The stress‐cracking of polycarbonate by a gaseous or liquid agent results from the diffusion of this agent into the polymer. The low molecular weight polymer fractions and the chain ends within the bulk of the polymer become more ordered during the diffusion and swelling process by their partial solubility in the crazing agent, causing crystallization. The creation of interfaces at areas of order‐disorder causes high shearing forces at this boundary and voids within the bulk of the polymer. These voids are then propagated as crazes or cracks at stresses much lower than the tensile strength of the polymer. Therefore, a stress‐cracking agent need not diffuse rapidly, but must be an effective environment for swelling and/or crystallization. Data from diffusion, density, thermal and molecular analyses are presented to support this mechanism.

The “living” polymer method was used to prepare block polymers of vinyl monomers and isocyanates at low temperatures in toluene–tetrahydrofuran mixtures. Vinyl monomers and diisocyanates, which have one hindered isocyanate group, as in 2,4‐toluene diisocyanate, form block polymers which contain pendant reactive isocyanate groups. These block polymers can be crosslinked with water, diols, diamines, etc. The polymerization is apparently limited to block polymer formation, since the polyisocyanate anion is incapable of initiating the polymerization of common vinyl monomers.

Miller and Rose 3357 610. s-Triaxolopyrirnidines. Part 1I.l Synthesis as Potential Therapeutic Agents By G. W. MILLER and F. L. ROSE 3-Amino-s-triazolo[4,3-c]pyrimidines and the isomeric 2-amino-s-triazolo-[2,3-c]pyrimidinesJ of the type described in Part I,l together with a number of a-substituted amino-derivatives, have been prepared , either directly or indirectly, from the corresponding 4-thioureido-, 4-semicarbazido-, and 4-thiosemicarbazido-pyrimidines. The circumstances under which compounds of the [4,3-c] series isoinerise into those of the [2,3-c] series are discussed, IN Part I is described the synthesis of a number of 3-amino-s-triazolo[4,3-c]pyrimidines (I; R3 = H) by the interaction of cyanogen chloride with the hydrazinopyrimidines (11), and the preparation is discussed of the isomeric 2-amino-s-triazolo[2,3-c]pyrimidines (I11 ;

SynopsisThe use of combined modes of thermal analysis by differential thermal analysis (DTA), thermomechanical analysis, and thermal depolarization analysis (TDA) presents a more illustrative picture of the phase transitions and behavior of the allotropes of sulfur.Volumetric changes suggest the T, of polymeric sulfur to occur at 35OC. Changes in depolarized light level and an abrupt change in volume a t 77°C identify this change as a firstorder fusion. The depolarized light behavior of sulfur, which has been quenched from 200°C to -190°C, illustrates the probable existence of a mesomorphic state of sulfur tietween 100" and 122'C. Polymericsulfur is thermally bleached near -150°C.

synopsisThe development of new and more sensitive techniques in thermal analyses aids in a more complete understanding of the contributions of individual components in urethane elastomers regarding their mechanical and thermodynamic behavior. The behavior of various segmenk of the elastomers reported in this work illustrates a clearer interpretation of reasons for changes in mechanical behavior caused by changes in heat capacity, volume and tensile properties; the gross changes previously reported for polyurethane properties as a function of temperature are also confirmed with a more exact definition of their origin.The sub-ambient temperature behavior and response of physical measurements near the melting point of the backbone polyol are largely a function of the so-called "soft block." The soft block does not contribute to the mechanical properties above the melting point of the polyol u n l w some urethane segments from the diisocyanate and extender are structured into the soft block, that is, excess diisocyanate and extender are added to build in the ''hard" block. The extender and isocyanate iduence for both low and high temperature properties is observed by the lack of molecular fit imparted to the backbone polyol as well as some crystallinity in the polymer hard block. The usual T, transition reported in urethane elastomers corresponds to a firsborder transition in the polyester or polyether backbone.