Massachusetts Mutual Life Insurance Co., the policyholder-owned company formed in 1851, is seeking to expand in the pension buyout business after hiring Lynn Esenwine from rival Prudential Financial Inc. “The pipeline is heavier than ever,” with employers looking to transfer pension risks, Esenwine said in a phone interview Thursday. “In 2012, some large-name, big companies decided to sell off some of their obligations, and I would say the story remains the same and, if anything, the interest is growing.” Insurers like MassMutual and Sun Life Financial Inc. are seeking to grow in a market that has been dominated by Prudential, which won more than $37 billion in pension deals last year.