E. S. Wilks, E. I. du Pont de Nemours and Co. (Inc.), P. O. Box 80014, Wilmington, DE 19880-0014
Introduction
Several important documents on systematic nomenclature for linear single- and multiple-strand polymers have been published since 1986.(1,2,3) Structure-based nomenclature is well documented for regular single-strand polymers,(4) irregular single-strand polymers,(5) regular double-strand (ladder and spiro) polymers,(6) and quasi-single-strand inorganic and coordination polymers.(7) Source-based nomenclature for copolymers(8) and graphic representations (chemical formulae) of polymers(9) were also covered. Chemical Abstracts Service (CAS) and International Union of Pure and Applied Chemistry (IUPAC) nomenclature and structure representations for linear polymers were compared recently.(10,11,12)
The last 15 years have witnessed such a rapid development of comb, crosslinked, dendritic, graft, star, star-branched, and other types of non-linear polymers that nomenclature for these has received little attention. Understandably, both chemists and nomenclature experts disagree on how to classify and name the bewildering variety of novel structures. Newkome et al.(13,14,15) and Baker and Young(16) have proposed a systematic, structure-based nomenclature specifically for dendritic and cascade polymers. Wilks has compared nomenclature and structure representations for non-linear polymers(17) and has discussed nomenclature and structure representations for hyperbranched polymers(18) and dendritic polymers.(19) CAS has not yet addressed the problem of indexing dendritic and hyperbranched polymers structurally. IUPAC has formed a working party to address dendritic polymer nomenclature.
Structure-Based versus Source-Based Representation for Dendritic Polymers
Structure-based representation for a polymer is generally acknowledged to be superior to source-based representation because it conveys structural information that is omitted by the latter. While it is possible to draw source-based representations for some dendritic polymers, e.g. polyamides or polyesters,(17) discussion here will be confined to the problems of structure-based representation and nomenclature. Space considerations prohibit discussion of end groups.
Relationship between Nomenclature and Structure-Based Representation
For virtually all synthetic polymer types encountered prior to the discovery of dendritic polymers, also called dendrimers,(20) structure has essentially ruled nomenclature. Thus, a name for a constitutional repeating unit (CRU)(21) is created based upon its structure; both CAS structural repeating unit (SRU) nomenclature rules(22) and IUPAC CRU nomenclature recommendations(4) are published.
Lack of space prohibits reproduction in toto of these rules and recommendations; the two most pertinent to this discussion are:
As examples of this problem, consider the Frechet dendrimer from 5-(bromomethyl)resorcinol (3,5-dihydroxy-benzyl bromide)(23) (Figure 1) and the Tyler and Hanson dendrimer from 3,5-bis(bromomethyl)phenol(24) (Figure 2).
The question arises: how should their CRU representations be drawn?
For the resorcinol of figure 1, figures 3, 4, and 5 show the logical possibilities. These are "phase-shift" variants of one another, i.e. all possible variants of the same intellectual CRU are drawn, but in each new figure the left and right-limiting brackets are moved one atom to the right from the previous one until all possibilities are covered. It is logical to create a rule that the single crossing bond(25) of A2B, A3B, etc. CRU representations are always drawn on the left. "Illogical" CRUs, e.g. figure 6 as a variant of figure 5, are excluded from discussion.
A fundamental difference is now observed between representations of linear polymers versus dendrimers. For a linear polymer, regardless of whether a CRU is drawn correctly or incorrectly phased per CAS and IUPAC rules, its molecular formula (MF) is invariable; in contrast, as shown in figures 3, 4, and 5, the MF of a dendrimer changes with CRU phasing. Therefore, what rules shall be used to select the "correct" CRU from the representations shown as figures 3, 4, and 5?
If the assumption is made that figures 3, 4, and 5 are all representations of regular polymers, as defined by IUPAC,(26a,26b) application of the IUPAC definition of a CRU(26c) to figures 3, 4, and 5 results in selection of figure 3 because it has the "smallest MF", i.e. the fewest number of atoms, and therefore represents the "smallest CRU".
Application of the CRU head atom rules cited above also lead to selection of figure 3 as the correct representation. Thus, the "smallest CRU" and "head atom" rules for determining a CRU all lead to the same selection. The IUPAC name for this CRU could be oxymethylenebenzene-1,3,5-triyl, and the corresponding polymer could be named poly(oxymethylenebenzene-1,3,5-triyl).(27)
Analogous treatment of the phenol of figure 2 results in three possible CRUs; these are shown as figures 7, 8, and 9.
However, when the rules applied above to figures 3, 4, and 5 above are now applied to figures 7, 8, and 9, a conflict arises; the "smallest CRU" rule leads to selection of figure 7 as the "correct" CRU, whereas the "head atom" rules lead to selection of figure 8.
There is currently no basis for selection of figure 7 versus figure 8 as the "correct" CRU. Compare possible names for the polymers corresponding to the CRUs shown as figures 7 and 8:
Figure 7: poly(methyleneoxybenzene-1,3,5-triyl)
Figure 8: poly[oxybenzene-1,3,5-triyl-3,5-bis(methylene)]
CAS rules(22) and IUPAC recommendations(4) state that a CRU must be defined before it can be named; ergo, a concise name, however tempting, should theoretically not be allowed to influence the creation of a CRU structure. It is very tempting to select the CRU of figure 7 in preference to that of figure 8 because (a) it follows the "smallest CRU" rule, and (b) the names of the CRU and its corresponding polymer are more concise.
This temptation is reinforced by the example shown as figure 10, which illustrates Tomalia's dendrimer from a bicyclic ether and pentaerythritol.(28)
The CRU possibilities are shown as figures 11 through 14. Note that the right-hand crossing bonds in figure 11 are three single bonds, not one triple bond.
As in the previous example, a conflict arises; the "smallest CRU" rule leads to selection of figure 11 as the "correct" CRU, whereas the "head atom" rules lead to selection of figure 12.
Plausible names for polymers corresponding to the CRU representations of figures 11 through 14 are as follows:
Figure 11: poly(methyleneoxyethane-1,2,2,2-tetrayl)
Figure 12: poly[oxypropane-1,2,2,3-tetrayl-2,2-bis(methylene)]
Figure 13: poly[propane-1,2,2,3-tetrayl-2,2-bis(methyleneoxy)-3-oxy]
Figure 14: poly[ethane-1,1,1,2-tetrayl-1,1-bis(methyleneoxymethylene)-2-(oxymethylene)]
These examples suggest strongly that as the molecular formula and degree of branching increase in dendritic CRUs, nomenclature quickly becomes complex to the point of confusion. Therefore, for dendrimers, would nomenclature considerations and use of the "smallest CRU" rule be preferable to rigid adherence to "head atom" rules? Once established, the new rules could be used also for hyperbranched polymers, in spite of their greater degree of imperfection versus dendrimers. Readers are invited to submit their suggestions, with supporting arguments, for choosing either the "smallest CRU" rule or the "head atom" rules as the principle by which dendritic and hyperbranched CRUs should be represented.
Please write to:
Dr. Edward S. Wilks
E. I. du Pont de Nemours and Co. (Inc.)
CR&D BMP 14-1288
P. O. Box 80014
Wilmington, DE 19880-0014, USA
Telephone (302)992-2718
FAX: (302)992-4288
email: wilks@esvax-a1.email.dupont.com or edward.s.wilks@usa.dupont.com
REFERENCES
1.IUPAC. Compendium of Macromolecular Nomenclature (The Purple Book); Metanomski, W. V., Ed.; Blackwell Scientific Publications, Oxford, UK, 1991.
2.Bikales, N. M. in Encyclopedia of Polymer Science and Engineering, 2nd Ed., Wiley, New York, 1987, Vol. 10, 191-204.
3.IUPAC. A Classification of Linear Single-Strand Polymers. Pure Appl. Chem. 1989, 61, 243-254. Reprinted as Chapter 8 in Ref. 1.
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.
6.IUPAC. Nomenclature of Regular Double-Strand (Ladder and Spiro) Organic Polymers. Pure Appl. Chem. 1993, 65, 1561-1580.
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.
8.IUPAC. Source-Based Nomenclature for Copolymers. Pure Appl. Chem. 1985, 57, 1427-1440. Reprinted as Chapter 7 in Ref. 1.
9.IUPAC. Graphic Representations (Chemical Formulae) of Macromolecules. Pure Appl. Chem. 1994, 66, 2469-2482.
10.Wilks, E. S. Polymer Nomenclature and Structure: A Comparison of Systems Used by CAS, IUPAC, MDL, and DuPont. 1. Regular Single-Strand Organic Polymers. J. Chem. Inf. Comput. Sci. 1997, 37, 171-192.
11.Wilks, E. S. Polymer Nomenclature and Structure: A Comparison of Systems Used by CAS, IUPAC, MDL, and DuPont. 2. Aftertreated (Post-treated), Alternating/Periodic, and Block Polymers. J. Chem. Inf. Comput. Sci. 1997, 37, 193-208.
12.Wilks, E. S. Polymer Nomenclature and Structure: A Comparison of Systems Used by CAS, IUPAC, MDL, and DuPont. 4. Stereochemistry, Inorganic, Coordination, Double-Strand, Polysiloxanes, Oligomers, Telomers. J. Chem. Inf. Comput. Sci. 1997, 37, 224-235.
13.Newkome, G. R.; Baker, G. R.; Young, J. K.; Traynham, J. G. A Systematic Nomenclature for Cascade Polymers. J. Polym. Sci., Part A, Polym. Chem. 1993, 31, 641-651.
14.Newkome, G. R.; Baker, G. R. Macromolecular Nomenclature Note No. 7. Polym. Prepr. 1994, 35(2), 6-9.
15.Newkome, G. R.; Moorefield, C. N.; V–gtle, F. in Dendritic Molecules: Concepts, Syntheses, Perspectives, Chapter 3 - Nomenclature. VCH, Weinheim, Germany, 1996, 37-47.
16.Baker, G. R.; Young, J. K. in Advances in Dendritic Macromolecules, Chapter 5 - A Systematic Nomenclature for Cascade (Dendritic) Polymers. Newkome, G. R., Ed., JAI, Greenwich, CT, 1994, 169-186.
17.Wilks, E. S. Polymer Nomenclature and Structure: A Comparison of Systems Used by CAS, IUPAC, MDL, and DuPont. 3. Comb/Graft, Cross-Linked, and Dendritic/Hyperconnected/Star Polymers. J. Chem. Inf. Comput. Sci. 1997, 37, 209-223.
18.Wilks, E. S. Macromolecular Nomenclature Note No. 11. Polym. Prepr. 1996, 37(2), 6-9.
19.Schultz, J. L.; Wilks, E. S. Dendritic and Star Polymers: Classification, Nomenclature, Structure Representation, and Registration in the DuPont SCION Database. J. Chem. Inf. Comput. Sci. 1998, 38, 85-99.
20.A. D. Tomalia acknowledges A. J. Vogel for coining the term dendrimer.
21.IUPAC prefers CRU, whereas CAS prefers SRU; the two are virtually synonymous. The abbreviation CRU is used in this communication.
22.Chemical Abstracts Index Guide; Chemical Abstracts Service, Columbus, OH, 1997; Appendix IV: Section 222.
23.FrÈchet, J. M. J.; Hawker, C. J.; Wooley, K. L. The Convergent Route to Globular Dendritic Macromolecules: A Versatile Approach to Precisely Functionalized Three-Dimensional Polymers and Novel Block Copolymers. J. Macromol. Sci., Pure Appl. Chem. 1994, A31, 1627-1645. FrÈchet et al. have published many other publications on this topic.
24.Tyler, T. L.; Hanson, J. E. New Approaches to the Synthesis of Poly(aryl ether) Dendrimers. Presented as paper PMSE-191, 210th ACS National Meeting, Chicago, IL, August 20-24, 1995.
25.A CRU crossing bond is defined as a bond that crosses the left- or right-limiting bracket or parenthesis of the CRU.
26.IUPAC. Basic Definitions of Terms Relating to Polymers. Pure Appl. Chem. 1996, 68, 2287-2311; (a) definition 2.15 - a regular polymer is a polymer composed of regular macromolecules; (b) definition 1.4 - a regular macromolecule is a molecule, the structure of which essentially comprises the repetition of a single constitutional unit with all units connected identically with respect to directional sense; (c) definition 1.15 - a constitutional repeating unit (CRU) is the smallest constitutional unit, the repetition of which constitutes a regular macromolecule.
27.IUPAC. A Guide to IUPAC Nomenclature of Organic Compounds. Panico, R., Powell, W. H., and Richer, J.-C., Eds.; Blackwell Scientific Publications, Oxford, UK, 1993. These IUPAC organic nomenclature rules recommend that locants (numerals and/or letters) be placed immediately before that part of the name to which they relate.
28.Wilson, L. R.; Tomalia, D. A. Synthesis and Characterization of Starburst* Dendrimers. Polym. Prepr. 1989, 30(1), 115-116. Padias, A. B.; Hall, H. K.; Tomalia, D. A. Starburst* Polyether and Polythioether Dendrimers. Polym. Prepr. 1989, 30(1), 119-120. Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Starburst* Dendrimers: Molecular-Level Control of Size, Shape, Surface Chemistry, Topology, and Flexibility from Atoms to Macroscopic Matter. Angew. Chem., Int. Ed. Engl. 1990, 29, 138-175. (*Starburst is a registered trademark of the Dow Chemical Company).
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