Molecular-orbital calculations polymerization

Simple resonance theory predicts that pentalene (48), azulene (49), and heptalene (50) should be aromatic, although no nonionic canonical form can have a double bond at the ring junction. Molecular orbital calculations show that azulene should be stable but not the other two, and this is borne out by experiment. Heptalene has been prepared but reacts readily with oxygen, acids, and bromine, is easily hydrogenated, and polymerizes on standing. Analysis of its NMR spectrum shows that it is... 

Recently, theoretical calculations were done on the olefin polymerization. In particular, an ab-initio molecular orbital calculation was used to optimize the geometry for the ground, transition and product states of model systems, based on gas-phase reactions ... 

The phosphorus analogue of pyrrole, phosphole, has a degree of aromatic character, according to molecular orbital calculations and nmr spectra (Brown, 1962 Chuchman et al., 1971). 1-Methyl-phosphole has a p/fg-value of 0-5 (Quin et al., 1969), much higher than that of pyrrole. It polymerizes rapidly in aqueous acid. The site of protonation of 1,2,5-triphenylphosphole is phosphorus according to the infrared spectra of some of its stable salts (Chuchman et al., 1971). 

This has been proposed by Imoto and Otsu (33, 34) and confirmed by the molecular orbital calculations of Tazuke et al. (76, 77). A similar explanation has been advanced for the increased reactivity of vinylpyri-dines and vinylimidazoles in the presence of zinc salts (76, 77). Super-delocalizability as a result of conjugation with the metal salt results in spontaneous thermal polymerization. 

Concerted two-electron transfer and reversible metal-metal bond cleavage in phosphine-bridged dimers have been investigated, and extended Hiickel molecular orbital calculations have shown that the redox-active orbital is a metal-metal antibonding orbital. A Ru-Ru-bonded dimeric cation [Ru(Cp)2]2++ has been prepared and characterized electrochemically. The electrochemistry of these dimers may give insight into more complex clusters and polymeric metals. 

Although cross-linked polymers were found for both PPy and PTh in later stages of polymerization reactions, the couplings in earlier stages take place predominantly at a and a -positions of these heterocycles. These positions were shown to have higher charge densities and were thus more favourable from molecular orbital calculations for radical cations produced in the initial stages of polymerization [26,27]. The positions where the two monomers couple for these molecules are more selective than those for aniline. 

Molecular orbital calculations support this. Polymerizations can also be carried out with less than stoichiometric ratios of the Lewis acids to the monomers. As an illustration, coupling of an acrylic or a methacrylic monomer with a Lewis acid can be shown as follows ... 

Of these four, the majority of the research tends to support the transition metal-carbon center model, as the mechanistic scheme of choice. Using a homogeneous catalyst (Cp2TiEt2) as a model, Breslow and Newburg proposed that polymerization growth occurred at the Ti—C center. Somewhat later, Cossee extended this concept into a more elaborate mechanism for supported catalysts, which he substantiated with molecular orbital calculations (see Ref. 179, Chapt. 13). 

Nucleophilic bimolecular ring-opening of ethylene oxide by the hydride anion has been investigated theoretically. Molecular orbital calculations of the interaction energy (a combination of coulomb, exchange, delocalization, and polarization interaction terms) were carried out. The kinetics and stereochemistry of base-catalysed polymerization of epoxides have been studied using optically active epoxide monomers. 

The Cossee-Arlman mechanism for the polymerization of olefins is the most widely accepted theory but as yet it is not complete. Cossee developed his early ideas of polyethylene growth at a titanium-carbon bond and supported the theory by molecular orbital calculations. The role of the alkyl aluminium co-catalyst was in the generation of the active species, via the alkylation of the titanium chloride bonds, and to remove impurities in both the gas stream and catalyst preparative procedure. There was also the suggestion that it might be involved in the insertion of each monomer molecule, and also in the regeneration of dormant sites or the formation of new active sites. 

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