Currently, the most commonly used clinical magnetic resonance imaging (MRI) contrast agents, Gd(III) chelates, have been found to be associated with nephrogenic systemic fibrosis (NSF) in renally compromised patients. Toxicity concerns related to Gd(III)-based agents prompted intensive research toward the development of safe, efficient, and long-cycle non-Gd contrast agents. Herein, three amphiphilic polymeric manganese (Mn) ligands (mPEG1k-P(L-a-HMDI)-mPEG1k, mPEG2k-P(L-a-HMDI)-mPEG2k and mPEG4k-P(L-a-HMDI)-mPEG4k) were synthesized, and then end-capped respectively with different molecular weights of polyethylene glycol monomethyl ether (mPEG 1 kD, 2 kD and 4 kD) to obtain amphiphilic polymer Mn ligands. After being chelated with Mn(II), these amphiphilic polymer Mn complexes show significantly higher T1 relaxivity than the small molecule Mn complex (MnL) at 0.5 T, 1.5 T and 3.0 T magnetic fields, respectively. Then, mPEG2k-P(MnL-a-HMDI)-mPEG2k with relatively high T1 relaxivities (23.2, 14.4 and 9.7 mM-1s-1 at 0.5 T, 1.5 T and 3.0 T, respectively), low CMC (4.7 mg L-1), reasonable size (48 nm) and excellent stability among these three polymer Mn complexes was selected for in vivo MR imaging of vascular vessels. The results suggest that mPEG2k-P(MnL-a-HMDI)-mPEG2k has an excellent and relatively long time-window vascular enhancement effect even at a low dose of 0.05 mmol Mn kg-1 BW, and could play a role in the diagnosis of vascular diseases (0.1 mmol Mn kg-1 BW). Therefore, mPEG2k-P(MnL-a-HMDI)-mPEG2k may be considered as a potential blood pool contrast agent.