Green tea has the same amount of caffeine as black.

Before I knew better, I brewed green tea for 5-6 minutes. It makes a
bitter tea with quite a bit of kick, but it takes alot of will to drink it.
Proper brewing is MUCH better.

My understanding is if you do the traditional "gongfu cha" brewing method
you get most of the caffeine out during the first steeping, subsequent
steepings have less caffeine. I have no idea about polyphenols, but I
suspect it's similar, as the polyphenols in green tea are 95 percent
absorbed by the water in about 3 minutes.

Also, some decaffination processes remove polyphenols, but there is
disagreement on this point in the literature I have read.