Polymers degradation initiation

In order to explain this intriguing phenomenon, we turned our attention to the possibility of polymer degradation initiated by light. To check this hypothesis, a two percent solution of the polymer in THF was irradiated with pyrex filtered U.V. light under argon. Interestingly, an insoluble gel was produced in approximately ten minutes. After an hour of irradiation, the gel fraction was recovered and was found to be approximately 60% of the initial polymer weight. 

Two other important commercial uses of initiators are in polymer cross-linking and polymer degradation. In a cross-linking reaction, atom abstraction, usually a hydrogen abstraction, occurs, followed by termination by coupling of two polymer radicals to form a covalent cross-link ... 

Most polymer degradation caused by the absorption of uv light results from radical-initiated autoxidation. 

NMR studies15 1 1 on polymers prepared with, 3C-labeled BPO have shown that the primary benzoyloxy and phenyl end groups formed by tail addition to monomer are thermally stable under conditions where the polymer degrades. They persist to > 50% weight loss at 300°C under nitrogen. Thus, these groups are unlikely to be directly responsible for the poor thermal stability of PS prepared with BPO as initiator. On the other hand, the secondary benzoate end groups, formed by head addition or transfer to initiator, appear extremely labile under these conditions. Their half life at 300°C is <5 min. 

There have been many studies on the thermal and thermo-oxidative degradation of PMMA.23 24 It is well established that the polymer formed by radical polymerization can be substantially less stable than predicted by consideration of the idealized structure and that the kinetics of polymer degradation are dependent on the conditions used for its preparation. There is still some controversy surrounding the details of thermal degradation mechanisms and, in particular, the initiation of degradation.31... 

As mentioned in the introduction, there are conflicting views as to the contributions made to polymer degradation by various initiating species. Among these species, in addition to ketones, hydroperoxides are some of the more important chromophores. As it is known, the photolysis of hydroperoxides yields alkoxy and hydroxy radicals. In polymers, in the presence of oxygen, these radicals lead to the secondary formation of peroxy radicals. The latter in turn are converted by hydrogen abstraction into new hydroperoxides (Scheme I) ... 

Oxidative degradation of polymers is initiated by radicals (R ) generated in the polymer by heat or mechanical shear during processing or by exposure to UV light. These radicals, in turn, react with 02 to form peroxy and hydroperoxide radicals that promote radical reactions. 

In their study of branched PSA, Maniar et al. (1990) found that the molecular architecture of branched polymers affects the release kinetics in a variety of ways. They found that the branched polymers degraded faster than linear PSA of comparable molecular weight (Maniar et al., 1990). They also noted that drug (morphine) release profiles were more characteristic of bulk erosion than surface erosion An initial lag time during which very little drug was released was associated with the time required for water to swell the polymer. This was followed by a period of relatively fast release, which tapered off as the device disintegrated. The polymer matrix lost its mechanical integrity before the release experiment was complete (Maniar et al., 1990). Despite the increase... 

This reaction is very exothermic (A// —180 to —200kJ mol-1) and, therefore, seems to be very probable from the thermochemical point of estimation. The pre-exponential factor is expected to be low due to the concentration of the energy on three bonds at the moment of TS formation (see Chapter 3). To demonstrate that this reaction is responsible for the oxidative destruction of polymers, PP and PE were oxidized in chlorobenzene with an initiator and analyzed for the rates of oxidation, destruction (viscosimetrically), and double bond formation (by the reaction with ozone) [131]. It was found that (i) polymer degradation and formation of double bonds occur concurrently with oxidation (ii) the rates of all three processes are proportional to v 1/2, (iii) independent of p02, and (iv) vs = vdbf in PE and vs = 1.6vdbf in PP (vdbf is the rate of double bond formation). Thus, the rates of destruction and formation of double bonds, as well as the kinetic parameters of these reactions, are close, which corroborates with the proposed mechanism of polymer destruction. Therefore, the rate of peroxyl macromolecules degradation obeys the kinetic equation ... 

Initiation sites, in VDC polymer degradation, 25 714-715 Initiator bonds, breaking, 14 278 Initiators. See also Anionic initiators Cationic initiators Free-radical initiators alkoxide, 14 259 copolymerization, 14 252 difunctional and trifunctional, 14 252— 254... 

Peroxide cure systems, in rubber compounding, 22 793-794 Peroxide decomposers, 3 111-114 Peroxide decomposition, 24 279-280 Peroxide formation, by VDC, 25 694. See also Hydrogen peroxide Peroxide initiators, 23 379-380 worldwide producers of, 24 303 Peroxide-ketazine process, 23 582-583 flow sheet for, 23 582 versus Raschig process, 23 583 Peroxide linkages, in VDC polymer degradation, 25 713... 

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