1.3- Diynes, metathesis polymerization

The acyclic diyne metathesis polymerization of dodeca-2,10-diyne is catalysed by W(=CEt)(OCMe3)3 see equation 73. But-2-yne is eliminated and the product is an off-white insoluble powder with the same Tm as the polymer prepared from cyclooctyne (Section XII.B)7. 

An alternative method to make PAEs is the acyclic diyne metathesis (ADIMET) shown in Scheme 2. It is the reaction of a dipropynylarene with Mo(CO)6 and 4-chlorophenol or a similarly acidic phenol. The reaction is performed at elevated temperatures (130-150 °C) and works well for almost any hydrocarbon monomer. The reaction mixture probably forms a Schrock-type molybdenum carbyne intermediate as the active catalyst. Table 5 shows PAEs that have been prepared utilizing ADIMET with these in situ catalysts . Functional groups (with the exception of double bonds) are not well tolerated, but dialkyl PPEs are obtained with a high degree of polymerization. The progress in this field has been documented in several reviews (Table 1, entries 2-4). Recently, a second generation of ADIMET catalyst has been developed that allows... 

The metathesis polymerization of diynes having four single bonds between the triple bonds (dipropargyl compounds) yields cyclopolymers. The structural units may contain a cyclohexene ring (equation 63) or a cyclopentene ring (equation 64) with the possibility... 

Alkyne metathesis is generally restricted to internal alkynes, and is driven to completion by the evolution of gaseous 2-butyne. Representative examples of alkyne cross-metathesis are depicted in Scheme 33, and include (i) dimerization of alkynylbenzoic acid derivative 287, (ii) alkyne cross-metathesis of 289 and bis(trimethylsilyl)acetylene, (iii) cross-metathesis of enyne 291 and various alkynes and metathesis dimerization of 291, and (iv) formation of high molecular weight alkyne-containing polymers through acyclic diyne metathesis (ADIMET) polymerization of acyclic diynes (e.g., 293) " using the molybdenum hexacarbonyl/2-chlorophe-nol system. 

Scheme 33 Representative examples of alkyne cross-metathesis and acyclic diyne metathesis (ADIMET) polymerization.

Zhang, X.-P. and Bazan, G.C. (1994) Regiospecific head-to-tail ring-opening acetylene metathesis polymerization of tetrasilacycloocta-3,7-diynes. Macromolecules, 27, 4627-4628. 

Figure 10-10 Polymerization of 1,6-diynes using a molybdenum alkylidene catalyst [Rp is (CFjljCHjC] [67]. The 1,6-diyne monomer is drawn in two different exaggerated conformations to illustrate that head-tail polymerization leads to six-membered rings, and tail-tail polymerization leads to five-membered rings. See Fig. 10-8 for a more mechanistic diagram of acetylene metathesis.

The diynes HC=C(CH2) C=CH ( = 2, 4, 6, 8) have also been polymerized. For n = 2, using NbCls as catalyst, the product is mainly trimer, but for n = 6 or 8, a polymer with a highly branched structure appears to be formed (Srinivasan 1988). For = 4, it is surprising to find that Mo(CO)6/HOC6H4-Cl-3 in toluene at 120°C causes polymerization rather than metathesis (cf. Section 10.2.1) the product is thought to be a cyclopolymer containing enchained cycloheptene rings (Vosloo 1991). 

The PPEs [14, 35, 36], which are dehydrogenated analogs of the poly(p-phenylene vinylene) s, display desirable optoelectronic properties that have found extensive use as emissive materials [37, 38]. PPE preparation is dominated by the Sonogashira Pd/Cu-catalyzed cross-coupling of aryl haUdes and terminal alkynes [39]. However, polymerizations under these conditions rarely achieve a high molecular weight, and defects such as diyne formation and alkyne crosshnking in the polymer backbone are endemic [14, 40]. Alkyne metathesis was envisioned as a complementary method to PPE synthesis, with much success. 

Well-defined Ru carbene catalysts, which are well known as very active catalysts for olefin metathesis reactions, have been elucidated to polymerize un- and substituted acetylenes such as a,co-diynes, propiolic acid esters, and DPAs. Grubbs 1st, 2nd-, and 3rd- (23-25, respectively. Figure 8) generation catalysts... 

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