Oxygen Polymerisation mechanism

Tetraneopentyltitanium [36945-13-8] Np Ti, forms from the reaction of TiCl and neopentyllithium ia hexane at —80° C ia modest yield only because of extensive reduction of Ti(IV). Tetranorbomyltitanium [36333-76-3] can be prepared similarly. When exposed to oxygen, (NpO)4Ti forms. If it is boiled ia ben2ene, it decomposes to neopentane. When dissolved ia monomers, eg, a-olefins or dienes, styrene, or methyl methacrylate, it initiates a slow polymerisation (211,212). Results from copolymerisation studies iadicate a radical mechanism (212). Ultraviolet light iacreases the rate of dissociation to... 

Cyclobutane has not been polymerised cationically (or by any other mechanism). Thermochemistry tells us that the reason is not thermodynamic it is attributable to the fact that the compound does not possess a point of attack for the initiating species, the ring being too big for the formation of a non-classical carbonium ion analogous to the cyclopropyl ion, so that there is no reaction path for initiation. The oxetans in which the oxygen atom provides a basic site for protonation, are readily polymerizable. Methylenecyclobutane polymerises without opening of the cyclobutane ring [72, 73]. 

There are many studies of the mechanism and kinetics of the polymerisation of cyclic oxygen compounds, but only relatively few of these are concerned with cyclic formals. In the present paper I will review this field, and I hope to show that formals have some special characteristics which distinguish their polymerisations from those of other cyclic oxygen compounds. Since it is not possible to deal with all aspects of this group of reactions in one lecture, I will concentrate attention on questions of chemistry and mechanism, and I will not deal with other aspects, such as the thermodynamics and kinetics of these polymerisations. Most of the published work has been done with 1,3-dioxolan (I) and there are only very few papers on any other cyclic formals, although the patent literature on the homo- and copolymerisation of (I) and other cyclic formals is quite extensive. 

In both PO activated structures, the electron density at the carbon atoms of the oxiranic ring decreases (in one case due to a neighbouring positive charge, in the second by coordination) and makes possible a nucleophilic attack of a weak nucleophile, such as the oxygen atom of hydroxyl groups. To conclude, the mechanism of PO polymerisation with DMC catalyst is based on the repeated nucleophilic attack of hydroxyl groups on the carbon atoms of PO, strongly activated by coordination (see Schemes 5.3 and 5.4). 

A very similar mechanism for PO polymerisation with DMC catalysts was investigated by Xiaohua and co-workers [65] and Chen and co-workers [66, 67]. They proved that the co-ordination number of Zn2+ increases from 3 to 5.7 in the process of activation with PO. One considered that 5 oxygen atoms co-ordinate the Zn2+ ion and the sixth position... 

The detailed mechanism of the reaction of ozone with olefins proposed by Criegee et involves initial attack upon the olefin to form a primary ozonide which suffers oxygen-oxygen and then carbon-carbon bond fission to form a reactive zwitterionic intermediate (III) which provides the normal ozonide , together with the products of rearrangement, polymerisation, or... 

Figure 21.1 Possible mechanisms for anionic ring-opening polymerisations of non-substituted lactones showing both acyl-oxygen scission (a) as well as alkyl-oxygen scission (b).

In the first scheme (Fig. 21.1, reaction a), the alkoxide attacks the ester group after which an acyl-oxygen bond is broken. In the other case [Fig. 21.1, reaction b), the alkoxide attacks the carbon atom adjacent to the alcohol residue of the ester, after which an alkyl-oxygen scission takes place. This last mechanism is of particular importance for the polymerisation of four-membered lactones (3-lactones] in which the first mechanism is disfavoured by stereo-electronic effects [32]. 

Laccases (benzenediohoxygen oxidoreductases, EC 1.10.3.2) are a diverse group of multi-copper enzymes, which catalyze oxidation of a variety of aromatic compounds. Laccases oxidize their substrates by a one-electron transfer mechanism. They use molecular oxygen as the electron acceptor. The substrate loses a single electron and usually forms a firee radical. The unstable radical may undergo further laccase-catalysed oxidation or non-enzymatic reactions including hydration, disproportionation, and polymerisation. ... 

An initial question in this complex reaction mechanism, which has been extensively investigated, is whether CO dissociates into C and O species prior to the formation of monomer CH intermediates or, alternatively, whether the CO bond remains without splitting and inserts, forming intermediate species containing oxygen. At present the polymerisation (chain prolongation) step seems to... 

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