Perester groups

Azoperoxydic initiators are particularly important due to their capacity to decompose sequentially into free radicals and to initiate the polymerization of vinylic monomers. The azo group is thermally decomposed first to initiate a vinyl monomer and to synthesize the polymeric initiator with perester groups at the ends of polymer chain (active polymer) [31,32]. 

It should be noted from the point of view of synthesis that the building up of tert-butyl perester groups in polymethacrylyl chloride proceeds with great difficulties, on account of the high steric hindrance which impedes the SN2 nucleophilic substitution reaction. 

Depending on the types of monomers used and on the predominant mechanism of termination, it is either the azo or the perester group which is decomposed first. This is illustrated by the following examples. 

Lastly, this third group also contains macromolecular poly-initiators which have been developed for a long time by A. E. Woodwards and G. Smets [43]. The principle of the reaction is the polymerization of a monomer (M,) with the phtaloyle peroxide —(OOOC—O—CO—O—CO—O—CO) —. The obtained homopolymer still exhibits perester groups which the authors used for initiating polymerization of a second monomer (M ) and for obtaining multiblock copolymers. 

In the case of 1 products containing perester groups could not be isolated, and in any event any peresters derived from 1 by hydrogen abstraction or radical coupling would be expected to be at least as reactive as their precursor 5. However the product distribution was quantitatively what would be expected from reaction of 1 without formation of 3, and no products uniquely ascribable to 3 were observed. Perhaps more convincingly radical 9, analogous to 1, could be formed at 25°C and did not undergo decomposition in the... 

Other examples of polymers containing peroxidic groups are noted in ref. 36. A recent example containing 2 kinds of perester groups and a vinyl group in shown in figure 4 ( ). ... 

Figure 4, Polymer containing 2 types of perester groups...

It has also been suggested that peroxy acid groups and perester groups [2315, 2316] formed during the photo-oxidation of isotactic polypropylene may participate in the photo-oxidative degradation of the polymer. Polypropylene films of low atactic content were found to undergo faster photooxidation than films of high atactic content, irradiated under identical conditions [1163]. 

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

I. 463 A) is close to the value for fert-butyl hydroperoxide (1) (95° and 104°, two crystallo-graphically independent molecules), which points to a comparatively small steric demand of the acyl group in peresters. Due to its structural similarity to diaUsyl peroxides, it is reasonable that the ortho ester 56 P2, 0—0 = 1.481 A, C—O—O—C = 145.1°) exhibits a significantly larger peroxide dihedral angle than 54 . 

A survey of the back side angles C—C(0)—of perester acyl groups in general provides values between 106.8-109.0° in case of aliphatic carboxylic acids (entries 2-7,... 

In this section we discuss the thermochemistry of some species with the general formulas, RC(=0)00H, RC(=0)00R and RC(=0)00C(=0)R. These species have the generic names peracids (peroxycarboxyUc acids), peresters (percarboxylate esters) and acyl peroxides. The enthalpy of formation values are in Table 3. Three formal reactions that are discussed here are conceptually the same as in the earlier sections. Because now there is a carbonyl group present, we rewrite equations 5, 6 and 9 as equations 14, 15 and 16. 

The final comparison is between the enthalpies of formation of di-t-butyl E,E-muconate (2,4-hexadienoate), —917.1 3.9 kJmol", and its diperoxy ester counterpart, —777.7 4.8 kJmol. The enthalpy of reaction 16 is 139.4 6.2 kJmol" or 69.7 3.1 kJmol per carboxylate ester group, nearly the same as the differences for the other peresters. 

Treatment of polymer films by reactive gases or reactive volatil compounds allows to easily modify polymers containing alcohols, hydroperoxides, carboxylic acids (or acids halides), double bonds or piperidine groups. New functional groups as organic nitrites, nitrates, iodides, acid halides (Cl, F), amides, esters, peresters and nitroxyl radicals can be generated by a single reaction or by combination of two consecutive treatments. The reactions are very efficient on thin films (ca 50-100 pm) and can be controlled by transmission and reflexion 1R spectroscopy. 

In keeping with findings in the hexuronic acid series (see Section 11,2), hexopyranose peresters having axial substituents at the anomeric center react appreciably more slowly than do compounds with such groups equatorial they give complex sets of products from which 5-bromo-a compounds have been isolated in only modest yields.26 34 As with the hexuronic acids, this observation can be attributed to steric inhibition of axial attack by bromine at C-5 or at H-5. 

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