Macromolecular Nomenclature Note No. 7

Macromolecular Nomenclature Note No. 7

[From the Nomenclature Committee of the ACS Division of Polymer Chemistry (W. V. Metanomski, chairman): In the previous Macromolecular Nomenclature Notes, the Committee publicized the existing nomenclature recommendations as published by the International Union of Pure and Applied Chemistry (IUPAC). With this issue of Polymer Preprints, we have opened the column for guest contributions. We present Prof. Newkome's proposal for a systematic nomenclature for dendritic and cascade polymers. Prof. Newkome and the Committee will welcome comments from the readers.]

G. R. Newkome and G. R. Baker
Center for Molecular Design and Recognition
University of South Florida

A systematic nomenclature of linear single-strand polymer systems has been in use for a number of years;(1,2,3) the "Compendium of Macromolecular Nomenclature" contains recommendations for the structure-based nomenclature of regular single-strand polymers, regular single-strand and quasi-single-strand inorganic and coordination polymers, and for source-based nomenclature for copolymers. The IUPAC Commission recently expanded these recommendations to cover regular double-strand (ladder and spiro) organic polymers.(4) Unfortunately, few recommendations have been suggested for the nomenclature of branched polymer systems.

We recently proposed a systematic nomenclature for dendritic or cascade polymers,(5,6) which are a subset of branched polymers. These macromolecules are highly symmetrical, discrete organic molecules possessing well defined repeat units. Many researchers in this field have coined trivial class names that reflect the arborescent branching of these molecular structures (e.g., arborols, dendrimers, molecular fractals).(7)

CASCADE (DENDRITIC) POLYMERS

Cascade polymers are macromolecules prepared by the attachment of a branched repeat unit to a central initiator core; larger macromolecules are derived by the addition of another layer (generation) of branched repeat units to the termini of an existing macromolecule. The nth generation of a symmetrical cascade polymer has the general line formula:

C[R₁(R₂( ... R_i( ... R_n(T)_{N_b_n} ... )_{N_b_i} ... )_{N_b₂} )_{N_{b_i}} ]_{N_c}

where C is the formula for the initiator core; Nc is the branch multiplicity of the initiator core; R_i is the formula for the formula of the ith generation repeat or branch unit; N_bi is the branch multiplicity of the ith repeat unit; and T is the formula for the terminal moiety. The number of terminal moieties (Z) for a given generation is easily calculated:

Z = N_{c_} _i-1Sⁿ N_{b_i}

The proposed nomenclature system was derived from this general line notation.

The need for a convenient and accurate nomenclature to describe cascade polymers was apparent from the difficulty and time required to name even the simplest of these structures using the rules for organic molecules. For example, the 36-Micellanol^TM and 36-Micellanoic Acid^TM cascade polymers(8) were named 22,22-bis[21-hydroxy-9,9-bis[12-hydroxy-9,9-bis(3-hydroxypropyl)dodecyl]-18,18-bis(3-hydroxypropyl)heneicosyl]-13,13,31,31-tetrakis[12-hydroxy-9,9-bis(3-hydroxypropyl)dodecyl]-4,4,40,40- tetrakis(3-hydroxypropyl)-1,43-tritetracontanediol and 4,4,40,40-tetrakis-(propylcarboxy)-13,13,31,31-tetrakis[12-carboxy-9,9-bis(3-propylcarboxy)dodecyl]-22,22-bis[21-carboxy-18,18-bis(propylcarboxy)-9,9-bis[12-carboxy- 9,9-bis(propylcarboxy)dodecyl]heneicosyl]tritetracontanedioic acid, respectively. These names obscure the number and type of terminal moieties, the constituency of the initiator core, and the structural similarity of the two molecules.

36-Micellanol^TM Cascade(left), 36-Micellanoic Acid^TM cascade(right)

The cascade (dendrimer) name begins with information detailing the number of terminal moieties (Z) and the molecular class descriptor ("cascade") , followed by name fragments that describe, in order, the initiator core, each of the branch repeat units, and finally the terminal functionalities. The majority of cascade macromolecules reported to date possess symmetrical branches and are easily described by this approach. Further descriptors are required to accommodate cascade macromolecules possessing: repetitions of combinations of repeat branch units (i.e., a block cascade polymer), nonidentical branches, or distinct segments emanating from the core (i.e., a segmental block cascade polymer).

The initiator core and terminal unit names resemble conjunctive nomenclature. Multiplicity of branching from the core is indicated by a bracketed numeral following the core unit name; any necessary locants are also enclosed within the brackets after the multiplicity numeral, separated by a hyphen.

A branch repeat unit consists of the molecular fragment extending from (but not including) a branch atom (or group) through the next cascade branching site. The parent chain of an intermediate (or terminal unit) always terminates at a cascade branching site. Example repeat branch units (and names) are shown below with branch multiplicities of two (2) and three (3). The illustrated replacement nomenclature is easy to use and readily indicates the length of the repeat unit, but masks the functionality. Alternatively, a repeat unit name [e.g., (3-methoxypropanamido)methylidyne] may be used; however, this approach indicates the functionality but obscures the chain length. The choice between styles for the repeat (or core or terminal) unit name has not been dictated and does not affect the general form of the proposed cascade nomenclature.

(1,4-diaza-5-oxoheptylidene) Repeat Unit (left), (3-oxo-6-oxa-2-azaheptylidyne) Repeat Unit (right)

The second generation alcohol- and acid-terminated Micellane^TM polymers shown earlier are named 36-cascade:methane[4]: (nonylidyne)2:propanol and 36-cascade:methane[4]:(nonylidyne)2:propanoic acid, respectively, via the proposed nomenclature. Multiple generations of a cascade polymer family can be easily described via a general name. A series of acid-terminated polyamido cascades (1: Z = 36; G = 2) have been prepared(9,10) and are readily described by the family name: Z-cascade:methane[4]:(3-oxo-6-oxa-2-azaheptylidyne):(3-oxo-2-azapentylidyne)G-1:propanoic acid, where Z is the number of terminal acid groups, and G is the number of generations. Furthermore, obvious structural similarities are easily noted for macromolecules where only the terminal functionalities differ.10 For example, 36-cascade:methane[4]:(3-oxo-6-oxa-2-azaheptylidyne):(3-oxo-2- azapentylidyne):propanoic acid (2) and 36-cascade:methane[4]:(3-oxo-6-oxa-2-azaheptylidyne):(3-oxo-2-azapentylidyne)G-1:propylamine (3).

Polyamidocascades Synthesized using modular Building Blocks

REFERENCES

1.International Union of Pure and Applied Chemistry, Commission on Macromolecular Nomenclature, Pure Appl. Chem. 1989, 61, 243.
2.Fox, R. B. Encyclopedia of Polymer Science and Engineering, Wiley, New York, 1986, Vol 9, pp. 336-344.
3.Bikales, N. M. Encyclopedia of Polymer Science and Engineering, 2nd Ed., Wiley, New York, 1987, Vol. 10, pp. 191-204.
4.International Union of Pure and Applied Chemistry, Commission on Macromolecular Nomenclature, Pure Appl. Chem. 1993, 65(7), 1561-1580.
5.Newkome, G. R.; Baker, G. R.; Young, J. K.; Traynham, J. G. J. Polym. Sci., Part A: Polymer Chemistry 1993, 31, 641-651.
6.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.
7.For an overview of dendritic macromolecules see: Moorefield, C. N.; Newkome, G. R. "A Review of Dendritic Macromolecules" in Advances in Dendritic Macromolecules, Newkome, G. R., Ed., JAI, Greenwich, CT, 1994, Chapter 1.
8.Newkome, G. R.; Moorefield, C. N.; Baker, G. R.; Johnson, A. L.; Behera, R. K. Angew. Chem., Int. Ed. Engl. 1991, 30, 1176-1178; Newkome, G. R.; Moorefield, C. N.; Baker, G. R.; Saunders, M. J.; Grossman, S. H. Angew. Chem., Int. Ed. Engl. 1991, 30, 1178-1180.
9.Newkome, G. R.; Young, J. K.; Baker, G. R.; Potter, R. L.; Audoly, L.; Cooper, D.; Weis, C. D.; Morris, K. F.; Johnson, C. S., Jr. Macromolecules 1993, 26, 2394.
10.Young, J. K.; Baker, G. R.; Newkome, G. R.; Morris, K. F.; Johnson, C. S., Jr. Macromolecules 1994, 27, in press.

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