The Diene Polymers

Although conjugated diene monomers are capable of exhibiting a functionality of four in polymerization reactions, the important examples are linear or branched polymers with residual unsaturation (Appendix 16.E). Natural rubber (cis-l,4-polyisoprene) 

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Early work on the microstructurc of the diene polymers has been reviewed.1 While polymerizations of a large number of 2-substituted and 2,3-disubstituted dienes have been reported,88 little is known about the microstructure of diene polymers other than PB,89 polyisoprene,90 and polychloroprene.91... 

Mechanisms depending on carbanionic propagating centers for these polymerizations are indicated by various pieces of evidence (1) the nature of the catalysts which are effective, (2) the intense colors that often develop during polymerization, (3) the prompt cessation of sodium-catalyzed polymerization upon the introduction of carbon dioxide and the failure of -butylcatechol to cause inhibition, (4) the conversion of triphenylmethane to triphenylmethylsodium in the zone of polymerization of isoprene under the influence of metallic sodium, (5) the structures of the diene polymers obtained (see Chap. VI), which differ. both from the radical and the cationic polymers, and (6)... 

Studies with model alkenes indicate that the action of accelerators is to increase the extent of sulfur substitution (crosslinking) at the allylic positions of the diene polymer [Skinner,... 

Termination of these polymerizations with dichlorodimethylsilane followed by hydrolysis of the protecting group generated polymeric diamines with functionalities of 1.7-1.9 and relatively broad MW distributions (1.49-2.22). The authors considered that the titrimetric method was less reliable for the higher molecular weight polymers and perhaps is a reason for the apparent ineficiency of this amination procedure. A major limitation of this method is the fact that the initiator is insoluble in hydrocarbon solvents and therefore most of the diene polymerizations were carried out in mixtures of hexane and ether which has a deleterious effect on the microstructure of the diene polymers. 

Polymerization of Dienes Using Organo-alkalis. The diene polymer structures that have so far been discussed were all those of polymers prepared in bulk using alkali metals. 

If instead of the alkali metals one uses organo-alkali metal compounds, the microstructure of the diene polymers remains very much the same if the polymerization is carried out... 

Conjugated olefins, like styrene, butadiene, and isoprene, can be caused to polymerize by cationic and anionic as well as by free-radical processes because the active site is delocalized in all cases. The most practical ionic polymerizations for these species are anionic, because such reactions involve fewer side reactions and better control of the diene polymer microstructure than in cationic systems. Free-radical polymerization of styrene is preferred over ionie proeesses, however, for cost reasons. 

Alkali and acid treatments have also been used to modify surface properties of polymers sulfonated polyethylene films treated first with ethylenediamine and then with a terpolymer of vinyhdene chloride, acrylonitrile, and acrylic acid exhibited better clarity and scuff resistance and reduced permeabihty. Permanently amber-colored polyethylene containers suitable for storing light-sensitive compoimds have been produced by treating fluorosulfonated polyethylene with alkali. Poly(ethylene terephthalate) dipped into trichloroacetic/chromic acid mixture has improved adhesion to polyethylene and nylons. Antifogging lenses have been prepared by exposing polystyrene films to sulfonating conditions. Acid and alkali surface treatments have also been used to produce desired properties in polymethylmethacrylates, polyacrylonitrile, styrene-butadiene resins, polyisobutylene, and natural rubber. Surface halogenation of the diene polymers natural rubber and polyisobutylene resulted in increased adhesion to polar surfaces. 

Recently, Ballard and co-workers discovered a novel path to prepare linear polyphenylene, based on the polymerization of derivatives of 5,6-dihydroxycyclohexa-1,3-diene. The advantage of this method is that the diene polymer remains soluble in a variety of solvents, because of the presence of solubilizing groups, such as ester functions. Because of this, molecular weight determinations can be made before the final pyrolysis to remove the attached groups. Ballard et al. reported that the final product is a high molecular weight poly(p-phenylene). 

Cyclized natural rubber has been of some use in providing stiffness to diene rubber vulcanizates without materially increasing the density. For this purpose it is in some competition with the high styrene resins. Furthermore there are today a number of elastomers available, alternative to the diene polymers blended with cyclized rubber or high styrene resins, such as certain block copolymers and polyurethanes which possess this property without the need to add such stiffening fillers. 

Interest in the reaction of diene polymers and maleic anhydride was no doubt first prompted by the Diels-Alder reactions occurring between simple conjugated dienes and activated unsaturated compounds, such as maleic anhydride, even though the polyunsaturation in the diene polymer was not conjugated. 

In the first case (reaction (A)) this could form the initiation stage of a polymerization reaction through the double bonds of the diene polymer. In turn this reaction could involve those other reaction mechanisms associated with double bond polymerization such as chain transfer to solvent, to monomer and to polymer chain termination and inhibition by free radicals including those from antioxidants. 

More recently considerable interest has been shown in polymers obtained by ring-opening of cyclic olefins to give products analogous to the diene polymers obtained by double-bond polymerization. 

The limited amount of cure sites and reactivity in EPDM requires higher levels of curatives than do the diene polymers ENB grades require 0.5-2 phr sulfur or equivalent amounts of sulfur donors (e.g., DTDM). They can be cured by sulfur and thiazoles/sulfena-mides, but with these systems the cure rate may be unsatisfactory (except for fast curing grades). 

In order to discriminate between the vinyl and the diene polymer nature, a combined approach was developed as crucial feature of the study involving poly-BFl. 

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