AN ACR B757-200 CREW TOOK OFF FROM A NON TWR ARPT UNICOM WITHOUT AN IFR RELEASE. ERROR ADMITTED.

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RECEIVED ENRTE CLRNC FROM SJU CTR. MONITORED UNICOM 123.0 FOR LCL TFC. THERE WERE 2 ACFT IN THE TFC PATTERN. AFTER THE FIRST ACFT LANDED AND THE SECOND ACFT CALLED DOWNWIND; I CALLED FOR VFR TKOF RWY 8 AT BQN. ONCE AIRBORNE; I CONTACTED SJU CTR FOR IFR CLRNC. CTR ASKED IF I HAD RECEIVED MY IFR RELEASE. I REPLIED THAT WE HAD DEPARTED BQN VFR. RADAR CONTACT WAS ESTABLISHED AND WE RECEIVED OUR ENRTE CLRNC. IT IS MY RECOLLECTION THAT CTR DID ASK US TO HOLD AT BQN AND AWAIT IFR RELEASE. I WAS DISTRACTED BY THE VFR TFC AND THOUGHT THAT I COULD DEPART VFR AND PICK UP MY ENRTE CLRNC AIRBORNE. SUPPLEMENTAL INFO FROM ACN 345611: IN COMPLETING CHKLIST; FORGOT WE HAD BEEN TOLD TO STAND BY FOR RELEASE. REALIZING OUR ERROR; WE MAINTAINED VMC UNDER VFR RULES; CONTACTED SAN JUAN; AND QUICKLY PICKED UP OUR IFR CLRNC. WERE WE TECHNICALLY LEGAL TO TKOF AND PICK UP OUR CLRNC AIRBORNE? I'M NOT SURE. CALLBACK CONVERSATION WITH RPTR OF ACN 345440 REVEALED THE FOLLOWING INFO: BQN IS A NON TWR ARPT WITH A UNICOM. THE RPTR WAS IN CONTACT WITH UNICOM WHICH GAVE TAXI INSTRUCTIONS. THE RPTR AND 2 ACFT IN THE PATTERN WERE IN CONTACT. THE RPTR ADMITTED HIS ERROR TO TJZS AFTER TKOF. TJZS SENT THE RPTR A LETTER STATING THAT THERE WAS NO VIOLATION; BUT IT WAS NOT POLICY TO TKOF WITHOUT AN IFR RELEASE. THE RPTR'S CHIEF PLT REMINDED THE RPTR THAT 'HE WAS NOT FLYING A CESSNA ANYMORE.' THERE HAS BEEN NO FURTHER ACTION. B757-200.