Styrene and a-Methylstyrene

Hydrocarbon resins (qv) are prepared by copolymerization of vinyltoluene, styrene, and a-methylstyrene in the presence of a Eriedel-Crafts catalyst (AlCl ). These resins are compatible with wax and ethylene—vinyl acetate copolymer (197). 

In comparison with the platinum catalysts, rhodium catalysts are much more reactive to effect addition of bis(catecholato)diboron even to non-strained internal alkenes under mild reaction conditions (Equation (5)).53-55 This higher reactivity prompted trials on the asymmetric diboration of alkenes. Diastereoselective addition of optically active diboron derived from (li ,2i )-diphenylethanediol for />-methoxystyrene gives 60% de (Equation (6)).50 Furthermore, enantioselective diboration of alkenes with bis(catecolato)diboron has been achieved by using Rh(nbd)(acac)/(A)-QUINAP catalyst (Equation (7)).55,56 The reaction of internal (A)-alkenes with / //-butylethylene derivatives gives high enantioselectivities (up to 98% ee), whereas lower ee s are obtained in the reaction of internal (Z)-alkenes, styrene, and a-methylstyrene. 

High yields are obtained with terminal alkenes (styrene and a-methylstyrene) and with the strained acenaphthylene, whereas stilbene and -methylstyrene are barely reduced. 

Many substituents stabilize the monomer but have no appreciable effect on polymer stability, since resonance is only possible with the former. The net effect is to decrease the exothermicity of the polymerization. Thus hyperconjugation of alkyl groups with the C=C lowers AH for propylene and 1-butene polymerizations. Conjugation of the C=C with substituents such as the benzene ring (styrene and a-methylstyrene), and alkene double bond (butadiene and isoprene), the carbonyl linkage (acrylic acid, methyl acrylate, methyl methacrylate), and the nitrile group (acrylonitrile) similarly leads to stabilization of the monomer and decreases enthalpies of polymerization. When the substituent is poorly conjugating as in vinyl acetate, the AH is close to the value for ethylene. 

Tetrahydrofuran and tetrahydropyran were refluxed for 24 hr over Na-K alloy and distilled onto fresh alloy. Before use the solvents were distilled once more onto SrS2 and subsequently distilled under vacuum into ampules. An all-glass apparatus equipped with breakseals was used for the latter operation. Styrene and a-methylstyrene were distilled under vacuum and dried twice over CaH2 under high vacuum. Styrene was further purified by distilling it in the presence of SrS2, while a-methylstyrene was further dried over Na-K alloy. 

A seed latex consisting of styrene and a-methylstyrene was prepared by Mestach et al. (4) and used to polymerize styrene and methylacrylate. 

Only a few examples of alkenylbenzene hydroformylation are available. Styrene and a-methylstyrene were reported to lead to hydrogenation as the main reaction when Co2(CO)8 was used. 

Ito O, Matsuda M (1982) Polar effects in addition reactions of benzenethiyl radicals to substituted styrenes and a-methylstyrenes determined by flash photolysis. J Am Chem Soc 104 1701-1703 Janata E, Veltwisch D, Asmus K-D (1980) Submicrosecond pulse radiolysis conductivity measurements in aqueous solutions. II. Fast processes in the oxidation of some organic sulphides. Ra-... 

Conjugated dehydrogenation of isopropylbenzene was planned [87] by the method of experiments with the minimal number of tests. Total yields of styrene and a-methylstyrene per injected isopropylbenzene were taken for optimized parameters, because these monomers are equally valuable. At the gradient motion (T = 640 °C) the highest yield of the target products (styrene + a-methylstyrene) equaled 56.8% with about 90% selectivity. Further gradient motion was of no practical interest due to a process selectivity decrease down to 80%. 

The transient absorption of the radical anions observed in pulsed styrene and a-methylstyrene were extremely sensitive to water they were greatly diminished or sometimes not observed at all if a small amount of water, even moisture in the atmosphere, was introduced into the sample. Similar phenomena have been observed in the pulse-irradiated monomers in cyclohexane solutions [16, 17]. Addition of ethanol, methylene chloride, chloroform, carbon tetrachloride, and n-butyl amine also reduced the yield of the anions [18]. 

High energy radiation generates ionic and free radical intermediates in styrene, and both ionic and radical polymerizations take place simultaneously. Therefore, the observation of neutral radicals related to the radical polymerization had been expected. The absorption of radicals with a maximum at 320-330 nm was actually observed in the pulsed styrene and a-methylstyrene it decayed over a time interval of several hundred microseconds without being affected by the presence of water [12 14]. This absorption was attributed for the main part to a radical with a benzyl-type structure. The similar absorptions were observed in the cyclohexane solutions [21], Swallow suggested that this intermediate would be formed by an ionic process, probably by the protonation of a... 

The dimer cation was supposed to have a sandwich structure in which the orbitals of one molecule overlapped with those of the other molecule. The band at 450 nm (B) is due to the bonded dimer cation (St—St T) the formation of this species corresponds to the initiation step of the polymerization. The bonded dimer cation may be formed by the opening of the vinyl double-bonds. Egusa et al. proposed that the structure was a linked head-to-head type I or II, by the analogy of the dimeric dianions of styrene and a-methylstyrene. Table 1 summarizes the assignment of absorption bands observed in pulse radiolysis of 1,1-diphenylethylene in dichloromethane, which is a compound suitable for studying monomeric and dimeric cations [28],... 

Apart from the relevance to the radiation-induced polymerizations, the pulse radiolysis of the solutions of styrene and a-methylstyrene in MTHF or tetrahy-drofuran (THF) has provided useful information about anionic polymerization in general [33]. Anionic polymerizations initiated by alkali-metal reduction or electron transfer reactions involve the initial formation of radical anions followed by their dimerization, giving rise to two centers for chain growth by monomer addition [34]. In the pulse radiolysis of styrene or a-methylstyrene (MS), however, the rapid recombination reaction of the anion with a counterion necessarily formed during the radiolysis makes it difficult to observe the dimerization process directly. Langan et al. used the solutions containing either sodium or lithium tetrahydridoaluminiumate (NAH or LAH) in which the anions formed stable ion-pairs with the alkali-metal cations whereby the radical anions produced by pulse radiolysis could be prevented from rapid recombination reaction [33],... 

Frank and coworkers (14) dimerized butadiene, styrene, and a-methylstyrene with finely dispersed sodium metal to give sebacic acid and substituted adipic acids, respectivdy, in good yields. In the case of butadiene, a portion of the product was a substituted suberic acid. Styrene and a-methylstyrene (14a) gave, on termination with water, about 90% yields of 1,4-diphenylbutane and 2,5-diphenylhexane, respectively. 

The structure of the dimers of methyl vinyl sulfone, styrene, and a-methylstyrene indicate that the preferred orientation of the ion-radical formed follows what would be expected from polarization of the 7t electrons of the bond by the attached groups. 

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

The formation of the styrene and a-methylstyrene anion under pulsed irradiation of monomers in the pure state and in dilute solutions has been clearly indicated from the absorption spectra obtained. 

The formation of a styrene and a-methylstyrene anion, absorbing at about 390 m/x, was shown by Schneider and Swallow (23, 24, 25), even for conventionally purified monomers which had not been treated to remove traces of water. The nature and reactions of this transient were studied in more detail in dilute solutions of styrene in different solvents. A solution (10 3M) of styrene in cyclohexane showed, immediately after the pulse, a spectrum with a distinct absorption band at 390 m/x (Figure 1), similar to that observed by Keene et al. (14). However, the intensity of the absorption was irreproducible during a series of experiments, probably because of the varying moisture content of the air and of the... 

The role of cations in the pulse radiolysis of styrene and a-methyl-styrene is not yet clear. Using frozen glasses, Shida and Hamill (26) and Williams (27) have seen absorptions with peaks at 350 and 650 m/x for styrene and a-methylstyrene. Bands have also been seen previously (24) at 460 or 475 m/x. Perhaps the species responsible for the shoulders seen by Schneider and Swallow between 340 and 370 m/x is a cation, whose absorption overlaps that of the anion radical. This could explain the diminution of the absorption intensity round 390 m/x when n-butylamine was added. However, there was no evidence for an absorption band at longer wavelengths. 

Table I summarizes some of the results on the pulse radiolysis of styrene and a-methylstyrene obtained by the different authors. Although...

Table II. Maximum Viscoelastic Relaxation Times for Block Copolymers of Styrene and a-Methylstyrene...

Few examples of the homogeneous diblock-incompatible homo-polymer behavior have been reported. One that has received considerable attention is the system polystyrene-poly-a-methylstyrene (2). Block copolymers of styrene and a-methylstyrene exhibit a single loss peak in dynamic experiments (2,3) and have been shown to be thermorheologi-cally simple (4) hence they are considered to be homogeneous. Mechanical properties data on these copolymers also has been used to validate interesting extensions of the molecular theories of polymer viscoelasticity (2,3,4). 

Monomeric styryl cations were observed recently at 315 and 325 nm during flash photolysis of styrene and a-methylstyrene, respectively, in trifluoroethanol [20]. Absorbance of these model compounds at wavelengths lower than those of the propagating species may be due to the... 

There is some spectroscopic evidence that aromatic compounds complex carbenium ions [42]. For example, the complexation equilibrium constant between trityl ions and hexamethylbenzene is K = 68 mol-1 L at 0° C [43]. Complexation should be stronger with more electrophilic carbenium ions such as those derived from styrene and a-methylstyrene. On the other hand, the monoalkyl-substituted phenyl rings attached to the polymer chain are weaker nucleophiles than hexamethylbenzene. A complexation constant K = 4 mol 1 L was reported for trityl cation and styrene [43]. Similar complexes have been proposed to explain the red color observed in inifer systems based on l,4-bis(I-chIoro-l-methyl-ethyl)benzene and BCI3 in CH2C12 at low temperature [44],... 

As outlined in Section III. A.3. a, the strength of the Lewis acid with mixed chloride and alkoxy derivatives decreases as the number of chloride ligands are replaced with alkoxy groups. Titanium chloride with one alkoxy group polymerizes styrene and a-methylstyrene Lewis acid with two alkoxy groups is too weak to initiate polymerization of styrene, but will initiate polymerization of a-methylstyrene and vinyl ethers. The Lewis acidity of titanium chloride derivatives with three alkoxy groups are so low that only vinyl ether polymerizations reach reasonable conversions. 

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