Diene polymerization cross-linking

NR, styrene-butadiene mbber (SBR), polybutadiene rubber, nitrile mbber, acrylic copolymer, ethylene-vinyl acetate (EVA) copolymer, and A-B-A type block copolymer with conjugated dienes have been used to prepare pressure-sensitive adhesives by EB radiation [116-126]. It is not necessary to heat up the sample to join the elastomeric joints. This has only been possible due to cross-linking procedure by EB irradiation [127]. Polyfunctional acrylates, tackifier resin, and other additives have also been used to improve adhesive properties. Sasaki et al. [128] have studied the EB radiation-curable pressure-sensitive adhesives from dimer acid-based polyester urethane diacrylate with various methacrylate monomers. Acrylamide has been polymerized in the intercalation space of montmorillonite using an EB. The polymerization condition has been studied using a statistical method. The product shows a good water adsorption and retention capacity [129]. 

Ethene/propene/diene monomer rubbers (EPDM) are elastomeric terpoly-mers used in the production of sealants, tubing and gaskets and, in the USA, is used in roofing applications. As the name suggests they are prepared by the polymerization of mixtures of ethene, propene and diene monomers, to form cross-links. By far the most common diene used is 5-ethylidene-2-norbomene (ENB). 

Nonlinear structures may arise in vinyl polymerizations through chain transfer with monomer or with previously formed polymer molecules, but such processes usually occur to an extent which is scarcely significant. A more common source of nonlinearity in the polymerization of a 1,3-diene is the incorporation in a growing chain of one of the units of a previously formed polymer molecule. The importance of both branching by chain transfer and cross-linking by addition of a polymer unit increases with the degree of conversion of monomer to polymer. 

The commonly observed, and technically troublesome, formation of gel in the polymerization of dienes testifies to the occurrence of cross-linking processes during their polymerization. According to Eq. (13), gel should begin to form when p reaches the critical value pc = l/y , where is the weight average degree of polymerization of the primary molecules. Thus... 

I.4.2.I. Synthesis and Modification of Polymers Unstable bis(nitrile oxide), generated by dichloroglyoxime dehydrochlorination, polymerizes in solution to give poly(furoxan) or (in the presence of 1,3-dienes) gives rise to their being cross-linked (500). Polymerization of terephthalonitrile dioxide and its... 

Conjugated dienes such as 1,3-butadiene very readily polymerize free radically. The important thing to remember here is that there are double bonds still present in the polymer. This is especially important in the case of elastomers (synthetic rubbers) because some cross-linking with disulfide bridges (vulcanization) can occur in the finished polymer at the allylic sites still present to provide elastic properties to the overall polymers. Vulcanization will be discussed in detail in Chapter 18, Section 3. The mechanism shown in Fig. 14.3 demonstrates only the 1,4-addition of butadiene for simplicity. 1,2-Addition also occurs, and the double bonds may be cis or trans in their stereochemistry. Only with the metal complex... 

Gheneim R, Perez-Beramen C, Gandini A. Diels-Alder reactions with novel polymeric dienes and dienophiles synthesis of reversibly cross-linked elastomers. Macromolecules 2002 35 7246-7253. 

The random copolymer of propylene and ethylene (EP) lacks the good symmetry of it PP and is a flexible elastomer. Since this copolymer is used as an elastomer, it is customary to add a small amount of a diene, such as ethylidene norbomene, to the monomer reactants before polymerization to allow subsequent cross-linking or curing. 

Kuchanov and Pismen (94) have also applied their method to batch polymerization of a diene in which cross-linking by co-polymerization of double bonds in the polymer occurs as would be expected, the second moment diverges more rapidly than when this mode of reaction is absent. 

Benzofurans and dihydrobenzofurans have been prepared on polymeric supports by the palladium-mediated reaction of 2-iodophenols with dienes or alkynes (Entries 1 and 2, Table 15.9). This reaction is closely related to the synthesis of indoles from 2-iodoanilines, and probably proceeds via an intermediate palladacycle (Figure 15.3). Benzofuran and isobenzofuran derivatives have also been prepared on cross-linked polystyrene by intramolecular addition of aryl radicals to C=C double bonds and by intramolecular Heck reaction. 

RESINS (Acrylonitrile-Butadiene-Styrene). Commonly referred to as ABS resins, these materials are thermoplastic resins which are produced by grafting styrene and acrylonitrile onto a diene-rubber backbone. The usually preferred substrate is polybutadiene because of its low glass-transition temperature (approximately —80°C). Where ABS resin is prepared by suspension or mass polymerization methods, stereospedfic diene rubber made by solution polymerization is the preferred diene. Otherwise, the diene used is a high-gel or cross-linked latex made by a hot emulsion process. 

The rationale in using these particular dienes is that only the strained double bond of dicyclopentadiene and the terminal double bond of 1,4-hexadiene undergo polymerization with Ziegler catalysts. Consequently the polymer chains contain one double bond for each molecule of dicyclopentadiene or 1,4-hexadiene that is incorporated. These double bonds later can be converted to cross-links by vulcanization with sulfur (Sections 13-4 and 29-3). 

The ADMET polymerization of sugar-based monomers is much less explored than the ROMP approach, and only a few examples have been reported to date. Bui and Hudlicky prepared a,oo-dienes derived from a biocatalytically synthesized diene diol, from which chiral polymers (up to 20 kDa) with D-c/uro-inositol units were prepared via ADMET in the presence of 1 mol% of C4 [169]. Furthermore, several ot,co-dienes containing D-mannitol, D-ribose, D-isomannide, and D-isosorbide have been synthesized by Enholm and Mondal [170]. Also in this study, C4 was used to catalyze the ADMET polymerizations at 1 mol% catalyst loading. As pointed out by the authors, the viscosity increased as the reactions progressed and vacuum had to be applied to efficiently remove the released ethylene. Unfortunately, the polymers obtained were not further analyzed. As already mentioned above, Fokou and Meier have also reported the ADMET polymerization of a fatty acid-/D-isosorbide-based a,co-diene [126]. Furthermore, Krausz et al. have synthesized plastic films with good mechanical properties by cross-linking fatty esters of cellulose in the presence of C3 [171-173]. 

Scheme 70 Polymer-bonded lanthanide-based diene polymerization catalysts. PS = DVB-cross-linked polystyrene (2% divinylbenzene), PE = polyethylene (adopted from [293])...

Butadiene could also be trimerized to give cyclododecatriene. The trimer is again used by Hulls to manufacture nylon 12 and Vestamid . The codimerization of butadiene and ethylene is used by DuPont to manufacture 1,4-hexadiene, one of the monomers of EPDM (ethylene, propylene, diene, monomers) rubber. The role of the diene monomer in EPDM rubber is to provide with two double bonds of different reactivities. The more reactive, terminal double bond takes part in the polymerization with ethylene and propylene. The less reactive internal one is used later on for cross-linking. These important catalytic reactions are shown in Fig. 7.6. 

Cross-linking occurs early or late in the copolymerization depending on the relative reactivities of the two double bonds of the divinyl compound or diene. By the proper choice of a monomer with two double bonds it is possible to reduce the reactivity of one double bond just enough so that it will not enter polymerization under the same conditions as the other, but can be made to react under more drastic conditions. This leads to postpolymerization cross-linking reactions of which vulcanization reactions are an example. 

Chain-growth polymerization exhibits a preference for head-to-tail addition. Branching affects the physical properties of the polymer because linear unbranched chains can pack together more closely than branched chains can. The substituents are on the same side of the carbon chain in an isotactic polymer, alternate on both sides of the chain in a syndiotactic polymer, and are randomly oriented in an atactic polymer. The structure of a polymer can be controlled with Ziegler-Natta catalysts. Natural rubber is a polymer of 2-methyl-l,3-butadiene. Synthetic rubbers have been made by polymerizing dienes other than isoprene. Heating mbber with sulfur to cross-link the chains is called vulcanization. 

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