Metathesis polycondensation reaction

Acyclic diene metathesis (ADMET) is a step-growth polycondensation reaction for the polymerization of o -dienes 729 The process is catalyzed by the same metal alkylidene initiators used for ROMP, and is driven by the removal of ethylene from the system (Scheme 13). Both molybdenum and ruthenium-based initiators have been used to prepare a variety of materials including functionalized polyethy-... 

Metathetical polycondensation of acyclic dienes has not been successful with conventional catalysts used for the ring-opening metathesis polymerisation of cycloolefins, which is due to the fact that Lewis acids are usually present, and produce deleterious side reactions [13,16,17]. Only Lewis acid-free, well-defined catalysts have been successfully applied for acyclic diene metathesis polycondensation the key success has been to choose catalysts that obviate other pathways not involving the metathesis mechanism [18-20]. It was Wagener et al. [16,21] who first were able to convert an acyclic a, co-diene (1,9-decadiene), by using an acid-free metal alkylidene catalyst, to a high molecular weight... 

Lewis acid-free catalysts for acyclic diene metathesis obviate the formation of carbocations, which in turn completely eliminates competing reactions, mostly involving cationic oligomerisation via 1-alkene bonds. Thus, metathesis polycondensation occurs quantitatively to yield high molecular weight poly(l-alke-nylenejs with vinyl end groups and ethylene as a byproduct. 

The ruthenium-based catalyst provided by Grubbs et al. [19] also promotes acyclic diene metathesis polycondensation, although with higher concentrations being required to achieve reasonable reaction rates [24,25] ... 

The participation of metallacyclobutane rings in metathesis polycondensation is the reason why the aforementioned sterically encumbered acyclic diene monomers hardly undergo reaction [7-9] in such cases, steric influences at particular positions in the monomer hinder the formation of the necessary metallacyclobutane ring, thus inhibiting polymer formation [1],... 

Telechelic dienes for metathesis polycondensation, containing functional groups such as those in alcohols, esters, carboxylic acids and imides, can be obtained via acyclic diene metathesis depolymerisation [64,65]. They can then be used in further reactions to create hydrophobic polyurethanes and other special-purpose polymers [1]. 

To prove the point, model studies were done to demonstrate that crosslinking via cationic initiation was indeed the reaction that competed with metathesis polycondensation. Substituted styrenes were used to test the hypothesis of cation formation, since styrene is a well-known catioiuc polymerizable alkene that gives an easily characterized product. When varions styrenes were treated with classic Lewis acidic catalyst systems like WCl6/EtAlCl2, only polystyrene was produced, whereas metathesis products (substituted stilbenes) were not observed (Scheme 4)." ... 

The mechanism snfficiently explains how prodnctive condensation metathesis occms since terminal alkenes are contained in every step of the productive cycle (Scheme 5). In this kinetically controlled regime (high concentration of terminal alkene present), alkylidene reactions with unsubstituted (terminal) alkenes are favored over interchain redistribution reactions or other degenerate eqnilibria. However, when terminal alkene concentration decreases near the end of the polycondensation reaction, competing... 

Given these rigorous demands for the success of a polycondensation reaction, there are several reasons why olefin metathesis is particularly well suited for polycondensation. The first is that although olefin impurities are detrimental to ADMET, other more common synthetic impurities are not, unless they react with the catalyst. Secondly, like most polycondensations, ADMET is a reversible process and removal of the condensate will shift the equilibrium towards higher molecular weight polymer. The condensate molecule for ADMET, ethylene, is a gas at room temperature and is thus readily removed from the reaction mixture to drive the reaction to high conversion (Scheme 6.4). 

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. 

Let us emphasise that the driving force for acyclic diene metathesis, which is a step-growth condensation polymerisation, is the release and removal of a small condensate molecule. The polycondensation is performed preferably under bulk conditions (no solvent used), since acyclic diene metathesis is thermally neutral and there is no need to remove the heat of the reaction, in contrast to exothermic cyclic olefin ring-opening metathesis polymerisation. 

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]. 

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