Redistribution reaction, synthesis

In many cases, these cyclic siloxanes have to be removed from the system by distillation or fractionation, in order to obtain pure products. On the other hand, cyclic siloxanes where n = 3 and n = 4 are the two most important monomers used in the commercial production of various siloxane polymers or oligomers via the so-called equilibration or redistribution reactions which will be discussed in detail in Sect. 2.4. Therefore, in modern silicone technology, aqueous hydrolysis of chloro-silanes is usually employed for the preparation of cyclic siloxane monomers 122> more than for the direct synthesis of the (Si—X) functional oligomers. Equilibration reactions are the method of choice for the synthesis of functionally terminated siloxane oligomers. 

The first step for the synthesis of a melt spinnable polysilane is the alkoxylation and distillation of the residue (Figure 1). 1,2-dimethyltetramethoxydisilane and 1,1,2-trimethyltrimethoxydisilane are mixed in a special ratio and a poly silane will be obtained by a catalytic redistribution reaction. Catalysts for this reaction are alkali alkoxides like sodium methoxylate. Phenylmethoxydisilanes [22] or phenylchloride are used as additives. A mixture of methyltrimethoxysilane and dimethyldimethoxy-silane distils off as a byproduct of the redistribution reaction. Figure 2 shows the mechanism of the catalytic redistribution. 

Jain, V. K. In Synthesis, Reactivity and Redistribution Reactions of Homo- and Heterobinuclear Palladium(II) and Platinum(II) Complexes. Adv. Organomet. Proceedings Indo-Sov. Symp. Organomet. Chem., 1st. ed. Jain, D. V. S., Ed. Indian Natl. Sci. Acad New Delhi, India 1989, 143-162. 

In a recent synthesis of a mixed zinc/cadmium telluride alloy, a mixture of dimethyl-cadmium and diethylzinc was employed. The original authors interpreted the results by assuming that the redistribution reaction 3 is essentially thermoneutral since the difference between the enthalpies of formation for the symmetrical dialkylzincs is small. 

Although most known BH X3 adducts are stable and most known BX Y3 adducts are unstable to redistribution of substituents about boron, this need not reflect a major difference in behavior of the two types of adduct but may, instead, reflect the interests of the investigators. The BH X3 adduct studies have emphasized synthesis and, hence, systems that are stable to redistribution, whereas mixed boron trihalide adduct studies have emphasized redistribution reactions, which are easier to observe when the donor is weak and redistribution is rapid. 

Summary The heterogeneous catalytic redistribution reaction of methylchlorodisilanes provides spinnable poly(methylchlorosilanes/-carbosilanes). Especially copolymers like poly(methylchlorosilanes-co-styrenes) are suitable polymers for melt spinning. The high reactivity caused by Si-Cl bonds enables oxygen free curing methods of the melt spun polymer filaments with ammonia. The synthesis is achieved without the employment of highly reactive metals and any solvents. The thus produced SiC fibers exhibit oxygen contents lower than 1 wt. %. 

The Kocheshkov redistribution reaction between tetra-organotins and tin(rV) halides, in appropriate mole ratios, is a general method of synthesis of organotin halides (equations 59 and 60). It is a particularly useful reaction, since it is difficult to control the alkylation of SnX4 by organolithium or Grignard reagents to the desired extent. 

A cheaper approach to PSCs by means of a redistribution reaction of bis(tri-alkylsilyl)carbodiimides and dichlorosilanes or trichlorosilanes was described soon after in a patent by Klebe and Mnrray. °" Similar to the synthesis of polysilazanes by transsilylation of dichlorosilanes or trichlorosilanes with HMDS (Equation 18.12), the procednre takes advantage of cheap and commercially available starting compounds, simple processing (i.e., no solid by-product formation), and high polymer yields. 

The product in eq 4.17 results from dynamic ligand exchange ("redistribution reaction") of DSBM and DNBM. MgCl in eq 4.16 may be the insoluble by-product from synthesis of di-n-butylmagnesium in eq 4.15. This makes it possible to produce DBM in a "one-pot" synthesis. Though DBM in eq 4.17 is shown as if the n-butyl to sec-butyl ratio were 1, in practice the ratio is about 1.5. This minimizes the amount of costly sec-butyllithium that is needed. DBM is used to produce supported transition metal catalysts (discussed in sections 4.3.2 and 4.3.3). 

Scheme 1.1.1 Synthesis of organotin(fV) compounds based on alkylation of SnCU with an organometallic reagenf and the Kocheshkov redistribution reaction...

The n AMlNGOES program also falls into this category since the reactions are described as a cyclic bond redistribution. Besides synthesis planning, it may be used to study molecular rearrangements and to search for possible mechanisms of organic reactions. ... 

Diphenoquinones 36 can be used as initiators for the redistribution reaction. White found that equilibration of a diphenoquinone 36 with a poly (phenylene oxide) 2 would give a difiinctional polymer 37 as shown in Scheme 13. Low molecular weight PPO polymers were preferentially used since they are more free of imperfections. A simplified procedure for the synthesis of low molecular weight PPO by a precipitative polymerization reaction has recently been published. ... 

---