Maggie is right, if you are using a food grade plastic it should be fine.
I do have to say that what you are doing with the boiling water essentially an unnecessary step and may be actually increasing your chances of infection, not decreasing them. Hear me out.
Boiling water doesn't stay boiling very long once off the heat. You can't heat the bucket itself enough to be that effective because its plastic and can melt, plus plastic is a lousy conductor. Plus, if you are doing it prior to actually brewing well then its totally ineffective. Leave the sanitizer in the barrel until right before you are ready to pour in the cooled wort.
On top of that, when you rinse it with boiling water, you remove what little sanitizer was left clinging to the surface of the vessel. Any airborne critters then get a free ride as soon as you pour out the boiling water.
You must learn to trust your sanitizer. We all do. If sanitizer alone isn't enough, then we are screwed anyway.
Trust me, I know exactly how you feel. We all want sterilization, not sanitization and we have all gone to extraordinary lengths to try to get there. The rationalization is this: It couldn't hurt, it might help, and ultimately it makes us feel better.
But there is some good reasons to trust your sanitizer and not take steps like a boiling water rinse afterward.
First, it adds time. The longer brewing takes, the easier it is to rush and make a mistake. All that time boiling and handling (which is dangerous anyway) boiling water could be spent ensuring your sanitizer is measured properly, and that equipment gets the 1 to 2 minute soak required.
Second, it may have unintended consequences, like possible plastic leech and spilled boiling water on your feet.
Third, (this was the hardest for me) we all have to realize that steps like boiling water rinse have more to do with making ourselves feel good about sanitization than they do with any real benefits.
All the time spent doing that could be better spent making sure things are clean and sanitized. Lets face it, sanitizer is cheap. It is probably the cheapest thing on a per-gallon basis we buy for home brewing. Use lots of it, take the time to learn to get the concentrations right and know your proper soak time for your favorite sanitizer.
I hope this helps. Good luck!