What are the stress rules for pronouncing words in English that end "-ive"?

Everyone you ask will say it differently. It’s up to you.

I personally have never heard that word spoken.

According to the Cambridge English Pronouncing Dictionary, you are correct about the stress: PURposive.

I’m not sure about a general rule… I’m sure you’d be able to find enough loopholes to invalidate it—though for this example I’d say that the stress has more to do with the vowel sounds (rather than the consonents) in the second syllable of the base word: i.e. the schwa in purpose as opposed to the long o in explode.

Thanks, Andrew. That’s exactly what I was looking for. I knew there was some reference or other where I could find this. I’ll refer to the Cambridge English Pronouncing Dictionary in the future.