Sec Form 4 Filing - BAIN CAPITAL INVESTORS LLC @ Trinseo S.A. - 2016-09-23

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person BAIN CAPITAL INVESTORS LLC 2. Issuer Name and Ticker or Trading Symbol Trinseo S.A. [ TSE] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 200 CLARENDON STREET 3. Date of Earliest Transaction (MM/DD/YY) 09/23/2016 (Street) BOSTON, MA02116 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price ORDINARY SHARES 09/23/2016 S 10,669,567 D $ 54.05 0 I See Footnotes ( 1 ) ( 2 ) ( 3 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

BAIN CAPITAL INVESTORS LLC 200 CLARENDON STREET BOSTON, MA02116 Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other X BCIP Trust Associates IV-B, L.P. 200 CLARENDON STREET BOSTON, MA02116 X Signatures BAIN CAPITAL INVESTORS, LLC, By: /s/ Stephen Thomas, Managing Director 09/27/2016 Signature of Reporting Person Date BCIP TRUST ASSOCIATES IV-B, L.P., By: Boylston Coinvestors, LLC, its general partner, By: /s/ Stephen Thomas, Name: Stephen Thomas, Title: Authorized Signatory 09/27/2016 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Represents ordinary shares of Trinseo S.A. held by Bain Capital Everest Manager Holding SCA ("BCEM Holding"), whose general partner is Bain Capital Everest Manager S.a r.l. ("BCEM"). On September 23, 2016, BCEM Holding sold 10,669,567 Ordinary Shares. Following such sale, BCEM Holding held zero Ordinary Shares. ( 2 )All of the outstanding share capital of BCEM is held by Bain Capital Europe Fund III, L.P. ("Europe Fund III") and, in that capacity, Europe Fund III has the power to appoint the managers of BCEM. Bain Capital Investors, LLC ("BCI") is the general partner of Bain Capital Partners Europe III, L.P. ("Partners Europe III"), which is the general partner of Europe Fund III. BCI is also the general partner of Bain Capital Partners X, L.P. ("Partners X"), which is the general partner of Bain Capital Fund X, L.P. ("Fund X"). Boylston Coinvestors, LLC is the general partner of BCIP Associates IV, L.P. ("BCIP IV"), BCIP Associates IV-B, L.P. ("BCIP IV-B"), BCIP Trust Associates IV, L.P. ("BCIP Trust IV") and BCIP Trust Associates IV-B, L.P. ("BCIP Trust IV-B," and, together with BCEM Holdings, BCEM, Europe Fund III, Partners Europe III, Fund X, Partners X, BCIP IV, BCIP IV-B and BCIP Trust IV, the "Bain Capital Entities"). ( 3 )The governance, investment strategy and decision-making process with respect to the investments held by all of the Bain Capital Entities is directed by BCI's Global Private Equity Board. As a result, BCI may be deemed to share voting and dispositive power with respect to the shares of Trinseo S.A. held by BCEM Holding. BCI disclaims beneficial ownership of such securities except to the extent of its pecuniary interest therein.

Remarks: Form 2 of 2

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