<PAGE>
 
                                                          CONFIDENTIAL TREATMENT
                                                                   EXHIBIT 10.49

PORTIONS OF THIS EXHIBIT HAVE BEEN OMITTED AND FILED SEPARATELY WITH THE
SECRETARY OF THE COMMISSION PURSUANT TO REGISTRANT'S APPLICATION OBJECTING TO
DISCLOSURE AND REQUESTING CONFIDENTIAL TREATMENT UNDER RULE 24b-2; THE OMITTED
PORTIONS HAVE BEEN MARKED WITH BRACKETS.

                               AGREEMENT OF SALE
                               -----------------

This Agreement is entered into on the 1st day of January 1998 between:

1.   ARCO CHEMIE NEDERLAND, LTD. acting through its Dutch branch at Theemsweg
     14, Botlek, 3000 HD Rotterdam, The Netherlands ("Seller"); and

2.   STYROCHEM FINLAND OY having its registered office at P.O. Box 360, 06101
     Porvoo, Finland ("Buyer").

The parties agree as follows:

1.   SALE AND PURCHASE OF PRODUCT - SPECIFICATIONS
     ---------------------------------------------

     1.1  Seller shall sell, and Buyer shall purchase, the following product:
 
               Styrene Monomer T2

               Such product is hereinafter referred to as "Product".

     1.2  The Product shall meet the specifications attached at Annex 1 hereto.
 
2.   QUANTITY
     --------

     2.1  Buyer shall purchase and Seller shall sell the following quantities of
          Product +/- 5% at Buyer's option:

               1998                 [   ] KT
               1999                 [   ] KT
               2000                 [   ] KT
<PAGE>
 
               2001                 [   ] KT
               2002 and beyond      [   ] KT

     2.2  In the event that Buyer wishes to buy quantities of Product in excess
          of the quantities specified in section 2.1 in any one calendar year,
          Buyer shall place the additional order with Seller on or before [ ] of
          the preceding calendar year and Seller shall notify Buyer of its
          acceptance or rejection thereof on or before [ ] of the preceding
          calendar year.

     2.3  Buyer shall use its best efforts to take the Product in regular
          monthly quantities.

3.   DURATION
     --------

     This Agreement shall come into force on 1 January 1998 and shall continue
     for an initial period of five years.  It shall continue thereafter unless
     and until terminated by either party giving not less than one calendar year
     prior written notice to the other party expiring on 31 December 2002 at the
     earliest.

4.   PRICE AND REBATE
     ----------------

     4.1  The price for the Product is the lowest of:

          1)   ICIS contract low less [       ]%
                    or
          2)   the average of [      ]
                    or
          3)   ICIS contract low less [       ]% of Market Premium
                    or

                                      -2-
<PAGE>
 
          4)   USGC CMAI market contract low less [       ]% plus freight and
               duty
                    or
          5)   [                  ] plus freight and duty;

          the transatlantic freight and the freight from Rotterdam to Pori or
          Porvoo to be agreed during the first month of any calendar year by
          Buyer and Seller.  Seller shall send Buyer quotations from shippers
          prior to 14 January of any calendar year.  The freight shall be fixed
          at $[          ]/MT for 1998 USGC/Rotterdam.

          where: Market Premium = ICIS contract low less Cost.

          Cost = A + B + C + D + E

          A =  [       ] DM/MT capital recovery escalated with the rate of
               inflation in the Netherlands
 
          B =  [       ] DM/MT (L/L) fixed cash cost, where
 
               L    =  the latest wage index known on the last day of the
                       relevant month as published by the Dutch CBS under the
                       reference "Indexcijfers CAO lonen per month, inclusief
                       bijzondere beloningen (1990=100) -Chemische Industrie,
                       category adult workers"

               L/o/ =  the October 1997 "Indexcijfer CAO lonen per month
                       inclusief bijzondere beloningen" category adult workers =
                       118.8

          C =  [       ] DM/MT (NG/NGo) variable cash cost, where

              NG    =  N.V. Nederlandse Gasunie listed quarterly price
                       (Verrekenprijzen Grootverbruikers) for natural gas (zone
                       D) including any taxes, monthly fees or other charges
                       expressed in Dfl per Nm/3/of standard Groningen natural
                       gas with a calorific value of 35.17 MJ per Nm/3/

              NG/o/ =  the January 1, 1998 natural gas price, i.e.
                       Dfl.0.22559

                                      -3-
<PAGE>
 
          D = [       ] MT x market ethylene (contract)

          E = [       ] MT x market benzene (contract)

          Reference:
          ----------

          * ICIS - LOR European or CMAI United States Bimonthly publication
            (monomer market report) published closest prior to the bill of
            lading date.

          * Rates of exchange as published by the Financial Times on the Monday
            prior to the bill of lading date.

     4.2    Buyer shall be entitled to an annual volume rebate of [ ]% of the
            total amount invoiced upon achievement of the volumes set out in
            article 2.1, payable by credit note on or before [ ] of the
            subsequent calendar year.

5.   MODE OF DELIVERY
     ----------------

     The Product shall be delivered by vessel in 2500-3000 MT lots unless
     otherwise agreed prior to shipment.

6.   DELIVERY TERMS
     --------------

     6.1  The Product shall be delivered CIF Pori or Porvoo (Incoterms 1990 to
          apply) unless article 6.2 applies.

     6.2  If the Price for the Product is as set out in article 4.1, 4) or 5)
          above and Buyer and Seller fail to agree the freight charges, the
          Product shall be delivered FOB USGC (Incoterms 1990 to apply).

                                      -4-
<PAGE>
 
7.   TAXES
     -----

     The Buyer shall pay to the Seller in addition to the price herein provided,
     any sales tax and/or value added tax (BTW) tax or equivalent thereof that
     might currently or in the future be imposed by any government authority
     with respect to the sale of the Product by Seller to Buyer.

8.   PAYMENT - CREDIT
     ----------------

     8.1  At the time of shipment, Seller shall send Buyer a provisional invoice
          in Deutsch marks estimating the price for the relevant shipment.

     8.2  By the end of the calendar month in which delivery occurs, Seller
          shall invoice Buyer with a final invoice in Deutsch marks for
          deliveries made during that month.

     8.3  Payment must be made by [ ] by bank transfer.

     8.4  If Buyer does not pay any amount which Buyer owes on the basis of the
          above, Seller shall be entitled to payment of interest on the amount
          of all Seller's claims which are overdue in the amount of [ ]% per
          month or part of a month during which Buyer remains in default. This
          interest obligation is also effective for all interest which is due
          for longer than a year.

     8.5  If Buyer fails to pay Seller in accordance with the above terms, then
          Seller, at its option and without prejudice to its other rights and
          remedies, may (i) terminate this Contract forthwith and without
          notice, (ii) suspend deliveries until all indebtedness is paid in
          full, and/or (iii) place Buyer on a cash-on-delivery basis. In the
          event of default in payment, Buyer shall pay Seller's reasonable costs
          of collection, including, but not limited to,

                                      -5-
<PAGE>
 
          reasonable attorneys' fees.

     8.6  If in the sole and reasonable opinion of Seller the financial standing
          of Buyer is impaired or unsatisfactory, deliveries may be suspended or
          Buyer may be placed on cash-on-delivery status until arrangements are
          made for security satisfactory to Seller or, at Seller's option, until
          all of Buyer's indebtedness to Seller is paid in full.

     8.7  Except as provided in article 4.2, no prompt payment or other type of
          discount applies and all payments under this Contract shall be made at
          the full invoiced amount.

     8.8  Invoicing will be in Deutsch Marks. The exchange rates published in
          the Financial Times on the Monday prior to shipment shall be used. The
          Deutsch Mark shall be replaced by the Euro upon its introduction.

9.   SHIPMENTS
     ---------

     9.1  Buyer shall place individual orders and give shipping instructions at
          the latest [   ] days prior to the desired delivery date.  Buyer's
          shipping instructions shall include all pertinent information,
          including but not limited to desired date of delivery or loading.
          Seller shall confirm availability of Product within [      ] hours of
          receipt of the order and the delivery date within [      ] working
          days of receipt of the order.  Seller shall use reasonable endeavours
          to meet the desired delivery date.

          If Seller is unable to deliver an individual order accepted by Seller
          on or before the delivery date agreed upon for any reason except those
          described in article 12, Purchaser shall be entitled to place orders
          with a third party to the extent that such is necessary to prevent a
          reduction in their operations which cannot be recovered at a later
          date.  In such case, Seller shall be liable to Buyer for the amount,
          if any, that Buyer

                                      -6-
<PAGE>
 
          pays to such third party in excess of the price which would have been
          payable by Buyer under the terms of this Agreement and any such
          quantities purchased from that third party shall be deemed to have
          been supplied by Seller for the purposes of article 4.2.

     9.2  Buyer shall promptly receive and unload shipments and pay all
          demurrage, rental and other charges or damages resulting from Buyer's
          delay in receiving or unloading the Product.

     9.3  Shipments shall not be diverted or reconsigned by Buyer without the
          prior written consent of Seller other than to Neste Oy or Norlatex.

10.  TRANSFER OF TITLE
     -----------------

     Title to the Product shall transfer upon delivery to Buyer.

11.  RISK OF LOSS - LIABILITY
     ------------------------

     Risk of loss in connection with the Product shall pass from Seller to Buyer
     as specified in the Incoterms 1990.  Liability in connection with the
     Product shall pass from Seller to Buyer at the same time as the risk of
     loss.

12.  FORCE MAJEUR
     ------------

     12.1 When either party's ability to manufacture or deliver or receive
          Product or to otherwise perform under this Contract (other than
          Buyer's obligation or ability to make payment for Product delivered
          under this Contract) is impeded, restricted, or affected (A) by any
          cause beyond such Party's reasonable control such as, but not limited
          to, (i) fire, explosion, flood, storm, earthquake, tidal wave, war,
          military operation, national emergency, civil commotion, or other
          event of the type of the foregoing,

                                      -7-
<PAGE>
 
          (ii) any strike or other difference with workers or unions (without
          regard to the reasonableness of acceding to the demands of such
          workers or unions), (iii) any governmental law, regulation, decree,
          order, or similar act, or (iv) any shortage in supplies of, or
          impairment in the facilities of production, manufacture,
          transportation, or distribution of, either party attributable to (a)
          mechanical or other breakdown or failure, (b) the order, requisition,
          request, or recommendation of any governmental agency or acting
          governmental authority, or either party's compliance therewith,  (c)
          governmental proration, regulation, or priority, or (d) the inability
          of Seller to obtain, on terms deemed to be reasonable, any feedstock
          or other raw material (including energy) or (B) by any cause beyond
          such party's reasonable control, similar or dissimilar to any
          aforementioned cause, then the party whose ability is so impeded,
          restricted, or affected shall have the right, by notice to the other
          party, to reduce or suspend, in part or in full, deliveries or receipt
          of Product hereunder for the period during which such cause persists.

     12.2 For the purpose of the application and interpretation of the
          provisions of this Article, it is expressly deemed that all Product is
          to be produced at Seller's own facilities. If Seller's ability to
          supply Buyer with Product from Seller's facility is impeded,
          restricted, or affected by one or more of the aforesaid causes, then
          Seller shall use its reasonable efforts to supply Product from its
          worldwide production capacity. Seller shall not be under the
          obligation to purchase or obtain the Product for Buyer on the open
          market or from other producers or suppliers of the Product.

          However, in the event that Seller should, nevertheless, decide to
          purchase or obtain Product on the open market or from other producers
          or suppliers of Product, then any such purchase or obtaining of
          Product shall not create a continuing obligation for Seller to do so.

     12.3 Seller's obligation to sell Product is subject to modification and
          reduction

                                      -8-
<PAGE>
 
          in accordance with any present or future allocation programme of any
          governmental authority.

13.  CLAIMS
     ------

     13.1 All claims of Buyer with respect to the quality or quantity of
          Products sold and delivered pursuant to this Contract shall be deemed
          waived and forever barred unless Buyer notifies Seller of the nature
          and details of the claim in writing within thirty days after the
          defect could have reasonably been detected.  Any such claim that is
          not asserted as a claim, counterclaim, defence, or set-off in a
          judicial proceeding instituted within one year after Seller's written
          denial thereof shall be forever waived, barred and released.

     13.2 If the Product meets the specifications set out in Annex 1, Buyer
          assumes all risk and responsibility for the handling of any Product,
          for the results obtained by the use of any Product in manufacturing
          processes or otherwise, or for the results obtained by the use of any
          Product in combination with other substances, irrespective of the fact
          that such use or any handling of such Product may be in accordance
          with any description, advice, or suggestion of Seller. If any
          description, advice, or suggestion is given, it is given and accepted
          at Buyer's risk, and Seller shall not be responsible or liable
          therefor or for the results thereof.

14.  PRODUCT HAZARDS
     ---------------

     Buyer acknowledges receipt of Seller's Material Safety Data Sheets for
     Product and is aware of the hazards or risks in handling or using Product.
     Buyer shall fully inform its employees, agents, contractors, and customers
     who handle, use, buy, or may be exposed to any Product of such Product's
     hazards or risks and shall hold Seller harmless from all claims by its
     employees and such other third

                                      -9-
<PAGE>
 
parties for compensation for damages caused by the Product to the extent such
damage is addressed in the Material Safety Data Sheets.  Buyer shall provide
copies of Seller's Material Safety Data Sheets, and any updates furnished by
Seller,  to all such employees, agents, contractors, and customers; and Buyer
shall conspicuously post the Material Safety Data Sheets throughout the area of
Buyer's plants or premises where exposure to any Product may occur.

15.  WARRANTIES
     ----------

     15.1 Seller warrants that all Product shall meet the specifications
          established in this Contract.

     15.2 Seller warrants that all Product shall be delivered free of the
          rightful claim by anyone of infringement of any patent or other right.

     15.3 Seller makes no other express or implied warranty, statutory or
          otherwise, concerning any product, including, without limitation, any
          warranty of fitness for a particular purpose, any warranty of
          merchantability, or any warranty as to quality or correspondence with
          any description or sample. Seller does not warrant against any claim
          of infringement of any patent based on (1) combinations of any product
          with other materials or (2) the use of any product in the operation of
          any process.

16.  LIMITATION OF DAMAGES
     ---------------------

     Buyer's exclusive remedy for any and all losses or damages resulting from
     the sale of Product under this Contract, including, but not limited to, any
     allegation of breach of warranty, breach of contract, negligence, or strict
     liability, shall be limited, at Buyer's option to either the return of the
     purchase price or the replacement of the particular product for which a
     claim is made and proved.

                                      -10-
<PAGE>
 
17.  FURTHER LIMITATION OF DAMAGES
     -----------------------------

     In no event shall Seller be liable for any special, consequential,
     incidental, or indirect losses or damages attributable to the sale of
     Product under this Contract or to any other matter arising out of or in
     connection with this Contract except that Seller shall be liable for all
     reasonable expenses (including reasonable legal fees) incurred by Buyer in
     collecting amounts due from Seller under the terms of this Agreement or in
     enforcing any other rightful claims Buyer may have under the terms of this
     Agreement.

18.  DEFAULT
     -------

     18.1 If either party does not perform, does not timely perform or does not
          properly perform one or more of its material obligations within ten
          (10) days after written notification from the other party, is declared
          bankrupt, applies for a moratorium of payment, starts to liquidate its
          business, offers a settlement agreement to its creditors, or appears
          in any other way to be insolvent, the other party shall be entitled to
          suspend its obligations until arrangements are made for security
          satisfactory to such other party or until all indebtedness to such
          other party is paid in full or at such other party's option to
          terminate the contract by giving notice of termination without
          judicial intervention and without a term of notice and in addition, to
          claim compensation for costs, damages and interest.

     18.2 Seller's obligation to deliver will be suspended for so long as Buyer
          has not complied with any obligation towards Seller. Buyer's
          obligation to pay will be suspended for so long as Seller has not
          complied with any obligation towards Buyer.

                                      -11-
<PAGE>
 
19.  CONFIDENTIALITY
     ---------------

     Unless otherwise agreed between the Parties, any information exchanged
     between Buyer and Seller pursuant to this Agreement, including the content
     of this Agreement, is strictly confidential and shall neither be
     communicated to any third Party nor be used for other purposes than the
     performance of this Agreement.

20.  GENERAL
     -------

     20.1 This Contract shall be binding upon and inure to the benefit of the
          successors and assigns of Buyer and the successors and assigns of
          Seller, but shall not be assigned by either party, except to an
          affiliate, without the prior written consent of the other party.

     20.2 This Contract is to be construed in accordance with English law. Any
          dispute arising from this contract shall be submitted to the competent
          Court in the London, England.

     20.3 No amendment, addition to, modification, or waiver of all or any part
          of this Contract shall be of any force or effect unless in writing and
          signed by Seller and Buyer. If the provisions of this Contract and the
          provisions of any purchase order or order acknowledgment written in
          connection with this Contract conflict, then the provisions of this
          Contract shall prevail.

     20.4 The applicability of any conditions of Buyer or Seller other than
          those set forth herein is expressly rejected.

     20.5 Any notice required or permitted under this Contract shall be given by
          letter, telex, or telefax to the Parties as follows:

                                      -12-
<PAGE>
 
          To Buyer:  Styrochem Finland OY
                     Attention of:  Mr. Henrik Akermark
                     PO Box 360
                     FIN-0601 Porvoo
                     Finland

          with a copy to:
                     Styrochem U.S., Inc.
                     Attention of:  Mr. Thomas Springer
                     7980 West Buckeye Road
                     Phoenix, AZ  85043
                     U.S.A.

          To Seller: ARCO Chemie Nederland, Ltd.
                     c/o ARCO Chemical Europe, Inc.
                     Attention of:  Global Director Styrene
                     ARCO Chemical House
                     Bridge Avenue
                     Maidenhead
                     Berkshire SL6 1YP
                     United Kingdom

                                      -13-
<PAGE>
 
IN WITNESS WHEREOF, Seller and Buyer have executed this Contract in counterparts
as of the date first above written.

ARCO CHEMIE NEDERLAND, LTD.
By:    /s/ J.P. Benders
       ------------------------
Title:  MFG. Director Benelux
       ------------------------


STYROCHEM FINLAND OY
By:     /s/ Donald Rogalski
       ------------------------
Title:   SR VP Admin.
       ------------------------

                                      -14-
<PAGE>
 
                                    ANNEX 1


PRODUCT FOR RESALE SALES SPECIFICATIONS

 
                                                 Date        : January 1998
                                                 Reference   : JPP/PAR/S/STY01
                                                 Revision    : 02
 


                                STYRENE MONOMER


<TABLE>
<CAPTION>
<S>                              <C>                   <C> 
Property                         Specification Limits  Test Method
-------------------------------  --------------------  ------------

 
Purity, % wt                     99.7 min              ASTM D-5135

Inhibitor, ppm wt                10 - 15               ASTM D-4590

Polymers, ppm wt                 10 max.               ASTM D-2121A

Aldehydes, ppm wt                100 max.              ASTM D-2119

Peroxides, ppm wt                50 max.               ASTM D-2340

Chlorides, ppm wt                10 max.               UOP-779

Sulphur total, ppm wt            25 max.               ASTM D3120

Platinium-Cobalt Colour scale    10 max.               ASTM D-1209

Ethylbenzene, ppm wt             500 max.              ASTM D-5135

Benzene, ppm wt                  1 max.                GC

</TABLE>

                                      -15-