How do you break your opponent's grasp...
there are tons of techniques you can look up on youtube but generally i do one of two things, depending on whether i have read the catch coming or not
if you see it coming and can react in time, as your round kick lands and the opponent begins to encircle it you kind of continue your momentum and turn yourself to face somewhat away, turning the leg over so the knee is facing the floor, sole of foot towards the ceiling, and pull your leg out. Best way i can describe it. Try and imagine it all in one movement - its not like you're stood there for a minute with your knee pointing down etc coz that would just give ample time for your leg to be wrapped up. its an instant reaction
(you can also turn your knee out the other way, pointing outwards, and try to place your heel on the guys hip and try to push off; i am not too good with that one)
if your kick gets caught solidly, the catcher will basically guillotine your leg at the calf. Hop forward and push your leg through, driving the foot of the caught leg down to the floor as hard as you can, at the same time taking hold of of the catching arm's elbow with your palm and pulling it upwards as hard as you can, to break the grip. the combination of your leg and arm pressure in tandem should be enough to break the grip and allow you to get your foot back on the floor; you are then in a clinch situation
some guys also just unload punches and elbows when their kick is caught, taking advantage of the catcher having tied up one of his arms, but this generally isnt the case at the high level because catch turns to sweep pretty quick up there
...and prevent a sweep or takedown?
stiff-arm post on the catcher's jaw to push his head back and away, at the same time posting off your caught shin, pushing your hips in but your upper body away, you are kind of stalemated at this point as he cant sweep your standing leg as its too far away. You'll want to time soemthing here like a transition to another escape or into a jumping tech like a knee with your standing leg