Poly-spiroketal structure

Under certain conditions of propylene/carbon monoxide copolymerisation with certain catalysts, poly(spiroketal) structural units... 

Different mechanistic interpretations of the formation of an alternating propylene/carbon monoxide copolymer of poly(spiroketal) structure were considered [107, 478, 480, 481, 489]. Any reasonable proposal, however, needs to take into account the nature of the end groups in the copolymer chains. To date this has not been possible owing to the low solubility of the copolymer in solvents other than hexafluoroisopropanol however, this solvent, probably because of its acidic nature, causes transformation of the poly(spiroketal) structure into an isomeric poly(ketone) structure [489]. The formation of a cyclic polymeric structure could be favoured by minor entropy loss due to the intramolecularity of the process [480,481] and by the peculiar conformational situation of the poly(ketone) structure [491]. 

Under most conditions, only the simple polypropylene ketone) is formed in propylene/carbon monoxide alternating copolymerisation. Isomerisation of poly(ketone) to poly(spiroketal) can occur, and it may be assisted by cationic palladium species and protonic acids. It must be emphasised that a low reaction temperature favours the formation of a spiroketal structure [107]. At a temperature above the ceiling temperature, the poly(spiroketal) depolymerises to the more flexible and entropically favoured poly (ketone) [481]. 

Depending on the reaction conditions of PCO and OCO copolymerizations, polyketones are obtained either in a linear or in a spiroketal structure. The equilibrium of the keto- and ketal stmcture depends on the energetic difference between both forms, which is fairly small in OCO copolymers of higher olefins (the stability of the spiroketal form increases with the length of the a-olefin). Investigation of the dynamic behavior was carried out by NMR spectroscopy in solution and in the solid state. The spiroketal form can be converted into the linear poly( 1,4-ketone) at higher temperatures or by treatment with l,l,l,3,3,3-hexafluoro-2-propanol (HFIP) and precipitation (Figure 40). 

The possibility that olefin-carbon monoxide copolymers can exist in the isomeric poly-spiro ketal structure (Scheme 8.5) was recognized soon after the first synthesis of the copolymers of propene. Using dialkyl diphosphines as the modifying ligands [27], copolymers were obtained with blocks with the spiroketal form [28]. 

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