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Summary: Ultra-high-molecular-weight (UHMW) polymers can display outstanding properties; however, it is challenging to synthesize UHMW polymers by using external free radical initiators. To address this, here, we report the ration design and adoption of B-alkylcatecholborane (RBCat) as a radical initiator in controlled radical polymerization in the presence of a chain-transfer agent. The autoxidation of RBCat with one alkyl substituent generates only one alkyl radical. The oxygen is incorporated and stored in the oxidized form (ROOB-Cat) that slowly releases the alkyl radical. The rationally designed alkyl radical is proposed to preferentially activate the chain-transfer agent (CTA) and grow the polymer chain from CTA, so limited free polymer chains are formed from the initiating alkyl radicals. Polymerizations of (meth)acrylates are achieved with narrow molecular weight distribution (Р= 1.10) and predetermined molecular weights even in a UHMW range (up to 3 million). The successful chain extension of UHMW indicates the high degree of control of the polymerization.