Investigation of the Origin of Tensile Stress-Induced Rate Enhancements in the Photochemical Degradation of Polymers

Abstract

To investigate the effect of tensile stress on the photochemical degradation efficiencies of polymers, a modified PVC polymer with Cp(CO)3Mo-Mo(CO)3Cp (Cp = eta5-C5H5) units along the backbone was synthesized. The polymer is photochemically reactive because the Mo-Mo bonds are photolyzed with visible light and the resulting radicals are captured with Cl atoms from along the polymer backbone. Of most importance from a mechanistic standpoint, the photochemical degradation reaction occurs in the absence of oxygen, which eliminates the kinetically complicating effect of rate-limiting oxygen diffusion. Tensile stress initially caused the quantum yield of polymer degradation to increase, but, after a certain point, additional stress caused a decrease in the quantum yield. This dependence of quantum efficiency on stress is consistent with a hypothesis in which stress affects the ability of the photochemically generated radicals to recombine. At low to moderate stress, the effect of stress is to increase the separation of the radicals (by recoil), thus decreasing their recombination probability. As the stress increases, however, segments of different chains align, which induces a higher degree of orientation and crystallinity in the polymer, which in turn makes diffusion more difficult. The efficiency of degradation is predicted to decrease accordingly because of decreased radical and/or trap mobility in the ordered polymer. Infrared and X-ray data are presented, showing that the degree of orientation and crystallinity in the polymer does indeed increase with increasing stress.

Tetracarbonylbis(η5-cyclopentadienyl)bis(diphenylphosphine)dimolybdenum(Mo—Mo) hexane solvate

“…For related literature, see: Adams et al (1997); Chen et al (2004); Daglen et al (2007); Shultz et al (2008); Tenhaeff & Tyler (1991); Tyler (2003); Van der Sluis & Spek (1990); Wilson & Shoemaker (1957).…”

“…We previously reported the synthesis of photodegradable polymers that contain metal-metal bonds along the main chain (Tenhaeff & Tyler, 1991;Tyler, 2003). The metal-metal bond provides a convenient spectroscopic handle for monitoring the effect of external parameters as tensile stress (Chen et al, 2004) and temperature (Daglen et al, 2007) on the rate and onset of polymer backbone degradation. Recent work describes the preparation of phosphine-substituted dimeric molydenum complexes as precursors for step polymerization (Shultz et al, 2008).…”

From molecular mechanochemistry to stress-responsive materials

“…5 Historically, 1,6 the focus of polymer mechanochemistry has been on polymer degradation via homolytic bond scission reactions. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Studies of flow-induced mechanochemistry 5,16,[23][24][25][26][27][28][29][30] provided a firm theoretical and experimental foundation for the field. Mechanochemistry in polymers adsorbed to surfaces 31,32 or in the bulk is also known, and free radical generation due to stress-induced bond scission has been detected spectroscopically.…”

Tetracarbonylbis(η5-cyclopentadienyl)bis[(dec-9-en-1-yl)diphenylphosphine]dimolybdenum(0)(Mo—Mo) tetrahydrofuran disolvate

“…For related literature from our group in this area, see: Chen et al (2004); Daglen et al (2007); For other compounds containing Mo-Mo single bonds, see: Wilson & Shoemaker (1957); Shultz et al (2008). Selected bond lengths (Å ).…”

“…In an on-going mechanistic study in our lab we have examined the effects of external parameters, such as mechanical stress (Chen et al, 2004) and temperature (Daglen et al, 2007), on the rate and onset of polymer degradation using polymers with metal-metal bonds in the main chain. Such polymers are synthesized from functionalized dimeric metal carbonyl complexes.…”