process, utilizing suitable agents. The extraordinary attention that these living, or
more appropriately termed ‘‘controlled radical polymerizations’’ (as the ‘‘true livingness’’ criteria is not rigorously obeyed by these approaches), have gained during
the last decade is because of their relative simplicity and unparalleled functional
group tolerance that make them uniquely versatile [16]. Further discussions of living polymerization routes are outside the scope of this chapter and interested
readers may refer to the review articles that have appeared recently [16–22].
The standard molecular structural parameters that one would like to control in
block copolymer structures, especially in the context of polymeric nanostructures,
are the relative size and nature of the blocks. The relative size implies the length of
the block (or degree of polymerization, i.e., the number of monomer units contained within the block), while the nature of the block requires a slightly more
elaborate description that includes its solubility characteristics, glass transition
temperature (T g ), relative chain stiffness, etc. Using standard living polymerization
methods, the size of the blocks is readily controlled by the ratio of the monomer
concentration to that of the initiator. The relative sizes of the blocks can thus be
easily fine-tuned very precisely; to date the best control of these parameters in
block copolymers is achieved using anionic polymerization. The nature of each
block, on the other hand, is controlled by the selection of the monomer; for instance, styrene would provide a relatively stiff (hard) block while isoprene would
provide a soft one. This is a consequence of the very low T g of polyisoprene compared to that of polystyrene, which in simplistic terms reflects the relative conformational stiffness of the polymer chain.
To summarize, it is evident that control of the length and nature of the blocks in
block copolymers is readily achieved using living polymerization methods. Newer
living polymerization methods based on free-radical chain growth, although effective in generating block copolymers, have yet to attain the precision that the more
traditional anionic polymerization approach has achieved. The relatively wider distribution of block lengths, and homopolymer contamination, are the two main
factors, wherein the newer methods often fail to meet the stringent molecular
structural criteria that are often key to the development of nanostructured polymeric materials. Despite this drawback, these newer routes are likely to play a
crucial role in the future development of nanostructured polymeric materials because of the significantly wider range of functional block copolymers that they can
access.
15.3
Polymer Conformational Control
Although conformational control of synthetic macromolecules is a daunting proposition, given the enormous entropy cost that has to be borne, approaches to
achieve this are being actively researched. Typically, synthetic polymer chains adopt
a random coil conformation in solution, unlike biological macromolecules which
15 Nanostructured Polymers
480
more appropriately termed ‘‘controlled radical polymerizations’’ (as the ‘‘true livingness’’ criteria is not rigorously obeyed by these approaches), have gained during
the last decade is because of their relative simplicity and unparalleled functional
group tolerance that make them uniquely versatile [16]. Further discussions of living polymerization routes are outside the scope of this chapter and interested
readers may refer to the review articles that have appeared recently [16–22].
The standard molecular structural parameters that one would like to control in
block copolymer structures, especially in the context of polymeric nanostructures,
are the relative size and nature of the blocks. The relative size implies the length of
the block (or degree of polymerization, i.e., the number of monomer units contained within the block), while the nature of the block requires a slightly more
elaborate description that includes its solubility characteristics, glass transition
temperature (T g ), relative chain stiffness, etc. Using standard living polymerization
methods, the size of the blocks is readily controlled by the ratio of the monomer
concentration to that of the initiator. The relative sizes of the blocks can thus be
easily fine-tuned very precisely; to date the best control of these parameters in
block copolymers is achieved using anionic polymerization. The nature of each
block, on the other hand, is controlled by the selection of the monomer; for instance, styrene would provide a relatively stiff (hard) block while isoprene would
provide a soft one. This is a consequence of the very low T g of polyisoprene compared to that of polystyrene, which in simplistic terms reflects the relative conformational stiffness of the polymer chain.
To summarize, it is evident that control of the length and nature of the blocks in
block copolymers is readily achieved using living polymerization methods. Newer
living polymerization methods based on free-radical chain growth, although effective in generating block copolymers, have yet to attain the precision that the more
traditional anionic polymerization approach has achieved. The relatively wider distribution of block lengths, and homopolymer contamination, are the two main
factors, wherein the newer methods often fail to meet the stringent molecular
structural criteria that are often key to the development of nanostructured polymeric materials. Despite this drawback, these newer routes are likely to play a
crucial role in the future development of nanostructured polymeric materials because of the significantly wider range of functional block copolymers that they can
access.
15.3
Polymer Conformational Control
Although conformational control of synthetic macromolecules is a daunting proposition, given the enormous entropy cost that has to be borne, approaches to
achieve this are being actively researched. Typically, synthetic polymer chains adopt
a random coil conformation in solution, unlike biological macromolecules which
15 Nanostructured Polymers
480