Multi-modal polyisoolefin compositions and processes therefor

A process for producing a polyisoolefin-based polymer composition having a multimodal molecular weight distribution, the process comprising:

• polymerizing an isoolefin in a first step in the absence of a chain transfer agent;
• and, continuing to polymerize the isoolefin in a second step in the presence of a chain transfer agent to produce a polyisoolefin-based polymer composition comprising a multimodal molecular weight distribution comprising a low molecular weight fraction having a peak molecular weight (Mp) of less than about 100,000 g/mol and a high molecular weight fraction having a peak molecular weight (Mp) of greater than about 250,000 g/mol, the low molecular weight fraction present in the composition in an amount less than the high molecular weight fraction.