Trinseo PLC Shareholder Holdings Post NYSE Delisting

Following the delisting of Trinseo PLC's ordinary shares from NYSE on March 2, 2026, we are aware that the Depository Trust Company (DTC), the clearing house for Trinseo's shares, has taken the decision to remove Trinseo's shares from their system. DTC is a central securities depository and a clearing agency registered with the United States Securities and Exchange Commission. DTC facilitates the movement of securities ownership between buyers and sellers through a book-entry system instead of physically shipping paper stock certificates.

Shares previously held in DTC's system were transferred to the relevant DTC participants (brokers, custodians, and nominees). These shares are now reflected directly in book entry form on the register maintained by Computershare, Trinseo's transfer agent. Share ownership has not changed as a result of the NYSE delisting or DTC transfer of its book entries to Computershare. This is an administrative and custody-related process, not a corporate action affecting shareholder rights. Brokers remain the primary point of contact for beneficial shareholders. Computershare is supporting brokers to ensure accurate and timely handling.

What happened to shares held through brokers?

• Shares previously held through DTC have been distributed by Computershare to the applicable brokers, custodians, or nominees.
• Computershare issued holding statements to the legal registration address on file with DTC, which may reflect historical or transactional addresses rather than current broker headquarters.

Do shareholders need to take action?

• No immediate action is required by shareholders.
• Any next steps are determined by the broker, custodian, or nominee through which the shares were purchased.

Brokers, custodians, or nominees may choose to:

• Continue holding shares on behalf of clients, with no further action required; or
• Transfer shares directly to underlying clients, using appropriate Computershare documentation (individual stock transfer forms or mass issuance files).

Timing and support when transferring shares:

• Once valid documentation is received, share transfers generally complete within 5-7 business days, depending on volume. Brokers submitting mass issuance files with more than 25 holders are asked to notify Computershare in advance.
• Brokers needing assistance should contact Computershare at (866) 644-4127.

• Trinseo and Computershare cannot facilitate transfers initiated by or through brokers on behalf of individual shareholders.

Directly registered shareholders

Shareholders whose shares are already registered directly with Computershare and who have received a holding statement do not need to take any action. Directly registered shareholders may continue to hold shares, vote, or transfer shares using standard Computershare procedures.