[spqr0a1]

It's more than just the chain length. The most common synthetic rubbers are styrene-butadiene copolymers. Natural rubber is polyisoprene. While it is true that shorter chain synthetic polyisoprene is available, it is a much smaller part of the market than styrene-butadiene.

[dpeckett]

Does look like both natural and synthetic polyisoprene is at-least somewhat biodegradeable: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC92035/