protocols like cascade reactions) or obtaining high enantiomeric excesses to
measure its achievements. When looked upon from this angle, polymer synthesis
may appear simple. One could, of course, respond that polymer synthesis, in view
of the targeted materials usually being intended to have definite functions, must
be practical, high yielding, and, as an additional characteristic, combined with
appropriate processing to create a defined macroscopic state of matter. But, even
if one accepts that there is a rewarding chemistry beyond the dilute solutions of
classical organic chemistry and that a material synthesis is not always concluded
by obtaining a satisfactory solution NMR spectrum, some questions remain valid:
how complicated can polymer synthesis be, and must it necessarily be restricted to
straightforward one-pot methods? Some examples from the current cutting edge of
polymer science illustrate these points.
Metallocene-catalyzed polyolefin synthesis [22–26] has been developed to
obtain an impressive microstructural precision, and chain-growth polymerization of
activated olefins has become more and more controlled [27–30]. These techniques
have furnished increasingly demanding polymer topologies such as block copolymers
[31–35], star-polymers [36, 37] or core–shell polymer nanoparticles [38–43]. Stepgrowth polycondensation [44–47] not only differs from other polymerization
techniques from the point of view of its kinetics and plots of molecular weight versus
conversion, but also requires different experimental techniques. When considering
the state-of-the-art here, it is apparent that this field of polymer chemistry has
especially profited from the development of new monomeric building blocks,
new methods of catalysis, particularly those using transition-metal complexes,
and from deeper mechanistic insights [48, 49]. This is nicely illustrated by the
case of conjugated polymers with alternating donor (D) and acceptor units (A)
[50–65]. Such D–A polymers have acquired enormous importance as the active
components in solar cells and field-effect transistors and have defined new
benchmarks for high performance organic electronic polymers [66–72]. In the
first phase, an ever increasing number of new electron-rich and electron-poor
components have been incorporated into the polymers to tune their band gaps and
packing behavior [73]. Synthetic methods like Stille or Suzuki coupling have been
instrumental for ensuring the strictly alternating inclusion of different building
blocks [59, 74]. Obtaining high molecular weights and low polydispersities appears
to be crucial for improving device performance [75]. In order to establish the
required stoichiometries for optimized polycondensations, pure crystalline stannyl
or boronate components are needed, but further aspects such as the acceleration of
the reactions by microwave irradiation instead of conventional heating [76, 77],
appropriate choice of the catalytic metal–ligand complex [48, 78, 79], end-capping
to remove undesirable terminal functional groups [80–85], and rigorous purification
of the products could well be added to this list of improvements. Morphological
control of conjugated polymers [86, 87], for example, in designing bulk
heterojunction solar cells [58–61, 70], has demanded the synthesis of rod–rod
and rod–coil block copolymers and, in this context, chain-growth mechanisms
involving the living ends of the rod-like segments have become particularly important [81, 88–91].
64
K. Mu ¨llen
measure its achievements. When looked upon from this angle, polymer synthesis
may appear simple. One could, of course, respond that polymer synthesis, in view
of the targeted materials usually being intended to have definite functions, must
be practical, high yielding, and, as an additional characteristic, combined with
appropriate processing to create a defined macroscopic state of matter. But, even
if one accepts that there is a rewarding chemistry beyond the dilute solutions of
classical organic chemistry and that a material synthesis is not always concluded
by obtaining a satisfactory solution NMR spectrum, some questions remain valid:
how complicated can polymer synthesis be, and must it necessarily be restricted to
straightforward one-pot methods? Some examples from the current cutting edge of
polymer science illustrate these points.
Metallocene-catalyzed polyolefin synthesis [22–26] has been developed to
obtain an impressive microstructural precision, and chain-growth polymerization of
activated olefins has become more and more controlled [27–30]. These techniques
have furnished increasingly demanding polymer topologies such as block copolymers
[31–35], star-polymers [36, 37] or core–shell polymer nanoparticles [38–43]. Stepgrowth polycondensation [44–47] not only differs from other polymerization
techniques from the point of view of its kinetics and plots of molecular weight versus
conversion, but also requires different experimental techniques. When considering
the state-of-the-art here, it is apparent that this field of polymer chemistry has
especially profited from the development of new monomeric building blocks,
new methods of catalysis, particularly those using transition-metal complexes,
and from deeper mechanistic insights [48, 49]. This is nicely illustrated by the
case of conjugated polymers with alternating donor (D) and acceptor units (A)
[50–65]. Such D–A polymers have acquired enormous importance as the active
components in solar cells and field-effect transistors and have defined new
benchmarks for high performance organic electronic polymers [66–72]. In the
first phase, an ever increasing number of new electron-rich and electron-poor
components have been incorporated into the polymers to tune their band gaps and
packing behavior [73]. Synthetic methods like Stille or Suzuki coupling have been
instrumental for ensuring the strictly alternating inclusion of different building
blocks [59, 74]. Obtaining high molecular weights and low polydispersities appears
to be crucial for improving device performance [75]. In order to establish the
required stoichiometries for optimized polycondensations, pure crystalline stannyl
or boronate components are needed, but further aspects such as the acceleration of
the reactions by microwave irradiation instead of conventional heating [76, 77],
appropriate choice of the catalytic metal–ligand complex [48, 78, 79], end-capping
to remove undesirable terminal functional groups [80–85], and rigorous purification
of the products could well be added to this list of improvements. Morphological
control of conjugated polymers [86, 87], for example, in designing bulk
heterojunction solar cells [58–61, 70], has demanded the synthesis of rod–rod
and rod–coil block copolymers and, in this context, chain-growth mechanisms
involving the living ends of the rod-like segments have become particularly important [81, 88–91].
64
K. Mu ¨llen