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Abstract  

General purpose poly(styrene) prepared by conventional radical techniques contains a head-to-head unit as a consequence of polymerization termination by radical coupling. As has been previously demonstrated, thermal stress promotes homolysis of the bond linking the head-to-head components. The macroradicals generated depolymerize rapidly to generate styrene monomer. This decomposition during processing can lead to finished articles containing objectionable levels of styrene monomer, particularly for food packaging applications in which even low levels of monomer can promote objectionable taste and aroma. Polymer containing no head-to-head units should not be prone to this facile decomposition. In this instance, poly(styrene) has been prepared by nitroxyl-mediated polymerization of styrene monomer followed by reductive removal of nitroxyl end groups. Polymer prepared in this manner contains no head-to-head units and displays thermal stability much greater than that observed for conventional poly(styrene). A direct comparison of the stability for the two polymers is readily available by thermogravimetric techniques. A quantitative reflection of the difference in stability is available from the rate constants for the respective decomposition.

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Abstract  

General purpose poly(styrene) is a large volume commodity polymer used in a variety of applications. It is widely used in food packaging, particularly for baked goods. In this application, the presence of styrene monomer, which has a distinctive taste and aroma, cannot be tolerated. Processing of the polymer and forming of the food container at an unacceptably high temperature leads to the formation of styrene monomer and finished articles with unacceptable aroma characteristics. An examination of the thermal degradation of poly(styrene) has revealed the origin of monomer formation. The thermal decomposition of poly(styrene) has been widely studied. However, most studies have been carried out at high temperature (>300�C) where many processes are occurring simultaneously. Degradation at lower temperature, 280�C, occurs in two well-defined steps. The first is thermolysis of a head-to-head bond present in the mainchain as a consequence of polymerization termination by radical coupling. This generates macroradicals which smoothly depolymerize to expel styrene monomer. The nature of the degradation is readily apparent from kinetic analysis of the isothermal thermogravimetry (TG) data and the identity of the single volatile product may be readily established by gas chromatography/mass spectrometry (GC/MS) analysis of the effluent from the TG analysis.

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