Styrene anionic polymerization rates

However, the mechanisms by which the initiation and propagation reactions occur are far more complex. Dimeric association of polystyryllithium is reported by Morton, al. ( ) and it is generally accepted that the reactions are first order with respect to monomer concentration. Unfortunately, the existence of associated complexes of initiator and polystyryllithium as well as possible cross association between the two species have negated the determination of the exact polymerization mechanisms (, 10, 11, 12, 13). It is this high degree of complexity which necessitates the use of empirical rate equations. One such empirical rate expression for the auto-catalytic initiation reaction for the anionic polymerization of styrene in benzene solvent as reported by Tanlak (14) is given by ... 

Over 5.5 billion pounds of synthetic rubber is produced annually in the United States. The principle elastomer is the copolymer of butadiene (75%) and styrene (25) (SBR) produced at an annual rate of over 1 million tons by the emulsion polymerization of butadiene and styrene. The copolymer of butadiene and acrylonitrile (Buna-H, NBR) is also produced by the emulsion process at an annual rate of about 200 million pounds. Likewise, neoprene is produced by the emulsion polymerization of chloroprene at an annual rate of over 125,000 t. Butyl rubber is produced by the low-temperature cationic copolymerization of isobutylene (90%) and isoprene (10%) at an annual rate of about 150,000 t. Polybutadiene, polyisoprene, and EPDM are produced by the anionic polymerization of about 600,000, 100,000, and 350,000 t, respectively. Many other elastomers are also produced. 

Auguste S, Edwards HGM, Johnson AF et al. (1996) Anionic polymerization of styrene and butadiene initiated by n-butyllithium in ethylbenzene determination of the propagation rate constants using Raman spectroscopy and gel permeation chromatography. Polymer 37 3665-3673... 

The propagation rate constant and the polymerization rate for anionic polymerization are dramatically affected by the nature of both the solvent and the counterion. Thus the data in Table 5-10 show the pronounced effect of solvent in the polymerization of styrene by sodium naphthalene (3 x 1CT3 M) at 25°C. The apparent propagation rate constant is increased by 2 and 3 orders of magnitude in tetrahydrofuran and 1,2-dimethoxyethane, respectively, compared to the rate constants in benzene and dioxane. The polymerization is much faster in the more polar solvents. That the dielectric constant is not a quantitative measure of solvating power is shown by the higher rate in 1,2-dimethoxyethane (DME) compared to tetrahydrofuran (THF). The faster rate in DME may be due to a specific solvation effect arising from the presence of two ether functions in the same molecule. 

The need for solvation in anionic polymerization manifests itself in some instances by other deviations from the normal reaction rate expressions. Thus the butyllithium polymerization of methyl methacrylate in toluene at — 60°C shows a second-order dependence of Rp on monomer concentration [L Abbe and Smets, 1967]. In the nonpolar toulene, monomer is involved in solvating the propagating species [Busson and Van Beylen, 1978]. When polymerization is carried out in the mixed solvent dioxane-toluene (a more polar solvent than toluene), the normal first-order dependence of Rp on [M] is observed. The lithium diethylamide, LiN(C2H5)2, polymerization of styrene at 25°C in THF-benzene similarly shows an increased order of dependence of Rp on [M] as the amount of tetrahydrofuran is decreased [Hurley and Tait, 1976]. 

The nature of the active species in the anionic polymerization of non-polar monomers, e. g. styrene, has been disclosed to a high degree. The kinetic measurements showed, that the polymerization proceeds in an ideal way, without side-reactions, and that the active species exist in the form of free ions, solvent-sparated and contact ion pairs, which are in a dynamic equilibrium (l -4). For these three species the rate constants and activation parameters (including the activation volumes), as well as the rate constants and equilibrium constants of interconversion have been determined (4-7.) Moreover, it could be shown by many different methods (e. g. conductivity and spectroscopic methods) that the concept of solvent-separated ion pairs can be applied to many ionic compounds in non-aqueous polar solvents (8). 

When a mixture of styrene and 1,3-butadiene (or isoprene) undergoes lithium-initiated anionic polymerization in hydrocarbon solution, the diene polymerizes first. It is unexpected, since styrene when polymerized alone, is more reactive than, for example, 1,3-butadiene. The explanation is based on the differences of the rates of the four possible propagation reactions the rate of the reaction of the styryl chain end with butadiene (crossover rate) is much faster than the those of the other three reactions484,485 (styryl with styrene, butadienyl with butadiene or styrene). This means that the styryl chain end reacts preferentially with butadiene. 

The catalysts which are required for the polymerization of butadiene to the 1.2 structure are less anionic than those for the anionic polymerization of styrene. The relative rates of the copolymerization of butadiene and of styrene is dependent on this anionic requirement of the propagating ion. Butadiene is polymerized to the near exclusion of... 

Using the fact that the rate of anionic polymerization of a-methyl-styrene, initiated by potassium amide in liquid ammonia, is smaller than the rate of hydrogen exchange, it was possible to establish (Shatenshtem et al., 1962b) that hydrogen-deuterium exchange occurs... 

Another investigation along this line is the pulse radiolysis study of the electron transfer reactions from aromatic radical anions to styrene this type of reaction is commonly used to initiate anionic polymerization of styrene [35], The electron transfer rates from the unassociated biphenyl radical-anions to styrene derivatives in 2-propanol were found to increase along the... 

Vinyllithium initiates the polymerization of styrene and of similar monomers. However, the rate of initiation is fairly low, and the molecular weights exhibited by the polymers obtained are by one order of magnitude higher than the expected ones. This is not the only drawback of the method. A macromolecule obtained by anionic polymerization using vinyllithium carries an allylic double bond at the chain end, and this type of unsaturation is not adequate for the subsequent radical copolymerization because of its low reactivity towards radicals and of isomerization that can take place. 

The first quantitative kinetic study of an anionic polymerization was afforded by Higginson and Wooding (27), who studied the polymerization of styrene using potassium amide in liquid ammonia at — 33°. The rate expression was found to be... 

The aggregates are polymerization inactive their formation is accompanied by a corresponding decrease in the polymerization rate [96], Aggregate formation is not limited to siloxane monomers and polymers. Independently, and in the same year, associate formation during anionic styrene polymerization was described by Worsfold and Bywater... 

Continuous solution Anionic Pure styrene monomer Much recycled solvent Anionic initiators Polymerize to completion Low residual monomer High polymerization rate Good for spec, copolymer Sensitivity to impurities Initiator cost Color of product Cannot produce HIPS Not proven for high-volume GP... 

Figure 4.5 Propagation rate-retarding effect of adding dibutylmagnesium to butyl-lithium-initiated anionic polymerization of styrene...

Figure 10.6. Rate of anionic polymerization of styrene initiated by sodium naphthalene in 3-methyl tetrahydrofuran at 20°C. Left linear variation of rate with (C°)l/2 right inverse linear variation of rate with concentration of Na+ in presence of added sodium tetraphenyl borate. (Data from Schnitt and Schulz [79].)...

Equation (26) is the ideal copolymer composition equation suggested [203] early in the development of copolymerization theory but which had to be abandoned in favour of eqn. (23) as a general description of radical copolymerization. Only in this particular case are the rates of incorporation of each monomer proportional to their homopolymerization rates. It was shown that the reactivity of a series of monomers in stannic chloride initiated copolymerization followed the same order as their homopolymerization rates [202] and so eqn. (26) could be at least qualitatively correct for carbonium-ion polymerizations and possibly for reactions carried by carbanions. This, in fact, does not seem to be correct for anionic polymerizations since the reactivities of the ion-paired species at least, differ greatly. The methylmethacrylate ion-pair will, for instance, not add to styrene monomer, whereas the polystyryl ion-pair adds rapidly to methylmethacrylate [204]. This is a general phenomenon no reaction will occur if the ion-pair is on a monomer unit which has an appreciably higher electron affinity than that of the reacting monomer. The additions are thus extremely selective, more so than in radical copolymerization. There is no evidence that eqn. (26) holds and the approximate agreement with eqn. (25) results from other causes indicated below. 

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