Chain scission hydrolytic

Most ester-forming reactions are reversible. Depending on circumstances, these reactions may be either undesirable side reactions, for example hydrolytic chain scissions occurring during processing, or useful reactions when chemical modification or polymer recycling is considered. 

FIGURE 21 Semilog plot of the in vitro rate of hydrolytic chain scission of various copolymers of e-caprolactone, measured under in vitro conditions. (From Ref. 95.)... 

FIGURE 25 Reduction in the rate of hydrolytic chain scission of PCL achieved by ethoxylation of the carboxy end groups of the polymer. The experimental result is compared with calculated predictions of the effect of varying degrees of ethoxylation. (From RGf. 49.)... 

Note 2 Some main-chain scissions are classified according to the mechanism of the scission process hydrolytic, mechanochemical, thermal, photochemical, or oxidative scission. Others are classified according to their location in the backbone relative to a specific structural feature, for example, a-scission (a scission of the C-C bond alpha to the carbon atom of a photo-excited carbonyl group) and P-scission (a scission of the C-C bond beta to the carbon atom bearing a radical), etc. 

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]. 

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]. 

These studies suggest the hydrophilic nature of PEG that increases the accessibility of water to the polymeric matrix. Also, PCL has been known to degrade very slowly because of its hydrophobic structure that does not allow fast water penetration [88]. PCL degradation by random hydrolytic chain scission of the ester linkages was documented by Pitt et al. [89]. 

Acid-catalyzed hydrolytic degradation of cellulose proceeds according to the principles of chemical kinetics. Nonetheless, concepts of kinetics have not been widely applied in the literature concerning the conservation of cellulosic materials. Thirty years ago, McBurney (I) provided an excellent exposition of this subject. We will review the subject in the light of developments since that time (2) and will present examples from the literature and from our own work to illustrate ways in which an analysis of the kinetics of chain scission can help conservators better understand the deterioration of cellulose-based materials. 

Figure 6. Hydrolytic chain scission of PLA (Reproduced with permission from reference 3. Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA).

Hydrolytic chain scission of polyesters, polyamides and polycarbonates... 

Acid Catalyzed Hydrolysis. When photosensitive onium salts are incorporated into a film of a hydrolytically sensitive polycarbonate and exposed to ultraviolet irradiation, the acid which is produced catalyzes chain scission at random sites along the backbone (Equation 25). 

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