By Disproportionation

Solutions of organo tellurium trichlorides are in equilibrium with diorgano tellurium dichlorides and tellurium tetrachloride. 

The equilibrium favoring the organo tellurium trichloride can be shifted by removing tellurium tetrachloride via its reaction with copper or hydroxylated solvents.  

Bis[4-methoxyphenyl] Tenurium Dichloride In a 250 ml flask fitted with a reflux condenser and a stirrer are plaxxd 4 g (12 mmol) of 4-methoxyphenyl tellurium trichloride, 1.5 g (24 mmol) of copper powder, and 40 ml of benzene and the vigorously stirred mixture is heated under reflux for 2.5 h. The mixture is filtered, 350 ml of petroleum ether are added to the fil irate, and the precipitated product is recrystallized from benzene yield 1.62 g (67%) m.p. 181°. 

Bis 4-ethoxyphenyl] tellurium dichloride (m.p. 108°) was similarly prepared in 64% yield. 

Bis[2-chloro-2-phenylethenyl] Tellurium Dichlorideh The yellow solution of 1.5 g (4 mmol) of 2-chloro-2-phenyl ethenyl tellurium trichloride in glacial acetic acid is heated until it becomes colorless. The solution is cooled to crystallize the product which is filtered off and recrystallizcd from glacial acetic acid yield 0.80 g (83%) m.p. 205-215° (dec.). 

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Termination by disproportionation comes about when an atom, usually hydrogen, is transferred from one polymer radical to another ... 

This mode of termination produces a negligible effect on the molecular weight of the reacting species, but it does produce a terminal unsaturation in one of the dead polymer molecules. Each polymer molecule contains one initiator fragment when termination occurs by disproportionation. 

Combination and disproportionation are competitive processes and do not occur to the same extent for all polymers. For example, at 60°C termination is virtually 100% by combination for polyacrylonitrile and 100% by disproportionation for poly (vinyl acetate). For polystyrene and poly (methyl methacrylate), both reactions contribute to termination, although each in different proportions. Each of the rate constants for termination individually follows the Arrhenius equation, so the relative amounts of termination by the two modes is given by... 

The degree of polymerization in Eq. (6.41) can be replaced with the kinetic chain length, and the resulting expression simplified. To proceed, however, we must choose between the possibilities described by Eqs. (6.34) and (6.35). Assuming termination by disproportionation, we replace n, by v, using Eq. (6.37) ... 

Only one additional stipulation needs to be made before adapting the results that follow from Eq. (5.24) to addition polymers. The mode of termination must be specified to occur by disproportionation to use the results of Sec. 5.4 in this chapter, since termination by combination obviously changes the particle size distribution. We shall return to the case of termination by combination presently. 

This expression is plotted in Fig. 6.7 for several large values of p. Although it shows a number distribution of polymers terminated by combination, the distribution looks quite different from Fig. 5.5, which describes the number distribution for termination by disproportionation. In the latter Nj,/N decreases monotonically with increasing n. With combination, however, the curves go through a maximum which reflects the fact that the combination of two very small or two very large radicals is a less probable event than a more random combination. 

These various expressions differ from their analogs in the case of termination by disproportionation by the appearance of occasional 2 s. These terms arise... 

This contrasts with a limiting ratio of 2 for the case of termination by disproportionation. Since and can be measured, this difference is potentially a method for determining the mode of termination in a polymer system. In most instances, however, termination occurs by some proportion of both modes. Although general expressions exist for the various averages and their ratio when both modes of termination are operative, molecular weight data are generally not sufficiently precise to allow the proportions of termination modes to be determined in this way. 

The equations derived in Sec. 6.7 are based on the assumption that termination occurs exclusively by either disproportionation or combination. This is usually not the case Some proportion of each is the more common case. If A equals the fraction of termination occurring by disproportionation, we can write n = A[ 1/1 - p] + (1 - A)[2/(l - p)] and n /n = A(1 + p) + (1 - A)[(2 + p)/2]. From measurements of n and n /n it is possible in principle to evaluate A and p. May and Smith have done this for a number of polystyrene samples. A selection of their data for which this approach seems feasible is presented ... 

In these equations I is the initiator and I- is the radical intermediate, M is a vinyl monomer, I—M- is an initial monomer radical, I—M M- is a propagating polymer radical, and and are polymer end groups that result from termination by disproportionation. Common vinyl monomers that can be homo-or copolymeri2ed by radical initiation include ethylene, butadiene, styrene, vinyl chloride, vinyl acetate, acrylic and methacrylic acid esters, acrylonitrile, A/-vinylirnida2ole, A/-vinyl-2-pyrrohdinone, and others (2). 

The purple permanganate ion [14333-13-2], MnOu can be obtained from lower valent manganese compounds by a wide variety of reactions, eg, from manganese metal by anodic oxidation from Mn(II) solution by oxidants such as o2one, periodate, bismuthate, and persulfate (using Ag" as catalyst), lead peroxide in acid, or chlorine in base or from MnO by disproportionation, or chemical or electrochemical oxidation. 

Although primary and secondary alkyl hydroperoxides are attacked by free radicals, as in equations 8 and 9, such reactions are not chain scission reactions since the alkylperoxy radicals terminate by disproportionation without forming the new radicals needed to continue the chain (53). Overall decomposition rates are faster than the tme first-order rates if radical-induced decompositions are not suppressed. 

The minimum polydispersity index from a free-radical polymerization is 1.5 if termination is by combination, or 2.0 if chains ate terminated by disproportionation and/or transfer. Changes in concentrations and temperature during the reaction can lead to much greater polydispersities, however. These concepts of polymerization reaction engineering have been introduced in more detail elsewhere (6). 

Termination reaction with vinyl acetate is nearly exclusively by disproportionation (216), although there are reports that recombination increases in... 

End Groups and Branching. Both saturated and unsaturated end groups can be formed during polymerization by chain transfer to monomer or polymer and by disproportionation. Some of the possible chain end groups are... 

Lyondell and Sun Oil Co. are the main producers of benzene by disproportionation. Eiaa Oil Co. of Texas has developed the Eiaa T2BX process for toluene disproportionation usiag a proprietary catalyst. The new catalyst is claimed to reduce hydrogen consumption and is suitable for feeds containing small amounts of moisture (53). A commercial production unit was started up ia the fall of 1985. 

The existence of bismuthine was first demonstrated by using a radioactive tracer, Bi (8). Acid treatment of a magnesium plate coated with Bi resulted in the hberation of a volatile radioactive compound. In subsequent experiments, magnesium bismuthide [12048-46-3], Mg Bi, was treated with acid the yield, however, was only one part of bismuthine for every 20,000 parts of bismuth dissolved. Attempts to prepare bismuthine by reduction of bismuth trichloride with a borohydride have not been particularly successful. Experimental quantities ate best prepared by disproportionation of either methylbismuthine [66172-95-0], CH Bi, or dimethylbismuthine [14381-45-4], C2H. Bi (7) ... 

Dichloramine. The least stable chloramine, dichloramine [3400-09-7] has not been prepared in pure form. However, it has sufficient stabiUty in dilute organic or aqueous solutions for deterrnination of some physical and chemical properties. It has a pungent odor and can impart an odor or off-taste to water at concentrations above 0.8 ppm. Dichloramine can be produced by reaction of HOCl with a slight excess of NH in the pH range 4—7 or by disproportionation of NH2CI at pH 3.5—4.0 ... 

Pyrolysis Thermal decomposition of 1,1,1,2-tetrachloroethane produces tetrachloroethylene (by disproportionation), hydrogen chloride, and trichloroethylene via dehydrochlorination (111). The yield of the latter is increased in the presence of ferric chloride (112). Other catalytic materials include FeCl —KCl mixture (113), AlCl (6), the complex of AlCl with nitrobenzene (114), activated alumina (3), Ca(OH)2 (115,116), and NaCl (94). 

The number average degree of polymerisation x is defined as the average number of monomer units per polymer chain. Therefore if termination is by disproportionation r = jc, but if by combination r = x. 

Rosins and rosin derivatives. The resins more commonly used in rubber base adhesives are rosin esters, particularly glycerol and pentaerythritol esters, as well as rosins modified by disproportionation and hydrogenation. The glycerol ester of hydrogenated rosin has been reported to be an excellent tackifier for polychloroprene adhesives (see pp. 344-357 in [17]). 

The next member of the open-chain series P H - -2 is P3H5, i.e. PH2PHPH2, a colourless liquid that can be stored in the dark at —80° for several days. 0 an be made by disproportionation (2P2H4-----> P3H5 -)- PH3) but it is dif-... 

The modes of thermal decomposition of the halates and their complex oxidation-reduction chemistry reflect the interplay of both thermodynamic and kinetic factors. On the one hand, thermodynamically feasible reactions may be sluggish, whilst, on the other, traces of catalyst may radically alter the course of the reaction. In general, for a given cation, thermal stability decreases in the sequence iodate > chlorate > bromate, but the mode and ease of decomposition can be substantially modified. For example, alkali metal chlorates decompose by disproportionation when fused ... 

Since it is well known that chloroalkenes are often much less stable than the corresponding alkanes, olefinic unsaturation may be an important source of thermal instability in PVC. Chain-end unsaturation could arise by disproportionation during bimolecular reaction of polymer radicals Eq. (2). 

Depending on the termination reaction of the vinyl monomer, termination by disproportionation or termination by combination occurs. As a result, AB or ABA block copolymers might be obtained. 

The mode of chain termination affects the type of block copolymer formed. For example, if a MAI (based essentially on the first monomer A) possessing one central azo bond is decomposed in the presence of monomer B, the growing chain Bn can terminate either by disproportionation or combination, leading to AB and ABA type copolymers, respectively. 

Which mechanism of termination will be preferably applied depends largely on the monomer used. Thus, methyl methacrylate chains terminate to a large extent by disproportionation, whereas styrene chains tend to termination by combination. The ratios of termination rate constants 8 = ktJkic (for disproportionation, td, combination,, c) are 5 == 0 and 5 = 2 for styrene [95] and methyl methacrylate [96], respectively. In the case of styrene, however, the values of 8 reported in the literature are at variance. Berger and Meyerhoff [97] found 8 = 0.2, at 52°C. Therefore, it is possible that a fraction of styrene terminates by disproportionation. 

In general, the activation energies for both cationic and anionic polymerization are small. For this reason, low-temperature conditions are normally used to reduce side reactions. Low temperatures also minimize chain transfer reactions. These reactions produce low-molecular weight polymers by disproportionation of the propagating polymer ... 

ZrCl2 prepared by disproportionation of ZrCl3 without ball-milling was also active (about 80 g C2H4/g ZrCl2 hr at 20 kg/cm2 and 75°C) in ethylene polymerization (157, 159). 

Another concern, is the potential reactivity of 10 as a transfer agent under polymerization conditions (see 3.3.1.1.4).103 Tetramethylsuccinonitrile (9) appears to be essentially inert under polymerization conditions. However, the compound is reported to be toxic and may be a problem in polymers used in food contact applications.1" 30 Methacrylonitrile (MAN) formed by disproportionation readily copolymerizes.7"34 The copolymerized MAN may affect the thermal stability of polymers. A suggestion103 that copolymerized MAN may be a "weak link" in PS initiated with AIBN has been disputed.14... 

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