Chain scission ketone polymers

Attaching the ketone groups to the polymer backbone is more efficient on a chain scission/ketone basis because some of the light energy that the pendent ketone absorbs leads direcdy to chain scission via the Norrish type II mechanism, as well as photooxidation via the Norrish type I mechanism (see... 

Photodegradation may involve use of inherently photo-unstable polymers or the use of photodegradant additives. An example of the former are ethylene-carbon monoxide polymers in which absorption of light by the ketone group leads to chain scission. The polymer becomes brittle and forms a powder. Such materials are marketed by Dow and by Du Pont. Other examples are the copolymers of divinyl ketone with ethylene, propylene or styrene marketed by Eco Atlantic. 

When the polymers are exposed to ultraviolet radiation, the activated ketone functionahties can fragment by two different mechanisms, known as Norrish types I and II. The degradation of polymers with the carbonyl functionahty in the backbone of the polymer results in chain cleavage by both mechanisms, but when the carbonyl is in the polymer side chain, only Norrish type II degradation produces main-chain scission (37,49). A Norrish type I reaction for backbone carbonyl functionahty is shown by equation 5, and a Norrish type II reaction for backbone carbonyl functionahty is equation 6. 

Measures of the sensitivity were made in two ways, (l) Loss of ketone carbonyl was determined by FTIR on the exposed samples by measuring the relative absorbance A at 1700 cm-1. The ratio (Aq/A))7oo, was adjusted for film thickness using the styrene bands at 1600, 1495, and 1455 cm-1. This value is proportional to the rates of the Norrish type I and photoreduction processes in the copolymer (2). Changes in molecular weight result from scission in the backbone of the polymer chain. A measure, Z, of the sensitivity to main-chain scission can be derived as follows. 

The results of these experiments are summarized in Table V, which shows values of A0/A and Z for the ketone copolymer films. All of the polymers in the first section underwent significant damage and were very brittle. However, there was not much attenuation in the first section as indicated by the similar Z values of the PS-MIPK films placed at the beginning and the end of the section. It should be pointed out that the errors in determination of Mn by GPC are substantial — considerably greater than the FTIR measurements. Nevertheless, the Z values correlate quite well with A0/A, indicating the importance of the Norrish type I process in causing chain scission. 

The pyrolysis of polymers of alkali emd alkaline earth metal salts of PMAA was studied by McNeill and Zulfigar [33]. The first psrolysis reaction is the elimination of water as in Figure 15.5. Then, two distinct processes may be discerned in the breakdown of the alkali metal salts of PMAA namely, chain scission and carbonate formation. Chain scission leads to monomer and metal isobutyrate. The metal carbonate formation occurs by intramolecular reaction of adjacent salt units in the chain, resulting in the elimination reaction of unstable four-membered ring structure species which undergo various transformations to cyclic or acrylic ketones. 

If one wishes to prepare a positive photoresist it is important to obtain polymers vdiich undergo efficient chain scission in the solid phase. Recently we reported studies on a series of copolymers of styrene with a variety of ketone functional groups which were introduced by copolymerization with substituted vinyl ketone monomers. The copolymer structures are shown schematically in Table V. Two processes are responsible for the reduction in molecular weight in these polymers when irradiated with either UV light or electron beams. These are shown schematically below. 

Copolymers of vinyl chloride and methyl vinyl ketone undergo chain scission with concomitant rapid decreases in tensile strength and elongation when exposed to near ultraviolet li t and solar radiation. Free radicals formed by the homol3rtic scission of the acyl group apparently deplete the stabilizers used and lead to rapid discoloration of the polymer, presumably by the usual radical chain reaction involving the production of HCl and conjugated double bonds. 

Blends. The type I reaction produces free radicals which, in the presence of oxygen, initiates photooxidation which also results in a decrease in the polymer molecular wei t. An indication of the relative importance of the type I reaction in these systems can be estimated from the amount of chain scission induced in a blend of the copolymers with homopolymer polystyrene. For these experiments, one part of 5% vinyl ketone copolymer was blended with four parts of styrene homopolymer to retain an overall ketone monomer concentration of 1%. 

Poly(vinyl ketones) such as poly(ethylene-a//-carbon monoxide) CAS 111190-67-1, poly(methyl vinyl ketone) CAS 25038-87-3, and poly(methyl isopropenyl ketone) CAS 25988-32-3, also have practical applications. For example, poly(ethylene-a/f-carbon monoxide) is used in photodegradable plastics and in various copolymers. Several studies were reported regarding the thermal stability of these polymers. It has been shown that poly(ethylene-a/f-carbon monoxide) decomposes upon heating with chain scission generating small molecular weight alkenes and ketones. Some literature reports discussing the thermal decomposition of poly(vinyl ketones) are summarized in Table 6.5.5 [13]. 

Main-chain scission has been shown to occur in polyphenylvinylketone under UV irradiation (366 nm). Norrish type-2 scission due to reaction of the first n-II triplet state of the ketone has been shown to occur [422]. Under 7-irradiation, this polymer, which was expected to crosslink according to the Miller rule, was shown to undergo main-chain fracture with a G value of 0.35. Inhibition of the degradation in the presence of napthalene and diphenyldisulphide demonstrates the participation of radicals and excited n-II triplet states in the radiolysis [423]. 

The ketonic products can be expected to occur in trace amounts in pure polymers and are also components of the complex photo-oxidation products. Photolytic reactions of these ketones are important both in the very early stages of photo-oxidative degradation and in the continuation of an established photo-degradation reaction. The ketones may affect the source of the photodegradation by contributing to free radical processes, chain scission and energy transfer. 

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