Keep all your weight training fast and explosive like adam gb said as this builds power, doing reps slowly without full force will only build strengh which will actualy make punches weaker.

as for reps and sets:

Variable Strength Power Hypertrophy Endurance
Load (% of 1RM) 80-100 70-100 60-80 40-60
Reps per set 1-5 1-5 8-15 25-60
Sets per exercise 4-7 3-5 4-8 2-4
Rest between sets (mins) 2-6 2-6 2-5 1-2
Duration (seconds per set) 5-10 4-8 20-60 80-150
Speed per rep (% of max) 60-100 90-100 60-90 6-80
Training sessions per week 3-6 3-6 5-7 8-14

hypertrophy = Bulky muscles that arnt that powerful which is realy bad for boxing. hypertrophy is the training that bodybuilders do

1RM = The maximum weight you can lift for 1 rep.

So for speed and power you should stick to the power routine but you should also do some of the endurance training otherwise you will lose that speed and power when you get fatigued. Bodyweight calisthenic exercises are great for building endurance.

Plyometric training is also good for speed and power. The best thing for building punching power is the heavy bag, heavy bag training is no.1 on my list for developing a KO punch. When doing heavy bag training make sure you are not pushing the bag when you punch it as this will result in week punches and will also leave you off guard in a fight. If you are getting power in your punch the bag should jump up a few inches when you hit it, if it swings to much you are pushing it .