Chemistry Reference
In-Depth Information
through this built-up of the generations.
4
In light of what has just been discussed above
as the disadvantage of the macromonomer route, this sounds attractive. There is a
price to be paid for this attractiveness though, which is the lack of perfection of the
dendritic branchwork. Considering a polyethyleneiminewith an average of 500 repeat
units [8b] in the first growth step 500 reactions have to be performed per macro-
molecule, followed by thousands of such reactions for each of the following steps.
Since (i) there is no chemical reaction proceeding with 100% conversion and (ii) the
growth steps become increasinglymore critical as the generation increases because of
growing congestion among the peripheral functional groups, it is evident that such an
approach unavoidably leads to structural imperfection. Experimentally this showed in
a couple of cases in the open literature published from 1994 onwards, in which the
degree of coverage for such attach-to approaches was estimated and found to be far
away from the theoretically possible [9]. Despite quite some effort in different
research groups [10], no synthetically convincing solution was provided until
recently. The main deficiencies are (i) the practically exclusive use of very short
PG1
30-300) for which it is intrinsically easier to reach higher
coverage; (ii) the lack of a proper determination of the coverage achieved. Often the
coverage is just believed to be virtually quantitative; (iii) the occasional failures to
provide essential experimental conditions like reaction scale. This unsatisfactory
situation has nourished the perception that basedonsyntheticeffortandachievable
structure perfection the attach-to protocol is premature and cannot compete with
the macromonomer route, though it is highly attractive on paper and seemingly the
only method capable of furnishing dendronized polymers with long main chains
and at the same time high-generation dendrons. Thus, there was a demand for
synthetic development.
Two key observations made this need even more obvious so that the authors
undertook a serious effort to improve on this situation by developing a reliable and
robust attach-to route. Both these observations relate to charged dendronized poly-
mers, the hundreds of peripheral charges of which render them polyelectrolytes with
an unusually high linear charge density. First, it was found that charged third- and
fourth-generation dendronized polymers at a certain concentration in aqueous
medium self-organize into a three dimensional network, each segment of which
consists of exactly two tightly connected strands [11]. Second, it was discovered that
in a lower concentration range a second and a third-generation charged dendronized
polymer folds back on itself to give a tight duplex [12]. At first glance both findings
look counterintuitive. Why should chains with like charges approach each other to
give a network or a back-folded duplex? A coarse-grained theoretical treatment of this
matter showed that dendronized polymers have a responsive thickness. There is
enough free volume that allows the branches to change their conformations if an
external stimulus makes this attractive [13]. Let's consider the back-folding process
main chains (e.g., P
n
ΒΌ
4
Typically the dendrons are used in considerable excess that can result in losses of polymer duringwork-up.
These losses can be associated with inadvertent fractionations leading to a preferred depletion of the long
chains. Nevertheless, even in complicated cases truly long chains (P
n
's of several thousands) of high
generation polymers were obtained according to the attach-to protocol in the authors' laboratory.
