Vinyl chloride carbon-centered radicals

Allyl radical is relatively stable due to the conjugation of a vinyl group with the carbon-centered radical. Allyl chloride and bromide (1-11, X = Cl, Br) were thus employed as initiators for controlled radical polymerization of styrene in conjunction with CuCl/ L-l and CuBr/L-1, respectively.161 The molecular weights agreed well with the calculated values assuming the formation of one living polymer chain per initiator and increased in direct proportion to monomer conversion. The MWDs were narrower with bromide than with chloride (MJMn 1.2 vs 1.3). 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

The configuration of an asymmetric carbon in a polymer is determined at the time of monomer addition to the propagating center. To visualize the situation for the free radical-initiated polymerization of vinyl chloride, approaching monomer may produce either the same or the opposite configuration for the chlorine-substituted carbon in the adding monomer, as already present in the adjacent unit of the propagating center (Eqs. 22.43 and 22.44). 

In practice, however, vinyl chloride polymerized commercially does not have regular tacticity because the growing end of the chain is a free radical at an sp hybridized carbon center which can freely rotate. This rotation rate varies with temperature by polymerizing vinyl chloride at very low temperature the rotation can be slowed significantly, preferentially trapping certain conformations and allowing for polymerization of a more syndiotactic polymer. 

Reactions at acid centers to create 5 -2-(trimethylsilyl) ethanethiolesters are also common. Carboxylic 5-thiolesters are formed in high yield by the DCC/DMAP mediated coupling of 2-(trimethylsilyl)ethanethiol and carboxylic acids or by thiol substitution on a carboxylic acid chloride. 5 -2-(Tiimethylsilyl) ethyl p-toluenethiolsulfonate is formed by treating 2-(trimethyl-silyl)ethanethiol with tosyl bromide (eq 4). The product is a useful electrophile for carbon nucleophiles, allowing the introduction of the 2-(trimethylsilyl)ethylthio unit by an alternative mechanism. Independent of the thiol, aryl and alkyl 2-trimethylsilylethyl thioethers may be prepared by the radical addition of the appropriate arene- or aikanethiol to vinyl trimethyl-silane in a reaction comparable to that of eq l7 ... 

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