Kinetics of the Polymerization of Styrene by Friedel–Crafts Catalysts
ALTHOUGH the mechanisms and kinetics of the free-radical catalysed polymerization of styrene have been fairly extensively investigated1,2, the polymerization employing Friedel Crafts catalysts has received much less attention. Williams3 has investigated the stannic chloride catalysed polymerization of styrene in carbon tetrachloride solution and shown that the overall rate of monomer consumption can be represented by the expression: where m is initial styrene concentration, x is styrene consumed in time t, b is initial catalyst concentration. He has also shown that induction periods are present. We are studying the aluminium chloride catalysed polymerization of styrene in carbon tetrachloride solution from a kinetic point of view, and it is proposed to summarize here the principal conclusions reached to date.
SOME ASPECTS OF THE MECHANISM OF ADDITION POLYMERIZATION
It has long been recognized and is now generally accepted that, aa a rule, the process of addition polymerization differs fundamentally from condensation polymerization. The latter involves only types of reactions with which the chemist is quite familiar, with the important characteristic feature that the monomer molecules are so constructed that the condensation process is capable of indefinite repetition to build up a polymeric. product. The reaction proceeds in a regular, stepwise manner, yielding material of steadily increasing degree of polymerization throughout the course of the reaction. This mode of formation contrasts sharply with that involved in typical addition polymerizations, which are characterized by the fact that the polymer molecules first formed are essentially the same size aa the rest. In other words, the polymer molecules do not grow gradually, but each polymer molecule grows rapidly to a certain size and is then stabilized. This behavior, coupled with the uusceptibility of these reactions to catalysis and inhibition, supports the suggestion that these reactions are chain reactions involving activation of the monomer (initiation) followed by rapid successive additions of monomer molecules to the active intermediate (propagation) until the activated polymer becomes stabilized (cessation).
One-pot synthesis of annulated pyrido[2,3-d:6,5-d]dipyrimidine derivatives using nitrogen based DBU catalyst in aqueous ethanol medium
Abstract We have reported DBU as an efficient organocatalyst for one-pot three-component synthesis of annulated pyrido[2,3- d :6,5- d ]dipyrimidine derivatives using aromatic aldehyde, thio-barbituric acid and 6-amino-uracil in aqueous ethanol under reflux. The attractive advantages of this process are mild reaction conditions, environment friendly procedure, short reaction times, easy isolation of products, and excellent yields (72–89%).