Excitation and Scission of a Chain

Figure 1. Adiabatic potential curves in the main chain scission of a model compound of poly(isobutylene) 2,2-, 4,4-tetramethylpentane (4). AE3l(=0.61eV), aET,(—0.35eV), and AEf (=2.05eV) are the activation energies of the main chain scission in the lowest singlet excited state (S,), the lowest triplet state (T,), and the ground state, respectively.

By the effect of ionization irradiation side substituents are abstracted from excited macromolecules in parallel to the scission of main chains. Elimination of side substitutions such as hydrogen and halogen atoms may be the main primary process. For instance, in polyethylene, although the C —H bond is stronger than the C—C bond, excited macromolecules eliminate hydrogen atoms preferentially through a C —C bond cleavage of a macromolecular backbone. Part of the... 

These intra- and inter-molecular deprotonations of the cation radical readily occur, so that the cation radical is unstable even at 4 K. The direct scission of a C-H bond in the excited state PMMA cannot be excluded as an additional source of either of the main-chain radical or the side-chain radical. The H atom generated from the excited state PMMA or from the recombination of a proton with an electron will recombine with the polymer radical or abstract an hydrogen to generate also the main-chain or the side-chain radical as... 

Fig. 15 Top-. Sono-scission of a polymer chain which includes a 1,2-dioxetane mechanophoie to form an adamantanone-excited species with blue light emission. Bottom Luminescence spectra obtained during the mechanical stretching of polymers containing organic acceptors show different colours and intensities. Reproduced with permission from [80]. Copyright 2012 Macmillan Publishers Ltd...

Note 2 Some main-chain scissions are classified according to the mechanism of the scission process hydrolytic, mechanochemical, thermal, photochemical, or oxidative scission. Others are classified according to their location in the backbone relative to a specific structural feature, for example, a-scission (a scission of the C-C bond alpha to the carbon atom of a photo-excited carbonyl group) and P-scission (a scission of the C-C bond beta to the carbon atom bearing a radical), etc. 

They assumed that the primary cation radical of PMMA spontaneously and quickly dissociated to form carbocation, which then recombined with the liberated electron to form an excited radical with a ferr-alkyl structure. This excited radical was thought to be the precursor of the scission of the main chain. This reaction model interpreted well their observation that the G value for the scission of the side chain was close to that of the main chain and that the mercaptan added to scavenge electrons suppressed the main-chain scission efficiently without affecting the formation of volatile products from the ester side chain. The above reaction model motivated us to apply the ESE method to the study of radicals in irradiated PMMA. The model now seems inadequate, because it cannot accommodate some recent ESE results as mentioned later. 

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