LIMITED POWER OF ATTORNEY
                      FOR SECTION 16 REPORTING OBLIGATIONS

      The  undersigned  hereby  makes, constitutes and appoints each of Scott M.
Storer,  Marie T. Washburn, Bruce A. Leicher, Alejandra Carvajal, Christopher S.
Kiefer  and  Andrea J. Paul, signing singly and each acting individually, as the
undersigned's  true and lawful attorney-in-fact with full power and authority as
hereinafter described to:

      (1)   execute  for  and on behalf of the undersigned, in the undersigned's
            capacity as a director and/or an officer of Momenta Pharmaceuticals,
            Inc.  (the  "Company"),  Forms 3, 4, and 5 (including any amendments
            thereto) in accordance with Section 16(a) of the Securities Exchange
            Act  of  1934,  as  amended, and the rules thereunder (the "Exchange
            Act");

      (2)   do and perform any and all acts for and on behalf of the undersigned
            that  may be necessary or desirable to prepare, complete and execute
            any  such  Form  3,  4,  or  5,  prepare,  complete  and execute any
            amendment  or  amendments  thereto, and timely deliver and file such
            form  with  the United States Securities and Exchange Commission and
            any stock exchange or similar authority;

      (3)   seek  or  obtain,  as  the  undersigned's  representative and on the
            undersigned's  behalf,  information  regarding  transactions  in the
            Company's  securities  from  any  third  party,  including  brokers,
            employee   benefit   plan   administrators  and  trustees,  and  the
            undersigned  hereby  authorizes  any such person to release any such
            information  to  such attorney-in-fact and approves and ratifies any
            such release of information; and

      (4)   take  any other action of any type whatsoever in connection with the
            foregoing  that,  in the opinion of such attorney-in-fact, may be of
            benefit  to,  in  the  best interest of, or legally required by, the
            undersigned, it being understood that the documents executed by such
            attorney-in-fact on behalf of the undersigned pursuant to this Power
            of  Attorney  shall be in such form and shall contain such terms and
            conditions   as   such   attorney-in-fact   may   approve   in  such
            attorney-in-fact's discretion.

      The undersigned hereby grants to each such attorney-in-fact full power and
authority  to  do  and perform any and every act and thing whatsoever requisite,
necessary,  or proper to be done in the exercise of any of the rights and powers
herein granted, as fully to all intents and purposes as the undersigned might or
could  do  if personally present, with full power of substitution or revocation,
hereby  ratifying  and  confirming  all  that  such  attorney-in-fact,  or  such
attorney-in-fact's  substitute  or substitutes, shall lawfully do or cause to be
done  by  virtue  of  this  Power  of  Attorney and the rights and powers herein
granted.  The  undersigned acknowledges that the foregoing attorneys-in-fact, in
serving in such capacity at the request of the undersigned, are not assuming nor
relieving,  nor  is the Company assuming nor relieving, any of the undersigned's
responsibilities  to comply with Section 16 of the Exchange Act. The undersigned
acknowledges that neither the Company nor the foregoing attorneys-in-fact assume
(i)  any  liability  for  the  undersigned's  responsibility  to comply with the
requirement  of  the Exchange Act, (ii) any liability of the undersigned for any
failure  to  comply with such requirements, or (iii) any obligation or liability
of  the  undersigned for profit disgorgement under Section 16(b) of the Exchange
Act.

      This  Power  of  Attorney  shall remain in full force and effect until the
undersigned  is no longer required to file Forms 3, 4, and 5 with respect to the
undersigned's  holdings of and transactions in securities issued by the Company,
unless  earlier  revoked by the undersigned in a signed writing delivered to the
foregoing  attorneys-in-fact.  This Power of Attorney supersedes any prior power
of attorney in connection with the undersigned's responsibilities to comply with
Section  16  of  the  Exchange  Act  in the undersigned's capacity as a director
and/or  an officer of the Company. This Power of Attorney shall expire as to any
individual attorney-in-fact if such attorney-in-fact ceases to be an employee of
the Company.

      IN  WITNESS  WHEREOF, the undersigned has caused this Power of Attorney to
be executed as of this 22nd day of September, 2017.

                                       /s/ Santiago Arroyo
                                       -----------------------------------------
                                       Santiago Arroyo