It is likely that Momenta has an insider trading policy. I did not see it in their website. A random Google search produced a polic y for another company, Overseas Shipping Group, that I know nothing about except that it is traded on the NYSE under the symbol OSG. This policy specifically addresses a variety of issues including option exercises and provides that the policy does not apply to exercises::
Transactions Under Company Plans Stock Option Exercises. The Company's insider trading policy does not apply to the exercise of an employee stock option. The policy does apply, however, to any sale of stock as part of a broker-assisted cashless exercise of an option, or any other market sale for the purpose of generating the cash needed to pay the exercise price of an option.