UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (date of earliest event reported): June 10, 2016

WESTLAKE CHEMICAL CORPORATION

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (713) 960-9111

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

On June 10, 2016, Westlake Chemical Corporation (“Westlake”) and Axiall Corporation (“Axiall”) published a joint press release announcing the execution of an Agreement and Plan of Merger, dated as of June 10, 2016, by and among Axiall, Westlake and Lagoon Merger Sub, Inc., a wholly owned subsidiary of Westlake. A copy of the joint press release is attached hereto as Exhibit 99.1 and is incorporated by reference herein.

On June 10, 2016, Westlake published a communication to Westlake employees regarding the proposed transaction. A copy of the employee communication is attached hereto as Exhibit 99.2 and is incorporated by reference herein.

On June 10, 2016, Westlake also provided supplemental information regarding the proposed transaction in connection with a presentation to investors. A copy of the investor presentation is attached hereto as Exhibit 99.3 and is incorporated by reference herein.

Forward-Looking Statements

This Current Report on Form 8-K 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”) proposed transaction to acquire Axiall Corporation (“Axiall”) (including financing of the proposed transaction and the benefits, results, effects and timing thereof), all statements regarding Westlake’s and Axiall’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, estimated synergies from the proposed transaction 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 Axiall (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 or Axiall 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 and/or Axiall’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 and/or Axiall are unable to predict or control, that may cause Westlake’s and/or Axiall’s actual results, performance or plans 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 and/or Axiall’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 the proposed transaction between Westlake and Axiall, (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 the proposed 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 will have the accretive effect on Westlake’s earnings or cash flows that are expected, (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, (xi) potential adverse effects on Westlake’s ability to operate Westlake’s business due to the increase in Westlake’s overall debt level contemplated by the proposed transaction, (xii) potential diminished productivity due to the impact of the potential transaction on Westlake’s and/or Axiall’s current and prospective employees, key management, customers, suppliers and business partner and (xiii) 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 and/or Axiall’s plans, results or stock price are set forth in Westlake’s and Axiall’s respective Annual Reports on Form 10-K and reports on Forms 10-Q and 8-K.

Many of these factors are beyond Westlake’s and/or Axiall’s control. Westlake and Axiall caution investors that any forward-looking statements made by Westlake and/or Axiall are not guarantees of future performance. Westlake and Axiall do not intend, and undertake no obligation, to publish revised forward-looking statements to reflect events or circumstances after the date of this communication or to reflect the occurrence of unanticipated events.

Additional Information and Where To Find It

With respect to the 2016 Axiall annual meeting of stockholders, Axiall has filed a definitive proxy statement and other documents regarding the 2016 annual meeting of stockholders with the SEC and has mailed the definitive proxy statement and a WHITE proxy card to each stockholder of record entitled to vote at the 2016 annual meeting. With respect to the proposed merger, Axiall expects to announce a special meeting of stockholders soon to obtain stockholder approval in connection with the proposed merger between Westlake and Axiall. In connection with the special meeting, Axiall expects to file with the SEC a preliminary proxy statement and other relevant documents in connection with the proposed merger. INVESTORS OF AXIALL ARE ENCOURAGED TO READ THE DEFINITIVE PROXY STATEMENT FOR THE 2016 ANNUAL MEETING OF STOCKHOLDERS, THE PRELIMINARY PROXY STATEMENT FOR THE SPECIAL MEETING OF STOCKHOLDERS RELATED TO THE PROPOSED MERGER AND ANY OTHER RELEVANT DOCUMENTS FILED WITH THE SEC WHEN THEY BECOME AVAILABLE BECAUSE THOSE DOCUMENTS CONTAIN IMPORTANT INFORMATION. Investors and security holders may obtain the documents free of charge at the SEC’s website, www.sec.gov, and from Axiall at its website, www.axiall.com, or 1000 Abernathy Road NE, Suite 1200, Atlanta, GA 30328, Attention: General Counsel.

Participants in the Merger Solicitation

Axiall and certain of its respective directors, executive officers and other persons may be deemed to be participants in the solicitation of proxies in respect of the annual meeting of stockholders and the special meeting of stockholders. Information regarding Axiall’s directors and executive officers is available in Axiall’s proxy statement filed with the SEC on April 12, 2016 in connection with its 2016 annual meeting of stockholders. Other information regarding persons who may be deemed participants in the proxy solicitations and a description of their direct and indirect interests, by security holdings or otherwise, will be contained in the definitive proxy statement related to the proposed merger and other relevant materials to be filed with the SEC when they become available.

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Date: June 10, 2016

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