SEC Form DEFA14A filed by Trinseo PLC

Your Vote Counts! *Please check the meeting materials for any special requirements for meeting attendance. At the meeting, you will need to request a ballot to vote these shares. Smartphone users Point your camera here and vote without entering a control number For complete information and to vote, visit www.ProxyVote.com Control # V71486-P30915 TRINSEO PLC RIVERSIDE ONE SIR JOHN ROGERSON’S QUAY DUBLIN 2 DUBLIN, D02 X576 IRELAND TRINSEO PLC 2025 Annual General Meeting Vote by June 23, 2025 11:59 PM ET You invested in TRINSEO PLC and it’s time to vote! You have the right to vote on proposals being presented at the Annual General Meeting. This is an important notice regarding the availability of proxy materials for the shareholder meeting to be held on June 25, 2025. Vote in Person at the Meeting* June 25, 2025 11:30 A.M., I.S.T. InterContinental Dublin Simmonscourt Road Ballsbridge, Dublin D04A9K8 Get informed before you vote View the Notice and Proxy Statement and Form 10-K online OR you can receive a free paper or email copy of the material(s) by requesting prior to June 11, 2025. If you would like to request a copy of the material(s) for this and/or future shareholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to [email protected]. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. To vote and/or attend the meeting, go to the “Attend a Meeting” link at www.proxyvote.com.

THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming shareholder meeting. Please follow the instructions on the reverse side to vote these important matters. Vote at www.ProxyVote.com Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click “Delivery Settings”. V71487-P30915 Voting Items Board Recommends 1. To elect, by separate resolutions, nine (9) directors specifically named in the proxy statement, each to serve for a term of one year expiring at the close of the 2026 Annual General Meeting; Nominees: 1a. K’Lynne Johnson For 1b. Joseph Alvarado For 1c. Frank A. Bozich For 1d. Victoria Brifo For 1e. Jeffrey Cote For 1f. Jeanmarie Desmond For 1g. Matthew Farrell For 1h. Sandra Beach Lin For 1i. Henri Steinmetz For 2. To approve, on an advisory basis, the compensation paid by the Company to its named executive officers. For 3. To ratify, by non-binding advisory vote, the appointment of PricewaterhouseCoopers LLP as the Company’s independent registered public accounting firm for the year ending December 31, 2025 and to authorize, by binding vote, the Audit Committee of the Board to set its auditors’ remuneration. For 4. To approve a proposal to grant the Board authority to issue shares under Irish law. For 5. To approve a proposal to grant the Board authority to opt out of statutory pre-emption rights, with respect to up to 10% of issued ordinary share capital, under Irish law. For 6. To set the price range for the Company’s re-issuance of treasury shares, as described in the Company’s proxy statement. For 7. To approve amendments to the Company’s Amended & Restated 2014 Omnibus Incentive Plan. For 8. To allow the Company’s CEO, CFO and CLO to voluntarily forfeit stock options for no consideration, to be returned to the Company’s equity plan. For NOTE: Such other business as may properly come before the meeting or any adjournment thereof.