Exact linear analogs of dendritic polyether macromolecules: Design, synthesis, and unique properties

Full text of DOI:10.1021/ja972027x

J. Am. Chem. Soc. 1997, 119, 9903-9904
Exact Linear Analogs of Dendritic Polyether
9903
Macromolecules: Design, Synthesis, and Unique
Properties
Craig J. Hawker,*,† Eva E. Malmstro¨m,†
Curtis W. Frank,‡ and J. Patrick Kampf‡
Center for Polymeric Interfaces and
Macromolecular Assemblies
IBM Almaden Research Center,
650 Harry Road, San Jose, California 95120-6099
Department of Chemical Engineering
Stanford UniVersity, Stanford, California 94305
ReceiVed June 19, 1997
The study of dendritic macromolecules has progressed at such
a rapid pace in recent years that these novel materials are
becoming major areas of interest from both an industrial and
academic standpoint.1 Much of this research is based on the
assumption that dendrimers are inherently different from linear
macromolecules and it is this inherent difference that leads to
many of the observed changes in physical properties.2 Unfor-
tunately, this comparison is not valid since the dendritic
macromolecules are compared with polydisperse linear polymers
of significantly different structure. For example, the unique melt
viscosity behavior of dendritic polyethers was compared with
linear polystyrene and not with monodisperse linear analogs
containing the same number of polyether repeat units based on
3,5-dihydroxybenzyl alcohol.3 In this report, we describe a
novel synthetic approach to monodisperse linear analogs of
dendritic macromolecules and the use of this strategy for the
preparation of the exact linear derivatives of the well-studied
polyether dendrimers 1.4
In designing the synthesis of the linear analogs it became
immediately apparent that a traditional exponential growth
strategy5 involving the synthesis of dimers, tetramers, octamers,
etc. would not be applicable since the number of internal repeat
units of the dendrimers increased in a different numerical
sequence. For example, both the exact linear analog of the
fourth-generation polyether dendrimer 2 and the actual den-
drimer 1 must have 15 (24 - 1) internal repeat units which is
not consistent with an exponential (2n) approach (Figure 1). A
strategy for the accelerated synthesis of linear polymers with
1, 3, 7, 15, 31, (2n - 1) repeat units was therefore developed.
The basic strategy, outlined in Schemes 1 and 2 involves a
converging dual exponential growth strategy in which two series
of linear macromolecules, one with 2n repeat units and the other
with 2n - 1 repeat units, are prepared and then coupled to give
the desired linear analogs. To obtain the exact linear analogs,
the deprotected monophenolic derivatives (i.e., 9 or 14) are
coupled at each generation step with the linear oligomer
containing 2n - 1 repeat units and a single benzyl ether chain
end (i.e., 11 or 13). For example, reaction of the first-generation
bromide 11 with the monophenol 9 leads to the exact linear
analog 12 of the second-generation dendritic alcohol. Bromi-
nation of 12 gives the linear [G-2]-Br derivative 13 which can
then be coupled with the next generation linear monophenolic
derivative 14 to give the exact linear analog of the third
generation alcohol 15 which contains seven repeat units, eight
benzyl ether groups, and a single hydroxymethyl chain end.
Figure 1. Comparitive structures of the fourth-generation dendritic
alcohol 1 and its exact linear analog 2.
Repetition of this strategy leads to higher generation linear
analogs of the dendritic polyether macromolecules which have
exactly the same number of repeat units and pseudo-chain ends
as their dendritic equivalents. In essence they are macromo-
lecular isomers of dendrimers and the synthesis has been
continued up to the sixth-generation derivative. Using standard
techniques developed for dendritic macromolecules,6 the struc-
ture of the exact linear analogs and associated precursor
1
molecules could be confirmed using a combination of H and
13C NMR spectroscopy, GPC, and MALDI mass spectrometry.
As expected GPC analysis of the linear analogs showed a single
peak (polydispersity less than 1.01), while MALDI mass
spectrometry showed a single molecular ion corresponding to
the expected molecular weight on the basis of the synthetic
strategy.
The ability to prepare the exact linear analogs of the polyether
dendrimers permits an unprecedented opportunity to study the
actual difference between linear and dendritic macromolecules.
Initially the three-dimensional molecular size of the linear and
dendritic polymers were compared by gel permeation chroma-
tography. Interestingly, a discontinuous relationship between
the hydrodynamic volumes of the exact linear analogs and the
corresponding dendrimers was observed. For generations four
and below, the hydrodynamic volume of the linear and dendritic
macromolecules were essentially the same. However, a marked
increase in the hydrodynamic volume for the linear derivatives,
compared to the dendritic equivalents, was observed on going
to generation five and six. For example, both the fifth
generation dendrimer [G-5]-OH, 16, and the exact linear analog
17 with 31 repeat units have molecular formulas of C441H380O63
and nominal molecular weights of 6680. While these materials
show the same molecular ion by MALDI, the GPC traces show
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S0002-7863(97)02027-1 CCC: $14.00 © 1997 American Chemical Society

9904 J. Am. Chem. Soc., Vol. 119, No. 41, 1997
Communications to the Editor
Scheme 1
Scheme 2
Significant differences were also found when the solubility
and crystallinity of the linear and dendritic polyethers were
compared. The fifth generation dendrimer 16 is completely
amorphous and shows a glass transition temperature of 42 °C.
In direct contrast, the exact linear analog 17 is highly crystalline
and displays a strong melting transition at 150 °C. This high
level of crystallinity was also observed for all other linear
analogs and results in significantly different solubility behavior
when compared to the amorphous dendrimers. In fact, the linear
analogs can be crystallized from THF, acetone, or chloroform;
solvents in which the dendrimers are extremely soluble.
In conclusion, a double-exponential growth strategy has been
developed and successfully applied to the preparation of the
linear analogs of the well-studied polyether dendrimers and the
physical properties of the two series of macromolecular isomers
compared. Significantly, it is observed that there is essentially
no difference in hydrodynamic volume between the linear and
dendritic polyethers up to generation four; however, a marked
difference occurs for generation five and six with the linear
macromolecule having a significantly larger hydrodynamic
volume in tetrahydrofuran. It is also observed that the linear
analogs are highly crystalline and have low solubilities which
is in direct contrast to the amorphous and highly soluble
dendrimers.
Figure 2. GPC traces for the (a) fifth generation dendritic alcohol 16,
[G-5]-OH, and (b) the corresponding exact linear analog 17.
Acknowledgment. The authors express their gratitude to the NSF
Center for Polymeric Interfaces and Macromolecular Assemblies.
a significant difference in retention volume with the linear
analog 17, having an approximately 40% larger hydrodynamic
volume (Figure 2). This result is fully consistent with the more
compact globular structure of the dendrimer in comparison to
the random coil structure of its linear analog. It is of particular
interest that similar property changes, or discontinuities, between
generation four and fifth has been observed for a number of
other dendrimer systems and suggests a fundamental difference
between linear and dendritic macromolecules may only manifest
itself in higher generation materials.7
Supporting Information Available: Experimental data for com-
pounds discussed in this work (10 pages). See any current masthead
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JA972027X
(7) (a) Hawker, C. J.; Wooley, K. L.; Fre´chet, J. M. J. J. Am. Chem.
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Tomalia, D. A. Macromolecules 1990, 23, 912. (c) Naylor, A. M.; Goddard,
W. A., III; Kiefer, G. E.; Tomalia, D. A. J. Am. Chem. Soc. 1989, 111,
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