based macro-chain-transfer agent (CTA-2, Scheme 1)22in THF([CTA]:[1]:[AIBN] = 1:100:0.2) at 75 °C. The polymerizationwas quenched when the degree of polymerization (DP) of 1reached the required value, as monitored by1H NMRintegration of the reaction mixture. For all cases, the isolatedyields of polymers and block copolymers after purification were67−82% compared with the yield calculated by assuming thecomplete consumption of the chain-transfer agent.Gel-permeation chromatography (GPC) of the blockcopolymers 3−7 [N,N-dimethylformamide (DMF), 65 °C]showed unimodal peaks with narrow polydispersity indices(PDIs) free from the peak of the PEG macro-chain-transferagent, indicating successful chain extension from the PEGchain-transfer agent (Figure 2B). The molecular weight and DPof the PBOx block were estimated by1H NMR integrationusing the methylene peak of PEG as a standard (Table S1 andFigure S2 in the SI). The molecular weights obtained by GPCusing polystyrene standards were consistently larger than thosemeasured by1H NMR analysis. Attempts to obtain MALDI−TOF mass spectra were unsuccessful