The synthesis of block copolymers in a flow system

Abstract

Recent advances in macromolecular synthesis have been concerned with molecules containing two components in which each component is not distributed randomly as in a copolymer but segregated to a defmite part of the molecule. The provisional name of block copolymer has been applied to such a nonrandom type of structure. Linear molecules of this kind have been synthesized in two ways.' By utilizing a two-phase system of aqueous methacrylic acid and styrene it has proved possible to synthesize part of a molecule in the aqueous phase and part in the styrene phase. This method has obvious limitations since it is not easy to choose monomers which will be solely confined to the phases concerned. The second method consists in attaching to the end of a polymer (in this case polystyrene) a bromine atom, dissolving the polymer in a suitable monomer and then removing the bromine atom by photolysis. The polymer radical and the brordine atom each initiate polymerization and thus three types of polymer are present in the system. The polystyrene component can be extracted by suitable solvents and the block polymer separated from the remaining mixture by precipitation procedures using the turbidity of the solution to follow the precipitation.2Branched chain nonrandom copolymers can also be made by the process of grafting a new polymer chain to a polymer by conducting polymerization of monomer B in the presence of polymer originally made from monomer A.3 In doing this two structural features are incorporated in the molecule and therefore it is not easy to see how each of these features contributes to the properties of the resultant polymer. In attempts to deal with the branching factor methods have been developed for the synthesis of branched chain polymer and also grafting has been followed in homogeneous polymer by grafting on radioactive polymer of the same chemical composition as the original polymer.Since this new field in polymer chemistry is extensive and relatively complicated it is necessary to devise a number of new methods for the synthesis of block polymers so that a wide variety of new kinds of moleciiles may be synthesized and their properties examined. This paper describes another method for the synthesis of block polymers and incidentally another technique for measuring the lifetimes of polymer radicals.A selected monomer containing photosensitizer flows along a capillary at a defined rate into a suitable reservoir. A limited portion of the capillary is illuminated with very in- 461The principle of the method is as follows.

Continuous Photopolymerization of n-Butyl Acrylate Using a Narrow Channel Reactor

“…For [P] 0 values higher than 2.5% w/w BA, primary radical termination is believed to be the possible cause for the decrease in conversion. When the local concentration of the radicals is high, the radicals terminate bypassing the propagation step to yield dead fragments. , When the [P] 0 concentration was less than 2% w/w BA, the amount of polymer formed increased with an increase in the amount of the [P] 0 used. Since a [P] 0 value of 2% w/w BA was found to be the optimum, the same was chosen for experiments to study the influence of exposure times and light intensity.…”

“…Hence, the study of various parameters on the kinetics of polymerization will not be accurate. Continuous photopolymerization in a narrow channel flow system has been reported by Hicks and Melville to synthesize block copolymers . A channel diameter of 1 mm and length of 30 cm was used.…”

“…Continuous photopolymerization in a narrow channel flow system has been reported by Hicks and Melville to synthesize block copolymers. 9 A channel diameter of 1 mm and length of 30 cm was used. A length of 4 cm was used as the zone of illumination.…”

Photopolymerization of vinyl monomers

“…The flow method can be used to synthesize block copolymers whose composition can be controlled (149,150). The polymerization of one vinyl monomer is photoinitiated at one point in the flowing system and allowed to flow into a second monomer solution.…”

“…The advantage of this initiator is that it does not have a dark rate due to thermal decomposition at 25° ( 182). It has been used as a photoinitiator for kinetic studies of vinyl acetate (36,209) and in the flow system for the synthesis by photopolymerization of block copolymers (148,150). Azonitriles as photosensitizers have the obvious advantage over peroxides in that they absorb radiation of longer wavelengths.…”