Polyacetals, hydrolysis

Ethylene oxide-polyacetal Hydrolysis- methylation Py-GC Sequence distribution [213]... 

Amide urethane, and ester groups in the polymer chain, such as those present in nylons and polyesters may be hydrolyzed by acids to produce lower-molecular-weight products. Polyacetals are also degraded by acid hydrolysis, but ethers, such as polyphenylene oxide (PPO), are resistant to attack by acids. 

Of all the selective, deprotection procedures that are available to carbohydrate chemists, the partial hydrolysis of polyacetals is probably the most familiar. Articles by de Beider4,5 and Brady6 contained examples of this type of reaction for aldose and ketose derivatives, respectively, and an article by Barker and Bourne7 gave useful information from the early literature on the graded, acid hydrolysis of acetal derivatives of polyols. A discussion of the stereochemistry of cyclic acetals of carbohydrates was included in an article by Mills 70 and in one by Ferrier and Overend,76 and a survey of the formation and migration of carbohydrate cyclic acetals was made by Clode.7c... 

Less is known concerning the partial, acid hydrolysis of alditol polyacetals derived from ketones, compared to those derived from aldehydes. The acid hydrolysis of 1,2 3,4 5,6-tri-O-isopropylidene-D-mannitol to 3,4-O-isopropylidene-D-mannitol55,56 and of l,2 3,4-di-0-isopropylidene-L-rhamnitol to 3,4-O-isopropylidene-L-rhamnitol57 indicates an order of isopropylidene acetal stability of a-threo > a, and this order is supported by the partial hydrolysis of 2,3 4,5-di-O-isopro-pylidene derivatives of dialkyl dithioacetals of D-arabinose58 and D-xylose59 to 2,3-acetals. 

Maltose is a disaccharide (made of two sugar units) produced by the enzymatic hydrolysis of starch or cellulose, which are themselves polyacetals made up of a string of glucose units. 

Heller et al.201,202) showed that the polyacetals prepared by the condensation of a divinyl ether and a diol are only marginally useful for the release of norethindrone because hydrolysis of the acetal linkage at pH 7.4 is too slow. On the other hand the... 

When considered solely as the dialdehyde form, oxystarch is a polyacetal of D-erythrose and glyoxal (98), although it will actually contain hemiacetal and other linkages, mentioned in the introduction. After hydrolysis of oxystarch, the optical rotation was near that for the equilibrium value of d-... 

Inasmuch as the pentosan structure represents a polyacetal, the acid hydrolysis of pentosan corresponds to the hydrolysis of acetals [3]. 

Hydrolysis has been the main method used for the chemical recycling of other condensation polymers, such as polyacetals and polycarbonates. Hydrolysis of polyacetals leads back to the starting monomers, formaldehyde or trioxane. Polycarbonates are polymers synthesized by the reaction of phosgene and a dihydric phenol, commonly bisphenol A. Chemical recycling of polycarbonate... 

Polyacetal can also be stabilized against degradative conditions by copolymerizing trioxane with small amounts of ethylene oxide. This introduces a random distribution of -C-C- bonds in the polymer chain. Hydrolysis of the copolymer with aqueous alkali gives a product with stable hydroxyethyl end groups. The presence of these stable end groups coupled with the randomly distributed C-C bonds prevents polymer depolymerization at high temperature. 

An acetal (e.g., 3 Figure 13.1) can be prepared by the equilibrium reaction of two equivalents of an alcohol and one equivalent of a compound possessing an aldehyde (or ketone to generate a ketal). The hemiacetal intennediate 2 is hydrolytically labile to both base and acid, but the resulting acetal product 3 is only labile to hydrolysis at acidic pH values. When polyacetals are prepared by acid catalysis, it is important to remove or neutrahze any residual add to ensure the polymer is stable enough to isolate and for storage. 

Some aspects of these inherent characteristics were addressed a few years later in the early 1980s when the preparation of polyacetals was described using a single (A-B) monomer 7 comprised of a vinyl ether and a hydroxyl moiety that could be polymerized to give a polyacetal [30] (Figure 13.2b). However, the vinyl ether moiety in monomers such as 7 must still be protected from hydrolysis, and as will be seen, the approach in Figure 13.2a is more flexible and has been followed to prepare different polyacetals for potential biomedical use [23,31,34]. 

We also reported the fer-polymerization strategy with the intent to combine DES (diethylstilboestrol), which is a diol, into the polymer backbone [23]. Hydrolytic degradation studies with these bioresponsive polymers showed an enhanced rate of hydrolysis of the polymer backbone at pH 5.5, in which 65% of DES was released over 96h, in comparison to pH 7.4, where only 4% of DES was released during the same time [23]. This DES containing ter-polyacetal has continued to be developed by Vicent etal. [32,71]. 

More recently, camptothecin (CPT) was conjugated to poly(l-hydroxymethylethylene hydroxymethylformyl) (PHF or Eleximer ) and the conjugate 11 (XMT-1001) is currently in clinical development [72,73]. The polyacetal is derived from the exhaustive oxidation of dextran [74]. CPT is poorly soluble and prone to rapid inactivation through lactone ring hydrolysis in vivo [75]. As a cytotoxic compound, CPT has been used in many efforts to develop other CPT-polymer conjugates [45,76,77]. The sodium salt of CPT has also been examined in phase I trials, but only modest responses were observed while severe toxicities remain. 

Use of acetal/ketal monomers with reactive polymerizable functionality allows easier access to families of related polyacetals. This facilitates determination of structure-activity correlations that influence many properties including hydrolysis half-life. Hydrolytic stability is often a function of whether the degradable unit is a ketal or acetal, along with other features of the polymer structure (e.g., hydrophilicity or neighboring group effects). The use of a polymerizable bis-diene Michael acceptor as an A-A mono-... 

E. Schacht, V. Toncheva, K. Vandertaelen, J. Heller, Polyacetal and poly(ortho ester)-poly(ethylene glycol) graft copolymer thermogels preparation, hydrolysis and FITC-BSA release studies, J. Control. Release 116 (2006) 219-225. 

Polymers such as PA (Nylon) or polyesters are normally predried before compounding operations to prevent degradation by hydrolysis. It will also be necessary to dry the filler before addition to the polymer in compounding. Discolouration of polyacetals after compounding and under service conditions is usually initiated by additives, including antioxidants [21]. 

Water-soluble or -swellable synthetic polymers, such as poly(vinyl alcohol), polyacetals, poly(acrylic acid), and polyacrylamide, are all relatively stable at neutral pH, but undergo quite rapid chain scission under acidic or basic conditions. Typical commercial polymers can be classified according to their resistance to hydrolysis (158) as follows. 

A great number of polymers are subject to hydrolysis, such as polyesters, polyanhydrides, polyamides, polycarbonates, polyurethanes (PU), polyureas, polyacetals and polyorthoesters. Different mechanisms of hydrolysis have been extensively reviewed not only for backbone hydrolysis, but also for the hydrolysis... 

The hydrolysis resistance of polyacetals depends on their chemical structure and to a lesser extent on their stabilization. POM copolymers are more resistant than POM homopolymers, because POM homopolymers contain hydrolysis-sensitive end groups. 

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