<PAGE>   1
                                                                   EXHIBIT 10.25

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                           STANDBY PURCHASE AGREEMENT

                                 BY AND BETWEEN

                        STERLING CHEMICALS HOLDINGS, INC.

                                       AND

                                WILLIAM A. MCMINN













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                          DATED AS OF DECEMBER 15, 1998

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IN MAKING AN INVESTMENT DECISION INVESTORS MUST RELY ON THEIR OWN EXAMINATION OF
THE ISSUER AND THE TERMS OF THE OFFERING, INCLUDING THE MERITS AND RISKS
INVOLVED. THE SECURITIES PURCHASED HEREUNDER HAVE NOT BEEN RECOMMENDED OR
APPROVED BY ANY FEDERAL OR STATE SECURITIES COMMISSION OR REGULATORY AUTHORITY.
FURTHERMORE, THE FOREGOING AUTHORITIES HAVE NOT CONFIRMED THE ACCURACY OR
DETERMINED THE ADEQUACY OF THIS DOCUMENT. ANY REPRESENTATION TO THE CONTRARY IS
A CRIMINAL OFFENSE.

THE SECURITIES PURCHASED HEREUNDER ARE SUBJECT TO RESTRICTIONS ON
TRANSFERABILITY AND RESALE AND MAY NOT BE TRANSFERRED OR RESOLD EXCEPT AS
PERMITTED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND THE APPLICABLE STATE
SECURITIES LAWS, PURSUANT TO REGISTRATION OR EXEMPTION THEREFROM. INVESTORS
SHOULD BE AWARE THAT THEY MAY BE REQUIRED TO BEAR THE FINANCIAL RISKS OF THIS
INVESTMENT FOR AN INDEFINITE PERIOD OF TIME.



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                                TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                                                             Page
                                                                                                             ----
<S>               <C>                                                                                         <C>
Section 1         Definitions and Interpretation............................................................   2
Section 2         Standby Commitment........................................................................   5
Section 3         Closings..................................................................................   6
Section 4         Simultaneous Closings Under Other Purchase Agreements.....................................   7
Section 5         Use of Proceeds...........................................................................   7
Section 6         Representations and Warranties of the Company.............................................   7
Section 7         Representations and Warranties of the Purchaser...........................................   9
Section 8         Compliance with Securities Laws and Agreements............................................  10
Section 9         Changes in Capital Structure..............................................................  12
Section 10        Specific Performance......................................................................  12
Section 11        Notices...................................................................................  12
Section 12        Survival..................................................................................  12
Section 13        Benefit and Burden........................................................................  13
Section 14        No Third Party Rights.....................................................................  13
Section 15        Amendments and Waiver.....................................................................  13
Section 16        Severability..............................................................................  13
Section 17        Expenses..................................................................................  13
Section 18        Governing Law.............................................................................  13
Section 19        Entire Agreement..........................................................................  13

Schedule I        Purchasers
</TABLE>



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                                                                         MCMINN



                           STANDBY PURCHASE AGREEMENT

              THIS STANDBY PURCHASE AGREEMENT (this "Agreement") is made and
entered into as of December 15, 1998 by and between STERLING CHEMICALS HOLDINGS,
INC., a Delaware corporation (the "Company"), and WILLIAM A. MCMINN (the
"Purchaser").

                                       PRELIMINARY STATEMENTS

         A.       The Company and the Purchaser are entering into this Agreement
                  to evidence the Purchaser's agreement to purchase up to
                  333,333 shares of common stock, par value $0.01 per share, of
                  the Company ("Common Stock").

         B.       Simultaneously with the execution and delivery of this
                  Agreement, the Company and Harris Trust and Savings Bank, as
                  agent, entered into a Warrant Agreement dated as of the date
                  hereof (the "Warrant Agreement"). In order to induce the
                  Purchaser to enter into this Agreement, the Company has issued
                  to the Purchaser on the date hereof warrants to purchase
                  40,000 shares of Common Stock (the "Initial Warrants"). The
                  Initial Warrants are evidenced by a certificate in the form
                  attached as Exhibit A to the Warrant Agreement and are subject
                  to the terms and conditions set forth in the Warrant Agreement
                  and such certificate.

         C.       In connection with the issuance of the Initial Warrants, the
                  Company and the Purchaser entered into an Adoption Agreement
                  dated as of the date hereof (the "Adoption Agreement") whereby
                  the Purchaser became a party to the Stockholders Agreement,
                  the Registration Rights Agreement and the Tag-Along Agreement
                  (as such terms are defined below).

         D.       In connection with the issuance of the Initial Warrants, the
                  Company, the Purchaser and certain other parties entered into
                  an Amended and Restated Voting Agreement dated as of the date
                  hereof (the "Voting Agreement").

         E.       Simultaneously with the execution and delivery of this
                  Agreement, the Company is entering into separate Standby
                  Purchase Agreements, substantially identical to this
                  Agreement, with the other purchasers named in Schedule I (the
                  "Other Purchasers") to evidence the Other Purchasers'
                  agreement to purchase up to the number of shares of Common
                  Stock set opposite their respective names in Schedule I.

              NOW, THEREFORE, in consideration of the premises and the mutual
covenants and agreements set forth herein and for other good and valuable
consideration, the receipt and sufficiency of which is hereby acknowledged, the
parties hereto, intending to be legally bound, hereby agree as follows:


<PAGE>   5


              Section 1. Definitions and Interpretation. Capitalized terms used
in this Agreement shall, except where the context otherwise requires, have the
following respective meanings:

              "Acts" has the meaning specified in Section 7(e).

              "Adoption Agreement" has the meaning specified in the Preliminary
          Statements of this Agreement.

              "Agreement" means this Standby Purchase Agreement, as from time to
       time amended in accordance with the terms hereof.

              "Bankruptcy Event" means that:

                     (i) the Company or any Subsidiary has (A) made a general
       assignment for the benefit of creditors, (B) filed a voluntary bankruptcy
       petition, (C) become the subject of an order for relief or been declared
       insolvent in any federal or state bankruptcy or insolvency proceeding,
       (D) instituted a proceeding or filed an answer in a proceeding seeking to
       adjudicate itself insolvent or seeking reorganization, arrangement,
       composition, readjustment, protection, liquidation, winding-up,
       dissolution or similar relief of it or its debts under any Debtor Relief
       Law, (E) filed an answer or other pleading admitting or failing to
       contest the material allegations of a petition filed against it in a
       proceeding of the type described in subclauses (A) through (D) of this
       clause (i), (F) sought, consented to or acquiesced in an order for relief
       or the appointment of a trustee, receiver, liquidator or similar official
       for it or for any substantial part of such its assets or (G) taken any
       action in furtherance of any such actions; or

                    (ii) any proceeding of the type referred to in clause (i)
       above has been filed or commenced against the Company or any Subsidiary
       or the Company or any Subsidiary by any act has indicated its approval
       thereof, consented thereto or acquiesced therein, or an order for relief
       has been entered in an involuntary case under any Debtor Relief Law, or
       an order, judgment or decree has been entered appointing a trustee,
       receiver, custodian, liquidator or similar official or adjudicating it
       insolvent, or approving the petition in any such proceedings.

              "Business Day" means any day which is neither a Saturday or Sunday
       nor a legal holiday on which banks are authorized or required to be
       closed in New York, New York, or Houston, Texas.

              "Closing" has the meaning specified in Section 3(a).

              "Closing Date" has the meaning specified in Section 2(b).

              "Common Shares" means the shares of Common Stock issuable to the
       Purchaser under this Agreement.


                                      -2-
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              "Common Stock" has the meaning specified in the Preliminary
       Statements of this Agreement.

              "Company" has the meaning specified in the opening paragraph
       hereof.

              "Debtor Relief Laws" means the Bankruptcy Code of the United
       States, and any successor statute of similar import, and all other
       applicable dissolution, liquidation, conservatorship, bankruptcy,
       moratorium, readjustment of debt, compromise, rearrangement,
       receivership, insolvency, fraudulent transfer or conveyance,
       reorganization or similar debtor relief laws from time to time in effect
       affecting the rights of creditors generally.

              "Election Notice" has the meaning specified in Section 2(a).

              "Exchange Act" means the Securities Exchange Act of 1934.

              "Form 10-K Report" has the meaning specified in Section 6(d).

              "Governmental Authority" means (i) any nation or government, (ii)
       any federal, state, county, province, city, town, municipality, local or
       other political subdivision thereof or thereto, (iii) any court,
       tribunal, department, commission, board, bureau, instrumentality, agency,
       council, arbitrator or other entity exercising executive, legislative,
       judicial, regulatory or administrative functions of or pertaining to
       government and (iv) any other governmental entity, agency or authority
       having or exercising jurisdiction over any relevant Person, item or
       matter.

              "Initial  Warrants" has the meaning specified in the Preliminary
       Statements of this Agreement.

              "Laws" means all laws, statutes, rules, regulations, ordinances,
       orders, writs, injunctions or decrees and other pronouncements having the
       effect of law of any Governmental Authority.

              "Material Adverse Effect" means a material adverse effect on the
       business, properties, assets, financial condition or results of
       operations of the Company and the Subsidiaries, taken as a whole.

              "Other Purchasers" has the meaning specified in the Preliminary
       Statements of this Agreement.

              "Person" means any individual, corporation, partnership, limited
       liability company, firm, association, joint venture, Governmental
       Authority or other entity or enterprise.

              "Purchaser" has the meaning specified in the opening paragraph
       hereof.


                                      -3-
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              "Registration Rights Agreement" means the Registration Rights
       Agreement dated as of August 21, 1996, as amended, among the Company and
       the Persons named therein.

              "SEC" means the Securities and Exchange Commission.

              "Securities" means the Common Shares, the Warrants and the Warrant
       Shares.

              "Securities Act" means the Securities Act of 1933.

              "Stockholders Agreement" means the Sterling Chemicals Holdings,
       Inc. Stockholders Agreement dated effective as of August 21, 1996, as
       amended, among the Company and the stockholders of the Company parties
       thereto.

              "Subsequent Warrants" has the meaning specified in Section 3(f).

              "Subsidiary" means any subsidiary of the Company of which greater
       than 50% of the outstanding shares of capital stock having ordinary
       voting power for the election of directors is owned directly or
       indirectly by the Company.

              "Tag-Along Agreement" means the Tag-Along Agreement dated
       effective as of August 21, 1996, among the Company and the stockholders
       of the Company parties thereto.

              "Transaction Documents" means, collectively, this Agreement, the
       Adoption Agreement, the Registration Rights Agreement, the Voting
       Agreement, the Tag-Along Agreement, the Warrant Agreement and the
       Stockholders Agreement.

              "Voting Agreement" means an Amended and Restated Voting Agreement
       dated as of the date hereof among the Company and the Persons named
       therein.

              "Warrants" means the Initial Warrants and the Subsequent Warrants.

              "Warrant Agreement" has the meaning specified in the Preliminary
       Statements of this Agreement.

              "Warrant Shares" means the shares of Common Stock issuable to the
       Purchaser upon exercise of the Warrants.

              (b) In this Agreement, unless a clear contrary intention appears:

              (i) the words "hereof," "herein" and "hereunder" and words of
       similar import refer to this Agreement as a whole and not to any
       particular provision of this Agreement;

             (ii) reference to any gender includes each other gender and the
       neuter;


                                      -4-
<PAGE>   8


            (iii) all terms defined in the singular shall have the same meanings
       in the plural and vice versa;

             (iv) reference to any Person includes such Person's heirs,
       executors, personal representatives, administrators, successors and
       assigns; provided, however, that nothing contained in this clause (iv) is
       intended to authorize any assignment not otherwise permitted by this
       Agreement;

              (v) reference to a Person in a particular capacity or capacities
       excludes such Person in any other capacity;

             (vi) reference to any contract or agreement means such contract or
       agreement as amended, supplemented or modified from time to time in
       accordance with the terms thereof;

            (vii) all  references to Sections shall be deemed to be references
       to the Sections of this Agreement;

           (viii) all references to Exhibits and Schedules shall be deemed to be
       references to the Exhibits and Schedules attached hereto which are made a
       part hereof and incorporated herein by reference;

             (ix) the word "including" (and with correlative meaning "include")
       means including, without limiting the generality of any description
       preceding such term;

              (x) with respect to the determination of any period of time, the
       word "from" means "from and including" and the words "to" and "until"
       each means "to but excluding";

             (xi) the captions and headings contained in this Agreement shall
       not be considered or given any effect in construing the provisions hereof
       if any question of intent should arise;

            (xii) reference to any Law means such Law as amended, modified,
       codified, reenacted, supplemented or superseded in whole or in part, and
       in effect from time to time; and

           (xiii) no provision of this Agreement shall be interpreted or
       construed against any party solely because that party or its legal
       representative drafted such provision.

              Section 2. Standby Commitment. (a) Upon the terms and subject to
the satisfaction or waiver of the conditions set forth herein, the Purchaser
agrees to purchase from the Company such number of shares of Common Stock (not
to exceed 333,333 in the aggregate) as the Company may, in its sole discretion,
specify at any time or from time to time, in each case at a price per share,
payable in cash, of $6.00. If the Company desires to require the Purchaser to
purchase shares of Common Stock as aforesaid, the Company must give the
Purchaser not less than 15


                                      -5-
<PAGE>   9


days' prior written notice (an "Election Notice"). Each Election Notice shall
(i) be given in accordance with Section 11, (ii) refer to this Agreement, (iii)
specify the number of shares to be purchased, (iv) specify the applicable
Closing Date (as defined below), which may not be later than the third
anniversary of the date hereof, and (v) state that, in the good faith judgement
of the Company, the sale of the Common Shares referred to in such Election
Notice is necessary in order for the Company to maintain, reestablish or enhance
its borrowing rights under its revolving credit facilities and/or to satisfy any
obligation under the documentation governing its revolving credit facilities to
raise additional equity capital. Notwithstanding anything to the contrary
contained herein, in no event shall the Company be required to sell or issue any
Common Shares unless the Company has given to the Purchaser an Election Notice
with respect to such Common Shares pursuant to this paragraph (a) and then only
the number of Common Shares specified in such Election Notice. The Company may
not revoke any Election Notice given by it as aforesaid.

              (b) For purposes of this Agreement, "Closing Date" means, when
used with reference to any purchase of Common Shares, the date on which such
purchase is to be consummated; provided, however, that if such date is not a
Business Day, then the Closing Date for such purchase shall be automatically
extended to the next succeeding Business Day and, provided further, that each
Closing Date shall be at least 30 days after the immediately preceding Closing
Date, if any.

              Section 3. Closings. (a) The consummation of each purchase of
Common Stock pursuant to this Agreement (a "Closing) shall take place on the
applicable Closing Date at 10:00 a.m. (Houston time) unless a different time is
specified in the applicable Election Notice. Each Closing shall be held at the
corporate offices of the Company or at such other place as may be agreed upon by
the Company and the Purchaser.

               (b) At each Closing, the Company shall deliver to the Purchaser
(i) a duly executed certificate evidencing the Common Shares being purchased at
such Closing and (ii) a duly executed certificate of the chief executive
officer, the chief financial officer or the treasurer of the Company, dated the
applicable Closing Date, stating that the representations and warranties of the
Company contained in Section 6 are true and correct on and as of such Closing
Date with the same force and effect as though made on and as of such Closing
Date, except for any representations and warranties made as of a specified date,
which shall be true and correct in all material respects as of such specified
date, and stating that (A) no Bankruptcy Event has occurred and is continuing,
(B) the Company is generally paying its debts as they become due, (C) the
Company owns property having a value greater than the amount required to pay the
probable liability on its debts and (D) the Company has no plans or intentions
to initiate a Bankruptcy Event nor is it aware that any creditor of the Company
plans or intends to initiate a Bankruptcy Event.

              (c) If the first Closing occurs before March 1, 1999, then the
Company shall issue and deliver to the Purchaser at the first Closing, for no
additional consideration, a certificate (in the form attached as Exhibit A to
the Warrant Agreement) evidencing warrants to purchase that number of shares of
Common Stock (rounded to the nearest whole number) equal to the number of Common
Shares purchased by the Purchaser at the first Closing divided by 8.3333.


                                      -6-
<PAGE>   10


              (d) If the first Closing occurs before March 1, 1999, then the
Company shall issue and deliver to the Purchaser at the second Closing, for no
additional consideration, a certificate (in the form attached as Exhibit A to
the Warrant Agreement) evidencing warrants to purchase 40,000 shares of Common
Stock minus the number of Common Shares covered by the warrants issued to the
Purchaser at the first Closing.

              (e) If the first Closing occurs after February 28, 1999, then the
Company shall issue and deliver to the Purchaser at the first Closing, for no
additional consideration, a certificate (in the form attached as Exhibit A to
the Warrant Agreement) evidencing warrants to purchase 40,000 shares of Common
Stock.

              (f) All warrants issued to the Purchaser pursuant to this Section
3 are collectively referred to herein as the "Subsequent Warrants". The
Subsequent Warrants shall be subject to the terms and conditions set forth in
the Warrant Agreement and the certificates evidencing the same.

              (g) At each Closing, the Purchaser shall deliver to the Company
(i) an amount (in immediately available funds) equal to the number of Common
Shares being purchased by the Purchaser times $6.00 and (ii) a duly executed
certificate of the Purchaser, dated the applicable Closing Date, stating that
the representations and warranties of the Purchaser contained in Section 7 are
true and correct on and as of such Closing Date with the same force and effect
as though made on and as of such Closing Date, except for any representations
and warranties made as of a specified date, which shall be true and correct in
all material respects as of such specified date.

              Section 4. Simultaneous Closings under Other Purchase Agreements.
In no event shall the Purchaser be obligated to consummate any Closing unless
simultaneously with such Closing (a) the Company shall sell to the Other
Purchasers shares of Common Stock numbering, in the aggregate, not less than the
amount (rounded to the nearest whole number) determined by multiplying the
number of Common Shares being purchased by the Purchaser at such Closing times
6.5 and (b) the Company shall have received from the Other Purchasers payments
(in immediately available funds) of the purchase price for such shares, which
shall not be less than $6.00 per share.

              Section 5. Use of Proceeds. The Company shall use the proceeds
from the sale of Common Shares at each Closing and the proceeds from each sale
of Common Stock to the Other Purchasers as contemplated by Section 4 solely for
general corporate purposes.

              Section 6. Representations and Warranties of the Company. The
Company represents and warrants to the Purchaser as follows:

              (a) The Company is a corporation duly organized, validly existing
       and in good standing under the laws of the State of Delaware, and has the
       corporate power to own its property and to carry on its business as now
       being conducted.


                                      -7-
<PAGE>   11


              (b) The Company has all requisite corporate power to enter into
       and perform its obligations under the Transaction Documents and the
       Warrants and to consummate the transactions contemplated thereby. The
       Company has taken all corporate actions necessary to authorize it to
       enter into and perform its obligations under the Transaction Documents
       and the Warrants and to consummate the transactions contemplated thereby.
       The Transaction Documents have been duly executed and delivered by the
       Company and constitute legal, valid and binding obligations of the
       Company, enforceable against the Company in accordance with their terms,
       except as such enforcement may be limited by applicable bankruptcy,
       insolvency, reorganization, moratorium or similar Laws affecting
       creditors' rights generally and general equitable principles. Upon
       issuance in accordance with this Agreement and the Warrant Agreement, the
       Warrants will have been duly executed and delivered by the Company and
       will constitute legal, valid and binding obligations of the Company,
       enforceable against the Company in accordance with their terms, except as
       such enforcement may be limited by applicable bankruptcy, insolvency,
       reorganization, moratorium or similar Laws affecting creditors' rights
       generally and general equitable principles. The Common Shares have been
       duly authorized and, when issued and paid for in accordance with this
       Agreement, will be validly issued, fully paid and nonassessable. The
       Warrant Shares have been duly authorized and reserved for issuance and,
       when issued and paid for upon proper exercise of the Warrants, will be
       validly issued, fully paid and nonassessable.

              (c) Neither the execution, delivery or performance of any of the
       Transaction Documents by the Company, nor the consummation by the Company
       of the transactions contemplated thereby, will violate, or result in a
       breach of the terms of, or constitute a default under, the charter or
       bylaws of the Company or any agreement, instrument or other arrangement
       or obligation to which the Company is subject, except for such
       violations, breaches or defaults that would not have a Material Adverse
       Effect.

              (d) The Company has delivered to the Purchaser true and complete
       copies of (i) the Stockholders Agreement, the Registration Rights
       Agreement, the Tag-Along Agreement, the Warrant Agreement and the
       Company's charter and bylaws, in each case as in effect on the date
       hereof, and (ii) the Company's Annual Report on Form 10-K for the fiscal
       year ended September 30, 1998 (the "Form 10-K Report"). As of the date
       hereof, the Form 10-K Report complies in all material respects with all
       applicable requirements of the Exchange Act and the applicable rules and
       regulations promulgated thereunder and does not contain any untrue
       statement of a material fact or omit to state any material fact necessary
       in order to make the statements therein, in light of the circumstances
       under which they were made, not misleading. As of the date hereof, the
       consolidated financial statements of the Company included in the Form
       10-K Report comply as to form in all material respects with applicable
       accounting requirements and the published rules and regulations of the
       SEC with respect thereto. Such financial statements were prepared in
       accordance with applicable generally accepted accounting principles
       applied on a consistent basis during the periods involved.


                                      -8-
<PAGE>   12


              Section 7. Representations and Warranties of the Purchaser. The
Purchaser hereby represents and warrants to the Company as follows:

              (a) The Purchaser has all requisite power, authority and capacity
       to execute and deliver the Transaction Documents to which the Purchaser
       is a party (the "Purchaser Transaction Documents"), to consummate the
       transactions contemplated thereby and to perform its obligations
       thereunder. Each of the Purchaser Transaction Documents has been executed
       and delivered by the Purchaser and constitutes the legal, valid and
       binding obligation of the Purchaser, enforceable against the Purchaser in
       accordance with its terms, except as such enforcement may be limited by
       applicable bankruptcy, insolvency, reorganization, moratorium or similar
       Laws affecting creditors' rights generally and general equitable
       principles.

              (b) Neither the execution, delivery and performance by the
       Purchaser of the Purchaser Transaction Documents, nor the consummation by
       the Purchaser of the transactions contemplated thereby, will violate,
       result in a breach of the terms of or constitute a default under, any
       material agreement, instrument or other arrangement or obligation to
       which the Purchaser is subject.

              (c) The Purchaser has read and understands this Agreement,
       including the Schedules and Exhibits hereto, and acknowledges and
       understands that execution and delivery of this Agreement by the
       Purchaser creates an irrevocable obligation of the Purchaser, subject to
       the terms and conditions contained in this Agreement, to purchase the
       Warrant Shares.

              (d) The Purchaser is an "accredited investor" within the meaning
       of Regulation D under the Securities Act.

              (e) The acquisition of the Securities by the Purchaser is for the
       Purchaser's own account, is for investment purposes and is not with a
       view to, or for offer or sale for the Company in connection with, the
       distribution of any Securities in violation of the Securities Act, the
       Exchange Act or any state securities laws (collectively, the "Acts"). The
       Purchaser is not participating and does not have a participation in any
       such distribution or the underwriting of any such distribution, and has
       no present intention of selling or otherwise disposing of any of the
       Securities in violation of the Acts.

              (f) The Purchaser is aware that neither the SEC nor any state
       securities commission has approved or disapproved the Securities or
       passed upon the accuracy or adequacy of this Agreement or any of the
       other Transaction Documents.

              (g) The Purchaser (i) recognizes that an investment in the
       Securities involves a high degree of risk, (ii) has such knowledge and
       experience in financial and business matters as to be capable of
       evaluating the risks and merits of this investment and protecting the
       Purchaser's interests in connection with this investment and (iii) is
       able to bear the economic risk of an investment in the Securities,
       including the risk of the total loss of such investment.


                                      -9-
<PAGE>   13


              (h) The Purchaser has received copies of the Transactions
       Documents, the Form 10-K Report and the Company's charter and bylaws. The
       Purchaser has received all the information the Purchaser considers
       necessary or appropriate for deciding whether to enter into this
       Agreement or to acquire the Securities, and the Purchaser has had an
       opportunity to ask questions of and receive answers from the Company
       regarding the Company and the terms and conditions of the Securities.

              (i) The Purchaser understands that the Securities have not been
       registered under the Securities Act on the basis that the issuance or
       sale of the Securities is exempt from the registration provisions
       thereof, and that the Company's reliance on the exemption is predicated
       upon the representations of the Purchaser herein.

              (j) The Purchaser has read and understands the terms and
       provisions of the Transaction Documents and understands that, by
       executing the Adoption Agreement and the Voting Agreement, all Common
       Shares and all Warrant Shares issued to the Purchaser will automatically
       become subject to the terms and provisions of the Stockholders Agreement,
       the Tag-Along Agreement and the Voting Agreement. The Purchaser
       understands that other shares of Common Stock currently held or
       subsequently acquired by the Purchaser may also become subject to the
       terms and provisions of the Stockholders Agreement, the Tag-Along
       Agreement and the Voting Agreement pursuant to the terms thereof.

              Section 8. Compliance with Securities Laws and Agreements. The
Purchaser acknowledges, understands and agrees that the following limitations
and restrictions are applicable to the purchase, resale and distribution of the
Securities:

              (a) The Purchaser must bear the economic risk of its investment in
       the Company for an indefinite period of time because the Securities have
       not been registered under the Acts and, therefore, may not be
       subsequently offered, sold, transferred, pledged or otherwise disposed of
       unless and until they have been registered under the Acts or exemptions
       from registration thereunder are available, and the Purchaser further
       understands that only the Company can take action to register the
       Securities.

              (b) The Purchaser has been advised that the Company does not
       expect that Rule 144 under the Securities Act will be available to the
       Purchaser with respect to any of the Securities unless the Purchaser is a
       non-affiliate of the Company (and has not been an affiliate of the
       Company for at least three months) and has held such Securities for at
       least one year from the later of the date that they were issued by the
       Company or the date that they were acquired from an affiliate of the
       Company.

              (c) The certificates representing the Warrants issued pursuant to
       this Agreement will bear the legend provided in the Warrant Agreement.
       The certificates representing the Securities issued pursuant to this
       Agreement (other than the Warrants) will bear a legend in substantially
       the following form:


                                      -10-
<PAGE>   14


                    THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
                    REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR
                    ANY STATE SECURITIES LAW (COLLECTIVELY, THE "ACTS") AND MAY
                    NOT BE OFFERED, SOLD, TRANSFERRED, PLEDGED OR OTHERWISE
                    DISPOSED OF UNLESS MADE PURSUANT TO A REGISTRATION STATEMENT
                    UNDER THE ACTS OR PURSUANT TO AN EXEMPTION FROM THE
                    REGISTRATION REQUIREMENTS THEREOF. FURTHER, SUCH SECURITIES
                    HAVE BEEN ACQUIRED FOR INVESTMENT AND MAY NOT BE OFFERED,
                    SOLD, TRANSFERRED, PLEDGED OR OTHERWISE DISPOSED OF UNLESS
                    AND UNTIL (1) SUCH SECURITIES HAVE BEEN REGISTERED UNDER THE
                    ACTS OR (2) THE HOLDER OF SUCH SECURITIES PROVIDES THE
                    COMPANY WITH (A) AN UNQUALIFIED WRITTEN OPINION OF LEGAL
                    COUNSEL, WHICH COUNSEL AND OPINION (IN FORM AND SUBSTANCE)
                    SHALL BE REASONABLY SATISFACTORY TO THE COMPANY, TO THE
                    EFFECT THAT THE PROPOSED DISPOSITION OF SUCH SECURITIES MAY
                    BE EFFECTED WITHOUT REGISTRATION UNDER THE ACTS OR (B) SUCH
                    OTHER EVIDENCE AS MAY BE REASONABLY SATISFACTORY TO THE
                    COMPANY THAT THE PROPOSED DISPOSITION OF SUCH SECURITIES MAY
                    BE EFFECTED WITHOUT REGISTRATION UNDER THE ACTS.

              (d) In addition to the legends provided in paragraph (c) above,
       the certificates evidencing the Common Shares and the Warrant Shares will
       bear legends in substantially the following forms:

                    BY THE TERMS OF THE STOCKHOLDERS AGREEMENT AND THE TAG-ALONG
                    AGREEMENT, CERTAIN RESTRICTIONS HAVE BEEN PLACED UPON THE
                    TRANSFERABILITY OF THE SECURITIES REPRESENTED BY THIS
                    CERTIFICATE. THE COMPANY WILL FURNISH A COPY OF THE
                    STOCKHOLDERS AGREEMENT AND THE TAG-ALONG AGREEMENT TO THE
                    RECORD HOLDER OF THIS CERTIFICATE WITHOUT CHARGE UPON
                    WRITTEN REQUEST TO THE COMPANY AT ITS PRINCIPAL PLACE OF
                    BUSINESS OR REGISTERED OFFICE. NO REGISTRATION OR TRANSFER
                    OF ANY SECURITIES REPRESENTED BY THIS CERTIFICATE WILL BE
                    MADE ON THE BOOKS OF THE COMPANY UNLESS AND UNTIL SUCH
                    RESTRICTIONS HAVE BEEN COMPLIED WITH.

                    THE SECURITIES REPRESENTED BY THIS CERTIFICATE ARE SUBJECT
                    TO THE TERMS AND CONDITIONS OF AN AMENDED AND RESTATED
                    VOTING AGREEMENT, A COPY OF WHICH IS ON FILE WITH THE
                    SECRETARY OF THE COMPANY, AND ARE HELD AND MAY BE SOLD,
                    ASSIGNED, TRANSFERRED OR OTHERWISE DISPOSED OF ONLY IN
                    ACCORDANCE WITH SUCH AGREEMENT.


                                      -11-
<PAGE>   15


              (e) Stop Transfer Instructions. The Company's stock transfer
       records will contain stop transfer instructions with respect to the
       Common Shares and the Warrant Shares to provide notice of the
       restrictions on the resale or distribution thereof imposed by the Acts
       and the Stockholders Agreement, the Tag-Along Agreement and the Voting
       Agreement.

              Section 9. Changes in Capital Structure. In the event of any
change after the date hereof in the number of issued shares of Common Stock by
reason of any stock dividend, split-up, recapitalization, merger, combination,
conversion, exchange of shares or other change in the corporate or capital
structure of the Company, then there shall be appropriate and equitable
adjustments made in the number and kind of shares of stock or other securities
of the Company thereafter issued to the Purchaser pursuant to this Agreement.

              Section 10. Specific Performance. The parties agree that the
covenants and obligations contained in this Agreement relate to special, unique
and extraordinary matters and that a violation of any of the terms hereof would
cause irreparable injury in an amount which would be impossible to estimate or
determine and for which any remedy at law would be inadequate. As such, the
parties agree that if either party fails or refuses to fulfill any of such
party's obligations under this Agreement or to make any payment or deliver any
instrument required hereunder, then the other party shall have the remedy of
specific performance, which remedy shall be cumulative and nonexclusive and
shall be in addition to any other rights and remedies otherwise available under
any other contract or at law or in equity and to which such party might be
entitled.

              Section 11. Notices. Any and all notices, requests or other
communications hereunder shall be given in writing and delivered by (a) regular,
overnight or registered or certified mail (return receipt requested), with first
class postage prepaid, (b) hand delivery, (c) facsimile or electronic mail
transmission or (d) overnight courier service, to the parties at addresses or
facsimile numbers set forth below their respective names on the signature pages
hereof, or at such other address or number as shall be designated by either
party in a notice to the other party given in accordance with this Section 11.
Except as otherwise provided in this Agreement, all such communications shall be
deemed to have been duly given, (i) in the case of a notice sent by regular
mail, on the date actually received by the addressee, (ii) in the case of a
notice sent by registered or certified mail, on the date receipted for (or
refused) on the return receipt, (iii) in the case of a notice delivered by hand,
when personally delivered, (iv) in the case of a notice sent by facsimile or
electronic mail, upon transmission subject to telephone confirmation of receipt,
and (v) in the case of a notice sent by overnight mail or overnight courier
service, the date delivered at the designated address, in each case given or
addressed as aforesaid.

              Section 12. Survival. All representations, warranties and
covenants contained in this Agreement shall survive the execution and delivery
of this Agreement, the Closings, the delivery of any Securities to the Purchaser
and the death or disability of the Purchaser.


                                      -12-
<PAGE>   16


              Section 13. Benefit and Burden. This Agreement shall inure to the
benefit of, and shall be binding upon, the Company and its successors and
assigns and the Purchaser and the Purchaser's heirs, executors, personal
representatives, administrators, successors and assigns.

              Section 14. No Third Party Rights. Nothing in this Agreement shall
be deemed to create any right in any creditor or other Person not a party
hereto, and this Agreement shall not be construed in any respect to be a
contract in whole or in part for the benefit of any third party.

              Section 15. Amendments and Waiver. No amendment, modification,
restatement or supplement of this Agreement shall be valid unless the same is in
writing and signed by the parties. No waiver of any provision of this Agreement
shall be valid unless in writing and signed by the party against whom that
waiver is sought to be enforced. No failure or delay on the part of any of the
parties in exercising any right, power or privilege hereunder, and no course of
dealing between or among the parties, shall operate as a waiver of any right,
power or privilege hereunder. No single or partial exercise of any right, power
or privilege hereunder shall preclude any other or further exercise thereof or
the exercise of any other right, power or privilege hereunder. No notice to or
demand on either party in any case shall entitle such party to any other or
further notice or demand in similar or other circumstances or constitute a
waiver of the rights of any party to any other or further action in any
circumstances without notice or demand.

              Section 16. Severability. Should any clause, sentence, paragraph,
subsection or Section of this Agreement be judicially declared to be invalid,
unenforceable or void, such decision will not have the effect of invalidating or
voiding the remainder of this Agreement, and the parties agree that the part or
parts of this Agreement so held to be invalid, unenforceable or void will be
deemed to have been stricken herefrom as if such stricken part or parts had
never been included herein.

              Section 17. Expenses. The Company will pay, or reimburse the
Purchaser for the payment of, all reasonable expenses incurred by the Purchaser
prior to the date hereof in connection with this Agreement, including all legal
and accounting fees and disbursements. Except as aforesaid, each of the parties
shall pay its own expenses incident to this Agreement, including all legal and
accounting fees and disbursements.

              Section 18. Governing Law. THIS AGREEMENT SHALL BE GOVERNED BY,
CONSTRUED UNDER, AND ENFORCED IN ACCORDANCE WITH THE LAWS OF THE STATE OF
DELAWARE, WITHOUT REFERENCE TO THE CONFLICT-OF-LAWS PROVISIONS THEREOF.

              Section 19. Entire Agreement. This Agreement sets forth all of the
promises, agreements, conditions, understandings, warranties and representations
of the parties with respect to the transactions contemplated hereby, and
supersedes all prior agreements, arrangements and understandings between the
parties, whether written, oral or otherwise, with respect to the subject matter
hereof. There are no promises, agreements, conditions, understandings,
warranties or representations, oral or written, express or implied, between the
parties concerning the subject matter hereof except as set forth herein.


                                      -13-
<PAGE>   17
              IN WITNESS WHEREOF, the parties hereto have caused this Agreement
to be executed as of the date first above written.


                                  STERLING CHEMICALS HOLDINGS, INC.



                                  By:
                                      ------------------------------------------
                                  Printed Name:
                                                  ------------------------------
                                  Title:
                                          --------------------------------------

                                  Address:    Sterling Chemicals Holdings, Inc.
                                              1200 Smith, Suite 1900
                                              Houston, Texas 77002
                                              Attention: General Counsel
                                  Facsimile No.: (713) 654-9577
                                  E-Mail:  Delkins@sterlingchemicals.com


                                  PURCHASER:




                                  ----------------------------------------------
                                  William A. McMinn

                                  Address:
                                            ------------------------------------

                                            ------------------------------------

                                            ------------------------------------

                                  Facsimile No.:
                                                   -----------------------------

                                  E-Mail:
                                            ------------------------------------


                                      -14-
<PAGE>   18



                                   SCHEDULE I
                                   PURCHASERS

<TABLE>
<CAPTION>

PURCHASER                                                                NUMBER OF SHARES        PERCENTAGE ALLOCATION
---------                                                                ----------------        ---------------------
<S>                                                                      <C>                     <C>
Gordon A. Cain..................................................             1,333,333                  53.33%
William A. McMinn...............................................               333,333                  13.33%
James Crane.....................................................               250,000                  10.00%
Frank P. Diassi.................................................               166,667                   6.67%
Frank J. Hevrdejs...............................................               166,667                   6.67%
Koch Capital Services, Inc......................................               250,000                  10.00%
                                                                             ---------                 ------
         Total..................................................             2,500,000                 100.00%

</TABLE>





                                      -15-