Solid-stating accelerators

Figure 14.5 Effect of a commercial solid-stating accelerator (Irgamod 1425) on the rate of intrinsic viscosity enhancement of PET (data obtained from Ciba Specialty Chemicals)...

The relatively inexpensive and safe sodium borohydride (NaBH4) has been extensively used as a reducing agent because of its compatibility with protic solvents. Varma and coworkers reported a method for the expeditious reduction of aldehydes and ketones that used alumina-supported NaBH4 and proceeded in the solid state accelerated by microwave irradiation (Scheme 7) [50]. The chemoselectivity was apparent from the reduction of frarcs-cinnamaldehyde to afford cinnamyl alcohol. 

Transformations in the Solid State. From a practical standpoint, the most important soHd-state transformation of PB involves the irreversible conversion of its metastable form II developed during melt crystallization into the stable form I. This transformation is affected by the polymer molecular weight and tacticity as well as by temperature, pressure, mechanical stress, and the presence of impurities and additives (38,39). At room temperature, half-times of the transformation range between 4 and 45 h with an average half-time of 22—25 h (39). The process can be significantly accelerated by annealing articles made of PB at temperatures below 90°C, by ultrasonic or y-ray irradiation, and by utilizing various additives. Conversion of... 

In addition to secondarv resistance control, other devices such as reactors and thyristors (solid-state controllable rectifiers) are used to control wound-rotor motors. Fixed secondary reactors combined with resistors can provide veiy constant accelerating torque with a minimum number of accelerating steps. The change in slip frequency with speed continually changes the effective reac tance and hence the value of resistance associated with the reactor. The secondaiy reactors, resistors, and contacts can be varied in design to provide the proper accelerating speed-torque curve for the protection of belt conveyors and similar loads. 

Isoindoles are reactive toward oxidizing agents, and precautions usually advocated in the preparation of these compounds to prevent their oxidation merit careful consideration. The end products of oxidation are most often colored, resinous materials of indeterminate structure. The oxidative reactions appear to be accelerated by light and occur much more rapidly in solution than in the solid state. In a separate but possibly related process, certain isoindoles undergo polymerization in the solid state to give resins which, according to... 

Control and Acceleration of Photoreactions via Freeze of an Equilibrium by Complexation and Irradiation in the Solid State... 

Because observed rate enhancements are usually small, or zero, nonthermal effects do not seem to be important in MW heated reactions in homogeneous media, except possibly in some reactions of polymers and reactions in nonpolar solvents. Relatively few studies have been conducted on MW-assisted reactions of polar reactants in nonpolar solvents. Also, since there is some disagreement as to whether or not these reactions are accelerated significantly by MW, in comparison with conventionally heated reactions at the same temperature, more research on the effect of MW irradiation on the rates of these reactions is required. Nonthermal effects may, however, explain the more substantial MW rate enhancements in solvent-free reactions on solid supports [44] (see Chapt. 5) and solid state reactions [68, 69]. 

The dispersion and solid-state ion exchange of ZnCl2 on to the surface of NaY zeolite by use of microwave irradiation [17] and modification of the surface of active carbon as catalyst support by means of microwave induced treatment have also been reported [18]. The ion-exchange reactions of both cationic (montmorillonites) and anionic clays (layered double hydroxides) were greatly accelerated under conditions of microwave heating compared with other techniques currently available [19.]... 

While in the unbound enediyne the c-d distance is 4.1 A, this distance is diminished upon metal complexation 3.3 A for Pt(II) and Pd(II), and 3.4 A for Hg(II). The Pt and Pd species cyclize in the solid state at only slightly elevated temperatures, and give Bergman products below ambient temperature in solution. While the change in reactivity was attributed to the change in distance between the alkyne termini, an accelerating influence of the metal cannot be ruled out. 

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