AbstractNatural rubber breaks down readily during mechanical mixing, and rubber processors have traditionally made good use of this property. Emulsion SBR rubber breaks down with greater difficulty and some polybutadiene rubbers, not at all. This paper describes a new class of solution elastomers, both polybutadiene and butadiene–styrene copolymers, designed to undergo controlled breakdown during processing. They can be made by reacting active polymer lithium molecules with tin compounds to produce star‐shaped molecules containing a central tin atom connected by tin–carbon bonds. When such a product is mixed with an organic (stearic) acid, some of the tin bonds are ruptured and the desired breakdown occurs. The extent of breakdown can be controlled by the mixing conditions and the amount of organic acid employed. There are numerous applications in which these rubbers process more easily and display better properties than conventional rubbers. Data on breakdown under various conditions and physical properties of the compounds are presented in this paper.In addition to the superior processing characteristics, these products introduce a new concept of elastomer breakdown. The breaking of a polymer chain at a carbon–tin bond by a chemical agent eliminates the need for breaking carbon–carbon bonds by mechanical energy or oxidative degradation. Accomplishing polymer breakdown by a controlled reaction, rather than by conventional means, has long been an unsolved problem of the rubber
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