Cychc olefin polymer

The olefins that undergo metathesis include most simple and substituted olefins cycHc olefins give linear high molecular-weight polymers. The mechanism of the reaction is beheved to involve formation of carbene complexes that react via cycHc intermediates, ie, metaHacycles. Industrial olefin metathesis processes are carried out with soHd catalysts (30). 

Due to restrictions imposed by the geometry of a simple cubic lattice and due to the inability [ 18,20] of the current ECT to treat polymer systems whose monomers contain closed rings, the cycHc olefin norbornene monomer is modeled by an approximate compact structure (Fig. 10) that is composed of seven united atom groups (5a = 7). The ethylene monomer is represented by a two-bead dimer with two CH2 groups, i.e., 5b = 2. Thus, this simple model incorporates the actual size disparity between these two types of monomers, a feature that influences the magnitude of the entropic component /s of the interaction parameter xsans> and it also affects the dependence of the energetic portion of xsans on the compositions x and y of the two blend components. 

Most Kaminsky catalysts contain only one type of active center. They produce ethylene—a-olefin copolymers with uniform compositional distributions and quite narrow MWDs which, at their limit, can be characterized by M.Jratios of about 2.0 and MFR of about 15. These features of the catalysts determine their first appHcations in the specialty resin area, to be used in the synthesis of either uniformly branched VLDPE resins or completely amorphous PE plastomers. Kaminsky catalysts have been gradually replacing Ziegler catalysts in the manufacture of certain commodity LLDPE products. They also faciUtate the copolymerization of ethylene with cycHc dienes such as cyclopentene and norhornene (33,34). These copolymers are compositionaHy uniform and can be used as LLDPE resins with special properties. Ethylene—norhornene copolymers are resistant to chemicals and heat, have high glass transitions, and very high transparency which makes them suitable for polymer optical fibers (34). 

Monomers 24 and 25 behave differently when exposed to catalyst 14, shown in Fig. 8.15. Divinyltetramethyldisiloxane 24 is found to be metathesis inactive due to similar steric inhibitions experienced with divinyldimethylsilane. Monomer 25 is synthesized with one additional methylene spacer unit between the silicon atom and the olefin moiety, which then is reacted with Schrock s [Mo] catalyst. Here, metathesis occurs quite readily, exclusively forming a seven-membered cychc molecule (26) instead of polymer. The formation of the cyclic product can be explained by tire Tliorpe-Ingold effect.15... 

AcycHc diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkyhdene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cycHc compounds, while the intermolecular reaction, called acycHc diene metathesis (ADM FT) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

Near-quantitative conversion of monomer to polymer is standard in these polymerizations, as few side reactions occur other than a small amount of cychc formation common in all polycondensation chemistry [41]. ADMET depolymerization also occurs when unsaturated olefins are exposed to pressures of ethylene gas [42,43]. In this case, the equilibriiun nature of metathesis is shifted towards low molecular weight products under saturation with ethylene. Due to the high catalytic activity of [Ru] and the abihty of [Mo] and [Ru] to create exact structures, ADMET has proven a valuable tool for production of novel polymer structures for material applications as well as model copolymer systems to help elucidate fundamental structure property relationships [5]. 

As with all polycondensation reactions, the formation of cychc ohgomers by ADMET is possible and has been demonstrated in a variety of cases [31-33]. This occurs by intramolecular back-biting metathesis of an active metal carbene with an internal olefin of the polymer (Scheme 6.5) to hberate cyclooctadiene, for example, from ADMET polybutadiene, although larger cychcs have also been observed. A related undesired cyclization is the intramolecular cyclization of the monomer by RCM. 

Ring-opening metathesis polymerization (Problem 26.29) An olefin metathesis reaction that forms a high molecular weight polymer from certain cychc aUtenes. 

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