Styrene/p-methylstyrene

TABLE 6-11 Effects of Solvent and Counterion on Copolymer Composition in Styrene-p-Methylstyrene Cationic Copolymerization"... 

Data from O Driscoll et al. [1996]. b Comonomer feed= 1 1 styrene-p-methylstyrene. 

The general characteristics of anionic copolymerization are very similar to those of cationic copolymerization. There is a tendency toward ideal behavior in most anionic copolymerizations. Steric effects give rise to an alternating tendency for certain comonomer pairs. Thus the styrene-p-methylstyrene pair shows ideal behavior with t = 5.3, fy = 0.18, r fy = 0.95, while the styrene-a-methylstyrene pair shows a tendency toward alternation with t — 35, r% = 0.003, i ii 2 — 0.11 [Bhattacharyya et al., 1963 Shima et al., 1962]. The steric effect of the additional substituent in the a-position hinders the addition of a-methylstyrene to a-methylstyrene anion. The tendency toward alternation is essentially complete in the copolymerizations of the sterically hindered monomers 1,1-diphenylethylene and trans-, 2-diphe-nylethylene with 1,3-butadiene, isoprene, and 2,3-dimethyl-l,3-butadiene [Yuki et al., 1964]. 

The change in surface acidity with water content was also demonstrated by the ability of kaolinite to promote acid-catalyzed polymerization (236). Styrene, p-methylstyrene, and p-methoxymethylstyrene polymerized vigorously on kaolinite that was dried at 110°C. At 0.2% wt water content, p-methylstyrene and p-methoxystyrene polymerized and at 0.6% wt water content, only p-methoxystyrene polymerized. The polymerization results are consistent with lower acidity at higher water contents since the susceptibility of these monomers to acid-catalyzed polymerization is in the order p-methoxystyrene > p-methylstyrene > styrene. 

Considerable efforts have been directed, primarily in Kennedy s group [3], to synthesize a series of block copolymers of isobutene with isoprene [90,91], styrene derivatives [92-104], and vinyl ethers [105-107]. Figure 7 lists the monomers that have been used for the block copolymerizations with isobutene. The reported examples include not only AB- but also ABA- and triarmed block copolymers, depending on the functionality of the initiators (see Chapter 4, Section V.B, Table 3). Obviously, the copolymers with styrene derivatives, particularly ABA versions, are mostly intended to combine the rubbery polyisobutene-centered segments with glassy styrenic side segments in attempts to prepare novel thermoplastic elastomers. These styrene monomers are styrene, p-methylstyrene, p-chlorostyrene, a-methylstyrene, and indene. 

The B values for addition of styrene, p-methylstyrene, and 2-vinylpyridine to living polystyrene demonstrate the importance of the polar character of the monomer. The addition of vinylpyridine is much faster than the addition of styrene, whereas addition of p-methylstyrene is slower. Hence, a decrease in the negativity of the C=C bond enhances the addition (vinylpyridine as compared with styrene), its increase having the opposite eflFect (p-methylstyrene as compared with styrene). On the other hand, the change in the polarity of the ion has a less pronounced effect as shown by the for the addition of styrene monomer to... 

Copolymerization studies involving the Cs salts of living polymers of p-substituted styrenes and 1,1-diphenylethylene in THF have enabled the reactivities of the corresponding free ions and ion pairs to be assessed. The method used for the calculation of rate constants appears to be satisfactory, and the values obtained in the case of poly-(p-methylstyryl) caesium and poly-(/ -methoxystyryl) caesium correlate well with the accepted values for the unsubstituted styrene system. For example, kp —) values in THF at 0 °C are 95 000, 220 000, and 1 000 000 M" s for the series styrene, p-methylstyrene,... 

Living cationic polymerizations have been carried out with a number of monomers, such as isobutylene, styrene, p-methylstyrene, p-methoxystyrene, A/-vinyl caibazole, and others. To achieve living conditions, it is necessary to match the propagating carbon cation with the counterion, the solvent polarity, and the reaction temperature. Some examples are presented in Table 3.2. 

The mechanism of photo-degradation of poly(p-methylstyrene) was studied by several investigators [544-547]. A gas evolution was observed during the irradiation with ultraviolet light. This gas contains hydrogen as its major portion and methane, ethane as the minor portions. There are also traces of styrene, p-methylstyrene, and toluene [101]. The gas evolution is accompanied by cross-linking. The start of the process is pictured as follows [547] ... 

Loffredo, E, Pranzo, A., Venditto, V., Longo, P., Gnerra, G. Clathrate phases of styrene/p-methylstyrene co-syndiotactic copolymers. Macromol. Chem. Phys., 204, 859-867 (2003). 

Waters61 have measured relative rates of p-toluenesulfonyl radical addition to substituted styrenes, deducing from the value of p + = — 0.50 in the Hammett plot that the sulfonyl radical has an electrophilic character (equation 21). Further indications that sulfonyl radicals are strongly electrophilic have been obtained by Takahara and coworkers62, who measured relative reactivities for the addition reactions of benzenesulfonyl radicals to various vinyl monomers and plotted rate constants versus Hammett s Alfrey-Price s e values these relative rates are spread over a wide range, for example, acrylonitrile (0.006), methyl methacrylate (0.08), styrene (1.00) and a-methylstyrene (3.21). The relative rates for the addition reaction of p-methylstyrene to styrene towards methane- and p-substituted benzenesulfonyl radicals are almost the same in accord with their type structure discussed earlier in this chapter. 

Morphology of the anionically synthesized triblock copolymers of polyfp-methyl-styrene) and PDMS and their derivatives obtained by the selective chlorination of the hard segments were investigated by TEM 146). Samples with low PDMS content (12%) showed spherical domains of PDMS in a poly(p-methylstyrene) matrix. Samples with nearly equimolar composition showed a continuous lamellar morphology. In both cases the domain structure was very fine, indicating sharp interfaces. Domain sizes were estimated to be of the order of 50-300 A. 

Indeed, cumyl carbocations are known to be effective initiators of IB polymerization, while the p-substituted benzyl cation is expected to react effectively with IB (p-methylstyrene and IB form a nearly ideal copolymerization system ). Severe disparity between the reactivities of the vinyl and cumyl ether groups of the inimer would result in either linear polymers or branched polymers with much lower MW than predicted for an in/mcr-mediated living polymerization. Styrene was subsequently blocked from the tert-chloride chain ends of high-MW DIB, activated by excess TiCU (Scheme 7.2). 

Styrene (Fisher), p-methylstyrene (Mobil), and t-butylstyrene (DOW) were purified by passing through a column of activated alumina and then carefully degassed to remove all traces of 0. Further purification by vacuum distillation from dibutyl magnesium resulted in anionically pure monomers. 

The results of this work are not limited to just S-b-MM and S-b-tBM, but may be extended to include styrene derivatives such as p-methylstyrene and p-t-butylstyrene 1). In addition to t-butyl methacrylate, other alkyl esters capable of stabilizing a carbonium ion, such as benzyl methacrylate and allyl methacrylate, should exhibit similar reactivity toward acidic hydrolysis and TMSI. In contrasting the hydrolysis of tBM blocks with TsOH and their reaction with TMSI, it should be noted that the hydrolysis is reportedly catalytic in nature (7-10), whereas the reaction with TMSI is stoichimetric. Therefore the latter approach may allow one to more easily "dial in" a desired level of methacrylic acid or metal methacrylate. 

The cis alkenes are more reactive and more selective than their trans counterparts. As with the Evans system, this reaction is not stereospecific. Acyclic cis alkenes provide mixtures of cis and trans aziridines. cis-p-Methylstyrene affords a 3 1 ratio of aziridines favoring the cis isomer, Eq. 67, although selectivity is higher in the trans isomer. A fascinating discussion of this phenomenon, observed in this system as well as the Mn-catalyzed asymmetric oxo-transfer reaction, has been advanced by Jacobsen and co-workers (83). Styrene provides the aziridine in moderate selectivity, Eq. 68, not altogether surprising since bond rotation in this case would lead to enantiomeric products. 

Ever since 1962, when Williams, Okamura, and their associates started to publish propagation rate-constants k+p for the cationic bulk polymerization of cyclo-pentadiene, isobutene, styrene, a-methylstyrene and isopropylvinyl ether by ionizing radiations, these constants have been accepted as the best, most likely, values for the k+p of unpaired cations in a medium of low-polarity, and those obtained subsequently by Stannett and his collaborators, using similar methods, enjoyed the same status, (The loci classici are Bates et al. (1962), Bonin et al. (1964), Taylor Williams (1969) and the three papers by Ueno et al. (1967), Hayashi et al. (1967) and Williams et al. (1967).)... 

Epoxidation of c/.v-P-mcthylstyrcnc and trara-P-methylstyrene produce the epoxides with retention of configuration [36], The reaction is first order in styrene and Ru02 and the reaction is faster when the styrene molecules contain electron donating substituents. This shows that the oxo attack at the alkene is an electrophilic attack [37], Epoxidation of 1-alkenes gives aldehyde as the... 

Under identical conditions the aromatic olefins styrene and p-methylstyrene give the vic-dimethoxy adducts as the sole products. Methoxytellurenylated adducts are formed, however, as minor by-products from different substituted olefins (p-chlorostyrene) or exclusively (from styrene) when the amount of H2SO4 is reduced. 

Olefin styrene, a-methylstyrene, frans-p-methylstyrene, 1-hexene, c/s and frans-4-octene, c-pentene, c-hexene, c-heptene, indene, 1,2-dihydronaphthalene... 

Among the most extensive studies of monomer reactivity have been those involving the copolymerization of various meta- and para-substituted styrenes with other styrene monomers (styrene, a-methylstyrene, and p-chlorostyrene) as the reference monomer [Kennedy and Marechal, 1983], The relative reactivities of the various substituted styrenes have been correlated by the Hammett sigma-rho relationship ... 

Steric effects similar to those in radical copolymerization are also operative in cationic copolymerizations. Table 6-9 shows the effect of methyl substituents in the a- and 11-positions of styrene. Reactivity is increased by the a-methyl substituent because of its electron-donating power. The decreased reactivity of P-methylstyrene relative to styrene indicates that the steric effect of the P-substituent outweighs its polar effect of increasing the electron density on the double bond. Furthermore, the tranx-fl-methylstyrene appears to be more reactive than the cis isomer, although the difference is much less than in radical copolymerization (Sec. 6-3b-2). It is worth noting that 1,2-disubstituted alkenes have finite r values in cationic copolymerization compared to the values of zero in radical copolymerization (Table 6-2). There is a tendency for 1,2-disubstituted alkenes to self-propagate in cationic copolymerization, although this tendency is low in the radical reaction. 

The stoicheiometric reagent fra -Ru(0)j(por )/CH2Cl2 epoxidised several aUc-enes with good e.e. while fra 5-Ru(0)2(por )/PhlO/CH2Cl2 epoxidised styrene to the (R) oxide, and cw-P-methylstyrene to (lR,25)-cti-P-methylstyrene oxide with an e.e. of 91% [613] fran -Ru(0)2(por )/02 (8 atm)/CH2Cl2 also asymmetrically epoxidised aUcenes [612]. Second order rate constants for a number of such reactions were measured, as were Hammett plots for oxidation of para-substituted styrenes. Oxygen-atom transfer via the unstable Ru" (0)(por ) was suggested [613, 614]. 

Ru(0)(biqn)(tmtacn)](C10 )2 and [Ru(0)(diopy)(tmtacn)](C10 )2 (biqn=C2 symmetric 1,T-biisoquinoline, diopy=(R,R)-3,3 -(l,2-dimethylethylenedioxy)-2,2 -bipyridine) aremadefrom [RuCl(L)(tmtacn)] + (L=biqn, diopy) and(NH )2[Ce(N03)J with Li(ClO ). Electronic and IR spectra were measured (v(Ru=(0) bands lie at 760 and 795 cm" respectively). The (diopy) complex is paramagnetic with 2.88 B.M. As stoich. [Ru(0)(biqn)(tmtacn)] + and [Ru(0)(diopy)(tmtacn)] VCH3CN they oxidised alkenes (styrene, cis and fran.y-P-methylstyrenes, fran -stilbene, nor-bomene, cyclohexene) to mixtures of aldehydes and epoxides. Conttary to expectation the (diopy) complex did not effect enantioselective epoxidations except with fran -stilbene, for which a moderate e.e. of 33% was observed [623]. 

RuCl(dmso)(bpy)2]Cl and [RuCl(SOMePh)(bpy) ]Cl, called respectively rac-Ru-1 and A-Ru-2 by the authors, are made by microwave irradiation of cis-RuCljtbpy) with the the sulfoxide ligands. Trani-stilbene, styrene and R-styrene (R=p-MeO, a and P-methyl) were epoxidised by [RuCl(dmso)(bpy)3]Vaq. Ph(IOAc)2/CH3Cl3/40°C, while with [RuCl(SOMePh)(bpy)3]V TBHP/water-CH Cl fran -stilbene, fran -P-methylstyrene gave the (R,R) epoxides [947]. 

The complexes Ru(pydic)(tpy), Ru(pydic)(pybox-R ) (pydic=pyridine-2,6-dicarboxylate, pybox-Rj=chiral bis(oxazolinyl)pyridines with R=PP, Ph (Fig. 1.37) [105] epoxidised trani-stilbene (as complex/PhlO, Ph OAc), TBHP or OJ CHjClj). Asymmetric oxidations of trani-stilbene were similarly achieved in toluene, benzene and CH Cl with e.e. values from 40-80% cf mech. Ch. 1) [53, 54, 81,97]. Asynunetric epoxidations of rranx-stilbene, styrene, tranx-fl-methylsty-rene and 1-hexene were catalysed by [RuCl(SOMePh)(bpy)j] /TBHP or Ph(IOAc)y CHjCy40°C e.e. values of 33-94% were obtained of the (R. R) forms of the epoxides of tra i-stilbene, tranx-P-methylstyrene [52]. The system Ru(CO)(TPP)/ (CljpyNO)/HBr/C H epoxidised fullerene (C ) to 1,2-epoxy[60]fullerene with 1,2 3,4 di-epoxy and 1,2 3,4 9,10 h- 1,2 3,4 11,12 tri-epoxy species [106]. 

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