Polyesters, main-chain cleavage

Imaging via Main-chain Cleavage of Polycarbonates. Polyesters and Polvethers. 

Scheme 1 shows the reaction which occurs when an aromatic polyester such as [I] is subjected to UV irradiation (5). The polymer first undergoes main-chain cleavage with subsequent rearrangement to polymer [II] which is photostable and can be used as a thin coating to protect efficiently other substrates which are normally photodegradable. 

In the case of polyamides and polyesters the most important photolytic reactions are the Norrish I and II reactions (see Scheme 1). The Norrish I reaction leads to chain cleavage and radicals that might initiate oxidation, the Norrish II reaction only leads to chain cleavage. The main question for these polymers is What is the relative importance of photolysis and photo-oxidation ... 

Another interesting pathway to telechelics is to first synthesize polymers containing thermal or light sensitive groups such as azo groups in the main chain. The polyester (70) resulting from the polycondensation of AIBN with diols yields such a structure. Decomposition of the azo function in the presence of styrene yields (AB) block copolymers which can be converted into a,G -dihydroxy (PS) by reductive cleavage of the ester bonds (Scheme 60). 

The main underlying chemistry of biodegradation for polyesters is the hydrolysis reaction. In the reaction, water molecules react with ester bonds of the polymer backbone or side groups to cleavage the polymer into smaller chains. An example of the chain scission in poly lactic acid (PL A) is shown in Fig. 15.6. 

The milling of poly(ethylene terephthalate) generally causes fracture of the heterobonds of the glycol chain units [20,21]. Simionescu and co-workers [21], in a complete study on poly(ethylene terephthalate), confirmed that homolytic bond cleavage occurs mainly at the weakest links, which were thought to be the heteroatomic bonds. Electron spin resonance has been widely employed for the study of rupture in heterochain polymers. If a chain has an aliphatic ether structure, for example, poly(ethylene oxide), rupture takes place mainly at the —C—C— bond [22]. The breakdown of aromatic polyesters and ethers occurs at the relatively weak —C—C— bonds around the aromatic nuclei [23]. In general, the bond most susceptible to rupture is beta-to-chain functionality as in polyesters and nylons. 

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