Date: Aug 19, 2004
Free-radical initiators have been misunderstood under the Toxic Substances Control Act (TSCA) with respect to the chemical identity of polymers. This misunderstanding was fostered both by confusion on the part of the chemical industry as well as by the U.S. Environmental Protection Agency (EPA) who administers TSCA. In 1989, this confusion lead to what has become known as the free-radical initiator policy.1
With regard to chemical identity, persons who currently submit premanufacture notices (PMNs) or other section 5 notices under TSCA for polymers must include, among other information:
It was not always so. The TSCA Inventory Reporting Regulations5 required only that monomers of all polymers be reported.6 Perhaps, when TSCA was enacted, regulators thought that the term monomers was inclusive of all precursors of polymers, i.e., that polymers were made up of only monomers. Later, the term monomers was expanded to monomers and other reactants. Even then, free-radical initiators and other non-monomers still were not considered monomers or other reactants.
Sometimes, free-radical initiators were considered classical catalysts, i.e., merely influencing the rate of a reaction (or causing the reaction to occur), not as precursors to polymers. In that context, whether or not free-radical initiators were chemically incorporated into the products being produced was either not addressed or often ignored.
A free-radical initiator is a chemical substance that undergoes a unimolecular, homolytic, bond cleavage to form free radicals upon exposure to heat, radiation, or collisions with subatomic particles. Free radicals are atoms or molecular fragments with one or more unpaired electrons. These unpaired electrons make these species generally highly reactive, transitory intermediates. Free-radical initiators that thermolytically decompose are widely used as direct precursors to free radicals that initiate (and sometimes terminate) the addition polymerization of unsaturated monomers. Generally, a free-radical fragment of a free-radical initiator directly adds to an unsaturated monomer and becomes chemically incorporated into a new radical that is comprised of the free-radical fragment and the monomer molecule. This new radical usually adds to another monomer molecule in turn. This is known as the propagation of the polymer molecules. One way that propagation ends is when two radicals combine. Thus, the free-radical fragment becomes chemically incorporated into the polymer's molecules at least at the beginning of the polymer molecule and sometimes at the end as well. In all such cases, the free-radical initiator is a precursor to the polymer. Therefore, a free-radical initiator ordinarily plays the role of other reactant in the manufacture of a polymer.
Generally, every direct precursor that is intentionally charged at any level to manufacture a new polymer must be reported to describe that polymer under TSCA. Every precursor to a new polymer that is used or incorporated at more than two weight percent (whichever is less) must be considered for inclusion in the formal chemical description of the polymer. Every precursor to a polymer that is used or incorporated at two weight percent or less (whichever is less) may be considered for inclusion in the formal chemical description of the polymer, but the submitter may elect to omit such precursors from that consideration.7
In cases where one or more precursors are omitted from consideration, the polymer is named as though those precursors were not charged - even if the resulting polymer description is not an accurate chemical description of the polymer. However, a polymer's chemical description generally is not significantly affected if omission is elected. The election carries with it other chemical nomenclature consequences discussed in the context of the reporting provision known as the "two-percent rule."8
The chemical identification of polymers initiated by chemicals that became free radicals sometimes omitted the fact that one or more free-radical initiators were used to effect polymerization. In such cases, the identity and use levels of free-radical initiators were not disclosed. However, those responsible for the TSCA Inventory at the EPA sometimes ignored the identity of free-radical initiators - even when they were reported at levels substantially above two weight percent. Free-radical initiators were regarded as if they did not become chemically a part of the polymers that were formed. They were treated as if they were catalysts, not as if they were intended to be incorporated into polymer molecules. Often, free-radical initiators are used at levels below two weight percent to manufacture new polymers; therefore, the notifier could omit them. On the other hand, in some cases in the past, free-radical initiators were recognized as precursors to polymers, and their identity and use levels were reported for new polymers and, where appropriate, they were included in the chemical identification of such polymers. This inconsistent handling of free-radical initiators, lead to internal discussions at the EPA that eventually resulted in a free-radical initiator policy.
On June 28, 1989, the EPA published a "clarification" of the Inventory listings for polymers made with free-radical initiators used at greater than two percent. 54 Fed. Reg. 27174. The clarification states that free-radical initiators used at greater than two-weight percent in the manufacture of a polymer must be included in the Inventory description of a polymer. In addition, the EPA will apply this policy only to polymers added to the Inventory after July 28, 1989, the effective date of the clarification. In essence, this means that polymers listed on or before July 28, 1989, can be manufactured using any amount of any free-radical initiator.
The EPA has identified the polymers that were listed on the Inventory as of July 28, 1989, by placing a so-called flag in the database known as CHEMLIST, which is part of the collection of databases known as the Scientific and Technical Network (STN). The flag set for these polymers is N in the inventory (INV) field of these records. If a polymer was not listed as of the effective date of this policy, and a free-radical initiator is used or incorporated at more than two weight percent (whichever is less), the free-radical initiator must be considered for inclusion in the formal chemical description of the polymer and most likely will be reflected in the polymer's formal chemical name (the Chemical Abstracts Service Index Name).
The EPA's statement of clarification and its decision to take corrective action regarding previously reported polymers was prompted by the Agency's awareness that the vast majority of polymer listings on the Inventory did not comport with the Agency's then extant interpretation of the Inventory reporting requirements and the premanufacture reporting requirements. Although the Agency does not admit any fault in dealing with polymers where free-radical initiators were used or incorporated at more than two weight percent (whichever is less), the EPA's decision to grandfather polymers chemically incorporating free-radical initiators onto the Inventory as of the effective date of the clarification is tacit acknowledgment of that their handling of such polymers was incorrect.
An interesting point in the clarification is the statement that the Inventory Reporting Regulations did not define the term monomer and the implication that the term monomer included non-monomers that chemically became a part of the polymeric composition. The implication stems from this clarification going on to say9 that in 1977 the EPA published an instruction booklet that "amplifies" the Inventory Reporting Regulations by the inclusion of the language that,
The polymer description should identify only monomers and other reactive ingredients such as chain-transfer or crosslinking substances. Other additives, such as emulsifiers and plasticizers, which are not chemically a part of the polymeric composition should not be identified in the description of the polymer, and their weight should not be included in estimating the 'dry' weight of the polymer.
This is a noteworthy admission for the Agency to make with respect to a document that explicitly states that it is "neither a substitute for, nor a supplement to, the regulations" and was not included in the rulemaking record. Finally, it should be noted that the June 28, 1989, clarification was issued without benefit of notice and comment.
The EPA's clarification statement does not expressly extend to chain-transfer agents omitted from polymer listings on the Inventory. However, the scope of the policy was not fully defined. In a 1989 letter, we asked Henry Lau, Chief of EPA's Chemical Inventory Section, whether non-monomeric reactants such as alcohols and ketones, which can act as chain-transfer agents, fall within the scope of the free-radical initiator policy. In his response, Mr. Lau states that such substances are not included in the free-radical initiator policy. The reasons given are (1) free-radical initiators and chain-transfer agents function differently; and (2) chain-transfer agents have always been included in the Inventory descriptions of polymers when used at greater than two-weight percent. No factual support was offered for the second statement, and no attempt was made to justify the present distinction between chain-transfer agents and free-radical initiators, considering that they were equally excluded from the Initial Inventory reporting requirements of 40 C.F.R. § 710.5(c).
154 Fed. Reg. 27174 (Jun. 28, 1989).
2For the purposes of this requirement, the weight percentage must be based either upon (1) the weight of the monomer or reactant actually charged to the reaction vessel; or (2) the minimum weight of monomer or reactant theoretically required to account for the actual weight of monomer, reactant molecules, or their fragments that are chemically combined in the polymeric substance manufactured. 40 C.F.R. § 720.45(a)(2)(iii). Certain records must be kept to support the latter determination. 40 C.F.R. § 720.45(a)(2)(iv).
3Although the terms monomer and other reactant are not defined in Part 720, they are defined in the 1995 polymer exemption (40 C.F.R. § 723.250(b)), and are discussed in the Instructions Manual for the PMN form.
440 C.F.R. § 720.45(a)(2).
542 Fed. Reg. 64572-64596 (Dec. 23, 1977).
640 C.F.R. § 710.5(c).
7In ex post guidance, the EPA has changed the application of the "two-percent rule" in ways that make the application different under the polymer exemption. Not only is the guidance not in the 1995 polymer exemption regulation, it is ambiguous on certain points and a response has been pending for some time to an inquiry to the EPA seeking clarification.
8See 40 C.F.R. § 720.45(a)(2)(iii) and (iv).
954 Fed. Reg. 27174 (Jun. 28, 1989).
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