Studies On Rate Enhancement Of Atom Transfer Radical Polymerization Of Styrene

Combining various merits of free radical polymerization and living polymerization, such as mild reaction condition, wide range of monomers and robust method for preparing well-defined polymers with controlled molecular weight and narrow polydispersities, Atom Transfer Radical Polymerization has become one of the most heated domains in polymer science and a focus for industry. Nevertheless, relative high reaction temperature and slow rate of polymerization inhibits its prevailing application in industry. Hence, the pressing problem that should be addressed for its industrial application is to improve its polymerization rate. This paper mainly concentrates on accelerating ATRP of styrene by introducing a variety of additives.At the beginning, four malononitrile derivatives were introduced as promoter for ATRP of styrene. The results showed that malononitrile (MN) and acetylmalononitrile (ACMN) were both able to remarkably enhance the polymerization. However, compared with MN in the presence of which controlled moleculare weight and narrow polydispersity polystyrene was obtained, ATRP of styrene using ACMN as rate promoter loses its controllability. Futher study informs us that the optimum ratio of MN to initiator was 4:1. The conversion of styrene reached 86.2% in 6 hours at 75℃in the presence of MN, while that of typical ATRP of styrene in the similar condition was only 30.7%. Excellent controllability was obtained with the indication of first order kinetical curves, linear increase of Mn with conversion, good agreement between Mn_exp. and Mn_theo. , and well-defined polymer with narrow distribution (PD=1.09). Ddependences of rate accelerating efficiency of MN on temperature were observed and overall rate constants were improved with a multiple of 1.8, 2.4 and 2.67 at 65℃, 75℃and 85℃respectively. The preliminary mechanism for rate enhancement of MN was proposed basing on ¹HNMR method and uv-vis spectrum of mixtures of MN, ligand, and catalyst. The possible mechanism was that interreaction between MN, ligand, and catalyst weakens the coordination of catalyst and ligand resulting in facilitating the transfer process of bromine atom and enhancing sigle electron transfer between copper ions in different oxidation states.ATRP of styrene was also investigated in the presence of 2,4-pentanedione derivatives including acetylacetone(AAT), 3-methyl-2,4-pentanedione(MPD), 1,1,1,5,5,5-hexafluoroacetylacetone(HFA). Among these derivatives, MPD was the optimum rate accelerating additive. A conversion of 71.6% was achieved in the presence of MPD in 6 hours at 80℃and the ratio of MPD to initiator providing remarkable rate enhancement and good controllability was 4:1. Positive dependences of rate accelerating efficience of MPD on terperature were also observed. The overall rate constant irrespectively increases with a multiple of 1.34, 1.79 and 2.25 at 70℃, 80℃and 90℃respectively. Results of ¹HNMR and Uv-vis indicate that interaction between MPD and catalyst is likely to enable sigle electron transfer between high and low oxidation state of catalyst to process more quickly.Malonic esters, such as Diethyl malonate (DEM), diethyl methylmalonate (DEMM), diethyl tert-butylmalonate (DEBM) and diethyl diethylmalonate (DEDEM) were also investigated to promote ATRP of styrene. DEM, DEMM and DEMM were found to enhance remarkably polymerization rate, and corresponding conversions are 67.8%, 81.7% and 70% at 80℃for 6 hours. Unfortunately, only in the presence of DEDEM can polymerization obtain polystyrene with narrow polydispersity when accelerating polymerization. When the ratio of DEDEM to initiator was 0.5, good controllability was achived. What should be pointed out is that electron donating groups in malonic esters and 2,4-pentanedione derivatives facilitate the polymerization rate , while the introduction of electron withdrawing groups and groups with steric hinderance directly inhibit polymerization rate of styrene.Finally, three kinds of inorganic compounds including boric acid (BA), aluminum hydroxide (AH), and 2-methylpropylboric acid (MPBA) were found to be effective to accelerate ATRP of styrene. Conversions were respectively improved to 77.8%, 79.1% and 83.0%. Furthermore, the avaible range of molecular weight is investigated under these inorganic compounds with different designed molecular weight. When designed molecular weight was more than 20 000, efficiency of enhancement of BA and AH decline sharply, while a effective enhancement of MPBA on polymerization is still observed even through designed molecular weight was 40 000. BA is the most efficient promoter among these inorganic compounds.