Filed by Westlake Chemical Corporation pursuant to Rule 425 under the Securities Act of 1933 and

deemed filed pursuant to Rule 14a-12 under the Securities Exchange Act of 1934

Subject Company: Axiall Corporation

Filer’s Commission File Number: 001-32260

Subject Company’s Commission File Number: 001-09753

The following email was sent to Westlake Chemical Corporation (“Westlake”) employees by the President and Chief Executive Officer of Westlake, Albert Chao, on February 16, 2016.

Internal Email

Dear Employees,

I want to update you on an important step we have taken as part of our effort to combine our business with Axiall Corporation.

In a press release issued today, we reported that we have notified Axiall of our intention to nominate ten independent and highly-qualified individuals to Axiall’s Board of Directors who we believe will serve the best interests of Axiall shareholders. This important step is consistent with our commitment to begin a dialogue with Axiall’s Board regarding our offer.

We continue to believe that the combination of Westlake and Axiall will make a stronger and more efficient company that is better positioned to succeed and grow in this challenging environment. We look forward to concluding a transaction and welcoming Axiall’s employees to the Westlake family.

While I understand this news has the potential to be distracting, I ask you to remain focused on the safe and reliable operation of our facilities and providing our customers with the exceptional service and high quality products that they have come to expect of us.

We also want to remind you of our policy not to speak with members of the media or the investment community unless authorized to do so. Please refer investor relations and financial inquiries to Steve Bender, Senior Vice President and CFO, and media questions to David Hansen, Senior Vice President or Erika Soechting, Manager, Corporate Communications. In response to questions from customers or other third parties, please refer them to today’s press release.

As promised, we will continue to keep you informed as new developments occur. I encourage you to visit www.WestlakeAxiall.AcquisitionProposal.com for the most up to date information, including the recent press release and information about the proposed directors.

Thank you for your continued hard work and dedication.

Sincerely,

Albert Chao

President and Chief Executive Officer

Forward Looking Statements

This communication contains forward-looking statements within the meaning of the federal securities laws. These forward-looking statements include, but are not limited to, statements regarding Westlake Chemical Corporation’s (“Westlake”) proposal to acquire Axiall Corporation (“Axiall”) (including financing of the proposed transaction and the benefits, results, effects and timing of a transaction), all statements regarding Westlake’s (and Westlake’s and Axiall’s combined) expected future financial position, results of operations, cash flows, dividends, financing plans, business strategy, budgets, capital expenditures, competitive positions, growth opportunities, plans and objectives of management, and statements containing the use of forward-looking words, such as “may,” “will,” “could,” “would,”

“should,” “project,” “believe,” “anticipate,” “expect,” “estimate,” “continue,” “potential,” “plan,” “forecast,” “approximate,” “intend,” “upside,” and the like, or the use of future tense. Statements contained herein concerning the business outlook or future economic performance, anticipated profitability, revenues, expenses, dividends or other financial items, and product or services line growth of Westlake (and the combined businesses of Westlake and Axiall), together with other statements that are not historical facts, are forward-looking statements that are estimates reflecting the best judgment of Westlake based upon currently available information. Statements concerning current conditions may also be forward-looking if they imply a continuation of current conditions.

Such forward-looking statements are inherently uncertain, and stockholders and other potential investors must recognize that actual results may differ materially from Westlake’s expectations as a result of a variety of factors, including, without limitation, those discussed below. Such forward-looking statements are based upon management’s current expectations and include known and unknown risks, uncertainties and other factors, many of which Westlake is unable to predict or control, that may cause Westlake’s actual results, performance or plans with respect to Axiall to differ materially from any future results, performance or plans expressed or implied by such forward-looking statements. These statements involve risks, uncertainties and other factors discussed below and detailed from time to time in Westlake’s filings with the Securities and Exchange Commission (the “SEC”).

Risks and uncertainties related to the proposed business combination transaction include, but are not limited to: (i) the ultimate outcome of any possible transaction between Westlake and Axiall, including the possibility that Axiall will not accept a transaction with Westlake, (ii) the ultimate outcome and results of integrating the operations of Westlake and Axiall if a transaction is consummated, (iii) the ability to obtain regulatory approvals and meet other closing conditions to any possible transaction, including any necessary stockholder approvals, (iv) potential adverse reactions or changes to business relationships resulting from the announcement, pendency or completion of the proposed transaction, (v) competitive responses to the announcement or completion of the proposed transaction, costs and difficulties related to the integration of Axiall’s businesses and operations with Westlake’s businesses and operations, (vi) the inability to obtain, or delays in obtaining, cost savings and synergies from the proposed transaction, (vii) uncertainties as to whether the completion of the proposed transaction or any transaction will have the accretive effect on Westlake’s earnings or cash flows that it expects, (viii) unexpected costs, liabilities, charges or expenses resulting from the proposed transaction, (ix) litigation relating to the proposed transaction, (x) the inability to retain key personnel, and (xi) any changes in general economic and/or industry-specific conditions.

In addition to the factors set forth above, other factors that may affect Westlake’s plans, results or stock price are set forth in Westlake’s Annual Report on Form 10-K and in its reports on Forms 10-Q and 8-K. Many of these factors are beyond Westlake’s control. Westlake cautions investors that any forward-looking statements made by Westlake are not guarantees of future performance. We do not intend, and undertake no obligation, to publish revised forward-looking statements to reflect events or circumstances after the date of this document or to reflect the occurrence of unanticipated events.

Additional Information

This communication relates to a proposal which Westlake has made for a business combination transaction with Axiall. In furtherance of this proposal and subject to future developments, Westlake (and, if a negotiated transaction is agreed, Axiall) may file one or more registration statements, proxy statements, tender offer statements, prospectuses or other documents with the SEC. This communication

is not a substitute for any proxy statement, registration statement, tender offer statement, prospectus or other document Westlake and/or Axiall may file with the SEC in connection with the proposed transaction. INVESTORS AND SECURITY HOLDERS OF WESTLAKE AND AXIALL ARE URGED TO READ THE PROXY STATEMENT(S), REGISTRATION STATEMENT(S), TENDER OFFER STATEMENT(S), PROSPECTUS(ES) AND OTHER DOCUMENTS FILED WITH THE SEC CAREFULLY IN THEIR ENTIRETY IF AND WHEN THEY BECOME AVAILABLE AS THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED TRANSACTION. Any definitive proxy statement(s) or prospectus(es) (if and when available) will be mailed to stockholders of Westlake and Axiall, as applicable. Investors and security holders will be able to obtain copies of these documents (if and when available) as well as other filings containing information about Westlake and Axiall, without charge, at the SEC’s website, http://www.sec.gov. Those documents, when filed, as well as Westlake’s other public filings with the SEC, may be obtained without charge at Westlake’s website at http://www.westlake.com.

Participants in Solicitation

Westlake and its directors, executive officers and certain other members of management and employees may be deemed to be participants in any solicitation with respect to the proposed transaction under the rules of the SEC. You can find information about Westlake’s executive officers and directors in Westlake’s definitive proxy statement filed with the SEC on April 1, 2015. You can obtain free copies of these documents from Westlake using the contact information above. Investors may obtain additional information regarding the interest of such participants by reading any proxy statement/prospectus regarding the proposed transaction when it becomes available.

This document shall not constitute an offer to sell, buy or exchange or the solicitation of an offer to sell, buy or exchange any securities, nor shall there be any sale of securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction. No offering of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the U.S. Securities Act of 1933, as amended.

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