While they will sometimes split or the root balls divide naturally when being transplanted, I'd hesitate to do so intentionally -- you are never guaranteed of a successful result. If given the choice, I'd opt for a small, new plant rather than risking the mother plant.

Agreed unless it is one of the non vining Integrifolia types. I've split several, easily.

Jackmanii isn't in that group though.

Many times I've successfully taken a division from an established large flowered clematis, though this must be done most carefully so as not to damage the mother plant. As gardengal48 has mentioned, older plants will often naturally divide, though unless needing to move a plant, I really dislike disturbing a clematis, as these can be a tremendous pain to dig because of the vast root system most will develop. I agree it being best to purchase a new plant that can often be obtained at a very reasonable price.

Thanks, a new one is on my spring list...