In this paper, we firstly synthesized comb polymers with crystallizable poly(ethyleneimine) (PEI) side chains via a combination of reversible addition–fragmentation chain-transfer (RAFT) polymerization, cationic ring-opening polymerization (CROP), and further hydrolysis. At first, poly(4-chloromethylstyrene) (PCMS) and poly(4-chloromethylstyrene)-block-polystyrene (PCMS-b-PSt) as macroinitiators were synthesized by employing cumyl dithiobenzoate-mediated RAFT polymerization. Then, comb polymers with poly(2-methyloxyzoline) (PMOZ) side chains were prepared using PCMS and PCMS-b-PSt as macroinitiators in the presence of an excess of potassium iodide in benzonitrile, and the PMOZ side chains on the comb polymers were subsequently hydrolyzed into PEI. Furthermore, we demonstrated that the crystalline aggregates of these PEI-bearing comb polymers (i.e., comb-like PEI and comb-like PEI block with a PSt block) formed via crystallization-driven self-assembly in different aqueous media such as water, water/methanol, water/acetone and water/DMF could act as catalytic templates in silicification to give silica with well-controlled morphologies.