Addition polymerizations involving soluble organometallic species have received intensive attention in recent years with special reference to the type of counterion and solvent. An anionic mechanism is proposed for those systems where there is good reason to assume that the metal is strongly electropositive relative to the carbon (or other) atom at the tip of the growing chain. Hence, the metal, e.g. lithium, becomes a cation either in the free state or coupled with the growing carbanion. Under the appropriate experimental conditions, spontaneous termination is avoidable in many of those systems when one of the metals of Group I is used as the counterion. The alkali metals sodium and potassium were revealed to be polymerization initiators of isoprene in the disclosures of Matthews and Strange in 1910 and Harries in 1911. The first unambiguous report of the use of lithium in reactions with diolefins appears to be that of Ziegler and coworkers in 1934. Their work consisted of an investigation of the reaction between the alkali metals (lithium, sodium) or alkyllithium species and butadiene, isoprene, 2,3-dimethylbutadiene, or piperylene.

No abstract

Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

No abstract

No abstract

at the present time. Preliminary X-ray studies of the triblock polymers under stress indicate very marked changes in all of the scattering peaks. However, the interpretation is no longer as simple as in the above experiments.

A study has been carried out of the association phenomena which occur during the polymerization of styrene, isoprene, and butadiene by organolithium initiation. This has been done by means of viscosity measurements of these systems after the conclusion of the polymerization reaction. It has been found that, in general, the polymerlithium species, in hydrocarbon systems, are associated in pairs. This applies to the styrene system in benzene and to the isoprene and butadiene systems in n‐hexane. In tetrahydrofuran solution, none of these systems show any polymer association. For the polyisoprenyl lithium in n‐hexane, the equilibrium constant for the dissociation of the associated pairs at 30°C. is of the order of 10−6, and the heat of dissociation is about 37 kcal./mole. By means of this viscosity technique, it has also been found possible to show that tetrahydrofuran forms a monosolvate with the polyisoprenyl lithium (RMiLi · THF) and that the heat of solvation is about 18 kcal./mole. These findings are in accord with the proposed mechanism in which the active species, in hexane, is represented by the unassociated polymer‐lithium, RMi,Li, while, in THF, it is the solvated species, RMiLi · THF. On this basis the following propagation rate constants have been deduced for isoprene polymerization: kp(hexane) = 3.4 × 103 exp {−4100/RT} = 8.9 at 60°C.; kp(THF) = 1.5 × 104 exp{ −6800/RT} = 0.4 at 60°C.

A study has been carried out on block polymers of the A‐B‐A type, the thermoplastic elastomers, where A represents polystyrene and B polybutadiene or polyisoprene. The objective was to relate the mechanical properties of these elastomers to their molecular architecture. For this purpose a series of styrene‐butadiene and styrene‐isoprene block polymers were synthesized by means of organolithium initiators, in which the block lengths of the polystyrene and polydiene were varied. It was found that the stress‐strain properties of styrene‐butadiene polymers are mainly dependent on the polystyrene content, regardless of the block sizes, and that the center, elastic block does not appear to behave as the “molecular weight between crosslinks” of these networks. However, the monodispersity of the chain length of these elastic blocks does appear to contribute substantially to higher tensile strengths. The polystyrene appears to be aggregated in small domains, of the order of several hundred angstrom units, and these undergo an irreversible deformation under stress, which is, however, completely recoverable above the Tg of the polystyrene. At high polystyrene contents (∼40%) these elastomers exhibit an irreversible yield point at very low elongations, and this is ascribed to the presence of a continuous phase of connected polystyrene domains, which can be re‐formed by raising the temperature above the Tg of the polystyrene. Although the stress‐strain curves are not affected by the thermal history of the sample, the tensile strength, especially at high styrene contents, is strongly dependent on “annealing” of any frozen stresses in the polystyrene phase. The useful range of block molecular weights is about 10,000–20,000 for polystyrene and 40,000–80,000, respectively, for polybutadiene. The lower limit is probably governed by the minimum polystyrene chain length required to insure the formation of a heterogeneous phase; while the upper limit is set by the high viscosity of both blocks, which might seriously hamper domain formation.