Main-chain carbonyl polymers

Other Compounds Main-Chain Carbonyl Polymers... 

The Norrish I and II reactions may occur from the excited singlet (S ) or triplet (T ) states however the triplet state is much more favoured because of its longer lifetime (Table 1.1). Both Norrish reactions are responsible for the photodegradation of polymeric ketones (cf. section 3.2.1) and polymers containing main chain carbonyl groups. 

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 above examples of free-radical ring-opening polymerization, which have been explored by Bailey and Endo, produce polymers containing ketonic carbonyl and/or ester groups in the main chain. In addition, these cyclic monomers can be copolymerized with vinyl monomers by free-radical mechanism. Thus, the variety of the polymers produced by radical polymerization has been enlarged. 

Polyamides are an important example of polymers which do not contain pseu-doasymmetric atoms in their main chains. The chain conformation and crystal structure of such polymers is influenced by the hydrogen bonds between the carbonyls and NH groups of neighboring chains. Polyamides crystallize in the form of sheets, with the macromolecules themselves packed in planar zigzag conformations. 

Since pure PP does not Incorporate chromophorlc groups, it is clear that photolnltlatlon of radical degradation processes must Involve chromophorlc impurities. There has been a great deal of discussion of this in the past and hydroperoxides or carbonyl structures formed by oxidation of the parent polymer and transition metal residues from the polymerization catalyst seem to be the most likely candidates. It is not appropriate to discuss this aspect in the present paper, suffice it to say that the association of methane with photolnltlatlon, but not thermal Initiation, suggests that photolnltlatlon Involves C-CH3 bond scission to form chain side radicals in contrast to thermal Initiation which involves scission of the C-C bond In the main chains. 

The hydroxylated copolymers, poly(acenaphthylene-co-vinyl alcohol), were reacted with the following acyl chlorides 1-naphthoyl, 2-naphthoyl, benzoyl and cinnamoyl. The latter polymers contain an ester group as a spacer between the main chain and one of the fluorophores. The choice of an ester group as a connecting unit between the polymer chain and fluorophore is probably a poor one since it is known that carbonyl groups easily form triplets and photo-Fries rearrangements can occur (2). 

The mechanism of carbonyl group formation in the main chain of the polymer is not entirely elucidated. The following are some of the suggested mechanisms. 

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