Stopper, catalytic

Polymerization. The polymerization of aziridines takes place ia the presence of catalytic amounts of acid at elevated temperatures. The molecular weight can be controlled by the monomer—catalyst ratio, the addition of amines as stoppers, or the use of bifimctional initiators. In order to prevent a vigorous reaction, the heat Hberated during the highly exothermic polymerization must be removed by various measures, ie, suitable dilution, controlled metering of the aziridine component, or external cooling after the reaction has started. 

The 1-1. centrifuge bottles (Coming No. 1280) carrying No. 6 rubber stoppers, as used for catalytic hydrogenation, are suitable for carrying out reactions under pressures up to at least 3 atmospheres. The submitter used a heavy, selected 2-1. round-bottomed flask instead of the four bottles specified in the procedure above. 

As in all catalytic reductions, the purity of the starting material is of great importance. Redistilled furan, b.p. 31-32 , prepared by the method of Wilson (Org. Syn. Coll. Vol. i, 269) is quite satisfactory. It has been reported that furan prepared by the method of Gilman and Louisinian should be dried over calcium chloride and fractionated carefully. It is advisable to redistil the furan shortly before use and to avoid contact with rubber stoppers. 

Analysis of NROR by plasma emission spectroscopy reveals that the enzyme does not contain any metals in significant amounts (>0.1 g-atom/mol) and flavin appears to be its only prosthetic group. The enzyme is quite thermostable, with the time required for a 50% loss in catalytic activity when it is incubated at 80° and 95° of 12 and 1.6 hr, respectively. These values are determined by maintaining the purified enzyme (0.4 mg/ml) in 60 mM EPPS, pH 8.0) in serum-stoppered glass vilas at the desired temperature and periodically removing samples to determine residual BVNOR activity at 80°. 

EtOH, then again three times with absolnte EtOH. Store the Ni catalyst in bottles which are filled with absolnte EtOH to the brim and stopper them. Keep them in a cold store room. Yield of Raney Ni is usually 75g. The metal should be kept moist, or under hquid, at all times because the dry solid is highly pyrophoric. The more active the catalyst, the more readily it ignites. For this reason it is best to spoon the catalyst out from the settled material. When prepared as above it contains 0.6g of Ni per ml of settled soUd, and a level tablespoonful contains 3g of Ni. [Mozingo Org Synth Coll Vol III 181 7955]. The activity of Raney Ni deteriorates slowly with time, but if prepared properly and stored as above it should be quite active for about 6 months. Its activity can be enhanced immensely by addition of a very small amount of triethylamine and chloroplatinic [Levering Lieber Chem Soc 71 1515 1949], It has been used both as a catalytic oxidation catalyst (with an appropriate donor), as well as a reduction catalyst [Kleiderer Kornfeld J Org Chem 13 455 1948. ... 

In all instances only starting material was isolated with the and C-NMR confirming the absence of any triazole signals. This clearly illustrates that the dipolar cycloaddition reaction only takes place in the presence of cucurbituril, which has a dual function to catalyze the cycloaddition reaction and in the process become inextricably threaded onto the polymer chain. The sequence of events involved in this catalytic self-threading process is presented in Fig. 1.45. The combination of using monomers with bulky in-chain stopper groups and the catalyst self-threading onto the polymer backbone limits this catalysis to a turnover of exactly 1. 

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