No abstract

The filtrate from (II) was concentrated under reduced pressure, acidified, and the oily solid recrystallized from toluene to give 5.6 g. of an approximately equimolar mixture of Ncarbobenzoxy-D-o-fluorophenylalanine and N-carbobenzoxy-DL-o-fluorophenylalanine. Fractional recrystallization from toluene gave 1.0 g. of Ncarbobenzoxy-D-o-fluorophenylalanine (IV); m. p. 103-105°( cor.

A study has been made on the effect of thiols and disulfides on lowering the molecular weight of polysulfide polymers. It was shown that in the presence of aqueous sodium disulfide there is an interchange of chain segments. This effect is observable if a simple organic disulfide is present in the system, resulting in a lower molecular weight. Co‐polymers can be prepared by heating together in the presence of sodium disulfide, aqueous dispersions of two different polysulfide polymers. When thiols are present in the system, interchange of disulfide groups occurs in the absence of sodium disulfide. The results show that disulfide/disulfide redistribution and thiol/disulfide interchange take place in polysulfide polymers. The occurrence of these same reactions has recently been recognized by workers in the biochemical field. A literature survey of thiol/disulfide reactions showed that results formerly attributed to oxidation/reduction reactions could be also explained by an interchange mechanism.

Incorporation of elastomers into polystyrene has been used in recent years to overcome its inherent brittleness. Mechanical mixing of rubber with polystyrene has been largely superseded by dissolving the elastomer in the styrene before polymerization. Impact-resistant polystyrene produced by either method is a two-phase system with polystyrene the continuous phase and elastomer the discrete phase. Polymerization of styrene in the presence of an unsaturated elastomer converts the elastomer to a graft copolymer. The graft copolymer is formed by reaction of the polymerization initiator with the rubber to form an active site thereon, which can initiate polymerization of styrene to form a side chain of polystyrene. The graft copolymer is crosslinked in the process. The size of the gel particles must not be too large for a satisfactory product, but can be controlled by shear and by chain transfer agents. The rubber chiefly used with polystyrene has been SBR, but use of polybutadiene is growing because of improved impact resistance at low temperatures. The resistance to impact increases with increasing amounts of elastomer, but surface gloss and hardness decrease. The chief advantage of graft copolymerization over poly-blending is more efficient utilization of the elastomer. The increase in resistance to impact has been attributed to the energy absorbing character of the elastomer. The two-phase system is necessary so as not to lose this basic property of elastomers. The rubber particles may also act to induce many small cracks rather than one large one, thus increasing the energy requirement for total failure. The greater efficiency obtained by a graft copolymer may be due to a more diffuse interface between rubber and polystyrene, leading to more absorption and less reflectance of a stress wave by the particle of rubber.

Polysulfide liquid polymers are a comparatively recent development conceived at the Thiokol Laboratories in 1943. The development was initiated by the problem of finding methods to reduce the molecular weight of a polysulfide rubber which was too tough to process on conventional rubber milling equipment. The problem was solved by reduction of a few of the disulfide links present in the polymer chain. It was soon found that the method was applicable to the preparation of polymers low enough in molecular weight to be liquids. The method produces dithiols of high purity which are extremely active in a wide variety of chemical reactions. Some formulations have been developed which depend on conversion from the liquid to rubber state at temperatures as low as 50° F in about 30 minutes. Most of the converting agents function through oxidation with hydrogen removal from the thiol and a linkage of sulfurs to reform the disulfide group. The converted polymers have the general properties of polysulfide polymers: good solvent resistance to a wide range of solvents, low diffusion rate of gases, good resistance to oxidation, ozone, and weathering, and a service temperature range from −65° to +250° F. (Some compounds can withstand intermittent temperatures as high as +350° F.) The low temperature properties are inherent in the polymer and do not depend on special compounding techniques.