Hydrolysis aliphatic polyesters

For an aliphatic polyester, poly(pivalolactone) has a rather high of 245°C and for such a an unexpectedly low of -10°C. It is also claimed to have good hydrolysis resistance for a polyester and this appears to be one of the reasons for its manufacture on an experimental scale by Shell with a view for use as both a fibre and as a thermoplastics moulding material. 

Lipase is an enzyme which catalyzes the hydrolysis of fatty acid esters normally in an aqueous environment in living systems. However, hpases are sometimes stable in organic solvents and can be used as catalyst for esterifications and transesterifications. By utihzing such catalytic specificities of lipase, functional aliphatic polyesters have been synthesized by various polymerization modes. Typical reaction types of hpase-catalyzed polymerization leading to polyesters are summarized in Scheme 1. Lipase-catalyzed polymerizations also produced polycarbonates and polyphosphates. 

Syntheses of aliphatic polyesters by fermentation and chemical processes have been extensively studied from the viewpoint of biodegradable materials science. Recently, another approach to their production has been made by using an isolated lipase or esterase as catalyst via non-biosynthetic pathways under mild reaction conditions. Lipase and esterase are enzymes which catalyze hydrolysis of esters in an aqueous environment in living systems. Some of them can act as catalyst for the reverse reactions, esterifications and transesterifications, in organic media [1-5]. These catalytic actions have been expanded to... 

Biodegradation of the aliphatic polyesters occurs by bulk erosion. The lactide/gly-colide polymer chains are cleaved by random nonenzymatic hydrolysis to the monomeric lactic and glycolic acids and are eliminated from the body through the Krebs cycle, primarily as carbon dioxide and in urine. 

Polyesters offer multiple options to meet the complex world of degradable polymers. All polyesters degrade eventually, with hydrolysis being the dominant mechanism. Degradation rates range from weeks for aliphatic polyesters (e.g. polyhydroxyalkanoates) to decades for aromatic polyesters (e.g. PET). Specific local environmental factors such as humidity, pH and temperature significantly influence the rate of degradation. 

The conventional synthesis of aliphatic polyesters based on adipic acid and a range of diols, such as 1,4-butanediol or 1,6-hexanediol, involves a high-temperature esterification reaction typically at 240-260 °C and an organometallic catalyst such as stannous octano-ate. The use of enzyme catalysis results in a much lower reaction temperature, but also the possibility of removing the esterification catalyst, giving the polyester significantly improved hydrolysis resistance. 

Hydrolysis of Polyester by Lipase. Aliphatic polyester, PEA and PCL were hydrolyzed by lipases from Achromobacter sp., C. cylindracea,... 

Lipases are enzymes of the hydrolase family and, in nature, hydrolyze fatty acid esters in aqueous environment. It is worth recalling that the hydrolysis of esters is a reversible reaction. Chemists thus often use lipases to catalyze the reverse reaction, i.e., the esterification and the ROP of lactones. In 1993, the groups of Kobayashi [91] and Knani [92] reported independently the hpase-catalyzed ROP of sCL and 8-valerolactone. The aliphatic polyesters were functionalized by a carboxylic group at one chain-end and a hydroxyl group at the other chain-end. Accordingly, the polymerization was initiated and terminated by water present in the reaction media. 

Hydrolytic degradation is especially important in polymers with hydrolyzable links between the CRUs. Thus, polyesters can be saponified to yield the starting materials from which they were formed. Acetal links in synthetic polymers such as polyoxymethylene, or in natural polymers such as cellulose, can be hydrolyzed with acids. However, the resistance to hydrolysis depends very much on the structure of the polymer for example, polyesters of terephthalic acid are very difficult to hydrolyze while aliphatic polyesters are generally easily hydro-... 

It is believed that chain scission occurs through simple hydrolysis, but the kinetics of this hydrolysis are influenced by anions, cations, and enzymes [190]. The process is autocatalytic and the products of hydrolysis such as carboxylic groups participate in the transition state. Water preferentially enters the amorphous parts but crystalline domains are also affected [125]. The degradation of aliphatic polyesters is believed to be dominated by a hydrolytic mechanism but it is also promoted by enzymatic activities [4,7,191-193]. 

There are two principal ways by which polymer chains can be hydrolyzed, passively by chemical hydrolysis or actively by enzymatic reaction. The latter method is most important for naturally occurring polymers such as polysaccharides and polyfhydroxy alkanoate)s, e.g., polyhydroxybutyrate and polyhydroxyvaler-ate [121,125]. Many synthetic aliphatic polyesters utilized in medical applications degrade mainly by pure hydrolysis [121]. 

Aliphatic polyesters degrade chemically by hydrolytic cleavage of the backbone ester bonds [38,92,93,143-145] and by enzymatic promotion [35,146]. Hydrolysis can be catalyzed by either acids or bases [38]. Polyester hydrolysis is schematically illustrated and exemplified for PLA in Fig. 5. Carboxylic end groups are formed during chain scission, and this may enhance the rate of further hydrolysis. This mechanism is denoted autocatalysis [147] and makes polyester matrices truly bulk eroding [38,43]. Degradation products are resorbed by the body with a minimal reaction of the tissues [8,15,95,148]. 

Polyether Polyols. The major polyols for preparing various urethane foams are polyether polyols. Polyester polyols are used only in specific applications. The advantages of polyether polyols are choice of functionality and equivalent weight the viscosities are lower than those of conventional polyesters production costs are cheaper than for aliphatic polyesters and resulting foams are hydrolysis-resistant. 

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