Acetylenic monomers

The polymerization of acetylene (alkyne) monomers has received attention in terms of the potential for producing conjugated polymers with electrical conductivity. Simple alkynes such as phenylacetylene do undergo radical polymerization but the molecular weights are low (X 25) [Amdur et al., 1978]. Ionic and coordination polymerizations of alkynes result in high-molecular-weight polymers (Secs. 5-7d and 8-6c). 

Patrick, Kinetics and Mechanisms of Polymerization Reactions, Chaps. 2-4, 7, Wiley, New York, 1974. 

Ullmann s Encyclopedia of Industrial Chemistry, Wiley, New York, 2002 (online version), 1996 (hardcopy). 

Bamford, C. H., Radical Polymerization, pp. 708-867 in Encyclopedia of Polymer Science and Engineering, Vol. 13, H. F. Mark, N. M. Bikales, C. G. Oveiberger, and G. Menges, eds., Wiley-Interscience, New York, 1988. 

Baumann, M. and G. Schmidt-Naake, Macromol. Chem. Phys., 202, 2727 (2001). 

Berlin, D., M. Destarac, and B. Boutevin, Controlled Radical Polymerization with Nitroxyl Stable Radicals, pp. 47-78 in Polymers and Surfaces. A Versatile Combination, H. Hommel, ed.. Research Signpost, Trivandrum, India, 1998. 

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Despite the name, the material is consistently represented as (CH) in most places, a formula that should definitely be replaced by (C2H2) not only to reflect correctly the name and genesis from acetylene monomer but also to facilitate comparison with... 

Rh complexes are examples of the most effective catalysts for the polymerization of monosubstituted acetylenes, whose mechanism is proposed as insertion type. Since Rh catalysts and their active species for polymerization have tolerance toward polar functional groups, they can widely be applied to the polymerization of both non-polar and polar monomers such as phenylacetylenes, propiolic acid esters, A-propargyl amides, and other acetylenic compounds involving amino, hydroxy, azo, radical groups (see Table 3). It should be noted that, in the case of phenylacetylene as monomer, Rh catalysts generally achieve quantitative yield of the polymer and almost perfect stereoregularity of the polymer main chain (m-transoidal). Some of Rh catalysts can achieve living polymerization of certain acetylenic monomers. The only one defect of Rh catalysts is that they are usually inapplicable to the polymerization of disubstituted acetylenes. Only one exception has been reported which is described below. 

Group 10 transition metal catalysts including Ni and Pd are known as a new class of catalysts for the polymerization of substituted acetylenes, but the reports treating these catalysts are still not many. Some of the reports in an early stage displayed that the group 10 catalysts rather induce cyclic and linear oligomerizations of acetylene monomers. Thus, only fragmental information is available in some of the papers. 

A number of Mo carbene catalysts, bearing various modified ligands, have been reported and proven to elegantly induce living polymerization of acetylene monomers. The first example is the cyclopolymerization of 1,6-heptadiynes catalyzed by Mo carbenes Mo carbenes ligated by bulky imido and alkoxy groups are quite effective. In... 

The DSC of primary acetylenic monomer 68 showed the presence of two distinct exotherm peaks. The first peak of 202 °C was attributed to the acetylene reacting separately with a second acetylene while the exothermic peak of 270 °C was proposed to be a benzocyclobutene reacting with a second benzocyclobutene. The resulting polymer was believed to be a highly crosslinked network and to some extent the Tg of 380 °C would support this contention. Interestingly, this homopolymer had poor thermal stability at 343 °C in air, with a retention of 41 % of its weight after 200 h. 

Replacing the hydrogen in 68 with a phenyl group leads to the secondary acetylenic monomer 70. It was believed that this disubstituted acetylene would suppress the reaction of acetylene with itself and insure that there was an acetylene functionality available for reaction with the o-quinodimethane at 200 °G The DSC of 68 showed the presence of a single exothermic peak at 263 °C which the authors felt was adequate evidence for the occurrence of a Diels-Alder reaction between the acetylene and benzocyclobutene. Unfortunately they did not report on any control experiments such as that between diphenylacetylene and simple benzocyclobutene hydrocarbon or a monofunctional benzocyclobutene in order to isolate the low molecular weight cycloaddition product for subsequent characterization. The resulting homopolymer of 68 had a Tg of 274 °C and also had the best thermooxidative stability of all of the acetylenic benzocyclobutenes studied (84% weight retention after 200 h at 343 °C in air). 

Fig. 20 Synthesis of cyclic polyethylene from acetylenic monomer...

Note that the transformation of metal carbene vice versa, during polymerisation occurs as in the case of cycloalk-ene ring-opening metathesis polymerisation. Considering the mechanism of metathesis polymerisation of acetylenic monomers, it is worth noting that catalysts containing a transition metal carbyne bond (Mt=C) can induce polymerisation only when this bond is transformed into the respective metal carbene bond (Mt=C) [39],... 

Acetylenic monomers also appeared to undergo polymerisation with conventional olefin metathesis catalysts. This relates to monosubstituted highly branched alkylacetylenes and arylacetylenes as well as disubstituted acetylenes (internal alkynes) [16-18], It has been demonstrated that acetylene itself may also be polymerised using olefin metathesis catalysts [19,20]. The polymerisation of alkynes [scheme (2)] involves a metathesis reaction [scheme (5) of Chapter 2] analogously to that of cycloolefins [21] ... 

The mechanism of polymerisation of alkynes with metathesis catalysts requires that the original triple bond of the acetylenic monomer becomes a single bond in the polymer [scheme (5) of Chapter 2], in contrast to the insertion mechanism of acetylene polymerisation with Ziegler-Natta catalysts, where the triple bond becomes a double bond [scheme (1)]. Ideas about the mechanism of metathesis polymerisation of cycloolefins suggested that isolable metal carbenes might promote the polymerisation of cycloolefins suggested that isolable metal carbenes might promote the polymerisation of alkynes, as indeed turned out to be true, as several metal carbenes were found [22-24] to cause alkyne polymerisation. 

Interesting features in the polymerisation of acetylenic monomers are displayed by rhodium-based catalysts they may be applied in metal akyl or hydride-activated systems, e.g. RhCl3-LiBH4 [49] and [(Cod)Rh]1 [[BPh4]... 

It is worth noting that suitable olefins added to the polymerisation system can act as chain transfer agents during the metathesis polymerisation of acetylenic monomers for instance, trimethylvinylsilane has been found [84] to be an effective chain transfer agent in the polymerisation of phenylacetylene in the presence of the WC16—SnPh4 catalyst. 

The cyclohomopolymerisation of 1,6-heptadiyne by using Shirakawa [85] and related [86] catalysts is a representative insertion polymerisation of acetylenic monomers with Ziegler-Natta catalysts ... 

It was proved that metal carbynes are sources of metal carbenes [e.g. scheme (9) in Chapter 6] promoting the polymerisation of acetylenic monomers. Therefore, related metal carbynes and carbenes appeared to catalyse the polymerisation of alkynes in the same way as regards the identity of the products, in particular as regards stereochemistry. For the terminal and internal alkynes, the Fischer carbyne acts much like the Casey and Fischer metal carbenes. The Fischer carbyne also promotes acetylene polymerisation, and it does this where the Fischer carbene fails and the Casey carbene is much less effective [22,143]. 

Interesting evidence supporting the mechanism of polymerisation of acetylenes via carbene species is provided by the block and random copolymerisation of acetylenic monomers with cycloolefins. For instance, block copolymers of acetylene and cyclopentene with the WC —AlEtCT catalyst [41] and block copolymers of acetylene and norbornene with the (MeA. Oj2W(=NAr)= CHMe3 catalyst [42] have been obtained moreover, random copolymers of phenylacetylene and norbornene with the WC16 catalyst have also been obtained [149, 150],... 

As in the case of the ring-opening metathesis polymerisation of cycloolefins, an important matter is the control of polymerisation to prepare acetylenic polymers having precise structures. A living polymerisation is of practical importance in the synthesis of monodisperse polymers, such as terminally functionalised polymers and block copolymers. The metathesis catalysts that promote the living polymerisation of acetylene [42] and acetylenic monomers include M0OCI4 SnBu EtOFkNbCls and Ta, Mo and W alkylidenes [84, 133, 152, 153]. 

Polycycloaddition reactions of acetylenic monomers have been used to synthesize hyperbranched polymers comprised of pure aromatic rings and heterocyclic units. 

The Diels-Alder reaction of 2-pyrones has been used in polymer syntheses (6, 13) (Reaction 4). In these reactions the 2-pyrone is difunctional in that it consumes two moles of dienophile (Reaction 16). A practical way of stopping at the 1 1 adduct is to use an acetylenic monomer (24) (Reaction 17). Although 2-pyrone itself polymerizes on standing (I), phenyl-substituted 2-pyrones, including monophenyl 2-pyrones, are stable at ordinary temperatures. 

Homodimerization of aryl iodides (9, 19, 21) with an acetylene bridge was then conducted at room temperature (PdCl2(PPh3)2, Cul, Et3N, 24 h.) to provide symmetrical dimer 15 (58%) accompanied by the acetylenic monomer 13 (27%). Attempts to provide the corresponding ortho and meta monoacetylene-bridged dimers 23 and 26 were troublesome since they were invariably contaminated with doubly bridged-dimers (25, 28, Scheme 5) as inseparable mixtures (2 1 and... 

Mesogen-containing acetylene monomers, (II), were prepared by Tang [2] and polymerized into polyacetylenes, (III). These monomers had excellent tracta-bility typically associated with polymers having flexible backbones. 

Some of the acetylenic monomers can be commercially obtained (e.g., Farchan Labs., USA), and others are prepared by the methods described in our original papers and in the literature 119 121). 

The following relationships between the structure of acetylenic monomers and the activity of catalysts are observed usually Ziegler catalysts can give rise to polymers from acetylene and monosubstituted acetylenes without bulky substituents. In contrast, Mo and W catalysts are very effective for monosubstituted acetylenes with bulky substituents and disubstituted acetylenes with less bulky substituents. Further, Nb and Ta catalysts are useful for various disubstituted acetylenes, including those with bulky substituents. 

Among Si-containing acetylene monomers, only (trimethylsilyl)acetylene (HC=CSi(CH3)3 la) has appeared in the literature, if our studies are excepted. 

Figure 7. Radiation treatment of HDPE with and without reactive acetylene monomer. (Reprinted from Ref. 7, copyright 1978, and Ref. 16, copyright 1977.)...

As shown in Table 3, cis- and trans-coni nU of poly acetylene prepared by the Ziegler-Natta catalysts depends strongly upon the polymerization temperature ss x ere are two possible explanations for this observation One is that the fundamental mechanism is the formation of cis double bonds by the cis insertion of acetylene monomer into the Ti—C bond of the catalyst. This fits the orbital interaction consideration for the role of the catalyst by Fukui and Inagaki, according to which the initially formed configuration of the double bond is cis as a result of the favorable orbital interaction between the inserting acetylene monomer and the active site of the catalyst. Because the cis double bond is thermodynami-... 

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