Chain copolymerization anionic reaction

Copolymerizations initiated by lithium metal should give the same product as produced from lithium alkyls. Usually the radical ends produced by electron transfer initiation have so short a lifetime they can have no influence on the copolymerization. This is true for instance in the copolymerization of isoprene and styrene (50). The product is identical if initiated by lithium metal or by butyllithium. With the styrene-methylmethacrylate system, however, differences are observed (79,80,82). Whereas the butyllithium initiated copolymer contains no styrene at low conversions, the one initiated by lithium metal has a high styrene content if the reaction is carried out in bulk and a moderate one even in tetrahydrofuran. These facts led O Driscoll and Tobolsky (80) to suggest that initiation with lithium occurs by electron exchange and that in this case the radical ends are sufficiently long-lived to produce simultaneous radical and anionic reactions at opposite ends of the chain. Only in certain rather exceptional circumstances would the free radical reaction be of importance. Some of the conditions required have been discussed by Tobolsky and Hartley (111). The anionic reaction should be slow. This is normally true for lithium based catalysts in hydrocarbon solvents. No evidence of appreciable radical participation is observed for initiation by sodium and potassium. The monomers should show a fast radical reaction. If styrene is replaced by isoprene, no isoprene is found in the copolymer for isoprene polymerizes slowly by free radical initiation. Most important of all, initiation should be slow to produce a low steady concentration of radical-anions. An initiator which produces an almost instantaneous and complete electron transfer to monomer produces a high radical concentration which will ensure their rapid mutual termination. 

A macromonomer technique was employed by Ishizu and Kuwahara to prepare miktoarm star copolymers of the (PS) (PI)m type.128 PS and PI macromonomers were prepared by coupling the living chains with />chloromethylstyrene. The PS and PI macromonomers (vinyl end-capped chains) were copolymerized anionically in benzene using n-BuLi as initiator. The products are comb-shaped copolymers, but they behave as miktoarm stars of the type A Bm. The reaction sequence is given in Scheme 58. 

Anionic copolymerization of lactams presents an interesting example of copolymerization. Studies of the copolymerization of a-pyrrolidone and e-caprolactam showed that a-pyrrolidone was several times more reactive than e-caprolactam at 70 °C, but became less reactive at higher temperatures due to depropagation210 2U. By analyzing the elementary reactions Vofsi et al.I27 concluded that transacylation at the chain end occurred faster than propagation and that the copolymer composition was essentially determined by the transacylation equilibrium and the acid-base equilibrium of the monomer anion together with the usual four elementary reactions of the copolymerization. 

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. 

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. 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

Homo- and copolymerizations involving the monomers depicted in Scheme 1 are chain reactions which can be initiated, at least potentially, by typical free-radical, cationic or anionic promoters. TTie object of the studies reported below is to establish first of all which monomers adapt best to each type of initiation, then what peculiarities (if any) are caused by the presence of the furan ring, compared to the known behaviour of the corresponding aliphatic and/or aromatic homologues and finally to establish the structure-properties relationships of the materials obtained. 

For any specific type of initiation (i.e., radical, cationic, or anionic) the monomer reactivity ratios and therefore the copolymer composition equation are independent of many reaction parameters. Since termination and initiation rate constants are not involved, the copolymer composition is independent of differences in the rates of initiation and termination or of the absence or presence of inhibitors or chain-transfer agents. Under a wide range of conditions the copolymer composition is independent of the degree of polymerization. The only limitation on this generalization is that the copolymer be a high polymer. Further, the particular initiation system used in a radical copolymerization has no effect on copolymer composition. The same copolymer composition is obtained irrespective of whether initiation occurs by the thermal homolysis of initiators such as AIBN or peroxides, redox, photolysis, or radiolysis. Solvent effects on copolymer composition are found in some radical copolymerizations (Sec. 6-3a). Ionic copolymerizations usually show significant effects of solvent as well as counterion on copolymer composition (Sec. 6-4). 

The initial product is essentially poly(methyl methacrylate) homopolymer. Little styrene is incorporated into copolymer chains unitl most or all of the methyl methacrylate is exhausted. Reports of significant amounts of styrene in products from anionic copolymerization of styrene-methyl methacrylate are usually artifacts of the particular reaction system, a consequence of heterogeneity of the propagating centers and/or counterion. 

One of the first detailed studies on these systems was that of Beaman (26), who showed that methacrylonitrile polymerizes by an anionic chain mechanism when treated with various bases, including Na in liquid ammonia at —75° C. He noted also that low molecular weight polymers are obtained from reaction of acrylonitrile with butylmagnesium bromide. Foster (56) extended the liquid ammonia method to copolymerization studies in which acrylonitrile was combined with styrene, with methyl methacrylate and with vinyl butyl sulfone. Satisfactory data were obtained only with the sulfone, in which case there was some tendency for alternation. 

The solution thus consists of different particles denoted as contact ion pairs, solvent-separated ion pairs and free ions. The fraction of the individual particles depends on the type of salt, type of solvent, polymerization system, temperature, and salt concentration. The catalytic effect of these particles may be very different as is evident in anionic polymerization of vinyl monomers. For instance, free polystyryl anion is 800times more reactive than its ion pair with the sodium counterion 60 . From this fact it follows that, although the portion of free ions is small in the reaction system, they may play an important role. On the other hand, anionic polymerization and copolymerization of heterocycles proceeds mostly via ion pairs. This is due to a strong localization of the negative charge on the chain-end heteroatom which strongly stabilizes the ion pair itself62. Ionic dissociation constants and ion contributions to the reaction kinetics are usually low. This means that for heterocycles the difference between the catalytic effect of ion pairs and free ions is much weaker than for the polymerization of unsaturated compounds. This is well documented by the copolymerization of anhydrides with epoxides where the substi-... 

At present, we can say that copolymerization initiated by various salts proceeds by an anionic mechanism, after dissociation of the initiators in the reaction medium. The primary step is the addition of the initiator anion to the epoxide. In the initiation by Lewis bases, i.e. by tertiary amines, initiation involves formation of a primary active centre of an anionic character. This active centre is probably generated by interaction of the tertiary amine with the anhydride and an allyl alcohol. The allyl alcohol can be formed by a base-catalyzed isomerization of the epoxide. In the presence of a proton donor, the formation of active centres is possible through interaction of tertiary amine, anhydride and proton donor without epoxide isomerization. Another way of initiation consists in a direct reaction of epoxide with tertiary amine yielding an anionic primary active centre. We believe that in both kinds of initiation in the strict absence of proton donors, the growing chain end has the character of a living polymer. The presence of proton donors, however, gives rise to transfer reactions. 

During the copolymerization of PO and C02, it was possible to control the formation of the cyclic carbonate, presumably by protonation of the anionic polymer chain when it became dissociated from the metal center. As a consequence, the reaction could be carried out at an elevated temperature (333 K), without the extensive production of propylene carbonate (—1096). 

Catalysts of the Ziegler-Natta type are applied widely to the anionic polymerization of olefins and dienes. Polar monomers deactivate the system and cannot be copolymerized with olefins. J. L. Jezl and coworkers discovered that the living chains from an anionic polymerization can be converted to free radicals by the reaction with organic peroxides and thus permit the formation of block copolymers with polar vinyl monomers. In this novel technique of combined anionic-free radical polymerization, they are able to produce block copolymers of most olefins, such as alkylene, propylene, styrene, or butadiene with polar vinyl monomers, such as acrylonitrile or vinyl pyridine. 

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