Step-reaction polymers, thermal degradation

An important step in the thermal degradation of many polymers such as PMMA involves depropagation. Here the polymer molecule or radical fragment rapidly unzips in such a way that produces mainly a monomer. This process may be modeled simply by a rapid reaction where an z-mer is converted to an (i - l)-mer plus a monomer P —> P x + Px. In reality, this is a gross oversimplification of the process and a more sophisticated model will be discussed subsequently and also later in the chapter in relation to the thermal degradation of PMMA. 

After many years of study, three distinct mechanisms of thermal degradation can he identified, although they are not mutually exclusive, and most polymers degrade hy more than one mechanism, depending on the temperature. Detailed mechanisms of degradation of a wide range of carbon-chain polymers have been reviewed (4) degradation of step-reaction polymers has also been reviewed (5). 

In the free-radical kinetic Scheme for the thermal degradation of polymers, it was considered that the termination reaction resulted in disproportionation so that there was no increase in molar mass of the polymer in the termination step. At moderate temperatures, particularly in polymers such as PE and PVC for which unsaturation plays an important role... 

The thermal decomposition of step-growth polymers cannot take place by a chain reaction like that of chain-growth polymers. As a result, these materials degrade in a random fashion, rupturing at the weakest bonds first. 

Product distribution, variable or stepped pyrolysis and kinetic studies have all been used to expose mechanistic events during polymer degradation. For example, the thermal degradation of polystyrene has been elucidated using the block copolymer poly(styrene-b-styrene-dg). Hybrid and homo monomers (styrene) and dimers (2,4-diphenylbut-l-ene) were detected but without hybrid trimers (2,4,6-triphenylhex-l-ene) (Figure 10). The amount of hybrid dimer far exceeds that which might arise from adjacent residues, while similar proportions of dimers were obtained when the two homopolymers were pyrolyzed together. Data indicate intermolecular reaction rather than the previously proposed 1,3-transfer. 

Polymers may be attacked by molecular oxygen, ozone, or by indigenous free radicals in the polymer. Thermal-oxidative degradation of polyolefins in air is autocatalytic, i.e., the rate is slow at first but gradually accelerates to a constant value. According to the three-step mechanism outlined below, the RO2 peroxy radicals formed (Step 1) are sufficiently reactive to attack some primary CH bonds of the chain R H (Step 2). The peroxy radical RO2 is thus reformed (Step 3) and can attack another CH bond. This chain reaction continues until termination occurs (Step 4) [1-11]. 

Where a polymer has functional groups in the main chain, typical of polymerization by step-reaction chemistry or chain poljunerization of heterocyclic monomers, the fimctional groups in the chain are often the weakest and may nndergo reactions which do not produce radicals. Thermal degradation by random scission, without significant yield of volatiles is typical of polymers such as polyesters, polyamides, and polyurethanes, all of which undergo thermolysis by reaction at the functional groups (5). 

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