My understand is that syncopation works in the same time signature, but polyrhythm introduces a new time signature.
Syncopation would be when a piece is in 4/4 and the lead soloist accents the 1 of 4, while the drummer accents the 2 of 4 and the bass player the 3 of 4. The result is effectively 3 different 4/4 rhythms interlocked with each other. The music over-all has a staggered but forward moving, easily discernible 4/4 pulse, making syncopated music very suitable for use as modern popular dance music.
Polyrhythm would be when the lead soloist is playing in standard 4/4 while the drummer is playing in 5/4 and the bass player is playing 7/4. The result is a staggered, often supspended and only vaguely forward motion - much more difficult to follow than syncopation, because of the assymetrical nature of the rythmic structure. Only a very talented dancer could successfully dance to polyrhythmic music.
Is my understanding correct?