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Why is synthetic isoprene rubber less strong in its raw state than natural rubber?
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Truck tires being removed from their molds.
Natural rubber is considered "self-reinforcing" because its molecular structure consists almost exclusively of the cis-1,4 polymer, which allows it to crystallize readily when stretched, thereby increasing its strength.12
In contrast, synthetic isoprene rubber (IR) is less strong in its raw, unvulcanized state because its structure is not as regular as that of natural rubber; it is at most 98 percent cis-1,4 polyisoprene. Due to this lack of structural regularity, synthetic isoprene rubber does not crystallize as easily as the natural material, which directly results in lower strength and tackiness before it undergoes vulcanization.12