Acyclic diene metathesis polymerisation

The first successful ADMET polymerisation was reported by Wagener and colleagues [28]. They polymerised 1,5-hexadiene and 1,9-decadiene to 1,4-polybutadiene [with a weight average molecular weight (Mw) of 28 kDa] and polyoctenylene (Mw = 108 kDa), respectively, using a tungsten-based catalyst that required extremely dry conditions to avoid side reactions. Recent advances in the development of very active and stable catalysts now allow the synthesis of various polymer architectures with relative ease. 

As stated by Cramail, [18] a disadvantage of such metathesis reactions is the possibility of side reactions (e.g., isomerisation of the unsaturation) to create a flaw in the polymer structure and thus decrease its thermo-mechanical properties. This 

Most of the studies in the last decade have concentrated on the ADMET polymerisation of fatty acids and its derivatives to obtain polyesters as the final materials. In 2008, Meier and Rybak [39] described the synthesis of an a,(0-diene by transesterification of 10-undecenoic acid with the corresponding alcohol (10-undecenol). ADMET polymerisation of this monomer was conducted to achieve a C20 unsaturated polyester. 

Cramail and co-workers [40] described the synthesis of two linear and branched a,(0-diene monomers from castor and vernonia oils to develop linear low-density polyethylene and very low-density polyethylene-like polyesters. Methyl 10-undecenoate and 12-hydroxystearic acid, obtained from ricinoleic acid, were employed for the synthesis of the branched a,ro-diene, and a second monomer of similar structure (but linear) was synthesised from methyl 10-undecenoate and 1,12-dodecanediol (obtained from vernolic acid). Further ADMET copolymerisation of the dienes led to a series of copolyesters with Mn values of 14-62 kDa, Tm of 13.2-87.4 °C, and with various degrees of branching depending on the feed ratio between the two a,(0-dienes. 

Another interesting work on the ADMET polymerisation of a castor-oil-based diene, l,3-di-10-undecenoxy-2-propanol, was conducted by the research team of Cadiz [41]. Authors also utilised 10-undecenol as a renewable comonomer to functionalise polymer chains and react them with 4,4 -methylene diphenyl diisocyanate (MDI) to yield a series of poljmrethanes (PU). 

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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. 

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... 

Figure 8.1 Schematic presentation of the mechanism for acyclic diene metathesis condensation polymerisation...

Catalyst and monomer development in acyclic diene metathesis remains a subject of interest, the goal being to obtain macromolecules with well-defined backbone structures and architectures by easily accessible and less expensive means. By the application of an appropriate design of monomers and a careful choice of catalysts, a variety of non-functionalised and functionalised dienes have been polymerised via metathesis condensation to high molecular weight polymers. 

In the first example, the research team of Meier described a ronte to the development of a,(0-dienes with ester and anhydride linkages (Scheme 6.11). These were polymerised further via acyclic diene metathesis (ADMET) polymerisation and thiol-ene click chemistry to compare the two processes for polyester/polyanhydride syntheses [33]. 

Acyclic a, co-diene metathesis is a method for the synthesis of poly(l-alkeny-lene)s, which are also available from the ring-opening metathesis polymerisation of cycloolefins [13-15]. 

Use of a symmetrical acyclic alkene limits the possible metathesis products to the desired diene (for example 45) and products formed from polymerisation of the cyclic substrate. Competing ROMP was suppressed in these reactions by using dilute conditions and a tenfold excess of hex-3-ene. By adding the cyclic substrate slowly to a solution of the catalyst and ris-hex-3-ene (which was significantly more reactive than the trans isomer), less than two equivalents of the acyclic alkene were used without causing a significant drop in the cross-metathesis yield. 

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