Polymerisation activity

Uninhibited chloroprene suitable for polymerisation must be stored at low temperature (<10° C) under nitrogen if quaUty is to be maintained. Otherwise, dimers or oxidation products are formed and polymerisation activity is unpredictable. Insoluble, autocatalytic "popcorn" polymer can also be formed at ambient or higher temperature without adequate inhibition. For longer term storage, inhibition is required. Phenothiasine [92-84-2] / fZ-butylcatechol [2743-78-17, picric acid [88-89-17, and the ammonium salt of /V-nitroso-/V-pheny1hydroxy1 amine [135-20-6] have been recommended. 

NHC-bearing ruthenium carbene complexes are used to design latent and switch-able initiators. In this research field emphasis is given to the development of initiators, which are inactive at room temperature in the presence of the corresponding monomer and can be activated upon a proper stimulus such as heat [130] or light [131]. Once activated, a high polymerisation activity is desired which is provided by NHC co-ligands [132]. 

Prior to the development of polymerisation catalysts based on the bis(imino)pyridine framework, the bis(imino)pyridine moiety was widely incorporated into macrocycles [163, 164], As an extension of this design strategy to polymerisation applications, several groups have been attempting to incorporate sterically bulky bis(arylimino) pyridine units into macrocycles. In a similar fashion, the introduction of the bis(imino)pyridine unit into polymeric chains in which polymerisation-active metal centres are bound has been the subject of study. 

A consensus on or explanation for the influence of the oxidation state of titanium on olefin polymerisation activity has not been reached. The absence of any insertion of the coordinating ethylene into the Ti-C bond in Ti(II) species is noteworthy instead, two ethylene molecules, which coordinate at two coordination sites at Ti(II) species, undergo an oxidative addition, and thus the respective metallacycle, titanacyclopentane, is formed [305], Such a reaction for dimethyltitaniumcomplexed by l,2-bis(dimethylphosphione)ethane [Dmpe] is as follows [305] ... 

Monocyclopentadienyl titanium derivatives are the most active precursors for catalysts possessing high syndiospecific polymerisation activity for styrene and ring-substituted styrenes. The polymerisation activity of biscyclopentadie-nyl titanium compounds activated with methylaluminoxane is lower than that of other soluble titanium-based catalysts [73]. 

Among catalysts based on cyclopentadienyl-substituted half-sandwich titanocenes, the IndTiCb—[Al(Me)0]x catalyst in particular has extremely high activity and stereospecificity. It is relatively more sensitive to polymerisation conditions. A minimum concentration of about 50 mmol of methylaluminoxane was required to obtain the desired polymerisation activity. The activity also increases as the [Al(Me)0]x concentration increases, reaching a maximum at an Al/Ti molar ratio of 4000. For instance, the productivity of the IndTiCU—[Al(Me)0]x catalyst under optimum polymerisation conditions... 

As previously demonstrated for the ROP of rac-LA, titanium complexes supported by benzotriazole-phenolate 23-28/ a mixture of rac- and meso-amino bis(alkanolate) 40/41 and anatrane 66-69 ligands proved to be efficient catalysts for the ROP of rac-BBL to give poly(3-hydro g butyrate) (PHB). The most notable features of these catalysts are the high polymerisation activity and productivity in solvent-free reactions at 80 °C (rac-BBL Ti = 200, 95-99% conv. after 9-48 min), combined in some cases with high stereoselectivity (i.e. in syndiotactic PHB, up to Pr = 0.73) and narrow PDIs (1.19-1.36). Note that up to now there is no report of isospecific polymerisation of rac-BBL using titanium catalysts. 

In the ATRP of methyl methacrylate, addition of an excess tricyclohexylphosphine to the reaction mixture (up to 10 equiv. PCy3 relative to RuCl2(=CHPh)(PCy3)2) only slightly decreased the polymerisation activity of the catalyst (Table 3). 

By reaction with the Fischer type carbyne complexes loose the surface chromium(II) atoms of the reduced Phillips their polymerisations activity for 1-alkenes (16). The surface chromium(II) atoms enhance the metathesis activity of the bimetallic catalysts (Table 1). In contrast to the original Fischer type carbyne complexes are the bimetallic catalysts stable at room temperature. They can be stored for a year at 25°C without changing their metathesis activity (14). 

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