B737 FLT CREW MUST DECLARE EMERGENCY TO DEVIATE AROUND THUNDERSTORMS WHILE DEPARTING THE AREA DUE TO ATC REFUSAL TO GRANT HEADING CHANGE.

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WE WERE CLBING OUT OF ZZZ. THERE WERE RADAR ECHOES (TSTMS) AT OUR 12 O'CLOCK POS. WE WERE ON AN ASSIGNED HDG OF 270 DEGS BY DEP CTL. APPROX 25 MI PRIOR TO THE WX; WE REQUESTED A DEV TO THE L. DEP DID NOT RESPOND TO OUR FIRST REQUEST. THE CTLR APPEARED TO BE 'COMBINED' AND WORKING MULTIPLE FREQS. JUST PRIOR TO ENTERING THE WX (STORMS) I ELECTED TO DEVIATE 30 DEGS TO THE L. WE AGAIN REQUESTED THE DEV CLRNC FROM ATC. WHEN THEY DID ANSWER US; THEY SUMMARILY DENIED THE REQUEST DUE TO TFC. I IMMEDIATELY SQUAWKED 7700 ON THE XPONDER AND ADVISED THE CTLR THAT WE WOULD IN FACT BE DEVIATING DUE TO THE WX. THERE WAS NO APPARENT TCAS TFC WITHIN ABOUT 40 MI IN THE DIRECTION OF THE DEV. THE CTLR THEN STATED THAT WE WERE NO LONGER IN HIS AIRSPACE AND THAT WE SHOULD CONTACT CTR. WE CALLED CTR AND ADVISED THE THAT WE WERE DEVIATING. THEY IMMEDIATELY SAID THAT WE WERE UNABLE TO DEVIATE DUE TO TFC. I GUESS ABOUT THEN HE SAW OUR XPONDER AND ASKED IF WE HAD DECLARED AN EMER. I ADVISED THAT WE HAD AND WERE DEVIATING 30 DEGS L. JUST ABOUT THAT TIME WE WERE CLRING THE WX. WE ADVISED THAT WE WERE THROUGH WITH THE DEV. WE WERE CLRED ON COURSE AND LANDED UNEVENTFULLY. I FEEL THE COMBINING OF FREQS HAMPERED OUR EFFORTS TO COORD A DEV IN A TIMELY MANNER. ADDITIONALLY; IT IS MY BELIEF THAT THE ATTITUDE OF BOTH CTLRS WAS DEFIANT INSTEAD OF HELPFUL. IT WAS MORE OR LESS LIKE; FINE; THAT'S NOT GONNA HAPPEN (THE DEV). 'YOU'RE GOING TO DO WHAT YOU'RE TOLD.' I FRANKLY BELIEVE IT IS UP TO THE PLTS TO NOT COMPROMISE THE SAFETY OF THE FLT BECAUSE OF POOR HANDLING. IT SHOULDN'T HAVE TO COME TO THIS; BUT IN CERTAIN INSTANCES; DECLARING AN EMER AND EXERCISING AUTH OVER THEM SEEMS TO BE THE ONLY REMEDY SHORT OF FLYING INTO A TSTM. IN FAIRNESS TO ATC IN GENERAL; MOST CTLRS DO THEIR UTMOST TO ACCOMMODATE OUR REQUEST. ZZZ IS A LITTLE UNIQUE DUE TO THE TFC VOLUMES.