Acetal interchange

Carboxy-hydroxy reactions, 63 Carboxyl endgroup chemical titration, 94 Carboxylic acid-aryl acetate interchange reactions, 62, 63... 

Competing side reactions in cationic polymerization of carbonyl monomers include cyclotrimerization and acetal interchange. Cyclotrimerization is minimized by low-polarity solvents, low temperatures, and initiators of low acidity. Acetal interchange reactions among different polymer chains do not occur except at higher temperatures. Acetaldehyde and higher aldehydes are reasonably reactive in cationic polymerization compared to formaldehyde. Haloaldehydes are lower in reactivity compared to their nonhalogen counterparts. 

Acetal-acetal interchange. Formaldehyde and acetaldehyde acetals can be prepared from methoxymethyl ethers and ethoxyethyl ethers, respectively, by reaction with TsOH in refluxing benzene. 

Hill and Carothers (4) investigated the acetal interchange reaction with di-n-butyl formal and glycols (Equation 4). When n was 3 or 4, cyclic formals were the principal products. Fentamethylene glycol (n = 5) gave a sirupy liquid polymer. The reaction with decamethylene gly-... 

The objectives of this study were to extend these synthetic methods to the preparation of low molecular weight, hydroxy-terminated polymers containing acetylenic bonds and evaluating these prepolymers in castable solid propellant formulations. Since 2-butyne-l,4-diol was commercially available, the formation of polyacetals from this glycol seemed to be an attractive route to the desired polymers. It was believed that the rigid triple bond would inhibit the cyclic acetal formation observed by the earlier workers with the lower members of the saturated glycol series. Thus, in an acetal interchange reaction a linear polymer (I) should be favored over the cyclic acetal (II) ... 

Similar acetal interchange reactions were carried out between butynediol and di-n-propyl acetal (Ilia) and butynediol and diethylpropional (Illb), to give low molecular weight, liquid polyacetals (IVa and IVb) ... 

Unlike anionic initiators or anionically growing alkoxide chains which can only grow (or terminate), cationic initiators (Lewis, Bronsted acids or preformed initiators) or the cationically growing chain may cause acetal-interchange reactions. These reactions are also called transacetalization and cause rearrangement in the molecular weight distribution in homopolymers. The rates of transacetalization are relatively slow compared to that of polymerization except at high temperatures. In the presence of cyclic ethers or cyclic formals, for example, dioxolane, polyformaldehyde can incorporate randomly the co-monomer polyoxyethylene units into the polymer under transacetalization conditions. 

The reaction of BejjO(OCOCH )g with diacid chlorides (4 ) gives low molecular weight polymers. On heating, interchange occurred and BejjO(OCCH-), could be sublimed at 110-l40 C at 10 torr pressure for tne product derived from adipyl chloride. The use of terephthalyl chloride appears to Increase the stability of the product such that a temperature of 340 C was required to obtain sublimed beryllium acetate. Interchange also slowly occurs at room temperature. 

Thioacetal—hemithioacetal—acetal interchange. Corey and Hase have disclosed a method for direct interchange of these functional groups under neutral conditions. The method is formulated for acetal interchange of heptanal. 

This acetal interchange is applicable also to trimethylene dithioacetals, as shown by conversion of the 1,3-dithane derivative (1) into the hemithioacetal... 

The basic principle of this polymerisation reaction is based on the acetal interchange illustrated in Scheme 5.17. 

Scheme 5.17 Acetal interchange reaction a) and the same principle applied to polymerisation by the acetal metathesis polymerisation mechanism b)...

Carothers was one of the first to exploit the polyacetal-forming reaction in his attempt to prepare polyformals by the reaction of formaldehyde [3] with glycols. He found that diols below 1,5-pentanediol led to cyclic formals whereas the principal product from 1,6-hexanediol and from higher molecular weight diols was a polyformal [3]. Similar products were formed by the acetal interchange reaction when appropriate diols were used [Eq. (4)] [4]. 

Hill and Carothers found that the acetal interchange reaction is carried out easily and that the reaction is reversible [3]. 

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