The tiger would need to execute a perfect ambush bite to the neck/back of head, and it would have to be enough to get through all the fur, fat, and big ass skull without the bear powering out. I just don't see it. Once that fails, the tiger has no way of landing a killing blow without getting wrecked. Polar bears have wicked big teeth and strong bites themselves that have been adapted for holding on to very large, slippery prey. Smaller, starved or young polar bears? A lot more feasible. Not the bigguns though.