P-Methoxy-a-methylstyrene

Copolymerization reactions with p-methoxy-a-methylstyrene were also attempted at 0°C, and a methanol-insoluble product having an Mjj of 1600 was obtained over a period of 36 h. From the 250-MHz 1h-NMR spectrum, it was found that p-methoxy-a-methylstyrene was present in the polymer to the extent of 14%. 

The most reactive monomers in cationic polymerizations, Af-vinyl carba-zole, vinyl ethers, and p-methoxy-a-methylstyrene provide the most stable carbenium ions. Carbenium ions generated from the first two monomers are stabilized by a-heteroatoms, whereas the latter monomer generates a tertiary carbenium ion with a strongly electron donating p-substituent. Carbenium ions with two alkoxy groups, such as 1,3-dioxo-lane-2-ylium cations and their acyclic analogs (Chapter 6) are stable at room temperature in the absence of moisture. 

Initiation is apparently slower than propagation. That is, the nucleophilic-ity of vinyl ethers is higher than their basicity. Other monomers such as p-methoxy-a-methylstyrene are apparently more basic and react rapidly with acid. In addition, the equilibrium monomer concentrations of a-meth-yl styrenes are relatively high ([M] 0.2 mol/L at —30° C). Because they can not polymerize at low concentration, they are ideal monomers for model studies [12,13]. The equilibrium constants of dimerization and tri-merization are much larger than that for the formation of high polymer. Therefore, dimers and trimers can be formed below [M] although high polymers cannot. 

Triflic acid homoconjugates with triflate anion, and two equivalents of acid are therefore necessary per cation. If only an equimolar amount of acid is used relative to p-methoxy-a-methylstyrene, then a dimeric cation M2+ is formed, along with small amounts of monomeric Mi+ cation and trimeric M3+ cation according to Eq. (7). No higher oligomers (n > 3) can be formed due to the low [M]0 ([M]0 < Ml ). 

Specific acid catalysis (by H ) was found [F. G. Ciapella and M. Kilpatrick, J. Am. Chem. Soc.. 70 (1948) 639] in the hydration of isobutene general acid-catalysis was not, and for this reason it was thought that the transition state comprised only a proton and the olefin. Schubert et a P, however, showed that formic acid itself could protonate p-methoxy-a-methylstyrene, and hence that general acid catalysis occurred. 

Methionine, / and d, SMA resin reaction, 430 I-Methoxy-1,3-butadiene MA copolymerization, 346, 348, 621 MA Diels-Alder reaction, 106, 139 p-Methoxy-a-methylstyrene, MA copolymerization, 371... 

Other reactions occur at even longer times. Indans and indanyl cations form first these are the final products for alkyl-substituted a-methylstyrenes. However, further reactions of the p-methoxy derivative leads to formation of spirobiindan and anisol [Eq. (11)]. 

As expected for an electrophilic reaction, the p values for oxymercuration of styrene (-3.16) and a-methylstyrene (-3.12) derivatives are negative. The positive deviation of the methoxy substituent, when treated by the Yukawa-Tsuno equation, is indicative of a modestly enhanced resonance component. The additional methyl substituent in a-methylstyrene is slightly activating and indicates that its electron-donating effect outweighs any adverse steric effect. 

In the bromination of styrene, a po-+ plot is noticeably curved. If the extremes of the curves are taken to represent straight lines, the curve can be resolved into two Hammett relationships with p = —2.8 for electron-attracting substituents and p = —4.4 for electron-releasing substituents. When the corresponding -methylstyrenes are examined, a similarly curved ap plot is obtained. Furthermore, the stereospecificity of the reaction in the case of the -methylstyrenes varies with the aryl substituents. The reaction is a stereoespecific anti addition for strongly electron-attracting substituents but becomes only weakly stereoselective for electron-releasing substituents, e.g., 63% anti, 37% syn, forp-methoxy. Discuss the possible mechanistic basis for the Hammett plot curvature and its relationship to the stereochemical results. 

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