can bipolar surround speaker on backwall with 5.1

I plan to build a 5.1 Home theater. However, the sidewalls of my living room, one side is window and the other is open space. So, I can't mount the dipolar speakers on the sidewalls. I am thinking to mount a pair of bipolar speakers (PSB imagine S) to the back wall, which is about 4 feet from the back of sofa. Does it work to create a surround evelopement? Or, do I need to change to uni-polar ones?
You can't mount a speaker which radiates both forward and backward on a wall; the back radiation would have nowhere to go and cancel out the forward radiation. In any case you would get no benefit from using bipolar radiators in regard to sound distribution when mounted in this manner. I have heard of them being mounted between rooms so they could radiate equally into each but these were full range speakers.
I don't think they will make most video people happy that way.If they were more like the S5,you might be able to pull it off.If you are right against the back wall,then it may get by.If the wall is extremely wide,it may get you in the field, but I doubt it.Others mat disagree.Link for them.[][]
My living room is 19 *12, and the back wall is at 12 side. In that case, is anyway to get a surround evelopement in my case? If bi-pole or di-pole will cancel their backward/forward with my original idea, would a pair of direct radiate speakers work?
Yes, but get ones intended for wall placement. Most intended for HT use should work. Most two channel speakers are designed to be used out into the room and the sound is affected if placed too close to wall. You are going to have trouble with the window side of your room, will be very reflective. Have you consideded an omni speaker like the Shanian Compass? Not 5.1 but would give you a large non directional sound that might work well in your room.
I'd really like to know your entire layout, the other speakers you are using (crucial), and overall acoustics of your space. Not knowing all of that, my first question would be to know what L/R/C speakers you're using? I'd then match to those. In your case, I'd default to maximizing the main front speaker setup - and integration of the subwoofer (all considering good acoustic fundamental practices) - and strongly consider doing in-ceiling speakers for rears.
I would be doing in-ceiling because the mains are most important in anchoring the soundstage, most of the dynamics, imaging, etc. You really want just some steering and ambience from the rears, regarless. If you put speakers behind you, you'll be having a big gap with transitions from front to back. Overhead is less localizable, you have more flexibilty, and will transition from front to back better than back wall only.
A better option for the money would be matching dipoler speakers mounted above and to the sides of your seating on the ceiling, just like in-ceiling. However, care and caution must be used in mounting on ceiling. So anchor wisely.
Yes, my final is to find MATCHING rears to your mains, and put em on the ceiling. I'd alway recommend here over behind with no sides...better envelopement, better stearing, less localization, more placement flexiblity for best sound, out of the way, etc. (if a little more work in installing)
Good luck
Thanks for everybody feedback. I plan to use PSB synchrony one as the L and R. Synchrony C as the center. Original idea was to use PSB imagine S as the surround speakers at the backwall, but some suggest that I can get platinum S2 with similar price in Audiogon.
In-ceiling speaker was my option. My wife, the CFO, says abolutely no no. She can't stand two "monsters" (to me, they are babies) mounted besides her lovely chandelier
If you haven't already purchased the receiver and speakers I highly recommend a 7.1 HDMI receiver and seven matching Sync One B's or smaller speakers and a sub. If your couch backs up to the back wall the rears would mount in the upper rear corners on the side wall pointing down at the listening position. Mount the the sides at ear level next to the couch. The L and R at ear level and on the same plane with no toe in as close to the display as possible with the C either over or under the display on plane with the L and R. With the calibration mic at the listening position let room correction do the rest.

No room is too small for 7.1 and matrixed 7.1 sounds better than 5.1.
You can mount a Dipole speaker for rear surround just fine its all up to you. The comment about not being able to use a dipole is silly, dipole surrounds radiate from both sides not front rear.
if the other half is picky enough to reject flush mounted, same color in-ceiling speakers, which aren't any more noticable than an HVAC vent, then How's she about the rest of the bulky speakers (including the ones you plan on mounting on back wall) and the rest of the electronics?
If that's the case, there's always the "Stealth" speakers and other invisible, surface integrated speakers, which completely dissapear into the wall!
I suggest ditching all the AV gear cluttered around the room, and simply go with an all in one sound-bar that's been excellently executed by numerous manufactures of late! They sound great, integrate all the necessary speakers, seriously reduce clutter and simplify, and look much more streamline with your monitor.
It just doesn't sound like all that gear strewn around the room is going to earn you any bonus points at your place, status quo.
Since I didn't read your well written room description closely my suggestion of surround and rear speaker placement is not correct.

My HT room is similar to yours except my 60" display is in a corner which would seem even more awkward for a good surround presentation but the Pioneer room correction is amazing. My description of the L,C, and R, are as I described above. My surrounds and rears are mounted very near the celling aimed at a 3' wide sweet spot. each of the surrounds and rears are at different distances from the listening position.

Previously I had a 5.1 Integra system which seemed to work well but upgrading to 7.1 with room correction has created an outstanding sense of surround which comes mostly from 5.1 media. Replacing the towers with matching speakers and adding a hidden sub will be much more fun with much deeper and controllable bass while giving the system a more sensible look.
Chadnliz, Queefee and Vicdamone. Thanks for your suggestion. Really appreciate it.
I went to a dealer during weekend and describe the floor plan of my living room. He, too, suggest me to use in-ceiling speakers. If this can't be done, he suggest me to use a pair of uni-polar speaker at back.