Sec Form 4 Filing - Chu Brian @ Trinseo S.A. - 2015-06-25

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 Chu Brian 2. Issuer Name and Ticker or Trading Symbol Trinseo S.A. [ TSE] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) BAIN CAPITAL INVESTORS, LLC, JOHN HANCOCK TOWER, 200 CLARENDON ST 3. Date of Earliest Transaction (MM/DD/YY) 06/25/2015 (Street) BOSTON, MA02116 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ 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 06/25/2015 A 3,236 ( 1 ) A $ 0 3,236 D Ordinary Shares 37,269,567 I See footnote ( 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

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Chu Brian BAIN CAPITAL INVESTORS, LLC JOHN HANCOCK TOWER, 200 CLARENDON ST BOSTON, MA02116 X X Signatures /s/ Brian Chu 06/29/2015 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Represents shares subject to restricted stock unit award that will vest in full on the earlier of (a) the first anniversary of the grant date, subject to the grantee's continued service as a member of the Board through such date, or (b) the termination of the grantee's service as a member of the Board as a result of his or her death. If the grantee's service as a member of the Board ceases for any reason other than death, the award, to the extent not already vested, will be automatically and immediately forfeited. ( 2 )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"). 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 Europe Partners III, L.P., which is the general partner of Europe Fund III. By virtue of these relationships, BCI may be deemed to share voting and dispositive power with respect to the shares held by BCEM Holding. In addition to being the general partner Bain Capital Partners Europe III, L.P., which is the general partner of Europe Fund III, BCI is the general partner of BCIP Associates IV, L.P. BCIP Associates IV-B, L.P., BCIP Trust Associates IV, L.P., and BCIP Trust Associates IV-B, L.P. ( 3 )Mr. Chu is an Operating Partner of BCI. By virtue of the relationships described in these footnotes, Mr. Chu may be deemed to share voting and dispositive power with respect to the shares of Trinseo S.A. held by BCEM Holding. Mr. Chu disclaims beneficial ownership of such securities except to the extent of his pecuniary interest therein.

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