Polymers degradation propagation

Propagation constants, 20 219 Propagation plant, brewer s yeasts, 3 581 Propagation rate constants (kp), in VDC polymer degradation, 25 715-716, 717t Propagation reactions, 14 274... 

Hindered amine light stabilisers (HALS) are the main stabiliser type (as a scavenger to inhibit free radical chain propagation), while organo-nickel compoimds (as a quencher to prevent initiation of polymer degradation) are used for UV stabilisation. 

Several polymers degrade primarily by a free radical depolymerisation, including polystyrene (PS) and polymethacrylates. When a free radical is produced in the backbone of polyethyl methacrylate, for example, the molecule undergoes scission to produce an unsaturated small molecule (ethyl methacrylate) and another terminal free radical. This radical will also cleave to form ethyl methacrylate and propagate the free radical. The net effect is often referred to as unzipping the polymer. 

FIGURE 4.5 Secondary unzipping degradation propagation reaction. (Redrawn from Xie, T. and Hayden, C. A. 2007. Polymer 4S 5497-5506.)... 

To some extent, the stability of VDC polymers is dependent on the nature of the comonomer present. Copolymers with acrylates degrade slowly [41, 42]. Apparent degradation propagation rates decrease somewhat as the acrylate content of the copolymer increases [41]. The polyene sequences generated by dehydrochlorination are limited in size by the level of acrylate incorporation that is, the acrylate molecules act as stopper units for the unzipping reaction. The impact of this chain stopping is that the... 

---