Trinseo S.A. 46A avenue John F. Kennedy L 1855 Luxembourg Grand Duchy of Luxembourg

Trinseo S.A. 46A avenue John F. Kennedy L 1855 Luxembourg Grand Duchy of Luxembourg The following information is being posted to our website as required under Luxembourg law in connection with our 2018 Annual and Extraordinary General Meetings of Shareholders (the "General Meetings") to be held on June 20, 2018. Further details regarding admission to the General Meetings as well as the business to be conducted at the meetings is more fully described in our Proxy Statement, filed on Form DEF14A with the U.S. Securities & Exchange Commission on April 30, 2018. Shareholders of record at the close of business on April 13, 2018 are entitled to vote at the General Meetings. On that date, there were 43,266,215 of our ordinary shares outstanding. Each ordinary share is entitled to one vote. Page Number Convening Notice of the 2018 Annual General Meeting of Shareholders... 2 Convening Notice of the 2018 Extraordinary General Meeting of Shareholders... 4 Draft Ordinary Resolutions of the Annual General Meeting... 6 Draft Extraordinary Resolutions of the Extraordinary General Meeting... 8 Report of the Board of Directors Re: Annual and Consolidated Accounts... 11 Report of the Board of Directors Re: Revise Authorization to Issue Shares... 12 A copy of the Luxembourg Annual Accounts and Consolidated Accounts for the year ended December 31, 2017 have been posted to our website and can be found by clicking the following link: http://s21.q4cdn.com/603930022/files/doc_downloads/global%20downloads/2018/5-6.- Trinseo-S.A._2017-Statutory-Accounts_Combined.pdf 1

Second, for the purpose of approving or authorizing any other business properly brought before the Annual General Meeting of Shareholders. Shareholders of record at the close of business on April 13, 2018 (the Record Date) are entitled to notice of, and entitled to vote at, the Meeting and any adjournments or postponements thereof. To attend the Meeting, you must demonstrate that you were a shareholder of the Company as of the close of business on the Record Date, or hold a valid proxy for the Meeting from such a shareholder. Each shareholder who plans to attend the Meeting in person is required to notify the Company in accordance with the Company s articles of association. We will again be using the Notice and Access method of providing proxy materials to shareholders via the Internet. We believe that this process provides shareholders with a convenient and quick way to access the proxy materials and vote, while allowing us to conserve natural resources and reduce the costs of printing and distributing the proxy materials. We will mail to most of our shareholders a Notice of Internet Availability of Proxy Materials containing instructions on how to access our proxy statement and Annual Report and vote electronically via the Internet. This notice will also contain instructions on how to receive a paper copy of the proxy materials. All shareholders who do not receive a notice will receive a paper copy of the proxy materials by mail or an electronic copy of the proxy materials by email. If you are a shareholder of record as of the Record Date, you may vote in person at the Meeting. If you do not wish to vote in person or if you will not be attending the Meeting, you may vote by telephone, or over the Internet, by following the instructions provided in the Notice of Internet Availability of Proxy Materials. If you requested a printed copy of the proxy materials, you may also complete, sign, and date your proxy card and return it in the prepaid envelope that was included with the printed materials. If you are a beneficial owner of shares and you wish to vote in person at the Meeting, you must obtain a proxy from your broker, bank, or other shareholder of record and present it to the inspector of election with your ballot. If you do not wish to vote in person or will not be attending the Meeting, you may vote by following the instructions provided in the Notice of Internet Availability of Proxy Materials you received from the shareholder of record of your shares. If you received a printed copy of the proxy materials, you should have received a proxy card and voting instructions from the shareholder of record of your shares. If you are a shareholder of record and submit a signed proxy card for the Meeting but do not fill out the voting instructions, the persons named as proxy holders will vote the shares represented by your proxy FOR the various items of the agenda mentioned above. Brokers are not permitted to vote your shares on any matter other than Proposal 8 (Ratification of the Appointment of the Independent Registered Public Accounting Firm). If your shares are held in the name of a broker or nominee and you do not instruct the broker or nominee how to vote with respect to the election of directors or if you abstain or withhold authority to vote on any matter, your shares will not be counted as having been voted on those matters, but will be counted as in attendance at the Meeting for purposes of a quorum. If you do not vote your shares, you will not have a say on the important proposals to be voted upon at the Meeting. By Order of the board of directors By: Christopher D. Pappas Title: President, Chief Executive Officer & Director 3

Trinseo S.A. Société anonyme Registered office: 46A avenue John F. Kennedy L 1855 Luxembourg Grand Duchy of Luxembourg R.C.S. Luxembourg: B 153.549 (the Company) CONVENING NOTICE Shareholders are cordially invited to attend an EXTRAORDINARY GENERAL MEETING of the shareholders of the Company to be held at the Sofitel Luxembourg Europe, 4, rue du Fort Niedergrünewald, BP 512 / Quartier Européen Nord, L-2015 Luxembourg, Grand Duchy of Luxembourg on Wednesday, June 20, 2018, at 3:15 p.m. local time or immediately following the Annual General Meeting (the Meeting). We are holding this meeting to solicit your approval of the following: First, for the purpose of approving extraordinary resolutions in order: 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; and 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. Second, for the purpose of approving or authorizing any other business properly brought before the Extraordinary General Meeting of Shareholders. Shareholders of record at the close of business on April 13, 2018 (the Record Date) are entitled to notice of, and entitled to vote at, the Meeting and any adjournments or postponements thereof. To attend the Meeting, you must demonstrate that you were a shareholder of the Company as of the close of business on the Record Date, or hold a valid proxy for the Meeting from such a shareholder. Each shareholder who plans to attend the Meeting in person is required to notify the Company in accordance with the Company s articles of association. We will be using the Notice and Access method of providing proxy materials to shareholders via the Internet. We believe that this process provides shareholders with a convenient and quick way to access the proxy materials and vote, while allowing us to conserve natural resources and reduce the costs of printing and distributing the proxy materials. We will mail to most of our shareholders a Notice of Internet Availability of Proxy Materials containing instructions on how to access our proxy statement and Annual Report and vote electronically via the Internet. This notice will also contain instructions on how to receive a paper copy of the proxy materials. All shareholders who do not receive a notice will receive a paper copy of the proxy materials by mail or an electronic copy of the proxy materials by email. If you are a shareholder of record as of the Record Date, you may vote in person at the Meeting. If you do not wish to vote in person or if you will not be attending the Meeting, you may vote by telephone, or over the Internet, by following the instructions provided in the Notice of Internet Availability of Proxy Materials. If you requested a printed copy of the proxy materials, you may also complete, sign, and date your proxy card and return it in the prepaid envelope that was included with the printed materials. 4

If you are a beneficial owner of shares and you wish to vote in person at the Meeting, you must obtain a proxy from your broker, bank, or other shareholder of record and present it to the inspector of election with your ballot. If you do not wish to vote in person or will not be attending the Meeting, you may vote by following the instructions provided in the Notice of Internet Availability of Proxy Materials you received from the shareholder of record of your shares. If you received a printed copy of the proxy materials, you should have received a proxy card and voting instructions from the shareholder of record of your shares. If you are a shareholder of record and submit a signed proxy card for the Meeting but do not fill out the voting instructions, the persons named as proxy holders will vote the shares represented by your proxy FOR the amendments to the Company's Articles of Association as described above. Brokers are not permitted to vote your shares on any of the proposals. If your shares are held in the name of a broker or nominee and you do not instruct the broker or nominee how to vote with respect to the election of directors or if you abstain or withhold authority to vote on any matter, your shares will not be counted as having been voted on those matters, but will be counted as in attendance at the Meeting for purposes of a quorum. If you do not vote your shares, you will not have a say on the important proposals to be voted upon at the Meeting. By Order of the board of directors By: Christopher D. Pappas Title: President, Chief Executive Officer & Director 5

TRINSEO S.A. 2018 ANNUAL GENERAL MEETING OF SHAREHOLDERS DRAFT ORDINARY RESOLUTIONS FIRST RESOLUTION The Meeting resolves to elect the following three individuals as Class I members of the Board of Directors, with effect as of the date of these resolutions: Mr. Joseph Alvarado, born on 19 July 1952 in East Chicago, Indiana, USA, with professional address at 2525 N. Pearl Street, Unit 1504, Dallas, Texas 75201 USA, for a period of three (3) years (until the annual general meeting of the shareholders to be held in 2021); Mr. Jeffrey J. Cote, born on 16 November 1966 in Lawrence, Massachusetts, USA, with professional address at 111 Milton Street, Milton, Massachusetts 02186 USA, for a period of three (3) years (until the annual general meeting of the shareholders to be held in 2021); and Mr. Pierre-Marie De Leener, born on 29 August 1957 in Antwerpen, Belgium, with professional address at 1 Route de Prafirmin, CH 1965 Saviese, Switzerland, for a period of three (3) years (until the annual general meeting of the shareholders to be held in 2021). SECOND RESOLUTION The Meeting resolves to approve the compensation paid to the named executive officers of the Company, as disclosed pursuant to the compensation disclosure rules of the Securities and Exchange Commission, including the Compensation Discussion and Analysis, compensation tables and narrative discussion. THIRD RESOLUTION The Meeting resolves to approve, after having heard the report of the Board and the report of the statutory auditor, 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. FOURTH RESOLUTION The Meeting acknowledges that the Company incurred a stand-alone loss in the amount of USD 8,981,870.34 for the financial year ended on December 31, 2017. The Meeting resolves to carry forward the foregoing loss to the next financial year. 6

FIFTH RESOLUTION The Meeting acknowledges that the Company made a profit in the amount of USD 77,699,304.81 for the financial year ended on December 31, 2016 and that, pursuant to Luxembourg law, 5% of the annual net profits must be allocated to the Company's legal reserve until the amount of the legal reserve reaches 10% of the Company's net profits. The Meeting therefore resolves to ratify the Company's compulsory allocation of USD 47,297 of these net profits to the Company's legal reserve. SIXTH RESOLUTION The Meeting resolves to give full discharge (quitus) to the members of the Board for the performance of their mandates for and in connection with the financial year ended on December 31, 2017. The Meeting resolves to give full discharge (quitus) to the external auditor (réviseur d entreprises agréé) of the Company for the performance of its mandate for and in connection with the financial year ended on December 31, 2017. SEVENTH RESOLUTION The Meeting resolves to approve the appointment of PricewaterhouseCoopers Société coopérative to be the Company s independent auditor for the year ending December 31, 2018 for all statutory accounts as required by Luxembourg law for the same period. EIGHTH RESOLUTION The Meeting resolves to ratify the appointment of PricewaterhouseCoopers LLP to be the Company's independent registered public accounting firm for the year ending December 31, 2018. 7

TRINSEO S.A. 2018 EXTRAORDINARY GENERAL MEETING OF SHAREHOLDERS DRAFT EXTRAORDINARY RESOLUTIONS FIRST RESOLUTION The Meeting resolves to approve an amendment to Article 7.1.3 of the Company's articles of association of the Company to declassify the board of directors, the full text of which is set forth on the attached Appendix A. SECOND RESOLUTION The Meeting resolves 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, the full text of which is set forth on the attached Appendix B. 8

APPENDIX A AMENDMENT TO ARTICLE 7.1.3 OF THE COMPANY'S ARTICLES OF ASSOCIATION 7.1.3 The General Meeting shall appoint Directors and determine their number, remuneration and term of office. Prior to the 2019 annual general meeting of shareholders, the Directors shall be and are divided into three classes designated as Class I, Class II, and Class III. Each director elected prior to the 2019 annual general meeting of shareholders shall serve for the full term to which such director was elected. Following the expiration of the term of (a) the Class I directors in 2021, (b) Class II Directors in 2019, and (c) the Class III directors in 2020, the directors in each such class shall be elected for a term expiring at the succeeding Annual General Meeting. Commencing at the 2021 annual general meeting of shareholders, all Directors shall then be elected at each Annual General Meeting for terms expiring at the succeeding annual general meeting of shareholders. Directors may be removed at any time, with or without cause, by a resolution of the General Meeting. 9

APPENDIX B AMENDMENT TO ARTICLE 5.5 OF THE COMPANY'S ARTICLES OF ASSOCIATION 5.5 The Board is authorised, for a period of five (5) years from the date of publication of these Articles as agreed to be amended on June 20, 2018, and without prejudice to any renewals, to: (a) increase the issued share capital, in whole or in part and on one (1) or more occasions, up to 20 percent of the existing share capital without preemptive rights and up to 100 percent of the existing share capital with preemptive rights, with the rights as set out in these Articles, against payment in cash or in kind or against a contribution of share premium, account 115, distributable reserves or retained earnings; (b) determine the place and date of the issue (or any successive issue) and the terms and conditions of the subscription for the Shares; (c) determine the allocation of the subscription price for the Shares to the share capital, share premium and/or any other reserve account of the Company; (d) limit and/or withdraw the preferential subscription rights of existing Shareholders in case of an issuance of Shares, as the case may be; and (e) record each such share capital increase by way of a notarial deed and amend the register of Shares to reflect the amendment accordingly. 10

Trinseo S.A. Société anonyme Registered office: R.C.S. Luxembourg: B 153.549 (the Company) 46A avenue John F. Kennedy L-1855 Luxembourg Grand Duchy of Luxembourg REPORT OF THE BOARD OF DIRECTORS OF THE COMPANY IN RESPECT OF THE ANNUAL ACCOUNTS PREPARED IN ACCORDANCE WITH ACCOUNTING PRINCIPLES GENERALLY ACCEPTED IN LUXEMBOURG FOR THE FINANCIAL YEAR ENDING ON DECEMBER 31, 2017 AND THE CONSOLIDATED ACCOUNTS OF THE COMPANY PREPARED IN ACCORDANCE WITH ACCOUNTING PRINCIPLES GENERALLY ACCEPTED IN THE UNITED STATES FOR THE FINANCIAL YEAR ENDING ON DECEMBER 31, 2017 TO THE GENERAL MEETING OF THE SHAREHOLDERS OF THE COMPANY Dear Shareholders, The board of directors of the Company (the Board) presents the annual accounts of the Company prepared in accordance with accounting principles genarally accepted in Luxembourg (the Annual Accounts) 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 (the Consolidated Accounts) for the financial year ending on December 31, 2017 (the Financial Year). During the financial year under review the Company made a loss of USD 8,981,870.34. The Board recommends to carry forward the loss to the following year. The accounts do not show any extraordinary figures. The Company has acquired its own shares (the Treasury Shares). The Company has no plans to open new branches. The Board suggests to the shareholders of the Company to approve the Annual Accounts and the Consolidated Accounts and to grant full discharge (quitus) to the Board and the company s statutory auditor for all their acts and duties in respect of, and in connection with, the Financial Year. The Board remains at the full disposal of the shareholders for any further information in relation to the above. 11

Trinseo S.A. Société anonyme Registered office: 46A avenue John F. Kennedy L-1855 Luxembourg Grand Duchy of Luxembourg R.C.S. Luxembourg B 153.549 (the Company) REPORT OF THE BOARD OF DIRECTORS OF THE COMPANY IN RESPECT OF THE PROPOSED AMENDMENT TO ARTICLE 5.5 OF THE COMPANY'S ARTICLES OF ASSOCIATION TO BE VOTED ON AT THE EXTRAORDINARY GENERAL MEETING OF THE SHAREHOLDERS OF THE COMPANY ON JUNE 20, 2018 Dear Shareholders, Under Luxembourg law, a Luxembourg incorporated company needs the authorization of its shareholders to issue shares. The current authorization for a period of five (5) years ends in May 2019. The board of directors of the Company (the Board) believes it is in the best interests of the Company to renew the Board s authority under Article 5.5 of its Articles of Association, and is therefore seeking authorization to issue its ordinary shares for a period of five (5) years from the date of the Extraordinary General Meeting, within the following limits: 20% of the existing share capital for issuances without preemptive rights. As of April 17, 2018, the Company had 48,777,934 shares issued; and 100% of the existing share capital for issuances with preemptive rights. This authorization would give the Board continued flexibility to issue ordinary shares with appropriate limits at such time or times as the Board in its discretion deems advisable, including, but in no way limited to, capital raising and financing, funding acquisitions, the settlement of shares under its employee incentive plans, and any other reason deemed advisable by the Board. Granting the Board this authority is a routine matter for companies incorporated in Luxembourg. This authority is fundamental to our business, as it is for all other companies incorporated in the United States, who have this authority without additional shareholder approval. We are not asking you to approve an increase in our authorized share capital. Approval of this proposal will only grant the Board the authority to issue shares that are already authorized under our Articles of Association on the terms described above. The full text of the proposed amendment to Article 5.5 of the Company s Articles of Association is set forth below: 12

5.5 The Board is authorised, for a period of five (5) years from the date of publication of these Articles as agreed to be amended on June 20, 2018, and without prejudice to any renewals, to: (a) increase the issued share capital, in whole or in part and on one (1) or more occasions, up to 20 percent of the existing share capital without preemptive rights and up to 100 percent of the existing share capital with preemptive rights, with the rights as set out in these Articles, against payment in cash or in kind or against a contribution of share premium, account 115, distributable reserves or retained earnings; (b) determine the place and date of the issue (or any successive issue) and the terms and conditions of the subscription for the Shares; (c) determine the allocation of the subscription price for the Shares to the share capital, share premium and/or any other reserve account of the Company; (d) limit and/or withdraw the preferential subscription rights of existing Shareholders in case of an issuance of Shares, as the case may be; and (e) record each such share capital increase by way of a notarial deed and amend the register of Shares to reflect the amendment accordingly. Under Luxembourg law, amendments to the articles of association of a company require an extraordinary resolution of shareholders. As such, the Amended and Restated Articles of Association will be adopted if approved by two-thirds of the votes cast by our shareholders at the Extraordinary General Meeting. If approved by our shareholders, the minutes of the Extraordinary General Meeting related to this proposal and the Amended and Restated Articles of Association will be notarized and filed with the Luxembourg trade registry (Registre de Commerce et des Sociétés) as public documents and will take effect immediately upon publication of the notarial deed recording the minutes of the Extraordinary General Meeting in the Luxembourg Official Gazette (Recueil Electronique des Sociétés et Associations). If the proposal is not approved, our current authority to issue shares, will remain in effect until expired. The Board remains at the full disposal of the shareholders for any further information in relation to the above. THE BOARD OF DIRECTORS By: Christopher D. Pappas By: Henri Steinmetz Title: Director Title: Director 13