Crosslinking quantum yield

The low quantum yield of the photografting process (0 = 2 X 10 ) provides a good opportunity to control the network formation (curing time control), and accordingly, the desirable properties of the crosslinked or grafted copolymer might be obtained. 

Table 2. Quantum yields for oolymer scission and crosslinking for some representative polysilanes (SiR R )n (a) Molecular weights were measured by GPC using polystyrene calibration standards (b) Ban, H Sukegawa, K. J. Polym. Sci., Polym. Chem. Ed., 1988, 26, 521...

During photolysis, the double bond content of the polysilane(P-l)(15mol% in this experiment) decreased to 10mol%, as measured by 1H-NMR spectroscopy. However, the ratio, quantum yield of scission(Q(S))/quantum yield of crosslinking(Q(X)), was not affected by the reaction of the double bond. West and his coworkers have reported that poly((2-(3-cyclohexenyl)-ethyl)methylsilane-co-methylphenylsilane) crosslinked upon irradiation(55). The difference between our results and West s may lie in the amount of the double bond and inhibitation of the radical closslinking by the phenol moiety. Polysilane with a halogen moiety, P-8, photodecomposed rapidly, compared with P-1 or P-3. The introduction of a chloride moiety was effective for the sensitization of the photodegradation. Similar results has already been reported(55). 

Hargreaves has suggested that the insolubilization of some closely related polymers is due to photolytic homolysis of the endoperoxide 0-0 bond and subsequent generation of carbon-centered radicals from the O radicals (19). There are several facts that make this an extremely unlikely explanation for the data described here these include the quantitative insufficiency of the maximum amount of endoperoxide reaction obtainable with a few hundred mJ/cm2 dose (homolysis quantum yield <0.5 (46), and extinction coefficient 1 (M cm)-1 (47)), and the synthetic utility of such homolysis reactions in related molecules in the presence of good hydrogen atom donors (implying facile epoxide formation) (48). Clearly the crosslinking observed under N2 is not accounted for by this mechanism. 

The quantum yields for scissioning ( >g) and crosslinking ( ) were determined for some representative polysilane derivatives both in solution and in the solid state (8). In all cases polymer scission is the predominant process and the values ranged from 0.2 to 1.0. For two cases, poly (methyl phenylsilane) and poly(cyclohexyl methylsilane) which were also examined in the solid state, the quantum yields were reduced by at least an order of magnitude from the solution values. 

One of the most interesting problems was evaluation of quantum yield for light induced crosslinking of poly(vinyl butyral) in the presence of diphenyl sulphoxide. Mathematical treatment of the molecular changes produced in polymers by light is based on that by ionizing radiation. In our case, as the changes above the gel point were examined, the only suitable approach... 

The photosensitized crosslinking of amorphous polymeric films containing dimerizable cinnamate moieties show quantum yields of dimerization of about 0.04 (4). Sensitization of such films by ketocoumarins proceeds via triplet-triplet energy transfer (2) (see Scheme V). The irradiations were performed at 405 nm at a wavelength... 

Two crosslinks per macromolecule are needed to make a polymer insoluble. Quantum yield of a diazide photolysis is ca. 0.5, therefore, the critical dose of the photochemical polymer insolubilization should be about 50mJ/cm. ... 

Nakamura, Sakata and Kichuchi 48> have compared the thermal and photochemical crosslinking of poly (vinyl cinnamate). The critical hardening value required is approximately 1.3 crosslinks per macromolecule possessing a degree of polymerization of 1400. The minute amount of crosslinking required to insolubilize poly(vinyl cinnamate) makes the study of the process difficult insight can be obtained only by inference. Furthermore, Nakamura and Kikuchi 44> reported that the quantum yield of the disappearance of the double bonds in poly(vinyl cinnamate) decreased rapidly with irradiation time. Extrapolation to zero time of exposure indicated a quantum yield of 0,34. It therefore seems hazardous to base conclusions on observations of products which result from long irradiation times. 

AI BN absorbs and decomposes +o radicals with a quantum yield of 0.43 (0.86 radicals per proton) (28K No photocross I inking occurred. This seemed to show that the crosslinking in this system is not radical-initiated. (It might be mentioned that the only possibility left open by this experiment for a radical process is that direct photo-initiation produces a radical of much greater reactivity than (CH3>2C-CN which is able to initiate cross I inking in the presence of the acceptor.)... 

The quantum yield for crosslinking, 0C L, can be calculated from the gelling dose (that is the dose for incipient gel formation) since this... 

Actually, with most polymers crosslinking is accompanied by main chain scission to a small extent. In these conditions the determination of quantum yields requires the measure of the soluble fraction as a function of dose. The following equation is useful, provided the initial distribution of molecular weights is random and crosslinking also occurs at random [7] namely,... 

When polystyrene films are exposed to ultraviolet radiation of 235.7 nm in vacuo, crosslinking occurs and the sample becomes partly insoluble [41]. Grassie and Weir [42] have performed a detailed investigation of the photolysis of polystyrene in vacuo as a preliminary step to the study of the photo-oxidation of that polymer. They found that the only gaseous product formed is hydrogen with a quantum yield of 4.3 x 10 2. The rate of the reaction increases linearly with the intensity of the incident light... 

The fate of the ally lie radicals is ultimately to recombine, to add to double bonds or to abstract hydrogen atoms from the chain. These reactions result simultaneously in the formation of crosslinks and chain scission. On exposure to 302 nm radiation in the presence of air, cross-linking has been shown to occur with a quantum yield of 4.5 x 10-3 [Ref. 50]. In the same conditions the quantum yield of chain scission is 2 x 10 3 [Ref. 51]. Moreover, oxygen is consumed with a quantum yield of 0.16 [52]. Carbonyl and hydroxyl groups are formed during the oxidation. 

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