Transition living polymerization

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. 

As will be discussed more thoroughly in Section 3.2.5, transition metal carbene complexes can mediate olefin metathesis. Because heteroatom-substituted carbene complexes are usually less reactive towards olefins than the corresponding nonheteroatom-substituted complexes, it is, e.g., possible to use enol ethers to terminate living polymerization or other types of metathesis reaction catalyzed by a non-heteroatom-substituted carbene complex. Olefin metathesis can also be used to prepare new heteroatom-substituted carbene complexes (Figure 2.15, Table 2.11). 

Block copolymers have been successfully synthesized because many metallocene polymerizations of MMA proceed as living polymerizations, and it is possible to have a single one-way crossover from carbanion (alkene) polymerization to MMA (enolate) polymerization with metallocene and related initiators, especially when group 3 transition metal initiators are used [Boffa and Novak, 2000 Desurmont et al., 2000a,b Jin and Chen, 2002 Yasuda et al., 1992],... 

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. 

The living polymerization prindples developed using the titanacene metall-acyclobutane catalysts have been extended to ROMP reactions catalyzed by other transition-metal complexes. These new systems include tantalacyclobutane complexes and preformed tungsten(VI) carbene complexes. 

Chains with monodisperse molecular weight distribution (Mw/Mn = 1.00) can occur in idealized conditions when all polymerizing centers initiate instantaneously and chain termination is absent. In these cases the catalyst is actually an initiator. These living polymerizations are quite rare among transition metal catalysts. More often, random chain termination leads to many chains formed per metal atom. A Schulz-Flory most probable distribution of polyalkene molecular weights (Mw/Mn = 2.00) is the result. In cases when more than one type of active site is present, bimodal or multimodal distributions of molecular weights result (Mw/Mn > 2.00). 

The standard molecular structural parameters that one would like to control in block copolymer structures, especially in the context of polymeric nanostructures, are the relative size and nature of the blocks. The relative size implies the length of the block (or degree of polymerization, i.e., the number of monomer units contained within the block), while the nature of the block requires a slightly more elaborate description that includes its solubility characteristics, glass transition temperature (Tg), relative chain stiffness, etc. Using standard living polymerization methods, the size of the blocks is readily controlled by the ratio of the monomer concentration to that of the initiator. The relative sizes of the blocks can thus be easily fine-tuned very precisely to date the best control of these parameters in block copolymers is achieved using anionic polymerization. The nature of each block, on the other hand, is controlled by the selection of the monomer for instance, styrene would provide a relatively stiff (hard) block while isoprene would provide a soft one. This is a consequence of the very low Tg of polyisoprene compared to that of polystyrene, which in simplistic terms reflects the relative conformational stiffness of the polymer chain. 

Multiple halogen compounds FI-31158 and FI-32328 seem interesting to attest the chemoselectivity of transition-metal catalysis, and the CF3 and the aromatic bromide therein, respectively, remain intact during the living polymerizations to afford a-end functions, though their utility might be limited. 

Keywords Transition metal complexes Living polymerization Rigid rod helical structure Optically active polymer Asymmetric polymerization... 

Coordination polymerization of dienes has progressed significantly within the last decade. Selective polymerization of 1,3-dienes is reinforced by conventional transition metal catalysts and by new organolanthanide catalysts. Nonconjugated dienes also polymerize selectively to produce polymers with cyclic units or vinyl pendant groups. Living polymerization of dienes has become common, which enabled preparation of block copolymers of dienes with alkenes and other monomers. Another new topic in this field is the polymerization of allenes and methylenecycloalkanes catalyzed by late transition metal complexes. These reactive dienes and derivatives provide polymers with novel structure as well as functionalized polymers. The precision polymerization of 1,2-, 1,3-, and l,n-dienes, achieved in recent years, will be developed to construct new polymer materials with olefin functionality. 

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