A challenge in PEGylation strategies is the site-specific conjugation of peptides, proteins, and antibodies onto a “clickable” well-defined PEG architecture whilst retaining the therapeutic potency of the parent molecule. Here, we investigated several activation processes of reversible addition–fragmentation chain transfer (RAFT) polymerization to prepare well-defined brush poly(poly(ethylene glycol) methyl ether acrylate) (PPEGA) with a long backbone of up to DP = 50 and a high fraction of living chains at near-total conversions and therefore potential applicability in PEGylation. The thermal and UV light activation methods used in the RAFT polymerization of PEGA in DMF showed the loss of control at high conversions (>70%), explained by the presence of chain transfer reactions. However, the use of the light irradiation (blue LED light) activation process with low energy intake for the RAFT polymerization of PEGA in DMF showed an unprecedented control (Đ ≤ 1.2). Blue LED light RAFT could be carried out without an exogenous initiator at room temperature, at various initial monomer concentrations at near-total conversions (ρ > 93%). The PEGylation potential of PPEGA obtained by blue LED light-initiated RAFT polymerization was highlighted by extensions of PPEGA at α- and ω-chain ends.