Copolymerisation of Tiiranes and Carbon Dioxide

Propylene sulphide was found [246] to undergo copolymerisation with carbon dioxide in the presence of the triethylaluminium pyrogallol (2 1) catalyst, yielding copolymers, polypropylene thioether-monothiocarbonate)s, containing propylene thiocarbonate units in an amount reaching 42 mol.-%, apart from the predominating propylene sulphide units (Table 9.4)  

Attempts to use the diethylzinc-pyrogallol (2 1) catalyst to copolymerise propylene sulphide and carbon dioxide failed, since the content of propylene thiocarbonate units in the copolymers formed was small and did not exceed 10 mol.-%. It has also been observed that the presence of carbon dioxide in this copolymerisation system causes a lowering of the molecular weight and yield of the copolymer formed. Thus, it has been suggested that propylene sulphide homopolymerisation was favoured over cross-propagation with carbon dioxide in the presence of a zinc-based coordination catalyst because of higher HSAB symmetry of the system in the former case. The zinc atom in the Zn-S unit of the catalyst is a rather soft acid and will prefer reaction with a soft base such as propylene sulphide rather than with hard carbon dioxide. The presence of a hard acid centre in the triethylaluminium-based catalyst should result in an increase in the affinity of the catalyst towards carbon dioxide [247], 

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