Syndiotactic poly hydrolysis

Because of acid-catalyzed hydrolysis of N-vinylpyrrolidone in water, polymerization was carried out in organic solvent - DMF. Three types of samples of poly(methacrylic acid) were used syndiotactic - obtained by radiation polymerization, atactic - obtained by radical polymerization, and isotactic - obtained by hydrolysis of isotactic poly(methyl methacrylate). It was found that in all cases the rate enhancement appeared in comparison with the blank polymerization (without template). The rate enhancement became more pronounced with increasing chain length and syndiotacticity of the template. According to the authors, the rate enhancement is connected with the stronger complex formation between poly(vinyl pyrrolidone) and syndiotactic poly(methacrylic acid) then with isotactic template. This conclusion was supported by turbimetric titration in DMF/DMSO system and by model considerations. It is worth noting, however, that... 

Poly(vinyl alcohol) is typically obtained by alcoholysis of poly(vinyl esters), for example from polyfvinyl acetate) and methanol in the presence of NaOH. The process can be completed or only partially conducted. In this latter case a copolymer (alcohol/ester) is obtained. Other synthetic procedures are used, most of them also based on the hydrolysis of poly(vinyl esters). Poly(vinyl alcohol) is typically used in the atactic form, but isotactic or syndiotactic poly(vinyl alcohols) also are known. 

Up to now the only way to get isotactic poly(acrylic acid) or poly(methacrylic acid) has been by hydrolysis of isotactic poly(acrylates) or poly(methacrylates) [476]. Direct routes to get isotactic polymers would be anionic and coordination polymerization. But these polymerizations are not practicable, because the acid function would destroy the initiator. Kargin et al. [477] prepared isotactic poly(acrylic acid) by reaction of isotactic poly(isopropylacrylate) in toluene as solvent with potassium hydroxide in propanol. Propanol and the formed isopropanal were removed after reflexing for 6 h by distillation. Complete hydrolysis was reached after 10 h. Aylward synthesized isotactic and syndiotactic poly(methacrylic acid) by quantitative hydrolysis of poly(trimethylsilyl methacrylate) [478]. 

Table III summarizes a variety of observations on the alkaline hydrolysis of several acrylic polymers. To be noted is that the difference in the rates of hydrolysis of isotactic and syndiotactic poly(methyl methacrylate) is sufficiently great that mixtures of such polymers may be separated on the basis of this behavior [18].

The alkaline hydrolysis of acrylic esters requires, as shown in Table III, considerable time. The procedure involves a heterogeneous reaction in its early stages and may actually be accompanied by oxidative degradation of the polymeric chains [20]. The acid hydrolysis, on the other hand, is a homogeneous reaction and is thought not to lead to chain scission [3]. Again, poly(acrylates) hydrolyze more rapidly than the poly(methacrylates), and the isotactic poly(methacrylates) are more easily converted to poly(methacrylic acids) than the syndiotactic poly (methacrylates) [18]. 

On the basis of a study of molecular models, it had been proposed that the polymerization of 2,4,6-triphenylbenzyl methacrylate would only result in the formation of syndiotactic polymer. However, upon hydrolysis of this sterically hindered polyester with phosphonium iodide, the resultant poly(acrylic acid) could be separated into an atactic and a syndiotactic fraction on the basis of differences in solubility. This method of hydrolysis is of particular interest since it evidently permits relatively rapid saponification of syndiotactic poly(esters). 

Figure 1 shows the 220 MHz PMR spectra of several syndiotactic methacrylic acid(A)-methyl methacrylate(m) copolymers prepared by the hydrolysis of syndiotactic poly(methyl methacrylate). Separate resonances are evident for the a-methyl protons centered in AAA, (MAA+AAM), MAM, AMA, (AMM+MMA) and MMM triads. Depending on reaction conditions, the relative intensities... 

Poly(acrylic acid) and Poly(methacrylic acid). Poly(acryHc acid) (8) (PAA) may be prepared by polymerization of the monomer with conventional free-radical initiators using the monomer either undiluted (36) (with cross-linker for superadsorber appHcations) or in aqueous solution. Photochemical polymerization (sensitized by benzoin) of methyl acrylate in ethanol solution at —78° C provides a syndiotactic form (37) that can be hydrolyzed to syndiotactic PAA. From academic studies, alkaline hydrolysis of the methyl ester requires a lower time than acid hydrolysis of the polymeric ester, and can lead to oxidative degradation of the polymer (38). Po1y(meth acrylic acid) (PMAA) (9) is prepared only by the direct polymerization of the acid monomer it is not readily obtained by the hydrolysis of methyl methacrylate. 

Free-radical polymerization in methyl ethyl ketone at 60°C Hydrolysis of poly(methacrylic anhydride) at 40°C Hydrolysis of esters having appropriate configurations 57% syndiotactic triads Atactic Syndiotactic... 

The first attempts to synthesize poly(acrylic acid) (PAA) or poly(methacrylic acid) (PMAA) were the hydrolysis of poly(acid derivatives) such as esters, acylchlorides, nitriles, or amides. The hydrolysis has to be quantitative otherwise, one obtains a copolymer of acid and derivative [472]. On the other hand, hydrolysis in boiling alkaline solution can diminish the molar masses [473]. One possibility is to polymerize methyl methacrylic ester and to hydrolyze the resulting PMMA in acetic acid by the addition of a small amount of -toluenesulfonic acid as a catalyst. The solution is kept at 120 °C for 18 h and the methylacetate fonned is removed by distillation [474]. The degree of hydrolysis depends strongly on the tacticity of the original polymer. Syndiotactic PMMA is hydrolyzed slowly, but isotactic polymer is hydrolyzed very rapidly [475]. The polymers examined had molar masses up to 125,000. 

It is interesting to reconsider the case of a poly(acrylic acid) dehydration reaction where two different steps can be distinguished for the atactic polymer the first step is five to six times faster than the second one and obviously corresponds to dehydration of meso dyads (isotactic sequences). Hydrolysis of syndiotactic... 

On the other hand, if the initial cyclopolymer is heated in presence of pyridine, isomerization of the syndiotactic form into the isotactic one is observed, probably by enolization of the position in a, whose hydrolysis gives rise to isotactic poly(acrylic acid) the latter can easily be transformed into polyanhydride by thionyl chloride ... 

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