Degradation of aliphatic polyesters

In vitro and in vivo degradation of aliphatic polyesters have been investigated However, in many studies these experiments have been 

Factors such as the size of the crystals within the polymer, pore size and pore volume within the implant, and the overall shape and size of the implant affect the mode of degradation within polymers, and these factors can thus be used to control the release profile in drug delivery applications.  

Simple chemical hydrolysis of the hydrolytically unstable backbone is the prevailing mechanism for polyester degradation, which depends on the pH of the solution and may be catalysed by an acid or a base.The acid-catalysed reaction mechanism is given in Fig. 5.2. The reaction is reversible and is displaced towards hydrolysis by an excess of H2O. This mechanism is characterised by acyl-oxygen cleavage, and the removal of the alcohol is the slow stage of the reaction. 

The base-catalysed ester hydrolysis reaction mechanism is outlined in Fig. 5.3. This reaction has an irreversible acyl-oxygen cleavage mechanism, and the slow stage is the attack of the ester by the OH . 

2 Acid-catalysed ester hydrolysis mechanism (based on Sykes, 1986 ). 

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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 rather few studies on how processing and processing additives affect the properties and degradation of aliphatic polyesters. Several types of processing additives are used in order to improve the properties of the product. Since... 

The solution is a combination of aliphatic polyesters and aromatic polyesters. This involves modifying the crystalline structure of PBT by incorporating aliphatic monomer (adipic acid) in the polymer chain in such a way that the material properties of the polymer would remain acceptable (e.g., melting point of the crystalline range still around 100 °C), but the polymer would also be readily compostable/biodegradable. In this way it was possible to combine the degradability of aliphatic polyesters with the outstanding properties of aromatic polyesters. 

Starnecker, A. Menner, M. (1996b). Kinetics of aerobic microbial degradation of aliphatic polyesters. In Biodeterioration and Biodegradation. Papers of the 10th International Biodeterioration and Biodegradation Symposium, DECHEMA Monographs, ed. W. Sand. Frankfurt Schon Wetzler, pp. 221-8. 

Hoshino, A. and Isono, Y Degradation of aliphatic polyester films by commercially available lipases with special reference to rapid and complete degradation of poly(L-... 

Nagata, M., 1996, Enzymatic degradation of aliphatic polyesters copolymerized with various diamines. Macromol Rap. Commun. 17 583-587. 

Some studies have addressed the possible interplay between sample size and degradation of aliphatic polyesters. Sample size is commonly interchanged with sample geometry. Production of variously shaped devices introduces a variety of proeessing routes, which results in further changes to polymer degradation. 

Degradation of aliphatic polyesters is generally regarded as a bulk phenomenon from the macroscopic viewpoint (Schakenraad et al., 1989 Helder et al., 1990 Lewis,... 

Antheunis, FI., van Der Meer, J.C., de Geus, M., Kingma, W., Koning, C.E., 2009. Improved mathematical model for the hydrolytic degradation of aliphatic polyesters. Macromolecules 42, 2462-2471. 

On the other hand, pores enhance the alkaline and enzymatic surface hydrolytic degradation of aliphatic polyesters because of the increased surface area per unit mass of porous materials compared to that of nonporous materials [307]. Similar to this, the pores formed by the removal of water-soluble polymers such as poly(p-vinyl phenol) from PLLA/ water-soluble polymer blends accelerate the alkaline hydrolytic degradation of PLLA as traced by weight loss [249]. 

TABLE 30.2 Degradability of Aliphatic Polyester Pellets Under Denitrifying Conditions in Laboratory-Scale Activated Sludge Reactors Fed with Synthetic Sewage... 

Nagata, M., et al, Hydrolytic Degradation of Aliphatic Polyesters Copolymerized with Polyfethylene glycoDs. Polymer International, A2 ) p. 33. 1997. 

Figure 7.3 Different enzymes involved in the degradation of aliphatic polyesters. d-PHB Poly(D-hydroxybutyrate) PBS polybutylene succinate and PBSA polybutylene succinate-adipate. Reproduced with permission from Y. Tokiwa and B.P. Calabia,of Polymers and the Environment, 2007, 15,...

According to literature, degradation of aliphatic polyesters in a living environment can result from enzymatic attack or from simple hydrolysis of ester bonds or both. 

One of the first problems encountered in overviewing the literature were the contradictory conclusions reached on the enzymatic degradation of aliphatic polyesters. Apart from the well-identified enzymatic degradation of biopolymers such as collagen, cellulose and PHB [10,13, 26, 27], there has been much debate about the involvement of enzymes in the degradation of synthetic poly-... 

From the macroscopic viewpoint, degradation of aliphatic polyesters has been regarded as homogeneous (bulk erosion) until recently although surface erosion was claimed in a few cases. 

The various factors which can affect the degradation of aliphatic polyesters are now considered. For the sake of clarity, each type will be discussed separately. 

Enzymatic degradation of aliphatic polyesters has been extensively investigated in the literature as discussed above. In contrast, little information is available on the effect of the presence of proteinic materials. 

Vert M, Li S, Garreau H, Mauduit J, Boustta M, Schwach G, Engel R, Coudane J. Complexity of the hydrolytic degradation of aliphatic polyesters. Angew Makromol Chem 1997 247 239-53. 

Albertsson AC, Varma IK (2002) Degradation of aliphatic polyesters. Adv Polym Sci 157 1-179... 

Boustta, M., Schwach, G., Engel, R., and Coudane, J. (1997) Complexity of the hydrolytic degradation of aliphatic polyesters./. Macromol. Mater. Eng,... 

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