In some cases, in which the initiator is very active, the less reactive monomer will be initiated to a small extent, but then the other monomer takes over... 

The copolymerization with alkyllithium to produce uniformly random copolymers is more complex for the solution process than for emulsion because of the tendency for the styrene to form blocks. Because of the extremely high rate of reaction of the styryl-lithium anion with butadiene, the polymerization very heavily favors the incorporation of butadiene units as long as reasonable concentrations of butadiene are present. This observation initially was somewhat confusing because the homopolymerization rate of styrene is seven times that for butadiene. However, the cross-propagation rate is orders of magnitude faster than either, and it therefore dominates the system. For a 30 mole percent styrene charge the initial polymer will be almost pure butadiene until most of the butadiene is polymerized. Typically two-thirds of the styrene charged will be found as a block of polystyrene at the tail end of the polymer chain ... 

It has been reported that vinylferrocene is polymerized by using a radical, cationic, anionic, or Ziegler system initiator [29 — 34]. In particular, higher molecular weight products can be obtained using radical-initiated bulk polymerization [31]. Indeed, both bulk copolymerization and solution copolymerization (in benzene) of 18 with vinylferrocene by using a radical initiator (AIBN) afforded the chiral polymers 19a —e (Scheme 3-13), which were purified by reprecipitation of the benzene solution with methanol. The ratios of the two comonomers were varied in copolymerization. The composition data of the copolymers obtained revealed nearly the same reactivity between 18 and vinylferrocene, which suggests that 19a—e are random copolymers. 

A number of block copolymers prepared with Ziegler-Natta catalysts have been reported however, in most cases the compositions may include significant amounts of homopolymer. The Ziegler-Natta method appears to be inferior to anionic polymerization for synthesizing carefully tailored block copolymers. Nevertheless, bock copolymers of ethylene and propylene (Eastman Kodak s Pofyallomers) have been commercialized. Unlike the elastomeric random copolymers of ethylene and propylene, these are high-impact plastics exhibiting crystallinity characteristics of both isotactic polypropylene and linear polyethylene. They also contain homopolymers in addition to block copolymers. 

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