AN ACR B737 FLC DEPARTED WITHOUT AN IFR ENRTE CLRNC.

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AS CAPT; WE PROCEEDED WITH THE CHKLISTS BEFORE PUSHING OUT OF THE GATE AT ORD FOR FLT TO CLT. WE COMPLETED THE BEFORE START CHKLIST (COMPLETING THE DEP REVIEW WITH MY UNDERSTANDING THAT A CLRNC WAS RECEIVED BY THE FO). WE CALLED FOR PUSHBACK; CONTACTED METERING; ORD GND CTL; AND TWR WITH A TKOF CLRNC. ON DEP THE CTLR INSTRUCTED US TO SQUAWK A XPONDER CODE. I THOUGHT THAT THE FO HAD JUST FAILED TO PUT IN THE CORRECT XPONDER CODE PRIOR TO DEP. WE PROCEEDED TO OUR DEST WITHOUT INCIDENT. CONTRIBUTING FACTOR WAS IN THE DEP REVIEW; I DID NOT SPECIFICALLY ASK IF THE CLRNC WAS RECEIVED AND WHAT WAS THE CORRECT/NEW XPONDER CODE. THIS WAS ALSO THE LAST LEG OF OUR 3-DAY TRIP HOME WHICH PERHAPS ADDED TO OUR COMPLACENCY. SUPPLEMENTAL INFO FROM ACN 399016: AS THE FO; I SET UP THE COCKPIT FOR FLT FROM ORD TO CLT. HAVING FLOWN IN AND OUT OF ORD A LOT; I SET UP EVERYTHING EXCEPT GETTING A CLRNC ABOUT 45 MINS BEFORE DEP. WE DID ALL CHKLISTS AND FAILED TO CATCH THAT I HAD NOT GOTTEN OUR CLRNC YET ON THE BEFORE START CHKLIST (ITEM-DEP REVIEW).