Hydrolysable linkages

Due to the fact that PET contains hydrolysable linkages in the backbone of the polymer chain, these results suggest that PET may be an inherently biodegradable polymer when exposed to appropriate conditions. This should lead to a much expanded use of the polymer, particularly for beverage containers and packaging films. 

Polymers with hydrolysable linkages in the backbone are very useful in a range of degradable materials. For disposable table-wares as cups or expendable packages many of them are still too expensive and do not exhibit the desirable combination of mechanical and chemical properties. Well-known synthetic hydrolysable polymers are polyesters [1], polycarbonates [2], polyanhydrides [2], polyamides [2] and poly(amino acids) [2]. Hydrolysable biopolymers may be cheaper than synthetically produced polymers (e.g. aliphatic polyesters such as polylactides) and many scientists today are looking for new possibilities using such traditional natural polymers as polysaccharides, proteins and lipids. Special interest is focused on poly(P-hydroxybutyrate) and its copolymers [3,4] (see Chapters 9 and 10). Well-known natural products such as Pullulan (a bacterial polysaccharide produced by Aerobasidium pullulans), cellulose acetate and starch, as well as synthetic polyvinyl alcohol are important degradable materials. 

Both the chemical solubility and the electrical properties are consistent with those expected of a lightly polar polymer, whilst reactivity is consistent with that of a polymer containing hydrolysable carbonate ester linkages partially protected by aromatic hydrocarbon groupings. The influence of these factors on specific properties is amplified in subsequent sections. 

Proteolytic enzymes - hydrolyse proteins selectively, either on terminal groups (exopeptidases) or internal linkages (endopeptidases), eg... 

As can be seen in Figure 6.12, penicillin G contains two amide functionalities, of which the (Madam linkage is extremely susceptible to basic and nucleophilic attack. Therefore, deavage of the phenylacetyl side chain could not be performed using classical base hydrolysis. The problem of selectivity was resolved by taking advantage of the fad that the amide bond to be hydrolysed is secondary rather than tertiary. 

The hydrolytic depolymerisation of PETP in stirred potassium hydroxide solution was investigated. It was found that the depolymerisation reaction rate in a KOH solution was much more rapid than that in a neutral water solution. The correlation between the yield of product and the conversion of PETP showed that the main alkaline hydrolysis of PETP linkages was through a mechanism of chain-end scission. The result of kinetic analysis showed that the reaction rate was first order with respect to the concentration of KOH and to the concentration of PETP solids, respectively. This indicated that the ester linkages in PETP were hydrolysed sequentially. The activation energy for the depolymerisation of solid PETP in a KOH solution was 69 kJ/mol and the Arrhenius constant was 419 L/min/sq cm. 21 refs. 

The latter products readily add chlorine at the olefinic linkage and are hydrolysed by aqueous sodium hydroxide to give (5) in good yield. 

Generally, these derivatives were stable in air, but the acetyl and benzoyl derivatives were readily hydrolysed at the imide rather than at the phos-phazene linkage ... 

Freudenberg, Kuhn and their co-workers showed that both the velocity constants and the courses followed by the hydrolyses of these various polymers can be accounted for by postulating that one or the other or both of the terminal linkages, a and h of Table VIII, in these various species hydrolyze more rapidly than the internal c linkages. All of the latter can be assumed to hydrolyze at the same rate. If, for example, one of the two terminal linkages, a or 6, in an x-mer reacts at a rate equal to cellobiose, 1.07 X10, and the rate for each of the other X —2 linkages corresponds to the initial average rate, 0.305 X10, of hydrolysis of the bonds in cellulose then the calculated... 

Hakomori methylation analysis, and preparation and GLC gf the PAAN derivatives (4), were used to determine the linkages. C NMR of partially hydrolysed samples confirmed the octasaccharide repeat unit, the beta linkages and the presence of the carboxylic acids. 

Enzymes that react with a specific type of ester linkage are known as general hydrolysing enzymes. Thus lipases hydrolyse a wide range of organic esters. Generally, phosphatases will break down phosphate esters into phosphoric acid and an alcohol. 

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