It's a bit simpler than that.

Syncopation is accenting the off beat, especially accenting in between beats. For example, "Superbad" or "Mother Popcorn" by James Brown. A single rhythm can be syncopated. It's just an unusual stress pattern.

Polyrhythm is playing two rhythms at once. For instance, on the piano, playing straight eighth notes (quavers) on the left hand while playing eighth note triplets on the right hand. In this case, at least two rhythms must be present. As you might logically conclude, generally at least one of the rhythms will come across as syncopated, since either the typical stress pattern or one of the other rhythms will seem to be unsyncopated. But both/all rhythms in a polyrhythm could be syncopated.

It's a bit simpler than that.

Syncopation is accenting the off beat, especially accenting in between beats. For example, "Superbad" or "Mother Popcorn" by James Brown. A single rhythm can be syncopated. It's just an unusual stress pattern.

Polyrhythm is playing two rhythms at once. For instance, on the piano, playing straight eighth notes (quavers) on the left hand while playing eighth note triplets on the right hand, or "Kashmir" by Led Zeppelin (the drums are playing straight 4/4 but the guitar and bass rhythm is in threes). In this case, at least two rhythms must be present. As you might logically conclude, generally at least one of the rhythms will come across as syncopated, since either the typical stress pattern or one of the other rhythms will seem to be unsyncopated. But both/all rhythms in a polyrhythm could be syncopated.