A polyisoolefin-based polymer composition having a multimodal molecular weight distribution and a level of unsaturation of greater than about 4 mol % is described, which can be prepared by co-polymerizing 70-96 vol % isoolefin monomer and 30-4 vol % β-pinene or multi-olefin monomer, based on volume of isoolefin monomer and β-pinene or multi-olefin monomer, in a solution comprising 0-30 vol % aliphatic hydrocarbon diluent, based on volume of the solution, and an alkylaluminum halide initiator system. The polyisoolefin-based polymer composition has a multimodal molecular weight distribution having a first molecular weight fraction having a peak molecular weight (Mp) of less than about 300 kg/mol, and a second molecular weight fraction having a peak molecular weight (Mp) of greater than about 1000 kg/mol or at least 3 times greater than the Mp of the first molecular weight fraction.