While it is possible to register hyperbranched polymers by source-based methods (i.e. in terms of the monomers), structure-based registration is superior in conveying structural information that is omitted from source-based registration.

Structure-based representations for hyperbranched polymers have been devised, and a comprehensive nomenclature system has been created. Four papers on systematic nomenclature for hyperbranched polymers (other than dendrimers) are in various stages of completion.(13) A brief overview is presented here of the method devised for structure-based representation of hyperbranched polymers and their accompanying systematic nomenclature. These are not ladder polymers; successive units of the generic structural repeating unit (SRU; see structure 1.1 and Note) fit together as shown in structure 1.2, not as shown in structure 1.3.

Note: The International Union of Pure and Applied Chemistry uses the term constitutional repeating unit (CRU).

A structure-based hyperbranched polymer may be defined as any polymer in which the SRU has a connectivity of more than two; this definition may be extended to hypercrosslinked polymers (in which macrocycles are present(14)), but not to ladder and spiro polymers. Because the registration and naming of symmetrical and unsymmetrical hyperbranched polymers present different problems, they are discussed separately.

Symmetrical "I"-Shaped Hyperbranched Polymers
Virtually all symmetrical three-crossing-bond SRUs, e.g. structure 2.1 (currently unregistrable by the CAS Registry System), can be "doubled" to create a four-crossing-bond SRU with two left- and two right-crossing bonds. A crossing bond is a bond that passes though the limiting parentheses or brackets of the SRU. This creates a symmetrical "I"-shaped SRU (see example 2.1 below).

The general structure rules are: the original SRU is "doubled"; intellectually superfluous atoms are eliminated; the bridge formed is kept as short and as unsubstituted as possible, oriented vertically, and placed as far to the left as possible; the new SRU is oriented with the head atom in the top leftmost position. The nomenclature rules are: the moiety in the center of the bridge is named; the doubling prefix "bis" is added; the rest of the SRU is named. If the bridge has a unitary name it is used. The method is described in detail elsewhere.(13b) Three examples are given.

Example 2.1 - hyperbranched poly-1,3,5-benzenetriyl15 (structure 2.1) "doubled" to poly-3,3',5,5'-biphenyltetrayl

Example 2.2 - resolution of the silsesquioxane problem

Silsesquioxanes, derived from hydrolytic polymerization of silanes such as RSiX3 or RSi(OR')3 (in which, typically, R = alkyl or aryl, R' = alkyl, and X = halogen), are registrable with structures resembling ladder polymers; thus, structure 2.2a circumvents the problem of the unregistrable SRU of molecular formula (R Si O1.5)n (structure 2.2d) with 1.5 oxygen atoms per silicon atom.

Because CAS software cannot currently accommodate ideal hyperbranched polymer representations (structure 2.2b), such structures are registered as ladder polymers (structure 2.2a). Structure 2.2a wherein R is methyl is named poly[(1,3-dimethyl-1,3:1,3-disiloxanediylidene)-1,3-bis(oxy)].

Thus, both the hyperbranched polymer (structure 2.2b) and the true ladder polymer (structure 2.2c) are registered in CAS File Registry as structure 2.2a, and it is currently impossible to distinguish between them.(16) This could easily be remedied by use of different textual descriptors for the two polymer types.

Example 2.3 - resolution of the 1,2,3-propanetriyltrioxy SRU problem

The original three-crossing-bond SRU (structure 2.3a), which has the molecular formula (C3 H5 O1.5)n, is unregistrable. The original SRU is "doubled" and superfluous oxygen atoms are omitted. The final result (structure 2.3b), now registrable by CAS software, has the DuPont name poly[oxybis(2,1,3-propanetriyl-1-oxy)].

Asymmetrical "I"-Shaped Hyperbranched Polymers
This type presents additional difficulties, both in structuring and nomenclature. For SRUs for which "doubling" is possible, the general structure rules are as for symmetrical hyperbranched polymers (see above).

There is an inherent difficulty in creating a nomenclature system for asymmetrical "I"-shaped SRUs, namely, how to deal with a junction moiety or unit, whether it is a ring or a single atom. Whether the complete name of the SRU begins with a linear moiety or atom sequence that connects to a junction unit, or with a central junction unit itself, merely naming the remaining moieties that emanate from the central junction unit is insufficient to indicate in which direction they go. Nomenclature must therefore include indications of directions. Two examples illustrate the method, which is described in detail elsewhere.(13c)

Example 3.1 - polymerization of diamines and dihalo compounds

Condensation of diamines and dihalo compounds produces mixtures of linear, branched, and hyperbranched SRUs; for this polymer class, CAS uses source-based registration.17 However, structure-based registration provides appreciably more information. Thus, for the condensation of 1,6-hexanediamine with a,a'-dichloro-p-xylene, five SRUs are possible (see structures 3.1a through 3.1e); one of the four-crossing-bond SRUs (structure 3.1e) is asymmetrical and needs novel nomenclature.

The complete polymer is named by alphabetizing the SRUs and separating them with oblique strokes: poly[1,6-hexanediylbis(nitrilomethylene-1,4-phenylenemethylene)/iminomethylene-1,4-phenylenemethyleneimino-1,6- hexanediyl/(<-L)nitrilo(R;D)-(R)(methylene-1,4-phenylenemethylene)(R->)-(D)(methylene-1,4-phenylene- methylenenitrilo)(L;R)-(L<-)-(R)1,6-hexanediyl(R->)/1,4-phenylenebis(methylenenitrilo-1,6-hexanediyl)/1,4-phenylenebis(methylenenitrilomethylene-1,4-phenylenemethylene)]

New directional symbols created for this novel nomenclature are:
(<-L) a bond enters the SRU as a crossing bond through a left-limiting bracket
(L<-) a bond exits as a crossing bond through a left-limiting bracket
(->R) a bond exits as a crossing bond through a right-limiting bracket
(D) a bond emanates downward from a junction point
(L) a bond emanates to the left from a junction point
(R) a bond emanates to the right from a junction point

A junction point always has at least two directions cited after it, e.g. (R;D). Rings that function as junction points need locants in addition to letters, e.g. (6R;4D). The symbol (R;D) following a junction point indicates two separate bonds that emanate to the right and downward. After the first junction point, which is always in the upper linear strand of the SRU, the rest of the upper strand is named before the vertical bridge and the lower linear strand.

Three-crossing-bond (i.e. branched) structures cannot, per se, be registered, but "intellectual mixing" of structures 3.1a through 3.1e can create them in the mind's eye.

Example 3.2 - this polymer is named: poly[(<-L)2,6,4-pyridinetriyl(6R;4D)-(R)(methylene-1,4-phenylenemethyl- ene)(R->)-(D)1,3,5-benzenetriyl(3L;5R)-(3L<-)-(5R)(methylene-1,4-phenylenemethylene)(R->)]

REFERENCES

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2.Bikales, N. M. in Encyclopedia of Polymer Science and Engineering, 2nd Ed., Wiley, New York, 1987, Vol. 10, 191-204.
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4.IUPAC. "Nomenclature of Regular Single-Strand Organic Polymers". Pure Appl. Chem. 1976, 48, 373-385. Reprinted as Chapter 5 in Ref. 1.
5.IUPAC. "Structure-Based Nomenclature for Irregular Single-Strand Organic Polymers". Pure Appl. Chem. 1994, 66, 873-889.
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7.IUPAC. "Nomenclature for Regular Single-Strand and Quasi-Single-Strand Inorganic and Coordination Polymers". Pure Appl. Chem. 1985, 57, 149-169. Reprinted as Chapter 6 in Ref. 1.
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10.Newkome, G. R.; Baker, G. R.; Young, J. K.; Traynham, J.G. J. Polym. Sci., Part A, Polym. Chem. 1993, 31, 641-651.
11.Baker, G. R.; Young, J. K. "A Systematic Nomenclature for Cascade Polymers" in Advances in Dendritic Macromolecules, Newkome, G. R., Ed., JAI, Greenwich, CT, 1994, Chapter 5.
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13.Schultz, J. L.; Wilks, E. S. (a) "Nomenclature and Structural Representation for Linear, Single-Strand Polymers aftertreated to Hyperconnected Networks" (submitted for publication); (b) "A Nomenclature and Structural Representation System for Symmetrical "I"-Shaped Hyperbranched Polymers" (submitted for publication); (c) "A Nomenclature and Structural Representation System for Asymmetrical "I"-Shaped Hyperbranched Polymers" (completed; awaiting clearance); (d) "A Nomenclature and Structural Representation System for Symmetrical and Asymmetrical Hyperbranched Polymers aftertreated to Crosslinked Networks" (in preparation).
14.Webster, O. W., E. I. du Pont de Nemours and Co., personal communication, 1995.
15.Webster, O.W.; Kim, Y.H.; Gentry, F.P.; Farlee, R.D; Smart, B.E. "Hyperbranched and Hypercrosslinked Rigid Polymers". Polym. Prepr. 1992, 33(1), 186-187.
16.Teague, S. J. Chemical Abstracts Service, personal communication, 1996.
17.STN International Registry File Polymer Class Terms; POLYAMINE Class (page 4), CAS6009-1192, January, 1993.