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Hexasubstituted ethanes

An example of the application of molecular mechanics in the investigation of chemical reactions is a study of the correlation between steric strain in a molecule and the ease of rupture of carbon-carbon bonds. For a series of hexasubstituted ethanes, it was found that there is a good correlation between the strain calculated by the molecular mechanics method and the rate of thermolysis. Some of the data are shown in Table 3.3. [Pg.129]

The reduction of diphenyl-2-thienylchloromethane gave the stable hexasubstituted ethane which shows no tendency to dissociate into radicals. The diphenyl 2-thienylmethyl cation was formed during the reduction. [Pg.92]

A third process involves use of the species (X-X) to generate (he stable radical in pairs and relies on the stable radical being able to react with monomer, albeit slowly, to generate P]X (Scheme 9.6). Polymerizations with dithiuram and other disulfides (Section 9.3.2.1) and hexasubstituted ethanes (Section 9.3.4) belong to this class. [Pg.460]

The first use of sterically hindered hexasubstituted ethanes [e.g. 33] as initiators of polymerization was reported by Bledzki et al.77,78 The use of related initiators based on silylatcd pinacols [e.g. 34, 35] has been reported by Crivcllo et a/.,7l>82 jjan( os et ai i anOther initiators of this class include 36 fi X/ and 37.The rates of decomposition of hexasubstituted ethanes and the derived macroinitiators are known to vary according to the degree of steric... [Pg.467]

The proposed polymerization mechanism is shown in Scheme 9.12. Thermal decomposition of the hexasubstituted ethane derivative yields hindered tertiary radicals that can initiate polymerization or combine with propagating species (primary radical termination) to form an oligomeric macroinitiator. The addition of the diphenylalkyl radicals to monomer is slow (e.g. k[ for 34 is reported as KT M"1 s l at 80 °C84) and the polymerization is characterized by an inhibition period during which the initiator is consumed and an oligomeric macroinitiator is formed. The bond to the Cl I formed by addition to monomer is comparatively thermally stable. [Pg.468]

As previously pointed out in the discussion of ring size effects on bond homolyses the largest steric acceleration by bulky substituents is expected for the thermal cleavage of C-C-bonds in tetra- or hexasubstituted ethanes 26. Im comparison to azoal-... [Pg.11]

In the polymerization of St, it was found that 12 scarcely induces living radical polymerization [111], because the C-C bond of the co-chain end is a pentasubsti-tuted ethane structure (23), while the co-chain end of the polymer produced from the polymerization of MMA is a dissociable hexasubstituted ethane structure (24). The non-dissociation properties of the co-chain end of the polymer produced in the St polymerization were also reported by Braun et al. [109,112-116]. Namely, the St polymerization with 12 was a dead-end type polymerization. The dissociation of the chain ends was also examined by the experiments using the oligomer (n=l-3in24) [117,118] or amodel compound of the chain-end structures, 25 [119]. The C-C bond length at the co-chain end is 1.628 A for 24 (n=l), which is longer than the ordinary C-C bonds [118]. [Pg.89]

The second example in Scheme 3.3 illustrates the reversibility of aldol additions. The starting bicyclic ketone is a vinylogous aldol which upon treatment with base undergoes retro-aldol addition by deavage of a strained, hexasubstituted ethane sub-... [Pg.37]

ATRP can be approached from both sides of the equilibrium, that is, beginning from an alkyl halide and a low oxidation state metal, or from a radical and the higher oxidation state metal this latter approach is termed reverse ATRP (rATRP) [81,189,190]. Qiu et al. used this technique to prepare block copolymers, also of MMA and St [ 191 ]. They used a hexasubstituted ethane thermal in-iferter, diethyl 2,3-dicyano-2,3-di (p-tolyl)succinate, which decomposes reversibly to form two radicals when heated. The new radical is either deactivated by the CuCl2/bpy complex or adds MMA monomer, followed by deactivation, both of which will produce the dormant species in the ATRP equilibrium. The rATRP... [Pg.59]

Disulfide derivatives and hexasubstituted ethanes may also be used in this context to make end-functional polymers and block copolymers. The use of dilhiuram disulfides as thermal initiators was explored by Clouet, Nair and coworkers. Chain ends are formed by primary radical termination and by transfer to the dilhiuram disulfide. The chain ends formed are thermally stable under normal polymerization conditions. I he use of similar compounds as photo-iniferters, when some living characteristics may be achieved, is described in Section 9.3.2.1.1. [Pg.377]

The first use of sterically hindered hexasubstituted ethanes [e.g. 33] as initiators of polymerization was reported by Bledzki et The use of related... [Pg.467]

The rotational barrier for all carbon-carbon sp -sp bonds is greater than that in ethane " , and to the extent that barriers broadly increase with steric congestion, hexasubstituted ethanes have higher barriers than pentasubstituted ethanes, which are in turn higher than tetrasubstituted ethanes and so on. It is attractive to think of rotational barriers in saturated hydrocarbons in terms of an intrinsic barrier of 2.9 kcal mol overlaid by a steric contribution that can be analysed in terms of the alkyl substituents on the bond, but some caveats are necessary. [Pg.113]

Several hexasubstituted ethanes with very long central bonds are known. For example, the diphenyltetrakis( -butyl)ethane of Figure 2.21 has a central bond of 1.64 A. A prototype of this family is hexaphenylethane. We discussed in Section 2.2.2 the fact that the triphenyl-methyl radical does not dimerize to hexaphenylethane, but instead makes the unsymmetri-cal dimer of Eq. 2.13. Finally, in 1986 a true hexaphenylethane was observed and structurally characterized. It has a very long C-C bond of 1.67 A [Figure 2.21, where Ar = 2,6-di(f-butyOphenyl]. [Pg.125]


See other pages where Hexasubstituted ethanes is mentioned: [Pg.229]    [Pg.841]    [Pg.229]    [Pg.467]    [Pg.91]    [Pg.3934]    [Pg.16]   


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