Loop particles with cc particular world

You could make it half the length you would like it, duplicate and pre-comp it, then reverse the new pre-comp ( cmd + option + R on a Mac
Ctrl + alt + R on a p.c.) so it plays back to the start point. worked for me 🙂

To make a seamless loop you need to start by making the first frame and the last frame identical, then you need to back up one frame and create your loop from there. This is extremely difficult to do with the particle world because of the randomness that is introduced. The best option is to duplicate your layer, pick a frame on layer 1 to use as the starting point, pick a spot to start transition using opacity, then move the duplicate of the first layer so that the same first frame starts after the transition. Then set the work area to the first frame and one frame before the first frame on the second layer, trim the comp to the work area, and loop from there. 

For example, set a marker at 5 seconds on a layer with CC Particle World, Duplicate the layer, move the duplicate so that the marker is now at 10 seconds plus one frame. Then create a dissolve transition from 7 to 8 seconds to hide the seam, set the work area to 5 seconds and 10 seconds, trim the comp to the work area, then pre-compose or render this 5-second comp to generate your loop. 

Just an idea... You could try making your composition one second longer than you need, rendering the first second, importing the rendered movie and layering it on top so it starts one second from the end, making a one second opacity cross dissolve to that layer, and then trimming one second from the front of your composition. You might notice the dissolve, but at least it wouldn't be a cut. I've never tried this so I'm not sure if it would work.