Is it a breach of contract if the disparagement is done before the existence of the contract?

Disparaging a company before you have a contractual obligation not to do so is not a breach of contract, but it may, if not disclosed prior to contracting, be a basis for the company rescinding the contract for fraud in the inducement.  There may even be a provision in the contract in which you must represent that you have not disparaged the company which, if false, could also be a basis for rescinding the contract.  Also, depending on what you said or wrote, it could constitute slander or libel.