Two hyperbranched polyglycerols (PG) (1a: M-n = 2000, 1b: M-n = 8000), both with 61% of the overall hydroxyl groups being transformed into trithiocarbonates (2a, 2b, with 16 and 65 OH groups being replaced by trithiocarbonates, respectively), can be used as a reversible addition-fragmentation chain transfer (RAFT) agent. Polymerization of styrene mediated by 2a/2b was in a living manner, resulting in star-like, amphiphilic, core-shell structured polymers with 16 arms (3a) and 65 arms (3b), respectively. It was found that for system mediated by 2b, more serious side reaction (radical-radical coupling) occurred than that by 2a, most probably due to their difference in shell congestion. A shell-crosslinked polymer could be prepared in the presence of divinylbenzene via a one-pot route at solid content up to 20%, but analysis proved the crosslinking was incomplete. Furthermore, the 65-arm-star polymer (3b) more tended to undergo macroscopic rather than intramolecular crosslinking than the 16arm-star polymer (3a) did. The phenomenon might again be attributed to their difference in shell congestion.