Acrylics, determination Ester groups

The high simple diastereoselectivities seen in Figures 15.29 and 15.30 are due to the same preferred orientation of the ester group in the transition states. The stereostructure of the cycloadduct shows unequivocally that the ester group points underneath the diene plane in each of the transition states of both cycloadditions and not away from that plane. Figure 15.31 exemplifies this situation for two transition states of simple Diels-Alder reactions of 1,3-butadiene A shows a perspective drawing of the transition state of the acrylic acid ester addition, and B provides a side view of the addition of ethene, which will serve as an aid in the following discussion. Both structures were determined by computational chemistry. 

Since the first preparation of stereoregular poly(methyl methacrylate) by Fox et al. and Miller et al. in 1958, a large number of papers have been published on the steieospecific polymerization of methyl methacrylate, while the NMR technique for the determination of microstructure developed by Bovey and Tiers and Nishioka et al. enabled us to accumulate the extensive information on this polymerization. Mostly anionic initiators have been used for the pdymerization. A review on the polymerization by lithium compounds was presented by Bywater In a recent review by Pino and Suter were discussed some of the factors which can influence the stereoregulation in the polymerization of vinyl monomers including a-substituted acrylate. A variety of magnesium and aluminum compounds can be utilized as stereospecific initiators. Besides methyl methacrylate, not only methacrylates with various ester groups, but also a-substituted acrylates, such as a-ethyl- or o-phenyl-acrylate, were also subjected to the stereospecific polymerization by anionic initiator. The stereospecificity in the copolymerization between the monomers described above is also a matter of interest. 

On the contrary to the value of a, the value of a can be determined experimentally by separating branches from the trunk polymer by cleaving ester groups in the case of poly(vinyl p-nitro-benzoate) and poly(p-nitrophenyl acrylate). 

It has been suggested by Bawn and Ledwith (1962) that the mechanism of alkyl acrylate polymerization is similar to that of methyl methacrylate, but with the size of the ester group determining the stereospecificity instead of the a-methyl group. This suggestion cannot be evaluated now owing to the lack of sufficient data however, recent observations by Schuerch et al, (1964) and Yoshimo et al. (1964) indicated that the nature of the catalyst is also important. 

Ester groups occur in a wide range of polymers (e.g., polyethylene terephthalate) and in copolymers such as, for example, ethylene vinyl acetate. The classical chemical method for the determination of ester groups, namely, saponification, can be applied to some types of polymer. For example, copolymers of vinyl esters and esters of vinyl esters and esters of acrylic acid, can be saponified in a sealed tube with 2 M sodium hydroxide. The free acids from the vinyl esters were determined by potentiometric titration or gas chromatography. The alcohols formed by the hydrolysis of the acrylate esters were determined by gas chromatography. Vinyl acetate ethylene copolymers can be determined by saponification with 1 N ethanolic potassium hydroxide at 80 C for 3 hours and back titration with standard acid or by saponification with p-toluene sulfonic acid and back titration with standard acetic acid [49, 50]. 

Haslam " has discussed infrared methods for the determination of ester groups in acrylic copolymers. NMR spectroscopy has been used for the determination of isophthalate in poly(ethylene terephthalate isophthalate) dissolved in 5% trichloroacetic acid. The NMR spectra of these polymers were measured on a high resolution NMR spectrometer at 80 C. A singlet at 7.74 ppm is due to the four equivalent protons attached to the nucleus of the terephthalate unit. The complicated signals which appear to 8.21, 7.90, 7.80, 7.35, 7.22 and 7.10 ppm are due to die four protons attached to the nucleus of die isophthalate unit. The content of the isophthalate unit can be calculated from the integrated intensities of these peaks. 

NMR has also been used to determine ethyl acrylate in ediyl acrylate-ethylene and vinyl acetate-ethylene copolymers. Measurements were made on 10% solutions in diphenyl ether at an elevated temperature. Resolution improved with increasing temperature and lower polymer concentration in the solvent. This technique has also been used to identify ester groups in acrylic copolymers and copolymers. 

METHOD 62 - DETERMINATION OF ACRYLIC ESTER GROUPS IN COPOLYMERS. HYDRIODIC ACID REDUCTION - GAS CHROMATOGRAPHY. ... 

This hydriodic acid reduction - gas chromatography procedure enables acrylic ester groups in copolymers to be determined in amounts down to 0.2%. 

This method enables methyl to butyl acrylate ester groups to be determined in acrylate copolymers in amounts down to 0.2% wifh an accuracy of + 3%. 

Determination of acrylic ester groups in copolymers. Hydriodic reduction-gas chromatography... 

The regioselectivity of the reaction appears to be determined by a balance of electronic and steric factors. For acrylate and propiolate esters, the carb-oxylate group is found preferentially at C3 of the carbazole product[6-8]. Interestingly, a 4-methyl substituent seems to reinforce the preference for the EW group to appear at C3 (compare Entries 4 and 5 in Table 16.2). For disubstituted acetylenic dicnophiles, there is a preference for the EW group to be at C2 of the carbazole ring[6]. This is reinforced by additional steric bulk in the other substituent[6,9]. 

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