<PAGE>   1
                                                                  EXHIBIT 10.16B

                       AMENDMENT TO PARTNERSHIP AGREEMENT

                                     BETWEEN
                               OLIN SUNBELT, INC.
                                       AND
                         1997 CHLORALKALI VENTURE, INC.

         THIS AMENDMENT TO PARTNERSHIP AGREEMENT is entered into as of this 23rd
day of December, 1997, between Olin Sunbelt, Inc., a Delaware corporation
("OSI"), and 1997 Chloralkali Venture, Inc. ("1997 CVI"), a Delaware
corporation.

         WHEREAS, OSI and 1997 CVI entered into a Partnership Agreement dated as
of August 23, 1996 (the "Partnership Agreement"), pursuant to which OSI and 1997
CVI (collectively, the "Partners") established the Sunbelt Chlor Alkali
Partnership a Delaware general partnership (the "Partnership"); and

         WHEREAS, OSI and 1997 CVI desire to amend the Partnership Agreement in
certain respects as set forth herein;

         NOW THEREFORE, OSI and 1997 CVI agree as follows:

         1. A new Section 1.12 is added as follows:

1.12 SENIOR DEBT
     -----------

(A) FOR PURPOSES OF THIS AGREEMENT, "SENIOR DEBT" SHALL MEAN THE OBLIGATIONS OF
THE PARTNERSHIP AS SET FORTH UNDER THOSE CERTAIN NOTE PURCHASE AGREEMENTS DATED
AS OF DECEMBER , 1997 BY AND BETWEEN THE PARTNERSHIP AND VARIOUS INSTITUTIONAL
INVESTORS, INCLUDING BUT NOT LIMITED TO THE NOTES (AS DEFINED THEREIN), AS SUCH
SENIOR DEBT MAY BE MODIFIED, AMENDED, OR RESTATED FROM TIME TO TIME. THE TERMS
"GUARANTEE" AND "GUARANTOR" USED HEREIN SHALL HAVE THE DEFINITIONS SET FORTH IN
THE ABOVE-REFERENCED NOTE PURCHASE AGREEMENTS. THE PROVISIONS OF THIS SECTION
1.12 SHALL APPLY, NOTWITHSTANDING ANY PROVISIONS TO THE CONTRARY ELSEWHERE IN
THIS AGREEMENT OR IN ANY OF THE ANCILLARY AGREEMENTS.

(B) IF A GUARANTOR EVENT OF DEFAULT WITH RESPECT TO A GUARANTOR (A "DEFAULTING
GUARANTOR") OCCURS (SUCH GUARANTOR EVENTS OF DEFAULT BEING AS DEFINED IN SUCH
DEFAULTING GUARANTOR'S GUARANTEE DATED AS OF DECEMBER _, 1997), THEN FOR SO LONG
AS SUCH GUARANTOR EVENT OF DEFAULT REMAINS UNCURED, AT THE OPTION OF THE PARTNER
WHICH IS NOT AN AFFILIATE OF THE DEFAULTING GUARANTOR (THE "NON-AFFECTED
PARTNER"), THE MANAGEMENT COMMITTEE MEMBER(S) APPOINTED BY THE PARTNER
AFFILIATED WITH SUCH DEFAULTING GUARANTOR (THE "AFFECTED PARTNER") SHALL BECOME
NON-VOTING MEMBER(S) OF THE MANAGEMENT COMMITTEE, AND THE REMAINING VOTING
MEMBER(S) OF THE MANAGEMENT COMMITTEE SHALL CONSTITUTE A QUORUM AT MANAGEMENT
COMMITTEE MEETINGS, AND SHALL BE FULLY AUTHORIZED TO TAKE ACTIONS, INCLUDING
WITHOUT LIMITATION TO APPROVE DECISIONS OF IMPORTANCE (EXCEPT AS PROVIDED IN (C)
BELOW), ON BEHALF OF 



<PAGE>   2

THE MANAGEMENT COMMITTEE, AND TO ACT BY RESOLUTION IN LIEU OF A MEETING. SUCH
OPTION SHALL BE EXERCISABLE IN THE SOLE DISCRETION OF THE NONAFFECTED PARTNER,
EFFECTIVE IMMEDIATELY UPON WRITTEN NOTICE TO THE AFFECTED PARTNER AND THE
PARTNERSHIP; PROVIDED HOWEVER THAT THE PARTNERS AGREE NOT TO EXERCISE SUCH
OPTION AT SUCH TIMES AS (A) THE ONLY OUTSTANDING GUARANTOR EVENTS OF DEFAULT
ARE, (I) WITH RESPECT TO THE GEON GUARANTEE, SOLELY A BREACH OF ITS COVENANTS
CONTAINED IN SECTIONS 4.1, 4.3, 4.8 AND/OR 4.9, OR (II) WITH RESPECT TO THE OLIN
GUARANTEE, SOLELY A BREACH OF ITS COVENANTS CONTAINED IN SECTIONS 4.3, 4.6
AND/OR 4.7; AND (B) NONE OF THE HOLDERS OF THE SENIOR DEBT HAVE DEMANDED
COMPLIANCE BY THE DEFAULTING GUARANTOR WITH SECTION 2.2 OF ITS GUARANTEE OR
STATED IN WRITING AN INTENTION TO REQUIRE SUCH COMPLIANCE OR TO ENFORCE SUCH
GUARANTEE.

         (C) NOTWITHSTANDING SUBSECTION (B) ABOVE, THE VOTING MEMBER(S) OF THE
MANAGEMENT COMMITTEE SHALL NOT AGREE, WITHOUT THE CONSENT OF THE AFFECTED
PARTNER, ACTING THROUGH ITS NON-VOTING MEMBER(S) OF THE MANAGEMENT COMMITTEE,
(I) TO SELL ALL OR SUBSTANTIALLY ALL OF THE ASSETS OF THE PARTNERSHIP, (II) TO
AN EXPANSION OF THE FACILITY TO BE FUNDED BY THE PARTNERSHIP UNDER SECTION 1.11,
(III) TO REQUIRE ADDITIONAL CAPITAL CONTRIBUTIONS BY THE PARTNERS, EXCEPT TO THE
EXTENT NECESSARY TO FUND EXPENDITURES REQUIRED UNDER SECTION 4.04 OF THE
OPERATING AGREEMENT, (IV) TO AMEND OR WAIVE ANY PROVISION OF ANY ANCILLARY
AGREEMENT TO WHICH THE PARTNERSHIP AND THE NON-AFFECTED PARTNER OR A PARTNER
AFFILIATE OF THE NON-AFFECTED PARTNER ARE PARTY, IN ANY MATERIAL RESPECT WHICH
IS COMMERCIALLY UNREASONABLE AND WILL MATERIALLY ADVERSELY AFFECT THE FINANCIAL
CONDITION OR OPERATIONS OF THE PARTNERSHIP, (V) TO DECIDE TO REPUDIATE OR BREACH
ANY PROVISION OF AN ANCILLARY AGREEMENT TO WHICH THE AFFECTED PARTNER OR A
PARTNER AFFILIATE OF THE AFFECTED PARTNER IS PARTY, EXCEPT TO THE EXTENT
PERMITTED UNDER SUCH ANCILLARY AGREEMENT OR AS AN EXERCISE OF RIGHTS AND
REMEDIES AVAILABLE TO IT AT LAW OR EQUITY, OR (VI) TO CAUSE THE PARTNERSHIP TO
INCUR OR ISSUE ANY DEBT FOR BORROWED MONEY (OTHER THAN A PARTNER ADVANCE, AS
DEFINED BELOW) OR OTHERWISE INCREASE THE DEFAULTING GUARANTOR'S LIABILITY FOR
THE SENIOR DEBT UNDER ITS GUARANTEE TO THE HOLDERS OF THE SENIOR DEBT, EXCEPT TO
THE EXTENT NECESSARY TO FUND EXPENDITURES REQUIRED UNDER SECTION 4.04 OF THE
OPERATING AGREEMENT.

         (D) IN THE EVENT THAT THE PARTNERSHIP INTEREST OF SUCH AFFECTED PARTNER
IS TRANSFERRED, ASSIGNED OR CONVEYED (WHETHER VOLUNTARILY, AS A RESULT OF
BANKRUPTCY PROCEEDINGS OR JUDICIAL PROCEEDING OR OTHERWISE BY OPERATION OF LAW)
OR THE DEFAULTING GUARANTOR CEASES TO OWN, DIRECTLY OR INDIRECTLY 100% OF THE
AFFECTED PARTNER, IN EACH CASE AT ANY TIME WHILE EITHER SUCH GUARANTOR EVENT OF
DEFAULT REMAINS UNCURED OR AS PART OF THE SETTLEMENT, SATISFACTION OR CURE OF
SUCH GUARANTOR EVENT OF DEFAULT, THEN THE VOTING AND NON-VOTING MEMBERS OF THE
MANAGEMENT COMMITTEE APPOINTED BY THE AFFECTED PARTNER SHALL CEASE TO BE MEMBERS
THEREOF AND THE TRANSFEREE, ASSIGNEE OR SUCCESSOR TO THE PARTNERSHIP INTEREST
SHALL HAVE NO RIGHT TO APPOINT VOTING MEMBER(S) OF THE MANAGEMENT COMMITTEE,
WHICH RIGHT SHALL BE DEEMED AUTOMATICALLY AND IRREVOCABLY EXTINGUISHED AND
PARAGRAPHS (B) AND (C) SHALL NO LONGER BE APPLICABLE. THIS SUBSECTION (D) SHALL
NOT APPLY IN THE CASE OF A PERMITTED ASSIGNMENT IN ACCORDANCE WITH ARTICLE 5 OF
THE PARTNERSHIP AGREEMENT, AND THE GUARANTOR EVENT OF DEFAULT IS CURED (OR
WAIVED BY THE HOLDERS OF THE SENIOR DEBT) AT OR PRIOR TO THE EFFECTIVE DATE OF
ASSIGNMENT.

         (E) UPON THE OCCURRENCE OF AN EVENT OF DEFAULT UNDER SECTION 11(1)(Y)
OF THE NOTE 


<PAGE>   3

PURCHASE AGREEMENTS (A "TRIGGERING EVENT") WITH RESPECT TO THE GUARANTEE OF THE
PARTNER AFFILIATE OF A PARTNER (THE "REPUDIATING PARTNER"), THEN THE
NONREPUDIATING PARTNER, DIRECTLY OR THROUGH AN AFFILIATE, SHALL HAVE THE RIGHT
TO PURCHASE THE REPUDIATING PARTNER'S INTEREST IN THE PARTNERSHIP FOR A PURCHASE
PRICE EQUAL TO ONE DOLLAR ($1.00). SUCH RIGHT SHALL BE EXERCISABLE AT ANY TIME
AFTER THE TRIGGERING EVENT, AND THE TRANSFER SHALL BE DEEMED TO OCCUR
IMMEDIATELY AND AUTOMATICALLY UPON EXERCISE OF SUCH RIGHT BY THE NON-REPUDIATING
PARTNER. THE REPUDIATING PARTNER SHALL DELIVER SUCH ASSIGNMENTS AND OTHER
DOCUMENTS OF TRANSFER AS THE NONREPUDIATING PARTNER REASONABLY REQUIRES TO
FURTHER EFFECTUATE THE TRANSFER AS PROMPTLY AS POSSIBLE (BUT IN ANY EVENT NO
MORE THAN FIVE (5) BUSINESS DAYS AFTER PRESENTATION OF SUCH DOCUMENTS BY THE
NON-REPUDIATING PARTNER). SUCH PURCHASE OF THE PARTNERSHIP INTEREST SHALL NOT
AFFECT ANY OTHER RIGHTS AND REMEDIES WHICH THE NON-REPUDIATING PARTNER, ITS
AFFILIATES, AND/OR THE PARTNERSHIP MAY HAVE AGAINST THE REPUDIATING PARTNER
AND/OR ITS PARTNER AFFILIATES, OR OTHERWISE BE DEEMED TO AFFECT OR EXTINGUISH
THE GUARANTEE OF THE PARTNER AFFILIATE OF THE REPUDIATING PARTNER.

         (F) IN THE EVENT A PARTNER DESIRES TO MAKE A TRANSFER OF ITS
PARTNERSHIP INTEREST PERMITTED UNDER ARTICLE 5 AND IN CONNECTION WITH SUCH
TRANSFER DESIRES THAT ITS PARTNER AFFILIATE'S GUARANTEE BE RELEASED, THEN THE
TRANSFERRING PARTNER SHALL NOTIFY THE OTHER PARTNER(S) AND THEREAFTER FOR A
PERIOD OF THIRTY (30) DAYS (OR SUCH LONGER PERIOD AS THE TRANSFERRING PARTNER IN
ITS SOLE DISCRETION MAY ELECT), THE TRANSFERRING PARTNER SHALL TAKE COMMERCIALLY
REASONABLE STEPS TO NEGOTIATE IN GOOD FAITH WITH THE HOLDERS OF THE SENIOR DEBT
TO SECURE SUCH RELEASE. IF SUCH EFFORTS ARE UNSUCCESSFUL, THEN SUCH PARTNER
SHALL NOTIFY THE OTHER PARTNER(S) AND THE OTHER PARTNER(S) SHALL, AT ITS OPTION,
HAVE AN ADDITIONAL THIRTY (30) DAY PERIOD TO CONDUCT NEGOTIATIONS WITH SUCH
HOLDERS REGARDING SUCH RELEASE. IF AT THE END OF SUCH NEGOTIATIONS SUCH HOLDERS
HAVE REFUSED TO RELEASE THE PARTNER AFFILIATE'S GUARANTEE, THEN THE PARTNERS
WILL CAUSE REPAYMENT OF THE SENIOR DEBT WITHIN THE FOLLOWING TEN (10) BUSINESS
DAYS, ON A 50/50 BASIS (OR OTHERWISE IN ACCORDANCE WITH THEIR THEN INTEREST IN
THE PARTNERSHIP PROVIDED IN SECTION 1.04); PROVIDED HOWEVER THAT ANY "MAKE
WHOLE" OR PREPAYMENT PENALTIES AND COSTS ASSOCIATED WITH SUCH REPAYMENT SHALL BE
PAID IN THE ENTIRETY BY THE TRANSFERRING PARTNER. THIS PARAGRAPH (F) SHALL NOT
BE APPLICABLE IF AN EVENT OF DEFAULT UNDER THE SENIOR DEBT OR ANY GUARANTEE HAS
OCCURRED AND REMAINS UNCURED (OTHER THAN AN EVENT OF DEFAULT WHICH IS SOLELY
APPLICABLE TO THE PARTNER AFFILIATE OF THE NON-TRANSFERRING PARTNER).

         (G) THE PARTIES AGREE THAT FOR PURPOSES OF THIS SECTION 1.12, A
GUARANTOR EVENT OF DEFAULT TRIGGERED BY A BANKRUPTCY SHALL NOT BE CURABLE.

         2. A new Section 2.13 is added as follows:

2.13     PARTNER ADVANCES
         ----------------

         IN THE EVENT THAT A PARTNER OR A PARTNER AFFILIATE OF SUCH PARTNER (I)
MAKES A LOAN TO THE PARTNERSHIP (WHICH LOANS SHALL REQUIRE THE CONSENT OF THE
MANAGEMENT COMMITTEE, AS PROVIDED IN SECTION 2.03 HEREOF), (II) ADVANCES FUNDS
ON BEHALF OF THE PARTNERSHIP TO SATISFY PARTNERSHIP OBLIGATIONS UNDER ANY OF THE
ANCILLARY AGREEMENTS (WHETHER SUCH OBLIGATIONS ARE TO A THIRD PARTY OR TO A
PARTNER OR A PARTNER AFFILIATE), THE SENIOR DEBT OR OTHER PARTNERSHIP
OBLIGATIONS, OR (III) HAS A RIGHT OF CONTRIBUTION OR REIMBURSEMENT FROM THE
PARTNERSHIP AND/OR


<PAGE>   4

THE OTHER PARTNER FOR PAYMENTS MADE BY SUCH PARTNER OR PARTNER AFFILIATE ON
ACCOUNT OF THE SENIOR DEBT OR OTHER OBLIGATIONS OF THE PARTNERSHIP TO THIRD
PARTIES (ANY AND ALL OF (I) THROUGH (III) BEING A "PARTNER ADVANCE"), THEN SUCH
PARTNER ADVANCE SHALL NOT BE CONSIDERED A CONTRIBUTION TO THE CAPITAL OF THE
PARTNERSHIP AND SHALL NOT INCREASE THE CAPITAL ACCOUNT OF THE ADVANCING PARTNER.
THE AMOUNT OF ANY SUCH PARTNER ADVANCE (PLUS INTEREST AT A RATE AGREED BY THE
PARTIES, BUT IN NO EVENT LESS THAN THE APPLICABLE RATE PROVIDED IN SECTION 1274
OF THE INTERNAL REVENUE CODE OF 1986) SHALL BE AN OBLIGATION OF THE PARTNERSHIP
WHICH IS PAYABLE BEFORE ANY FURTHER DISTRIBUTIONS ARE MADE UNDER SECTION 2.05
ABOVE. REPAYMENT OF SUCH PARTNER ADVANCES SHALL NOT BE DEEMED WITHDRAWALS FROM
THE CAPITAL OF THE PARTNERSHIP.

         3. Article 5 is amended as follows:

         (a) In the first sentence of Section 5.01, the words "DIRECTLY OR
INDIRECTLY THROUGH THE TRANSFER OF THE OWNERSHIP AND/OR CONTROL OF SUCH PARTNER
TO A NON-AFFILIATE" are added after the words "or any part of its interest in
the Partnership,".

         (b) Section 5.04(a) is amended by deleting the second sentence and
substituting the following therefor:

IF CHLORINE PRODUCTION IS CONTINUING AT THE OLIN PLANT AT THE TIME OF SUCH
ASSIGNMENT OR SHOT-GUN SALE, THEN OSI SHALL CAUSE THE OPERATOR OF THE OLIN PLANT
TO ENTER INTO A NEW OPERATING AGREEMENT BETWEEN THE PARTNERSHIP AND THE THEN
OPERATOR OF THE OLIN PLANT PURSUANT TO WHICH THE OPERATING AND OTHER SERVICES
AND RAW MATERIALS SUPPLIED UNDER THE OPERATING AGREEMENT SHALL BE PROVIDED AS A
UNITARY PACKAGE, TO THE EXTENT THE OLIN PLANT OPERATOR IS LEGALLY PERMITTED AT
THE TIME TO DO SO, ON THE EXACT SAME TERMS AND CONDITIONS AS THEN BEING
PROVIDED TO THE PARTNERSHIP UNDER THE OPERATING AGREEMENT.

         4. Except as amended herein, all other provisions of the Partnership
Agreement remain unchanged and in full force and effect.

         IN WITNESS WHEREOF, the Partners have executed this Amendment the day
and year first written above .

                                             OLIN SUNBELT, INC.

                                             /s/ Hassan Arabghani
                                             ---------------------------------
                                             By:      Name: Hassan Arabghani
                                                      Title: Vice President

                                             1997 CHLORALKALI VENTURE, INC.

                                             /s/ Jean M. Miklosko
                                             ---------------------------------
                                             By:      Name: Jean M. Miklosko
                                                      Title: Assistant Treasurer