Regioirregularity

The monomer 19 can also be polymerized using analogous methods of initiation to those employed in organic polymer science. Radical initiators afford regioirregular polymers, whereas anionic initiators add selectively to the phosphorus atom of the P=C bond and thus yield a regioregular polymer [85]. The product of the initial addition of MeLi across the P=C bond, Mes(Me)P-CPh2Li, was identified spectroscopically. The polymers obtained from anionic initiation are spectroscopically identical to those obtained from the thermolysis. Reasonable molecular weights (ca. 5000-10,000 g mol 0 are obtained when methyllithium is used as an initiator. 

Stereo- and Regioirregular High Molecular Weight Poly (Higher (XrOlefin)s... 

Syndiospecific catalytic systems based on metallocenes are highly regioreg-ular. As a consequence, their stereoselectivity in possible regioirregular insertions has been experimentally investigated for propene copolymers only.78,79 However, an analysis of the stereoselectivity of possible secondary propene insertions on syndiospecific catalytic models based on -symmetric metallocenes is reported here, also due to its relevance to the rationalization of the dependence of regiospecificity on the type of stereospecificity (see Section 3.1.4.1).80... 

Nonbonded energy interactions are able to rationalize not only the stereospecificities observed for different metallocene-based catalytic systems (isospecific, syndiospecific, hemi-isospecific, and with oscillating stereocontrol) but also the origin of particular stereodefects and their dependence on monomer concentration as well as stereostructures associated with regioirregular insertions. Nonbonded energy analysis also allowed to rationalize the dependence of regiospecificity on the type of stereospecificity of metallocene-based catalysts. 

Figure 9. Representation of primary (regioregular) and secondary (regioirregular) propene insertions into the Mt-C bond of group 4 polymerization catalysts, parts a and b, respectively.

Fig. 1 NMR spectra in the carbonate region in CDCI3 of poly(propylene carbonate) A regioselective 94% head-to-tail selective copolymer prepared using a (salan)CrCl catalyst B regioirregular random polymer commercially available from Aldrich Chemicals...

Byrnes Ml, Chisholm MH, Hadad CM, Zhou Z (2004) Regioregular and regioirregular oligoethercarbonates A C H NMR investigation. Macromolecules 37 4139 145... 

The optical band gaps (Eg) of all the polymer were determined from UV/Vis absorption onset. Polymers 624-629 have band gaps between 1.74 and 1.86 eV, which are smaller than rr-P3HT (1.90 eV). This low band gap can be due to the more coplanar structure of the polymer and the weak donor-acceptor effect of DTP and THs. Regioirregular 624-627 have a 1 1 ratio of TH/DTP units in the backbone. They have the band gaps between 1.74 and 1.79 eV. 

M-C as propagating species, 4, 1008 monomer coordination and insertion reactions, 4, 1010 monomer insertion regio- and stereochemistry, 4, 1015 overview, 4, 1005-1166 regioirregular insertions, 4, 1023 stereocontrol mechanism, 4, 1018 stereocontrol symmetry rules, 4, 1020 stereoregular polymers, 4, 1016 in Ru-Os heterodinuclear compounds, 6, 1046 in Ru-Os mixed-metal clusters, 6, 1064 semiconductor growth, conventional precursors, 12, 2 with silicon, 3, 514... 

Allene has been polymerised to high molecular weight linear polymers by various Ziegler-Natta catalysts based on compounds of such transition metals as Ti, V, Cr, Mn, Fe, Co and Ni as precursors and alkylaluminium compounds as activators [439-441], Crystalline and amorphous polymers have been produced, in various proportions, with each of the catalysts used. The crystalline polymers consist predominantly of 1,2-linked (head-to-tail) monomeric units formed by insertion polymerisation as in scheme (68), but some regioirregular-ities resulting from the 2,1-insertion, leading to head-to-head and tail-to-tail arrangements, may be present in the polymer chain ... 

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