A two-part adhesive composition includes: a Part A including a complex of an organoborane and an amine complexing agent, and reactive diluent selected from the group consisting of at least one vinyl ether, at least one vinylpolysiloxane, or combinations thereof; and a Part B. Part B includes a decomplexing agent that decomplexes the organoborane and the amine complexing agent, and at least one silicon-free free-radically polymerizable ethylenically-unsaturated compound selected from the group consisting of (meth)acrylates, (meth)acrylamides, and combinations thereof. The two-part adhesive composition is essentially free of organic polythiol compounds. A method of making an adhesive composition by mixing Part A with Part B is also disclosed.