If you get the Intelligence mastery on the Cunning tree with Zilean and max your CDR off at 45%, you can Q,w - wait 2 seconds for the first bomb - throw the next bomb - w when it gets off cool down-wait 1 second and throw the last bomb in the combo. This can be done further and faster for a 4 or 5 bomb stun chain. But my reflexes are too slow. for me its a 3 too 4.5 seconds stun. Zilean support meta incoming? 
Its not as easy as it seems in theory. First of all, the way you describe it, they are not "chain stuns", the stun only lasts 1.5 seconds, a proper chain stun is only possible from the first and second stun and is extremely hard to pull off on its own.
Disregarding the throw animation, for a chain stun, the first Q needs to be thrown at 0 seconds, the second one at 3 but the third one already at 4.5 seconds (because thats when the first stun ends), which makes it impossible to throw the fourth one at 6 seconds (for a tripple chain stun), because Rewind just isnt off cooldown yet (Remember Rewind has a Cooldown of 3.3 Seconds with 45% CDR).
I however prefer to maximize the Stun duration over time without the benefit of having the stuns chained together, in this variant it is possible to throw the first Q at 0 seconds, the second at 3, the third at 6, 9 and so on (which can be done up to 4 or 5 times depending on your cooldown reduction). Just remember these aren't chain stuns, I prefer to call the whole combo a "Stun Burst", because it cannot be repeated forever.
The trick is to let the Initial Bomb sit for as long as possible - in the best case exactly three seconds (remember the Bomb countdown starts as soon as the Bomb lands, not when you throw it). The hard thing here is to estimate the Bomb throw animation, which is kind of an undefined amount of time somewhere between 0.2 and 0.4 seconds.
The hard part is landing the first two
