Pendent vinyl group

Homopolymerization of butadiene can proceed via 1,2- or 1,4-additions. The 1,4-addition produces the geometrically distinguishable trans or cis stmctures with internal double bonds on the polymer chains, 1,2-Addition, on the other hand, yields either atactic, isotactic, or syndiotactic polymer stmctures with pendent vinyl groups (Eig. 2). Commercial production of these polymers started in 1960 in the United States. Eirestone and Goodyear account for more than 60% of the current production capacity (see Elastomers, synthetic-polybutadiene). 

In the mid-1970s there was a short period during which styrene was in very short supply. This led to the development of what were known as high-vinyl polybutadienes which contained pendent vinyl groups as a result of 1,2-polymer-isation mechanisms. These rubbers had properties similar to those of SBR and could replace the latter should it become economically desirable. 

A more complicated behaviour was obtained with divinyl ether due to the formation of both cyclic structures and pendent vinyl groups in the chain. The failure of such olefins as styrene and isopropenylbenzene to give copolymers with 2-fural-dehyde, and in fact to homopolymerize in its presence, was blamed on the strength of the complex formed between the initiator and the aldehyde, believed too stable to initiate polymerization. 

Ruthenium vinylidene species can be transformed into small carbocyclic rings via carbocyclization reactions. Ruthenium vinylidene complex 2, generated from the electrophilic reaction of alkyne complex 1 with haloalkanes, was deprotonated with "BU4NOH to give the unprecedented neutral cyclopropenyl complex 3 (Scheme 6.2) [5]. Gimeno and Bassetti prepared ruthenium vinylidene species 4a and 4b bearing a pendent vinyl group when these complexes were heated in chloroform for a brief period, cyclobutylidene products 5a and Sb formed via a [2 + 2] cycloaddition between the vinylidene Ca=Cp bond and olefin (Scheme 6.3) [6]. 

The macroradicals of natural rubber react with those of the styrene elastomer, due to the presence of the very reactive 1,2 pendent vinyl groups in the latter. This mechanism leads to a structure where the styrene rubber forms a gel network with grafted branches of natural rubber. 

Another example of rubber analysis is the determination of vinyl-butadiene in SBR. A PA-FTIR spectrum of a high-vinyl content SBR is shown in Figure 2.10. This determination by mid-IR methods invariably involves the use of the 910 cm"1 band due to the wagging motion of the pendent vinyl group of 1,2-butadiene. 

Network structure and reaction mechanisms in high pressure vulcanisation (HPV) and peroxide vulcanisation of BR was studied by 13C solid-state NMR [43]. Different samples of polybutadiene (51% trans, 38% cis, and 11 % vinyl) were peroxide cured with dicumyl peroxide on a silica carrier and by the HPV conditions of 250 °C and 293 MPa. The 13C NMR spectra from peroxide and HPV cures were compared to a control samples heated to 250 °C for 6 minutes under atmospheric pressure. Although no new isolated strong peaks were detected in either the peroxide or HPV vulcanisations, small increases in both spectra were observed at 29.5, 36.0, 46.5, and 48.0 ppm. These peaks compare favourably with calculated shifts from structures that arise from main chain radical addition to the pendent vinyl groups. These assignments are further reinforced by the observation that the vinyl carbon concentration is substantially reduced during vulcanisation in both peroxide and HPV curing. Two peaks at 39.5 and 42.5 ppm appear only in the peroxide spectrum. Cis-trans isomerisation was absent in both cures. 

Coupling Reaction 5b involves reacting living A-B polymer chains with divinylbenzene (DVB). The polymerization of DVB then results in a hub containing pendent vinyl groups that serve as branch points for the star-shaped polymer. This procedure has led to the... 

Low-molecular-weight PPE, functionalized with methacrylic groups may serve as a macromonomer with pendent vinyl groups. The PPE chains may have two end caps to provide two reaction sites for forming crosslinks. Mixtures of mono capped PPE and PPE chains bearing two or more end caps are also useful. PPE with such end caps can be copol5mierized with styrene and acrylonitrile monomers. Suspension polymerization has been demonstrated to be a suitable technique of polymerization. 

Figure 15.8 PI Precursors with Pendent Vinyl Groups ...

Evans JW, Ansari AM, Ziller WJ. Synthesis of zirconium aryloxide complexes eontain-ing pendent vinyl groups. Inorg Chem 1999 38(6) 1160-4. 

Both CIS- and trans-polybutadienes present a somewhat different picture since efficiencies much greater than unity have been observed. This high efficienc tas been found to increase with an increase in the vinyl (1,2-) content. J or example it has been found (Kraus, 1963) that whereas a poly butadiene with a 10% vinyl content had a cross-linking efficiency of about 2, a 98% 1,2- polymer had a value in excess of 100 It is reasonable to presume that this high efficiency is due to a polymerization process initiated by reaction (A) but it is to be noted that there is much evidence to show that this polymerization cross-linking occurs via main chain double bonds as well as on the pendent vinyl groups. As with accelerated sulphur vulcanization there are important, but not well understood differences between polybutadiene and polyisoprene. 

The peroxide vulcanization appears to proceed via these pendent vinyl groups and dominates the degradation reaction. 

Pendent vinyl groups in styrene-divinylbenzene copolymers have been analysed by IR and Raman spectroscopy [73-84] and by wet chemical methods [83, 85] after extents of reaction varying from before gelation to after nearly complete conversion of monomers. Periysamy and Ford [86] report a new analytical approach to the problem. Copolymers of styrene with methane- C-labelled p-divinylbenzene were analysed by liquid-state and solid-state cross-polarisation magic angle spinning (CP-MAS) i C-NMR methods. 

Table 9.6 Fraction of divinylbenzene repeat units with a pendent vinyl group ...

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