Polymerization of Substituted Acetylenes

This chapter surveys the metathesis polymerization of substituted acetylenes, focusing on research carried out during this decade. Monomers, polymerization catalysts, controlled polymerizations, polymers, and polymer properties are discussed. Readers are encouraged to also consult Chapter 3.11 of the first edition of Handbook of Metathesis [1] for more details on metathesis polymerization on acetylenes. Other monographs and review articles are also available concerning various aspects of chemistry of substituted polyacetylenes (PA)s [2-29]. 

Katz and Lee [34] supported this polymerization mechanism by accomplishing acetylene polymerization with metal carbenes as catalysts. Further, the 

Handbook of Metathesis VoL 3 Polymer Synthes, Second Edition. 

Although metathesis catalysts polymerize mono- and disubstituted acetylenes, they do not polymerize monomers having polar and/or protic groups because of their lack of tolerance of these groups. For the same reason, the polymerization with metathesis catalysts is carried out in relatively nonpolar solvents. 

Recent examples of various transition-metal catalysts used for acetylene polymerization are shown in Table 15.1. The central metals of effective catalysts are usually restricted to Mo, W, Nb, Ta, and Ru. All these catalysts can be classified into single-, two-, and three-component catalysts. In general, multicomponent catalysts show high activity but the system is complicated, whereas single-component catalysts, sometimes called well-defined catalysts, are often simpler. 

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Poly (acetylenes) [16], There are several catalysts available for polymerization of substituted acetylenes. Whereas Ziegler-Natta catalysts are quite effective for polymerization of acetylene itself and simple alkylacetylenes, they are not active towards other substituted acetylenes, e.g. phenylacetylenes. Olefin-metathesis catalysts (Masuda, 1985 Masuda and Higashimura, 1984, 1986) and Rh(i) catalysts (Furlani et al., 1986 Tabata, 1987) are often employed. In our experience, however, many persistent radicals and typical nitrogen-containing functional groups serve as good poisons for these catalysts. Therefore, radical centres have to be introduced after construction of the polymer skeletons. Fortunately, the polymers obtained with these catalysts are often soluble in one or other organic solvent. For example, methyl p-ethynylbenzoate can be polymerized to a brick-coloured amorph- See the Appendix on p. 245 of suffixes to structural formula numbers. 

Compared to early transition metals, the number of group 8-10 transition metal catalysts for the polymerization of substituted acetylenes has been relatively small except for Rh. However, unique aspects of these late transition metal catalysts have been revealed which cannot be seen in early transition metals and conventional Rh catalysts. 

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. 

Well-controlled polymerization of substituted acetylenes was also reported. A tetracoordinate organorhodium complex induces the stereospecific living polymerization of phenylacetylene.600 The polymerization proceeds via a 2-1 -insertion mechanism to provide stereoregular poly(phenylacetylene) with m-transoidal backbone structure. Rh complexes were also used in the same process in supercritical C02601 and in the polymerization of terminal alkyl- and arylacetylenes.602 Single-component transition-metal catalysts based on Ni acetylides603 and Pd acet-ylides604 were used in the polymerization of p-diethynylbenzene. 

Polymerization of substituted acetylenes has often been attempted by using radical and ionic initiators since a long time ago 5). In most cases, however, the products were linear oligomers whose molecular weights (MW s) were a few thousand (Eq. (2)). Also cyclotrimers often formed as by-products. Thus it was rather difficult to synthesize selectively polymers whose MW s are higher than ten thousand. 

Apart from the present polymerization of substituted acetylenes, olefin metathesis is known to be catalyzed specifically by W and Mo catalysts 67 69). In olefin meta-... 

We inferred in 1975 a metal carbene mechanism for the present polymerization of substituted acetylenes 66). That is, the propagation mechanism can be depicted by Eq. (9). The rationales for this reasoning are i) There are many catalysts effective... 

Two mechanisms have been proposed for acetylene and substituted acetylene polymerization by transition metal catalysts one is the metal-alkyl mechanism and the other is the metal-carbene mechanism. In general, it has been proposed that the polymerization of acetylenes by Ziegler-Natta catalysts proceeds by the metal-alkyl mechanism, while the metal-carbene mechanism has been accepted for the polymerization of substituted acetylenes by metathesis catalysts whose main components are halides or complexes of group 5 and 6 transition metals. The latter will be discussed in Section III. 

Metal carbenes 19 and 20 have been reported to be effective in the polymerization of substituted acetylenes. Since 19 has an olefin ligand that can be removed when an acetylene monomer approaches, it is more active than 18. Metal carbenes 20 and 21a induce living polymerizations of substituted acetylenes(see below). In general, metal carbene catalysts are not very active, but the initiation reaction thereby is simple, and hence they are useful for the investigation of kinetics, etc. 

Polymerization of substituted acetylenes has been carried out by a wide range of catalysts and condi-tions. Polymerization conditions include a homogeneous and heterogeneous Ziegler—Natta catalyst, transition metal complexes (Pd. Pt. Ru. W. Mo. Ni. etc.), free radical initiators such as 2.2 -azobis(isobu-tyronitrile) (AIBN). benzoyl peroxide (BPO). and di-tert-butylperoxide (DTBP). thermal polymerization, y-irradiation. cationic initiation with BF3. and anionic initiation by butyllithium. triethylamine. and sodium amide. 

It has been demonstrated that surface basic sites of PCPs successfully catalyze polymerization in the nanochannels. Thus, the pillared-layer complex [Cu2(pzdc)2bpy] acted as a host for spontaneous polymerization of substituted acetylenes in a specific manner (Fig. 5) [67]. In the case of acidic monosubstituted acetylenes, the basic oxygen atoms from carboxylate ligands in [Cu2(pzdc)2bpy] produced reactive acetylide species that subsequently initiated anionic polymerization in the nanochannel. Compared with a control experiment using a discrete model catalyst... 

Since many substituted polyacetylenes have unique properties (high 02-permeability, high Tg, good thermostability, etc. [18]), we became interested in the development of novel photoinitiators for the polymerization of substituted acetylenes (Scheme 9). It is known that certain substituted acetylenes can be polymerized upon UV irradiation of Mo(CO)6 or W(CO)6 [19]. However, the reaction can only be performed in CCI4, which, most probably, acts as a co-catalyst. Our goal was to develop a storage stable... 

The successful PROMP of cycloolefins, like 2-norbornene and dicyclopentadiene, with simple dialkyl-tungsten complexes by irradiation with UV- or visible light leads to the in situ formation of metal-carbenes by a Ha-abstraction reaction [20]. Metal carbenes have proven to be the active ROMP-initiators [21] and are supposed to be intermediates in the polymerization of substituted acetylenes as well [22]. Nevertheless, to the best of our knowledge, a one component catalytic system for the photopolymerization of mono- and disubstituted acetylenes has not yet been reported before our work [23]. 

In most cases, the polymerization of substituted acetylenes is performed using a flask equipped with a three-way stopcock under a dry, inert gas... 

A variety of transition metal catalysts have been found to polymerize substituted acetylenes. Effective catalysts range from Group 3 to Group 10 metals. Activity of catalysts greatly depends on monomer structure therefore, it is quite important to recognize the characteristics of each catalyst. Table 1 lists recent representative examples for the polymerization of substituted acetylenes with various transition metal catalysts, which will help readers to imderstand the general features of catalysts. 

An alternative metal carbonyl catalyst, (Mes)Mo(CO)3 (Mes = mesitylene), also catalyzes the polymerization of substituted acetylenes in CCI4 (143). Photoirradiation is unnecessary for this system the hgating mesitylene is readily released by heating, which allows the poljunerization to proceed without photoirradiation. In a similar way, photoirradiation can be omitted by using (CH3CN)3M(CO)3 as a catalyst (144). 

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