Vinyl, from 1,4-dienes

One of the very rare examples of a combination of a radical with a pericydic reaction - in this case a [4+2] Diels-Alder cycloaddition - is depicted in Scheme 3.83 [133]. The sequence, elaborated by Malacria and coworkers, is based on the premise that the vinyl radical 3-341 formed from the substrate 3-340 using tributyltin hydride exists mainly in the Z -form. This is reduced by a hydrogen atom to form a 1,3-diene, which can undergo an intramolecular Diels-Alder reaction via an exotransition state reaction (the chain lies away from diene). 

Trost et alJ2 also explored the compatibility of di-, tri-, and tetrasubstituted allenes with their intermolecular Alder-ene protocol. Multiple substituents present the opportunity for a mixture of products to arise from differing regio- and chemoselectivity. 1,1-Disubstituted allenes were coupled to methyl vinyl ketone with excellent chemo-selectivity only when one set of /3-hydrogens was activated by an cy-ester or amide (Equation (69)). If the /3-hydrogens were of similar acidity, a mixture of products was obtained, as in the coupling of allenol 103 with methyl vinyl ketone dienes 104 and 105 are produced in a 1.3 1 mixture (Equation (70)). 

The photo-cross-linkability of a polymer depends not only on its chemical structure, but also on its molecular weight and the ordering of the polymer segments. Vinyl polymers, such as PE, PP, polystyrene, polyacrylates, and PVC, predominantly cross-link, whereas vinylidene polymers (polyisobutylene, poly-2-methylstyrene, polymethacrylates, and poly vinylidene chloride) tend to degrade. Likewise, polymers formed from diene monomers and linear condensation products, such as polyesters and polyamides, cross-link easily, whereas cellulose and cellulose derivatives degrade easily. ... 

The most important representatives of anionic polymerization centres are formed from vinyl and diene monomers. The trivial schematic representation of a carbanion... 

Several papers have appeared in the literature in recent years showing that certain metal acetylacetonates can function as initiators for the polymerisation of vinyl and diene monomers in bulk and solution (1 - 12). Results for the kinetics of bulk and solution polymerisation are consistent with the view that the reaction occurs by a free-radical mechanism. The usual free-radical kinetics are operative, but an unusual feature is that, in some cases, certain additives such as chlorinated hydrocarbons have an activating effect upon the reaction by inducing more rapid decomposition of the initiator (2,11,12,13). Other additives which have been reported as promotors for the polymerisation include pyridlne(14) and aldehydes and ketones(15). The complexity of the reaction in the presence of such additives is evident from the fact that chloroform has been reported to be an inhibitor for the poly-merlsatlon(3). 

Both classes of polymers were attacked simultaneously, so that free-radical-initiated, self-propagating chain reactions and slow, endothermic step reactions were studied side by side. After the first results were attained, a grand strategy for practical applications developed quite naturally the vinyl- and diene-type addition polymers were pursued with the ultimate aim being the production of a synthetic rubber. The signals coming from the... 

In ATRP, the range of polymerizable monomers have been extended to vinyl acetate, dienes and vinyl chloride.Also, new metals (Mo, Os, Ti) and new ligands were successfully introduced. ATRP was successfully carried out in miniemulsion and microemulsion as well as in CO2 or in ionic liquids. New initiating systems were developed for ATRP, such as ARGET and ICAR. They start from oxidatively stable transition metal complexes (Cu") and are activated in the presence of reducing agents. Reduced amount of catalyst and its... 

Intramolecular Diels-Alder reaction of the vinyl sulphide diene (62) furnishes the corresponding fused ring tetrahydrothiophene as a mixture of the four possible diastereoisomers, where the major isomers (63) and (64) arise from the stereocontrol exerted by the silyloxy substituent.50... 

The synthesis of vinyl epoxides from dienes is mentioned in Chapter 19. The monoepoxide is formed first as the diene is more nucleophilic than the alkene in the monoepoxide. The main difficulty is that the monoepoxide rearranges with acid catalysis from the by-product, the carboxylic acid of the peroxyacid used in the epoxidation. The solution is simple the mixture must be buffered to keep the acidity low. 

The chemical structure of a repeat unit in a polymer produced from low molar mass precursors is dependent on the type of chemistry used to build up the chain structure. The monomers which are commonly used to construct polymer chains may be vinyl or diene compounds, ring compounds or functionalized compounds which require two or more coreactants in order to form a polymer. Some typical precursors are shown in Figure 1.4. 

Graft copolymers may be prepared in three general ways, namely transfer grafting, irradiation grafting and chemical grafting. Transfer grafting is most commonly free radical initiated. Typically, a vinyl or diene polymer is treated with a peroxide in the presence of a vinyl monomer. Transfer occurs between the polymer chain and radicals derived from the initiator the resultant polymer chain radical then initiates polymerization of the monomer, e.g. ... 

The methods described above are applicable to the preparation of block copolymers from vinyl and diene monomers and, in the case of anionic polymerization, to certain cyclic monomers such as ethylene oxide. These methods are not in general appropriate to the preparation of condensation-type and rearrangement-type block copolymers. For such materials a number of methods may be envisaged. As an example a polymer with carboxyl end-groups may be allowed to react with a polymer containing hydroxyl end-groups ... 

Propagation is a repetitive process which attaches vinyl monomers to the growing polymer chain end through (7-bonds. A wide range of applicable monomers (Scheme 2.20) are suitable for conventional radical polymerizations, ranging from dienes to styrenics to (meth)acrylates. 

---