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This is going to be a bit complicated, but let me try to explain things.
When you grapple someone, both you and the target gain the Grappled Condition which, among various penalties, also prevents you from performing any Attacks of Opportunities.
If you aren't grappling anyone and get an AOO, such as someone trying to grapple you without the improved grapple feat, you can either use your AOO to attack them once (and get a free grab if you hit) or to perform one of several combat manuevers. Trip is one of them, as is sunder, so you can trip if someone provokes an AOO.
Now, once you are grappling someone, if you choose to harm them as part of your maintaining a grapple (other options: reposition, pin), you do automatic damage equal to one attack from a natural weapon or one handed weapon. You cannot choose to sunder INSTEAD of the free damage because you are not making an attack.
However, since you have Greater Grapple, you can maintain with a move action (normal is standard), leaving you a standard action remaining after maintaining the grapple (you have to do this every turn or the target is automatically freed). You can use your standard action to sunder his armor or weapon, but you have to make the normal sunder check (don't forget the penalties you get for having the Grappled Condition).
Since you can grab and greater grapple, you can do this: Turn one move up and punch him, then grapple with the free grab. Next turn, use move action to maintain and pin him, then the standard action to make a sunder attempt on his stuff. Every turn afterwards, make a maintain check to harm him then sunder more stuff.
Alternatively, you can maintain to harm, free action to let go, then punch and grab, then just repeat every turn. You know, if you're bored and want to play with your prey, heh.