Trinseo (TSE) DEFA14A Additional proxy soliciting materials

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of
the Securities Exchange Act of 1934 (Amendment No.     )

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*** Exercise Your Right to Vote *** Important Notice Regarding the Availability of Proxy Materials for the Shareholder Meeting to Be Held on June 20, 2018. TRINSEO S.A. You are receiving this communication because you hold shares in the company named above. This is not a ballot. You cannot use this notice to vote these shares. This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. You may view the proxy materials online at www.proxyvote.com or easily request a paper copy (see reverse side). We encourage you to access and review all of the important information contained in the proxy materials before voting. proxy materials and voting instructions. E48297-Z72474 See the reverse side of this notice to obtain Meeting Information Meeting Type:Annual and Extraordinary General Meeting For holders as of:April 13, 2018 Date: June 20, 2018Time: 3:00 p.m. CEST/ 3:15 p.m. CEST Location:4 rue du Fort Niedergrünewald, BP 512 Quartier Européen Nord, L-2015 Luxembourg See following pages for separate Voting Items and control numbers for each of the Annual and Extraordinary General Meetings.

Before You Vote How to Access the Proxy Materials Have the information that is printed in the box marked by the arrow XXXX XXXX XXXX XXXX (located on the by the arrow XXXX XXXX XXXX XXXX (located on the following page) in the subject line. How To Vote Please Choose One of the Following Voting Methods marked by the arrow XXXX XXXX XXXX XXXX (located on the following page) available and follow the instructions. E48298-Z72474 Vote In Person: If you choose to vote these shares in person at the meeting, you must request a "legal proxy." To do so, please follow the instructions at www.proxyvote.com or request a paper copy of the materials, which will contain the appropriate instructions. Many shareholder meetings have attendance requirements including, but not limited to, the possession of an attendance ticket issued by the entity holding the meeting. Please check the meeting materials for any special requirements for meeting attendance. Vote By Internet: To vote now by Internet, go to www.proxyvote.com. Have the information that is printed in the box Vote By Mail: You can vote by mail by requesting a paper copy of the materials, which will include a voting instruction form. Proxy Materials Available to VIEW or RECEIVE: NOTICE AND PROXY STATEMENT AND ANNUAL REPORT How to View Online: following page) and visit: www.proxyvote .com. How to Request and Receive a PAPER or E-MAIL Copy: If you want to receive a paper or e-mail copy of these documents, you must request one. There is NO charge for requesting a copy. Please choose one of the following methods to make your request: 1) BY INTERNET:www.proxyvote.com 2) BY TELEPHONE: 1-800-579-1639 3) BY E-MAIL*:sendmaterial@proxyvote.com * If requesting materials by e-mail, please send a blank e-mail with the information that is printed in the box marked Requests, instructions and other inquiries sent to this e-mail address will NOT be forwarded to your investment advisor. Please make the request as instructed above on or before June 6, 2018 to facilitate timely delivery.

The Board of Directors recommends you vote FOR the following proposals: Annual General Meeting Proposals: 7. To ratify the appointment of PricewaterhouseCoopers Société cooperative to be the Company’s independent auditor for all statutory accounts required by Luxembourg law for the year ending December 31, 2018. Ordinary Resolutions: 1. To elect three Class I directors specifically named in the proxy statement, each to serve for a term of three years expiring at the 2021 annual general meeting. Nominees: 8. To ratify the appointment of PricewaterhouseCoopers LLP to be the Company’s independent registered public accounting firm for the year ending December 31, 2018. 1a. Joseph Alvarado NOTE: Such other business as may properly come before the meeting or any adjournment thereof. 1b. Jeffrey J. Cote 1c. Pierre-Marie De Leener 2. To approve, on an advisory basis, the compensation paid by the Company to its named executive officers. 3. To approve the Company’s annual accounts prepared in accordance with accounting principles generally accepted in Luxembourg for the year ended December 31, 2017 and its consolidated financial statements prepared in accordance with accounting principles generally accepted in the United States including a footnote reconciliation of equity and net income to International Financial Reporting Standards for the year ended December 31, 2017. 4. To approve the allocation of the results of the year ended December 31, 2017. 5. To approve an allocation to the Company's legal reserve. 6. To ap prov e t he g ra n ti ng an d di s ch ar ge o f t h e Company’s directors and auditor for the performance of their respective duties during the year ended December 31, 2017. E48299-Z72474 Voting Items

The Board of Directors recommends you vote FOR the following proposals: Extraordinary General Meeting Proposals: Extraordinary Resolutions: 1. To approve an amendment to Article 7.1.3 of the Company’s articles of association to declassify the Company’s board of directors. 2. To approve an amendment to revise the authority granted to the board of directors in Article 5.5 of the Company’s articles of association to issue shares from the Company’s authorized share capital. NOTE: Such other business as may properly come before the meeting or any adjournment thereof. E48300-Z72474 Voting Items

E48301-Z72474 Voting Instructions Mailed on or about May 11, 2018