Acetylenes living

Some transition metal catalysts induce the living polymerization of various acetylenic compounds.68,69 Such polymerizations of phenylacetylene catalyzed by rhodium complexes are used in conjunction with a quantitative initiation and introduction of functional groups at the initiating chain end (Scheme 16).70 The catalyst is prepared from an [RhCl(nbd)]2/Ph2C=C(Ph)Li/PPh3 mixture and proceeds smoothly to give quantitatively the polymer 54 with a low polydispersity ratio. 

Burris, R.H. The acetylene reduction technique. In Nitrogen Fixation by Free-living Micro-organisms, VoL 6, W.D.P. Stewart,... 

All the above-mentioned initiators are very sensitive towards substances with active hydrogen. Care must therefore be taken to exclude acids, water, thiols, amines, and acetylene derivatives. Oxygen, carbon dioxide, carbon monoxide, carbonyl compounds, and alkyl halides which can react with the initiator, also interfere with the reaction. Careful purification and drying of the starting materials and apparatus is, therefore, absolutely essential, especially when dealing with living polymers (see Example 3-19). 

Mo and W alkylidene complexes 4, the so-called Schrock carbenes, have explosively evolved the polymerization chemistry of substituted acetylenes. Although the preparation of these catalysts is relatively difficult because of their low stability, in other words, high reactivity, they elegantly act as living polymerization catalysts for substituted... 

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. 

A Ta vinylalkylidene complex 6, confirmed by a single crystal X-ray analysis, was revealed to polymerize 2-butyne in a manner of living polymerization.The initiation efficiency is quantitative, and the living end can be end-capped with aromatic aldehydes. As polymers from symmetric acetylenes are generally insoluble, soluble poly(2-butyne) is accessible if the degree of polymerization is suppressed below 200. The NMR analysis of living oligomers of 2-butyne clearly indicates that both cis- and 7ra r-structures exist in the main chain. 

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... 

Light emitted from flashrini dated explns of acetylene and 0 photo sensitized by N02 in a large quartz tube was studied photo-electricaLly. Presence of very short-lived Intense emission from the diat radicals was confirmed. Although main part of expln is homogeneous, 2 deton waves travel a short distance to the ends of the reaction vessel. Sharp peak is caused by the impact of one of these deton waves on the vessel window nearer the spectrograph Ref 5.A. Thrush, ProcRoySoc (London)... 

The simplest covalently linked systems consist of porphyrin linked to electron acceptor or donor moiety with appropriate redox properties as outlined in Figure 1. Most of these studies have employed free base, zinc and magnesium tetrapyrroles because the first excited singlet state is relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. Additionally, these pigments have relatively high fluorescence quantum yields. These tetrapyrroles are typically linked to electron acceptors such as quinones, perylenes , fullerenes , acetylenic fragments (14, 15) and aromatic spacers and other tetrapyrroles (e.g. boxes and arrays). 

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. 

Acetylenes copolymerize with each other and with cycloalkenes under the influence of olefin metathesis catalysts. With non-living systems it is possible to make statistical... 

When the propagating species are long-lived it is possible to make block copolymers of controlled chain length by sequential addition of monomers, for example with ClC=CC4H9/ClC=CCi4H29686 693 and with acetylene/norbornene396,628. 

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