<P> For example, polyethylene usually degrades by random scission--that is by a random breakage of the linkages (bonds) that hold the atoms of the polymer together . When this polymer is heated above 450 Celsius it becomes a complex mixture of molecules of various sizes that resemble gasoline . Other polymers--like polyalphamethylstyrene--undergo' specific' chain scission with breakage occurring only at the ends; they literally unzip or depolymerize to become the constituent monomers . </P> <P> Most polymers can be degraded by photolysis to give lower molecular weight molecules . Electromagnetic waves with the energy of visible light or higher, such as ultraviolet light, X-rays and gamma rays are usually involved in such reactions . </P> <P> Chain - growth polymers like poly (methyl methacrylate) can be degraded by thermolysis at high temperatures to give monomers, oils, gases and water . The degradation takes place by: </P> <Table> <Tr> <Th> Thermolysis type </Th> <Th> Added material </Th> <Th> Temperature </Th> <Th> Pressure </Th> <Th> Final product </Th> </Tr> <Tr> <Td> Pyrolysis </Td> <Td> </Td> <Td> Around 500 ° C </Td> <Td> Reduced pressure </Td> <Td> </Td> </Tr> <Tr> <Td> Hydrogenation </Td> <Td> Dihydrogen </Td> <Td> Around 450 ° C </Td> <Td> Around 200 bars </Td> <Td> </Td> </Tr> <Tr> <Td> Gasification </Td> <Td> Dioxygen and / or water </Td> <Td> </Td> <Td> Under pressure </Td> <Td> Carbon monoxide, Carbon dioxide and hydrogen </Td> </Tr> </Table>

Can silicone be broken down by a chemical change