You need to pre-compose the particles as well.
it does not work anyway!
Did you choose the option to move all the attributes into the new comp when you created it?
Yes, I've done it
If your particles do not move backwards then your workflow is wrong.
Try this test. Create a new comp and name it Test, copy your particle layer from your original comp, paste the particle layer in the Test comp. See if the particles do what you want them to do.
If they do then Create another new comp and name it Test 2. Drag the Test comp into Test 2. Apply Time Remapping to the nested comp. Go to the out point of your Test Comp and add a new keyframe using the little diamond in the AV Features column. Delete the last keyframe for Time Remapping, the one past the out point. Use the keyframe assistant to time reverse the keyframes. Your particles will now start apart and come together.
If that doesn't work please send us a screenshot and a complete description of your workflow. Here's a CS6 project that shows how it's done. Dropbox - backwardsParticles.aep (Note: Dropbox is probably going to add a .txt extension to the .aep file. Just delete it and the project should open.)
Hopefully the advice from the lads will get you started.
I have used Shatter to simulate reassembly of particles into solid items for many years. Using Time Remap on a precomp is easy. The hard part is setting up the physics in Shatter, or any other particle system, to appear to be assembling the item instead of simply running an explosion in reverse. That's far more difficult. Shatter wants to start fast and slow down and you must figure out a way to start it slowly and speed it up so the reverse looks organic and real. This is best done using both shatter spheres. The first is set to disrupt the image only slightly. The second is set to apply more explosive force.
I have also done this using two copies of Shatter so the viscosity in one can be dramatically slower than the rest of the physics in the other. Very convincing.
Try to have fun.