Don't know if rope is treated with chemicals or not. Just to be on the safe side, I wouldn't let him play with it, nor would I encourage him to be playing or eating of any threads, string, etc. All these can be swallowed and may cause an intestinal blockage which could result in expensive $$$ vet bill. Have heard of a cat that ingested dental floss with dire consequences. Not worth the chance, imo.
"We humans are indeed fortunate if we happen to be chosen to be owned by a cat." -- Anonymous