A Novel Controlled/living Radical Polymerization Via The Cycloketyl Group

The developments of polymer chemistry the past 20 years have involved a series of huge breakthroughs in the controlled/living radical polymerization(CLRP)field,including atom transfer radical polymerization(ATRP),reversible addition-fragmentation chain transfer polymerization(RAFT),stable free radical mediated polymerization(SFRP),single-electron transfer-LRP(SET-LRP),Te-,Sb-,and Bi-mediated polymerization,and reversible chain transfer catalyzed polymerization(RTCP).When it comes to their chemical principles,these CLRP methods rely on a dynamic equilibrium between the propagating radicals and the dormant chains via a reversible deactivation mechanism,and they show great potential for biological,optical and electronic applications thanks to a superior ability for the synthesis of structurally well-defined polymeric molecules.Nevertheless,since there exist some inherent drawbacks for the present methods,such as a high polymerization temperature,undesired color,odor and toxicity due to the use of metal and thiocarbonylthio as the radical deactivation reagent,it is of great importance to develop a new CLRP method that can overcome the aforesaid drawbacks.With this motivation,two years ago our group reported on a new Cycloketyl Radical Mediated Living Polymerization(CMP)method where,a compound,[9,9']bixantheny 1-9,9'diol(BIXAN)was first synthesized with xanthone(XT)as the raw material by a zinc coupling reaction;then under heat or UV light radiation,BIXAN broke into two cycloketyl(CK)radicals that could not only initiate the radical polymerization but also couple with the polymer chain radical as the dormant agent,and finally,the reversible activation/deactivation equilibrium was constructed.According to the mechanical analysis,the CMP system generally showed the following features:(1)the molecular weight of the polymers was higher at the beginning;then gradually approached the theoretical values,and(2)the molecular weight distributions gradually decreased with the polymerization time.However,there are also exists some obstacles for this CMP system to emplified into industrial field.Fisrt of all,the BIXAN molecular is not stable enough to storage.Secondly,this CMP system can be only achiveved in high monomer concentration in THF and the reaction condition is very strick to achive.Finally,due to the low initiate ability of the BIXAN molecular,this CMP system cannot initiate the polymerization quickly enough.Thus,we then put forward two new CMP systems to promote these drawbacks.One is the living polymerization via NHPI/XT redox reaction;the other is the Cycloketyl thioketone mediated living radical polymerization.In the first system,we describes the application ofNHPI/ketones as a redox initiating system that construct the CMP system with a single electron transfer reaction via NHPI/XTfor radical polymerization of three typical monomers,BA,MMA and styrene.Surprisingly,we found that the NHPI/XT system could initiate the radical polymerization efficientlyatlow temperatures of 70?,whereas the molecular weight could grow as the conversion increased.To confirm the livingness,the block polymer is also synthesized with this strategy.As far as we know,this is the first report of the direct use of NHPI/XT as an initiating system for C/LRP,thereby opening a new avenue for this field.The best reaction conditions can be concluding as follow:When this NHPI/XT redox system is ultilized in the polymerization behavior of MMA,the best temperature is 70?,the ratio of NHPI/XT is 1:1,the best solvent is toluene and the monomer concentration is 25%(wt).The molecular weight sharply increased with the growth of the concentration of the polymer.When this NHPI/XT redox system is ultilized in the polymerization behavior of styrene,the best temperature is 70?,the ratio of NHPI/XT is 1:1,the best solvent is toluene and the monomer concentration is 25%(wt).The molecular weight sharply increased with the growth of the concentration of the polymer.When this NHPI/XT redox system is ultilized in the polymerization behavior of BA,the best temperature is 70?,the ratio of NHPI/XT is 1:1,the best solvent is toluene and the monomer concentration is 25%(wt).The molecular weight sharply increased with the growth of the conversion of the polymer.It is obvious to see that,all the polymerization behavior is similar to that in the traditional CMP living radical polymerization system.These characters can be described as:the starting molecular weight is extremely high and the PDI decreased with the increase of the conversion of the polymer.Thus we can strongly confirm that this NHPI/XT redox LRP system belongs to the CMP system.Meanwhile,this new redox system shows great advantage in the synthesis of the giant molecular weight polymer.In the second system,we construct the living radical polymerization system with AIBN/cycloketyl thioketone and ultilized this system into many typical monomers.In this system,AIBN can initiate the polymerization well and the cycloketyl thioketone play the role as the dormant group to capture the main chain radical and fragment again to achive molecular weight increase.All the results show the living characters and the block polymer is also well synthesized.However,in this system,the starting molecular weight is very high and the PDI remains in 1.5 thus,we need a new strategy to accuire narrow distribution polymer.We then induced very tiny amount styrene into this system and surprisingly found that the starting molecular weight and the distribution in this cycloketyl thioketone mediated polymer are well controlled.The block polymer is also synthesized by this strategy.