Strain sterically induced

It is not possible to predict which mechanism is involved in a certain copolymerization. In the system a-methylstyrene-methyl methacrylate depolymerization of sequences of two monomer units seemed to occur as well as depolymerization of a-methylstyrene from longer sequences. In the system a-methylstyrene -acrylonitrile the sequence of two monomer units of a-methylstyrene is stable and does not depolymerize. The reversibility of the polymerizations of a-methylstyrene and methyl methacrylate can be explained by sterically induced strain in the polymer chain (13). In the copolymer a-methylstyrene-methyl methacrylate this strain involves the whole polymer chain whereas in the a-methylstyrene— acrylonitrile system the strain is broken by the acrylonitrile sequences and is built up again in the a-methylstyrene. This explains the difference in the depolymerization tendencies of sequences of two units of a-methylstyrene and longer sequences in this system. 

Fig. 3.31 Steric control in alternating ROMP Tendencies of norbomene and cyclooctene to give productive olefin metathesis upon coordination are illustrated by a thick arrow (preferred monomer) or a thin arrow (less favoured monomer) (a) only minor steric hindrance SlMes greatly favours the polymerisation of the strained norbomene (b) the rotating phenylethyl-group induces a steiically more congested active site, leading to preferred incorporation of the smaller cyclooctene (c) the flexible and small cyclooctene derived polymer fragment permits the incorporation of the bulky norbomene...

In the case of rram-(l-methyl-2-adamantylidene)-r-methyladamantane, 2, increased bromocarbenium character of the ionic intermediate is a consequence of steric strain present both in the starting olefin and in the corresponding bromonium ion. Introduction of the two tram-methyl groups at the allylic carbon atoms markedly increases the strain of the system, inducing significant torsion and out-of-plane bending of the double bond (7). 

Other disulfones like (32) and (33) were studied (see Ref [6]). They exhibit surprisingly stable anion radicals (and even a stable dianion with (32)). Such charged species could be used as redox mediators, tiowever, reduction of alkyl halides RX by (33) did not permit the formation of alkylated products presumably because of the strong steric strain induced by alkylation of the corresponding carbon atoms in the cyclobutene ring. 

The use of sterically hindered alkyl groups results in considerable distortion of the coordination sphere and enhanced tendency to homolysis. It is possible that binding of substrate induces a change in protein conformation, which will in turn introduce such steric strain around the cobalt. The advantage of this proposal is that the adenosyl radical would only be produced in the presence of substrate. 

The isomers of Cr(men)3 isomerize with half-lives (several hours) similar to the Cr(benz)3 complex. The rate of isomerization of the tris-(hydroxamate) complexes is therefore not particularly sensitive to the substituent of the hydroxamate nitrogen atom, since the men ligand contains an alkylated nitrogen atom, and the benz ligand contains an unsubstituted nitrogen atom. In the absence of an induced strain, the corresponding siderophore complexes must isomerize much more slowly because of the steric constraints of the ligand. 

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