Yeah, the Antarctic on average is 30 degrees Celsius colder than the Arctic, so we shouldn’t assume polar bears could live just anywhere down there.
Assuming this introduction could be accomplished with a minimum of impact, it deserves (almost wrote “bears”) consideration. It’s expected that the Arctic ice cap could become seasonal ice within the next few decades, which means polar bears could become extinct if something isn’t done.
The polar bear’s range has already shrunk in historic times. In pre-Columbian Europe they were known as “white bears” because they ranged much farther south than just the Arctic.
Temperature Cite:
http://www.visitandlearn.co.uk/deployment02-03/wilderness5.asp
Couldn’t find a cite about “white bears,” so I won’t swear to it.