This study explored the effect of radical size, chain length, and mass on the radical cage effect. Radical cage pairs of the type [(RCp)(CO)3M•, •M(CO)3(CpR)] (M = Mo or W; CpR = various substituted cyclopentadienyl ligands) were generated by photolysis (λ = 540 nm) of the metal−metal bonds in (RCp)2M2(CO)6. The cage recombination efficiencies (denoted as F cP) for the radical cage pairs were obtained by extracting them from quantum yield measurements for the photoreactions with CCl4 (a metal-radical trap) as a function of solvent system viscosity. For the series of molecules (R3SiOCH2CH2Cp)2Mo2(CO)6 (R = Me, i-Pr, n-Pr, n-Hx), the F cP values were linearly proportional to mass 1/ 2 /radius2, in agreement with the predictions of Noyes' cage effect theory. It is also demonstrated that the difference in the cage recombination efficiencies between the [(MeCp)(CO)3Mo•, •Mo(CO)3(CpMe)] and [(MeCp)(CO)3W•, •W(CO)3(CpMe)] cage pairs cannot be ascribed to the different masses of the radicals. Rather, the difference is shown to be attributable to differences in the metal−metal bond energies or to differences in the spin−orbit coupling. In another comparison, F cP at any viscosity for [(MeCp)(CO)3Mo•, •Mo(CO)3(CpMe)] was shown to be identical to that of [Cp*(CO)3Mo•, •Mo(CO)3Cp*] (Cp* = η5-C5Me5) in tetrahydrofuran (THF)/tetraglyme solution. Rotation of the MeCp ring is fast compared to the time scale of diffusive separation (k dP) and radical recombination (k cP), and hence the effective volumes of the radicals in the solvent cage are nearly identical, which leads to similar F cP values.

introductionAromatic poly(ether ketone) such as PEEK (3), has gained considerable importance due to its chemical resistance and excellent mechanical and thermal properties. Rose et al. ') were first to succeed in preparing high-molecular-weight 3, 4,4'-biphenyldiolbased poly(ary1 ether ketone), 4, and related semicrystalline polymers by choosing diphenyl sulfone as the reaction medium at a reaction temperature above 300°C.McGrath et al. *), and later Risse and Sogah3), reported the synthesis of an amorphous high-molecular-weight prepolymer containing removable tert-butyl substituents on the polymer chain. The bulky group could be cleaved to produce semicrystalline poly(ary1 ether ketone)s. Homogeneous hydrolysis of poly(ary1 ether ketimine) with trifluoromethanesulfonic acid was also reported by McGrath et al. ' ). The melting temperature, T,, of the resulting semicrystalline polymer was significantly lower than that of PEEK. The present paper describes a different approach to the hydrolysis of amorphous poly(ary1 ether ketimine)s 1 and 2. The required ketimine-to-ketone transformation was carried out under controlled heterogeneous hydrolysis conditions to afford semicrystalline poly(ary1 ether ketone)s. The polymers were characterized by differential scanning calorimetry (DSC) and the results were compared with the reported thermal transitions of PEEK and 4,4'-biphenyldioLbased poly(ary1 ether ketone) 4. Results and discussionSynthesis of poly(aryI ether ketirnine)s Polymerization of hydroquinone with appropriate ketimine functional difluoride was carried out in presence of potassium carbonate (excess) in N-methyl-2-pyrrolidone/ 0 1991, Hiithig & Wepf Verlag, Basel CCC 0173-2803/91/$01 .OO

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