I then sent these boitcoins to my Poloniex USD generated address.
Please check make sure you've sent the Bitcoins to a Bitcoin (BTC) address. Polo clearly tells in red "please make sure you send only to this address".
I have a hunch that you've sent to an address Polo doesn't handle.
You have a good point there, I now see what I did wrong.
In Poloniex I should had made the transfer to BTC address and not the USDT address. As in the main page where it says "BUY BTC" you must have USDT, I clicked "Deposit USDT" and it gave me a wallet address, I made the transfer to that address.
But shouldn't the transfer from Coinbase not happen due to an incorrect address? And will it be possible to undo the transaction?
I think coinbase does count USDT as valid address because i have heard somewhere that both bitcoin address and USDT adrress have similar structure and also please wait for poloniex reply may be they are the only one who can help you on this issue.
Meanwhile found this problem similar to yours, but OP didn't concluded his situation on last check