CO₂ Fixation By Meso-CHO And Corresponding Products Transformation Study

Fixating carbon dioxide by epoxides is merging as a feasible means for using carbon dioxide and saving fossil resources.meso-CHO is an inexpensive commercial epoxide and has been widely explored to fix carbon dioxide.meso-CHO possesses two chiral carbons and then generates two configurational products,cis-CHC and trans-CHC in cycloaddition reaction with CO₂.But being enthalpically favoured,cis-CHC is the dominating cycloaddition product.Besides cycloaddition,copolymerizing CO₂ with meso-CHO to give polycarbonate?PCHC?is another practical approach.Notably,chem-and regio-selectivity must be improved in the copolymerization accompanying two side reactions—backbiting of the polymer chain to give trans-CHC and inserting of sequential epoxide to give polyether fragments.A great number of CO₂-based polycarbonates being yielded arises worries on polymer degradation and recycling,which is also the most intractable problem in the world.PCHC can be depolymerized to produce trans-CHC which can in turn be a monomer for ring-opening polymerization and in this way,it makes polymer recycling come true.Although fixation CO₂ by meso-CHO has been widely investigated in the world,realizing neither three potential reactions,cycloaddition,polymerization and depolymerization nor conversion between the products via just one catalyst system has not been published yet.Hence,we focused on developing one catalyst capable to catalyze a)cycloaddition of CO₂ and meso-CHO to generate cis-CHC,b)alternative copolymerization of CO₂ and meso-CHO to give PCHC,c)degradation of the resultant PCHC to yield trans-CHC,pointedly and high selectively.Occasionally,we found that configurational inversion of the degradation product trans-CHC occurs giving more enthalpically favoured cis-CHC.A versatile catalyst accessing four reactions derived from meso-CHO and CO₂ has been developed.1)synthesize dinuclear?-diketiminato?BDI?zinc complex?BDI-ZnR?₂?R=methyl?and conduct copolymerization of CO₂ with meso-CHO.The effects of feed ratio,temperature,and CO₂ pressure were investigated.PCHC with absolute molecular weight Mn=30kDa,PDI=1.1 were yielded,showing both high chem-and regio-selectivity,both greater than 99%.It was tested that the PCHC is atactic polymer.2)conduct depolymerization of PCHC synthesized from the step 1)using complex?BDI-ZnR?₂ only.Feed ratio,reaction temperature,reaction time were investigated to realize high product selectivity and yield.co-catalyst benzyl alcohol BnOH was used to improve the catalyst activity and the product selectivity,and the degradation mechanism was investigated then.We also found that the degradation product trans-CHC would be transformed to its configurational counterpart cis-CHC after the polymer completely degraded.3)Subsequently,complex?BDI-ZnR?₂ was used for configurational inversion of trans-CHC and the reaction temperature was optimized.co-catalyst benzyl alcohol BnOH was used again to improve the product selectivity.co-catalyst ratio was investigated then to improve the catalyst activity and product selectivity.The configurational inversion mechanism was also explored.4)Anionic co-catalyst PPNCl?bis?triphenylphosphine?iminium chloride?,n-Bu₄NCl?tetrabutylammonium chloride?,n-Bu₄NBr?tetrabutylammonium bromide?,n-Bu₄NI?tetrabutylammonium iodide?were involved to catalyze the cycloaddition reaction of CO₂ with meso-CHO to give cis-CHC.co-catalyst ratio,reaction temperature was optimized to get high product yield and selectivity.The cycloaddition mechanism was investigated too.