Styrene polymers, scission

Quantum Yields for Scission in Styrene Polymers and Copolymers In DMM... 

In addition to providing fully alkyl/aryl-substituted polyphosphasenes, the versatility of the process in Figure 2 has allowed the preparation of various functionalized polymers and copolymers. Thus the monomer (10) can be derivatized via deprotonation—substitution, when a P-methyl (or P—CH2—) group is present, to provide new phosphoranimines some of which, in turn, serve as precursors to new polymers (64). In the same vein, polymers containing a P—CH group, for example, poly(methylphenylphosphazene), can also be derivatized by deprotonation—substitution reactions without chain scission. This has produced a number of functionalized polymers (64,71—73), including water-soluble carboxylate salts (11), as well as graft copolymers with styrene (74) and with dimethylsiloxane (12) (75). 

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. 

Hepuzer et al. [91] have used the photoinduced homolytical bond scission of ACPB to produce styrene-based MAIs. These compounds were in a second thermally induced polymerization transferred into styrene-methacrylate block copolymers. However, as Scheme 24 implies, benzoin radicals are formed upon photolysis. In the subsequent polymerization they will react with monomer yielding nonazofunctionalized polymer. The relatively high amount of homopolymer has to be separated from the block copolymer formed after the second, thermally induced polymerization step. 

Influence of Interpolymer Properties. As stated earlier, the physical and chemical properties of interpolymers markedly influence the reaction rate after the induction period. If the monomer present yields a polymer comparable in viscosity with the initial mixture the rate of scission will not accelebrate. For example, the polymerization rate of chloroprene on mastication with natural rubber does not increase as markedly with conversion (69), see Fig. 19, as with methyl methacrylate and styrene. The reason is the chloroprene-rubber system remained elastic and softer than the original rubber. 

In the poly(methyl methaerylate)-styrene system, less than 7% of the original polymer remained as homopolymer at total conversion (77). Over 85% of the product was non-branched, single-segment block copolymer. The difference for these two systems is in part due to the higher molecular weight of the initial poiy(methyl methycrylate) (2900000 versus 495000) and in part to the preferential scission of the poly(methyl methacrylate) chain. This point was confirmed by running tests on a mixture of the two homopolymers in the presence of a radical acceptor to prevent macroradical recombination, and on the isolated block copolymers. 

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. 

Breitenbach and Frank (5) showed that with styrene-divinylbenz-ene, no further additives (such as peroxides) are necessary for popcorn polymer formation. Breitenbach and Fally (6) found, in methyl acrylate polymerization, the possibility of crosslinking in the polymerization of a monovinyl compound. Miller and coworkers (7) developed the kinetics of the process Pravednikow and Medvedev (8) studied the chain scission, and assumed radical formation by that process as an important step. 

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. 

Solid alkalis might catalyse the cracking reactions of polymers as is the case with acidic catalysts. According to experimental work solid alkalis catalyse the degradation of polystyrene more efficiently than acidic catalysts [53]. This phenomenon could be explained by differences in the cracking mechanism of polymers. The main components in the oils obtained by solid acids were styrene monomer and dimer. Since cracking of hydrocarbons on solid acids has been explained in terms of P-scission of C-C bonds [19, 20], these were probably produced by P-scission of C-C bonds in the PS main chains as follows ... 

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