Catalyst systems obtained from the inexpensive, ligand-free and commercially available MgnBu2 and alcohols were used for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) under mild conditions. The catalytic system of MgnBu2/Ph2CHOH showed very high activity as compared with the system of MgnBu2/Ph3COH, which applied a more bulky methanol derivative. Interestingly, in the presence of excess amount of alcohol (Ph2CHOH), i.e., varying the OH-to-Mg ratio in a wide range from 10:1 to 800:1, the system MgnBu2/Ph2CHOH still remained highly active, and produced up to 800 polycaprolactone (PCL) chains per Mg center, suggesting a “living immortal” polymerization mode. The molecular weights of the obtained PCLs are accurately controlled when the ratio of [CL]0/[Mg]0 changed from 500 to 8000 together with narrow molecular weight distributions. Moreover, diblock PCL-b-PLA copolymers with narrow PDIs have been facilely achieved. Furthermore, the allyl- and propargyl-functionalized diphenylmethanols could be employed as the chain transfer agents (CTA) in this system for in situ construction of allyl- and propargyl-functionalized PCLs that were facilely modified further to PCLs with multiple functionality as building blocks for amphiphilic and topological microstructured PCLs via coupling and click reactions.