A R T I C L E S
Guo et al.
wide range of g and n values. Since the currently accessible
range of g is limited to g e 5, this goal can be attained by
variation of the functionality X thus allowing to explore a wide
range of n . 1 between n(X ) 6,g ) 5) ) 781 and n(X ) 3,g
) 5) ) 31. In implementing this approach it is desirable to
minimize the structural differences between junction monomers
of different X.
polymer chain. This immediately raises the issue of structural
fidelity, that is, the conversion fraction per dendronization step.
Though structural fidelity was not normally addressed in a
quantitative manner in the pertinent literature, there is a
precedent that such procedures can in fact be carried out in high
level of conversion.27 It was therefore a natural choice to use
this approach, with all its particular chemistry, in the present
case.
The step-by-step process also raises the question as to whether
the length of the starting polymer (PG1) has an impact on the
structural fidelity especially for the synthesis of high generation
polymers. This is an issue that has not yet been addressed
quantitatively in the literature, but it is useful to note that all
starting polymers used so far for which quantitative experiments
are reported have short chains with Pn e 400, thus much shorter
than the target of the present work. There is the one exception
with Pn ) 1100 and Pw ) 1300.6e,28 Obviously new grounds
had to be explored in regard to how, polymer chains with, for
example, Pn ≈ 7000 and weight average degrees of polymer-
ization Pw ≈ 13 000 behave in consecutive dendronization
reactions. Finally, the aspect of quantity needs to be addressed.
Typically, attach-to experiments are performed on a scale that
allows doing the final steps (PG3 to PG4) with a few tenths of
a milligram material.6 Sometimes the amounts obtained are not
even stated, which may indicate that they are rather small. For
studying polymer thickness effects, milligram quantities are
inadequate. It is thus important to establish a protocol enabling
the synthesis of each member of a homologous series at least
on the gram scale. All these considerations led us to devise the
route in Scheme 1.
The advantages of this strategy are manifold. Monomer MG1
and the two polymers PG1 and PG2 are known from earlier
work which facilitates their characterization.29 Also the depro-
tection chemistry, using neat trifluoroacetic acid, has been
employed numerous times and found to be highly reliable.30
The key building block for dendronization, compound 1
(Scheme 1), is new. However, its synthesis rests upon a
commercially available dendrons, 2a and 2b. Furthermore the
way it bonds to peripheral amines during the dendronization
event, succinidyl active ester amidation, has been quantified in
the few similar cases mentioned above. Under carefully
optimized conditions, coverages of up to 99.8% were repro-
ducibly achieved6d which obviously suggested this building
block for the present purpose. Finally, monomer MG1 can be
polymerized by controlled or, alternatively, free radical poly-
merization. This allowed obtaining starting polymers PG1 with
different chain lengths in the range of several hundred nanometers.
Considering the enormous increase in molar mass during the
homologization which may also be referred to as consecutive
dendronization or thickening, the molar mass of the starting
polymer PG1 had to be carefully chosen. High molar masses
may result in handling and characterization problems. The initial
focus was therefore placed on a “short” PG1 with Pn ≈ 1000
and L ≈ 250 nm. Assuming complete coverage throughout, its
corresponding PG4 had a molar mass of approximately 5.5 ×
III. Synthesis of Homologous Series
III.a. Conceptual Considerations. To systematically explore
the properties of homologous series of dendronized polymers
aiming at unraveling potential thickness effects, its representa-
tives must (i) have sufficiently long backbone chains, (ii) have
high structural fidelity, and (iii) be available in gram quantity.
These conditions fundamentally affect the kind of chemistry to
be used for their synthesis as will become evident from the
following considerations. With regard to scattering experiments
it is useful to distinguish between two situations: Polymers
whose contour length L is comparable to the persistence length
λ and those with L . λ. Polymers with L ≈ λ . req are expected
to behave as bendable rods while long polymers with L . λ
will behave as coils of L/λ segments of length λ each. In practice
L/λ g 10 is necessary for scattering experiments aiming at this
last regime.21 Current results suggest that λ of PG4 is in the
range 10 - 50 nm.24 Accordingly, the L ≈ 10λ criterion requires
polymers of length of several hundred nanometers up to a
micrometer. This condition rules out the use of the by far most
often used synthesis method, the so-called macromonomer route
(Figure 2),2b in which macromonomers already carrying the
dendrons of final size are polymerized. Even with the recent
improvement of the number average degrees of polymerization,
Pn, in the polymerization of macromonomers under supercriticial
carbon dioxide high pressure conditions,25 L > 500 nm and L >
40 nm are presently unrealistic for g ) 3 and g ) 4 polymers,
respectively. The attach-to route was therefore the only option
for g ) 4 polymers with L ≈ 10λ. It suggested itself strongly
also for g ) 3 polymers inasmuch as flask-type chemistry under
ambient conditions was preferable to synthesis under high
pressure.26 In this method the dendronized polymers are obtained
in a step-by-step process in which generation after generation
is added to an already existing starting polymer (Figure 2);
thousands of reactions have to be performed with each individual
(24) A theoretical study predicts persistence lengths λ of third and fourth
generation dendronized polymers to be 50 and 120 nm, respectively,
and lead to the “Janus chain” model for dendronized polymers: (a)
¨
Ding, Y.; Ottinger, H. C.; Schlu¨ter, A. D.; Kro¨ger, M. J. Chem. Phys.
2007, 127, 094904. Kro¨ger, M.; Peleg, O.; Ding, Y.; Rabin, Y. Soft
Matter 2008, 4, 18–28. Ding, Y.; Kro¨ger, M. Macromolecules 2009,
42, 576–579. For λ of dendronized polymers up to generation three
determined by SANS, see: (b) Stocker, W.; Schu¨rmann, B. L.; Rabe,
J. P.; Fo¨rster, S.; Lindner, P.; Neubert, I.; Schlu¨ter, A. D. AdV. Mater.
1998, 10, 793–797. (c) Fo¨rster, S.; Neubert, I.; Schlu¨ter, A. D.; Lindner,
P. Macromolecules 1999, 32, 4043–4049. A comprehensive LS and
SANS study on homologous series of dendronized polymers is
presently being performed: (d) Sigel, R.; Guo, Y.; Zhang, A.; Schlu¨ter,
A. D.; Schurtenberger, P. In preparation. For λ of dendronized
polymers up to the fifth generation determined by MALLS, see: (e)
Percec, V.; Ahn, C.-H.; Cho, W. D.; Jamieson, A. M.; Kim, J.; Leman,
T.; Schmidt, M.; Gerle, M.; Mo¨ller, M.; Prokhorova, S. A.; Sheiko,
S. S.; Cheng, S. Z. D.; Zhang, A.; Ungar, G.; Yeardley, D. J. P. J. Am.
Chem. Soc. 1998, 120, 8619–8631. (f) Ouali, N.; Me`ry, S.; Skoulios,
A.; Noirez, L. Macromolecules 2000, 33, 6185–6193. See also ref
6e.
(27) For studies in which structural defects were quantified by either UV-
and fluorescence spectroscopic or NMR-spectroscopic analysis, see
refs 6d, f and 6g.
(28) Reference 6e does not provide information on the quantity in which
these polymers were synthesized.
(25) Costa, L. I.; Kasemi, E.; Storti, G.; Morbidelli, M.; Walde, P.; Schlu¨ter,
A. D. Macromol. Rapid Commun. 2008, 29, 1609–1613.
(26) In certain special cases mixed strategies may be feasible alternatives
as well.
(29) Canilho, N.; Kasemi, E.; Mezzenga, R.; Schlu¨ter, A. D. J. Am. Chem.
Soc. 2006, 128, 13998–13999.
(30) For example, see Shu, L.; Schlu¨ter, A. D.; Ecker, C.; Severin, N.;
Rabe, J. P. Angew. Chem., Int. Ed. 2001, 40, 4666–4669.
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