To clarify, the addictiveness of nicotine ≠ the addictiveness of tobacco. Even aside from the additives used by the tobacco industry, tobacco naturally contains an array of MAO inhibitors and other compounds that work in harmony with nicotine causing it to be far more addictive than nicotine itself. Pure nicotine is much farther down the scale of addictiveness, classed as a “weak reinforcer” in studies.
If you are interested in the subject, I highly recommend reading the studies and posts by Maryka Quik, director of the Neurodegenerative Diseases Program at SRI International. I first found out about her in an interesting article published in Scientific American — LINK.