Tetrahydrofuran styrene copolymerization

Electron-withdrawing substituents in anionic polymerizations enhance electron density at the double bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to altemate. The reactivities of the monomers in copolymerizations and the compositions of the resultant copolymers are subject to solvent polarity and to the effects of the counterions. The two, just as in cationic polymerizations, cannot be considered independently from each other. This, again, is due to the tightness of the ion pairs and to the amount of solvation. Furthermore, only monomers that possess similar polarity can be copolymerized by an anionic mechanism. Thus, for instance, styrene derivatives copolymerize with each other. Styrene, however, is unable to add to a methyl methacrylate anion, though it copolymerizes with butadiene and isoprene. In copolymerizations initiated by w-butyllithium in toluene and in tetrahydrofuran at-78 °C, the following order of reactivity with methyl methacrylate anions was observed. In toluene the order is diphenylmethyl methacrylate > benzyl methacrylate > methyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > t-butyl methacrylate > trityl methacrylate > a,a -dimethyl-benzyl methacrylate. In tetrahydrofuran the order changes to trityl methacrylate > benzyl methacrylate > methyl methacrylate > diphenylmethyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > a,a -dimethylbenzyl methacrylate > t-butyl methacrylate. 

GopolymeriZation Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyUithium initiators produces a tapered block copolymer stmcture because of the large differences in monomer reactivity ratios for styrene (r < 0.1) and dienes (r > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkaU metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstmcture in diene polymerizations (57), the addition of small amounts of an alkaU metal alkoxide such as potassium amyloxide ([ROK]/[Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstmcture (58,59). 

There are few studies of the effect of temperature on monomer reactivity ratios [Morton, 1983]. For styrene-1,3-butadiene copolymerization by r-butyllithium in rc-hexane, there is negligible change in r values with temperature with r — 0.03, r2 = 13.3 at 0°C and n = 0.04, r% = 11.8 at 50°C. There is, however, a signihcant effect of temperature for copolymerization in tetrahydrofuran with r — 11.0, r2 = 0.04 at —78°C and r — 4.00, r2 = 0.30 at 25° C. The difference between copolymerization in polar and nonpolar solvents is attributed to preferential complexing of propagating centers and counterion by 1,3-butadiene as described previously. The change in r values in polar solvent is attributed to the same phenomenon. The extent of solvation decreases with increasing temperature, and this results in... 

As was stated above, the interpretation that the field affects the dis-sodation state of the growing chain ends was not uniquely substantiated by the experimental data, except those on copolymerizations. Thus it is interesting to investigate the field influence on much simpler systems than cationic homopolymerizations. For this purpose we have chosen living anionic systems in which only propagation steps are involved. The system first studied was a living anionic polymerization of styrene with n-butyllithium in the binary mixtures of benzene and tetrahydrofuran (17,24) and in the binary mixtures of benzene and dimethoxyethane (15). 

Fig. 5. Copolymerization of methyl methacrylate and styrene in tetrahydrofuran (O) and in N,N-dimethyl formamide ( ). Solid line represents copolymer composition produced by conventional free-radical initiators...

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. 

These two initiating sites might be considered to compete for monomer, but it appears that initiation is at the anionic site. In this study sodium benzophenone was used as initiator in tetrahydrofuran at 0°. Anionic polymerization was suggested by infrared evidence, by the rapid polymerization and by obtaining nearly pure polyacrylonitrile in attempted copolymerization with styrene. Inoue, Tsuruta and Furukawa (81) show that initiation is sensitive to specific reaction conditions. In their experiments sodium and potassium benzophenone gave polymer, but the lithium compound was ineffective. 

In some but not so rare cases, however, reactivity of macromonomers was found to be apparently reduced by the nature of their polymer chains. For example, p-vinylbenzyl- or methacrylate-ended PEO macromonomers, 26 (m=l) or 27b, were found to copolymerize with styrene (as A) in tetrahydrofuran with increasing difficulty (l/rA is reduced to one half) with increasing chain length of the PEO [41]. Since we are concerned with polymer-polymer reactions, as shown in Fig. 3, the results suggest that any thermodynamically repulsive interaction, which is usually observed between different, incompatible polymer chains, in this case PEO and PSt chains, may retard their approach and hence the reaction between their end groups, polystyryl radical and p-vinylbenzyl or methacrylate group. Such an incompatibility effect was discussed in terms of the degree of interpenetration and the interaction parameters between unlike polymers to support the observed reduction in the macromonomers copolymerization reactivity [31,40]. Similar observations of reduction of the copolymerization reactivity of macromonomers have recently been reported for the PEO macromonomers, 27a (m=ll) with styrene in benzene [42], 27b with acrylamide in water [43], and for poly(L-lactide), 28, with dimethyl acrylamide or N-vinylpyr-rolidone in dioxane [44]. 

Although the criterion for an anionic initiator given above is very useful, it is now known that other complicating factors can arise. O Driscoll and Tobolsky (79,80) have investigated the copolymerization of styrene and methyl methacrylate using lithium in bulk and in various ratios of tetrahydrofuran and heptane. In all of these systems the authors found considerably more than the less than 1% styrene reported for sodium and potassium. In fact as much as 33% styrene is... 

Kuntz (33) reported on the copolymerization of butadiene and styrene in n-heptane at 30° using n-butyl lithium. Although styrene homopolymerized six times faster than butadiene, the copolymerization rate was initially the same as that of butadiene homopolymerization and then increased markedly. It was found that about 80% of the styrene remained when 90% of the butadiene was consumed and that the increase in rate coincided with the almost complete consumption of butadiene. With added tetrahydrofuran, the rate of polymerization was faster and about 30% styrene was found in the initial copolymer. 

The copolymerization of monomer pairs such as butadiene and styrene follows a different course in hydrocarbon solvents than in more polar solvents such as ethers. The copolymerization in tetrahydrofuran is fairly straightforward, but the behavior in hydrocarbon solvents is often considered... 

Not all monomers are anionically pol5anerizable. Nevertheless, one can take advantage of the activity of the living ends to introduce reactive end groups at the extremity of homopolymers and then use arch end groups to initiate the polymerization of anionically non polymerizable monomers. This method has been applied to the synthesis of copolymere with polyvinyl and polylactone blocks and of copolymers with polyvinyl and polypeptide blocks One can at last use both anionic and cationic polymerization to prepare block copolymers of tetrahydrofuran with styrene or methylstyrene ... 

Fig. 20. Variation of the apparent propagation coefficients with reciprocal square root of concentration of active centres in the copolymerization of styrene (1) with p-methylstyrene (2). Solvent tetrahydrofuran. (o) kn, ( ) feji ( ) 22 ( ) 12 [146].

Problem 8.18 (a) Consider the cationic polymerizability of vinyl ethers, cyclic ethers (like tetrahydrofuran), cyclic acetals (like trioxane), and N-vinyl car-bazole. (b) Why do these monomers not copolymerize cationically with olefins like styrene or isobutene ... 

Copolymerizations of styrene with butadiene in hydrocarbon solvents using lithium alkyls initiators initially yield copolymers containing mainly butadiene. The amount of styrene in the copolymer increases considerably, however, in tetrahydrofuran solvent. 

The data in Table XV illustrate the problems encountered in such copolymerizations, since the use of polar solvents to assure a random styrene-diene copolymer of desired composition will, at the same time, lead to an increase in side vinyl groups (1,2 or 3,4) in the diene units (see Table XIV). This is of course quite undesirable, since such chain structures result in an increase in the glass transition temperature (Tg) and therefore to a loss of good rubbery properties. Hence, two methods are actually used to circumvent this problem (1) the use of limited amounts of polar additives such as tetrahydrofuran to... 

The final difference in the copolymerization of carbon monoxide with propene or styrene is the overall connectivity of the initial polymer generated under some conditions. The polymer generated from the copolymerization of carbon monoxide and propene in protic solvents consists of the fused tetrahydrofuran ketal structure shown in Figure 17.17. This polymer reopens to the polymer shown in Figure 17.13 upon addition of acid in alcohol. Several mechanisms for formation of this product have been proposed, and the origin of the ketal structure remains unresolved. Polymers formed in aprotic solvents form the acylic polymer. 

Nearly all the reported attempts at ionic copolymerization of vinyl ketones led to polymers containing very high ketone content, even when the comonomer was known to homopolymerize under the conditions. Copolymerization of phenyl vinyl ketone and styrene in bulk or in tetrahydrofuran initiated with n-butyllithium produced only poly(phenyl vinyl ketone) [341]. The non-incorporation of styrene in the anionic copolymerization was due to the phenyl vinyl ketone enolate anion being sufficiently nucleophilic to add the phenyl vinyl ketone monomer but not the styrene. 

The synthesis of block copolymers of controlled structures is most conventionally accomplished through the use of living anionic polymerization. One can easily imagine, however, desirable block copolymers derived from monomers which are inert to anionic polymerization conditions, or which do not share any common mode of polymerization. In a recent series of papers (24-34), Richards and coworkers have addressed this problem in a general way, and have developed methods which convert one kind of active center into another. Within the context of cyclic ether polymerizations, Richards has focused on the preparation of block copolymers of styrene and tetrahydrofuran (THF) several methods of accomplishing this copolymerization are described in the following paragraphs. 

The u-butyllithium-initiated polymerizations of myrcene proceed in a living manner in benzene (5-30°C) as well as in tetrahydrofuran (THF —30-15°C). Quantitative conversions can be obtained within 2 h (benzene, 30°C) or less than 1 h (THF, 15°C). The polymers have MWs in the range of 5-30 kg/mol, and PDl values are 1.4-1.6 (benzene) and 1.1-1.5 (THF). The polymyrcenes prepared in benzene consist of 85-89% 1,4 units and 11-15% 3,4 units, similar to those obtained by radical polymerization (see above). Increasing either polymerization temperature or initiator concentration causes an increase of the fraction of 3,4 units. On the other hand, polymyrcenes prepared in THF exhibit 40-50% 1,4 units, 39 14% 3,4 units, and 10-18% 1,2 units. Also here, the amount of 1,4 units is found to decrease with increasing polymerization temperature or initiator concentration. The copolymerization of myrcene and styrene results in the formation of block-like or tapered copolymers. The initial copolymers formed in benzene are rich in myrcene, and styrene is preferably incorporated at later stages of the reaction the situation is reversed when the copolymerization is performed in THF [38]. 

Ethylene oxide/styrene block copolymers have been further free-radical copolymerized with other ethylenically unsaturated compounds such as methyl methacrylate and methacrylic acid in benzene, tetrahydrofuran, and dimethylformamide (176). Correlations were made between reactivity ratio and solvent dielectric constant, as well as between solubility parameters of reaction solvent and growing polymer chains with marked effects apparent. Gel permeation chromatography of diblock and triblock copolymers based on polystyrene and poly(ethylene oxide) has revealed interesting molecular characteristics (177). Such block copolymers have an amphiphilic character. In aqueous solution, the polymers form spherical micells with a polystyrene core and a poly(ethylene oxide) outer sheath. The investigations used an aqueous-methanolic solution and were able to ascertain block copolymer structures and to estimate the impurities in the diblock copolymer. 

---