I purchased some Bitcoin on Coinbase with the intent of sending it to Poloniex. I accidentally sent my Bitcoin to my Bitcoin Cash address on Poloniex. Is it possible to retrieve the Bitcoin or is it gone?
You would need to contact Poloniex and hope that they are good enough samaritans that they will take the time to get the coins back to you, if you prove that you own the private keys of the origin of the BTC (you could sign the private keys you sent them from). Then again, you used Coinbase, so im not sure if Coinbase allows you to do that...
Most likely you lost them. Blame the BCash jackases for using the exact same format as BTC, but also blame yourself for not paying enough attention.