I was very surprised when open office's spell check underlined the word introductive. I checked a few dictionaries and only a few (including The Shorter Oxford English Dictionary, Oxford University Press, 1973) list it as an entry, whereas all contain the word introductory. I would like to know if the latter is preferable, since I used the former in an official document, and prefer to stick to it unless there are sound reasons not to.
I just checked the BNC and COCA for usage stats in British and American English and found zero for both. Checking Google's NGRAM viewer for these two, back in the early 1800s a few instances of introductive are recorded, but introductory is common.
There are sound reasons not to use introductive:
Despite its grammaticality, it's a stylistic zit. If you want your prose to be acned, then use it. If you want your prose to be smooth and appealing, then use either introductory or prefatory. I advise this as a writing teacher (40 years) and technical editor.