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Study On The Anion Of Quaternary Ammonium Salts As Cocatalysts For The Copolymerization Of CO2

Posted on:2009-05-03Degree:MasterType:Thesis
Country:ChinaCandidate:W J ZhaoFull Text:PDF
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Carbon dioxide is an atmospheric green house gas that contributes to global warming;thereby its emission into atmosphere is needed to reduce.The copolymerization of carbon dioxide and epoxides to produce polycarbonate has been intensively studied in the last decade as one of the most promising processes for CO2 utilization.After the use of ZnEtz/H2O as catalytic system in the copolymerization by Inoue in 1969,great efforts have been dedicated directly toward the development of catalytic system and mechanism of the copolymerization during the past forty years. Besides the catalyst,the cocatalyst also plays an important role in the copolymerization.At present,designing a novel cocatalyst with high activity is a hot topic in the field.In this dissertation,a series of quaternary ammonium salts had been introduced as cocatalyst for the copolymerization of CO2 with cyclohexene oxide catalyzed by (Salen)CrCl.The relationship between anionic properties and the activity of the cocatalyst was systematically investigated.The pronounced regularity is instructive for further design of cocatalyst with high activity to improve the copolymerization of CO2 with epoxides.The influence of simple anions such as Br-,OTs-,BF4- and PF6- on the copolymerization was discussed firstly.It seemed that anion with higher nucleophilicity,such as Br- was benefit for improving the activities and increasing the percentage of carbonate linkages than PF6- analogues.The utility of [C12H25(CH3)3N]Br as cocatalyst exhibited a turnover frequency(TOF)of 163 h-1, which is much higher as compared with[C12H25(CH3)3N]PF6(TOF 70 h-1).In addition,the carbonate linkages is dramatically reduced to 59%for [C12H25(CH3)3N]PF6,as compared with 99%for[C12H25(CH3)3N]Br. The performance of cocatalyst would change significantly by modifying the structure of the anion.The effects of dodecyltrimethyl ammonium salts with different acetyl anions by modifying the electrical property of substituents on the anionic structure on the copolymerization were investigated.The results showed that the electron-withdrawing group on anionic structure would markedly increase the catalytic activity,the use of[C12H25(CH3)3N][CCl3COO]as cocatalyst showed higher activity(TOF 160 h-1)than[C12H25(CH3)3N][CH3COO](TOF 137 h-1).On the other hand,the anion with electron-donating group such as CH3CH2COO- would decrease the catalytic activity(TOF 119 h-1).Furthermore,the anion with more electron-withdrawing group such as CF3COO- exhibited high activity(TOF 245 h-1), good selectivity(>99%),and high carbonate linkages(>99%).The rule was still work on the phenate.The results also revealed that the electro-donating group on the phenoxide anionic structure decreased the catalytic activity.For example,the TOF of 107 h-1for p-methylphenate was lower than that of 113 h-1for phenate,while the electro-withdrawing group such as nitryl group on phenoxide ligand would increase the catalytic activity from 113 h-1to 127 h-1.In addition,the more the electro-withdrawing group substituted on phenoxide ligand,the higher rate of copolymer production was exhibited.The turnover frequency of reaction was increased to 160 h-1when pentachlorophenate was used.On the basis of the previous achievements,a series of ammonium salts which include perfluoro-substituted anion,such as[(C4H9)4N][C2F5COO]and [(C4H9)4N][OBzF5]as cocatalyst were synthesized and employed for the copolymerization.These ammonium salts exhibited good performance.The utility of [(C4H9)4N][CF3COO]produced a TOF of 168 h-1,the TOF is 192 h-1for [(C4H9)4N][C2F5COO].[(C4H9)4N][C6F5O]was selected as a cocatalyst with(Salen)CrCl to study the effects of catalyst loadings,reaction time,and temperature on the catalytic performance.The results showed that higher molar ratio of monomer to catalyst increased the catalytic efficiency.In addition,the observed results also showed a pronounced dependence on the cocatalyst loadings when keeping the concentration of catalyst.The activities were substantially enhanced upon initially increasing the concentration of cocatalyst.For example,TOF was 206 h-1when 1 equiv,of cocatalyst was used,when the ratio of cocatalyst to catalyst increased to 5,the reaction activity increased to 274 h-1.However,the further increasing of the concentration of cocatalyst could reduce the reaction activities,for example,the obtained copolymers included a number of low molecular weight polymer when the ratio was 20.The operation temperature is an important issue for the copolymerization.At a lower temperature such as 65℃,the catalytic system can not catalyze the copolymerization.It seemed that the activities and carbonate linkages were enhanced upon increasing the temperature of copolymerization.However,the further increases of the temperature could increase cyclic carbonate,the reaction activities and the carbonate linkages also reduced.
Keywords/Search Tags:Carbon dioxide, Cyclohexene oxide, Copolymerization, (Salen)CrIIICl, Quaternary ammonium salts
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