3.4 Dendronized Polymers
Inspired by Staudinger’s vision [1], mimicking the size and eventually the function
of biomacromolecules has been a dream of chemists for decades [76]. This requires
not only giant molecular structures to be generated, whose dimensions are on the
order of tens and even hundreds of nanometers, but also that these man-made
objects should have a useful, predetermined shape. Last, but not least, at both the
periphery and the interior they should contain functionalities such as recognition or
catalytically active sites. Moreover, their interaction with solvents, in particular
water, should be controlled and exploited in their self-organization. It is evident that
successful projects in this direction will have considerable impact on both biological and materials sciences.
One approach along these lines is to incorporate building blocks such as amino
acids (see Sect. 3.2), generating bioinspired polymers [77]. A full synthetic approach
makes use of the enormous variety of dendrons and dendritic groups [78]; for recent
reviews see [76, 79]. The structure of dendritic groups can be varied in different ways,
e.g., by controlling their size by their generation, by generating amphiphilic character
by incorporating hydrophobic and hydrophilic building blocks, or by varying the
conformational freedom from completely rigid (polyphenylene) dendrimers [80] to
highly flexible as in hyperbranched polymers [81]. Linear polymers jacketed with
dendrons attached via their apex provide a conceptually simple class of dendronized
polymers. For such polymers with conventional backbone, poly(styrene) or poly
(methacrylate), the polymer shape can be controlled through the self-assembly of
flexible dendritic side-groups and the degree of polymerization (DP) [82]. For low
DP, spheres are observed, whereas for high DP, cylinders are obtained.
1
H and
13
C
solid state NMR on the latter have revealed details of the organization of the dendritic
groups within the supramolecular polymer [83]. The dendrons contain aromatic
moieties and flexible ethylene oxide linkers (Fig. 6). In the supramolecular assembly,
however, they largely lose their flexibility and exhibit dynamic order parameters S as
high as 40–80%, displaying a gradient of mobility that decreases from inside out. This
significant immobilization nicely demonstrates their role as structure-directing moieties displaying “edge-on” and “face-on” contacts between the ethylene units and the
aromatic rings, facilitating the formation of helices (see Fig. 6).
The shape of macromolecular objects can also be changed by external stimuli
[84]. For instance, thermoresponsive polymeric materials are of great interest owing
to their potential use in fields such as actuation, drug delivery, and surface modification [85]. Ever since the discovery by Wu and coworkers of the coil–globule transition
of single poly(N-isopropylacrylamide) (PNiPAAm) chains near the lower critical
solution temperature (LCST) [86], the collapse mechanism and the formation of
stable mesoglobules have been intense topics of research [84, 87]. Despite these
efforts, a molecular-scale picture of what happens when thermoresponsive polymers
start to dehydrate at a certain temperature, subsequently collapse, and then assemble
to mesoglobules, did not exist. This absence severely hampered rational
materials design. Dendronized polymers with amphiphilic dendritic groups based on
306
H.W. Spiess
Inspired by Staudinger’s vision [1], mimicking the size and eventually the function
of biomacromolecules has been a dream of chemists for decades [76]. This requires
not only giant molecular structures to be generated, whose dimensions are on the
order of tens and even hundreds of nanometers, but also that these man-made
objects should have a useful, predetermined shape. Last, but not least, at both the
periphery and the interior they should contain functionalities such as recognition or
catalytically active sites. Moreover, their interaction with solvents, in particular
water, should be controlled and exploited in their self-organization. It is evident that
successful projects in this direction will have considerable impact on both biological and materials sciences.
One approach along these lines is to incorporate building blocks such as amino
acids (see Sect. 3.2), generating bioinspired polymers [77]. A full synthetic approach
makes use of the enormous variety of dendrons and dendritic groups [78]; for recent
reviews see [76, 79]. The structure of dendritic groups can be varied in different ways,
e.g., by controlling their size by their generation, by generating amphiphilic character
by incorporating hydrophobic and hydrophilic building blocks, or by varying the
conformational freedom from completely rigid (polyphenylene) dendrimers [80] to
highly flexible as in hyperbranched polymers [81]. Linear polymers jacketed with
dendrons attached via their apex provide a conceptually simple class of dendronized
polymers. For such polymers with conventional backbone, poly(styrene) or poly
(methacrylate), the polymer shape can be controlled through the self-assembly of
flexible dendritic side-groups and the degree of polymerization (DP) [82]. For low
DP, spheres are observed, whereas for high DP, cylinders are obtained.
1
H and
13
C
solid state NMR on the latter have revealed details of the organization of the dendritic
groups within the supramolecular polymer [83]. The dendrons contain aromatic
moieties and flexible ethylene oxide linkers (Fig. 6). In the supramolecular assembly,
however, they largely lose their flexibility and exhibit dynamic order parameters S as
high as 40–80%, displaying a gradient of mobility that decreases from inside out. This
significant immobilization nicely demonstrates their role as structure-directing moieties displaying “edge-on” and “face-on” contacts between the ethylene units and the
aromatic rings, facilitating the formation of helices (see Fig. 6).
The shape of macromolecular objects can also be changed by external stimuli
[84]. For instance, thermoresponsive polymeric materials are of great interest owing
to their potential use in fields such as actuation, drug delivery, and surface modification [85]. Ever since the discovery by Wu and coworkers of the coil–globule transition
of single poly(N-isopropylacrylamide) (PNiPAAm) chains near the lower critical
solution temperature (LCST) [86], the collapse mechanism and the formation of
stable mesoglobules have been intense topics of research [84, 87]. Despite these
efforts, a molecular-scale picture of what happens when thermoresponsive polymers
start to dehydrate at a certain temperature, subsequently collapse, and then assemble
to mesoglobules, did not exist. This absence severely hampered rational
materials design. Dendronized polymers with amphiphilic dendritic groups based on
306
H.W. Spiess