End-chain scission

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. 

A new macroscopic degradation mechanism of polymers studied by Murata et al. [6] was suggested with two distinct mechanism in the thermal degradation of PE, PP and PS. One is a random scission of polymer links that causes a decomposition of macromolecnles into the intermediate reactants in liquid phase, and the other is a chain-end scission that caused a conversion of the intermediate reactants into volatile prodncts at the gas-liqnid interface. There are parallel reactions via two mechanisms. The random scission of polymer links causes a reduction in molecular weight of macromolecules and an increase of the number of oligomer molecules. The chain-end scission causes a dissipation of oligomer molecules and a generation of volatile products. 

Shih, C. (1995) Chain-end scission in acid catalyzed hydrolysis of poly(D,L-lactide) in solution. Journal of Controlled Release, 34, 9-15. 

In a study on PTT recycling, Ramiro and co-workers [24] noted that the structure of the polymer did not change despite lowering of molecular weight. This suggests that chain-end scission is probably the correct first step. They noticed that, despite the lack of new structures, the polymer did yellow considerably. This was a paradoxical observation unless the colour is attributable to small fragments or polymerised unsaturated species. 

Batycky et al. (1997) adopted population balances along with a pseudo-first order degradation kinetics to describe the behavior of eroding microparticles. The kinetic mechanism includes both random chain scission and chain-end scission. The change in matrix porosity is accounted for by considering the coalescence of small pores caused by the breakage of polymer chains. 

Figure 8.6 Time evolution of oligomer concentrations during poly(lactic acid) degradation in aqueous solution at 100°C. Symbols experimental data. Lines random chain scission model model dashed) and preferential chain end scission model continuous).

Recently, Sivalingham et al. [43] suggested that PCL underwent both random chain scission and specific chain end scission (elimination of monomer from the hydroxyl end of the polymer) simultaneously (a parallel mechanism) on non-isothermal heating and degraded by pure imzipping of the monomer from the lydroxyl end of the polymer chain on isothermal heating. 

Cullis and Hirschler proposed a detailed study on the mechanism of the thermal degradation of polymers [2, 40], which consists of two distinct reactions occurring simultaneously in the reactor. One is a random scission of links, causing a molecular weight reduction of the raw polymer, and the other is a chain-end scission of C-C bonds, generating volatile products [40]. 

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