And you never want the log to autogrow during index maintenance because it's such a slow process.
Instead, pre-grow the log to the total size you'll need. If you really must, you can shrink the log after the index rebuilds finish.
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial: "If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them."