Chemistry Reference
In-Depth Information
of anionic and cationic blocks forms the core. In addition to electrostatic interactions,
the increase in entropy attributable to the release of smaller counterions plays a key
role in micellar formation. These types of interactions were first observed between
PEO-b-polylysine and PEO-b-polyaspartate (Harada and Kataoka 1995). The combi-
nation of PEO-b-poly(sodium methacrylate) and poly(N-ethyl-4-vinylpyridinium) is
also another example of PIC formation. The interest in these types of polymers arises
from their ability to deliver charged biomolecules, such as DNA and enzymes at the
core. Block copolymer micelles containing a cationic block as the hydrophobic block
were capable of binding with charged biological molecules, especially with anionic
DNA molecules, forming PIC in the core between the cationic block and anionic
DNA molecules. These polymeric micelles are evolving as nonviral DNA delivery
vehicles for gene therapy (Katayose and Kataoka 1997; Kakizawa and Kataoka
2002). For example, poly(ethylene glycol)-b-poly(
L
-lysine) block copolymer has
been used to trap DNA in its ionic core through electrostatic interactions.
Another class of amphiphilic ionic block copolymers employs an ionic block as
the hydrophilic segment linked to a hydrophobic segment. Polystyrene-b-poly
(4-vinylpyrine) (PS-P4VP) and PS-P2VP were the first reported polymers of this
class in which PS is the hydrophobic segment and VP is the hydrophilic segment
(Marie et al. 1976; Gauthier and Eisenberg 1987). Gauthier and Eisenberg have esta-
blished PS-b-poly(acrylic acid) (PAA) based block copolymers that form spherical
micelles, called “crew-cut” micelles, in cases when the PS chain is long and the
PAA chain length is short. PS-b-PAA polymers have been used to obtain a
variety of other morphologies such as rods and vesicles by tuning the ratio of PS/
PAA by the addition of salt or by mixing organic solvents. They claim that the
factors that control these morphological changes include 1) the degree of stretching
in the core, 2) intershell (corona) interactions, and 3) core-shell interfacial energy.
A change in any of these factors affects the morphology of the assemblies that
results in translating the spherical micelles to rods and rods to vesicles (Allen et al.
1999; Choucair and Eisenberg 2003; Chen and Jiang 2005; Rodriguez-Hernandez
et al. 2005).
Nolte and others (Cornelissen et al. 1998) reported a new class of amphiphilic
ionic block copolymers in which a charged helical polypeptide chain was attached
to the hydrophobic PS. The charged helical peptide is poly(isocyanide) based
peptide derived from dipeptides, isocyano-
L
-alanine-
L
-alanine (IAA) and isocyano-
L
-alanine-
L
-histidine (IAH). From circular dichroism (CD) spectra they concluded
the formation of a left-handed helix for PS-b-PIAA and right-handed helix for
PS-b-PIAH (Chart 2.3). These polymers have been touted as superamphiphiles
forming various structures such as rods, vesicles, and bilayers. When the chain
length of PIAA is decreased, the rods change their morphologies to bilayers and ves-
icles. In another communication, Velonia et al. (2002) reported a protein-polymer
hybrid, called “giant amphiphiles,” that is based on a lipase enzyme attached to
PS. Rodlike micelles were observed for these polymers in tetrahydrofuran.
In addition to the micelle-type assemblies described above, there has been signifi-
cant interest in developing conditions for forming vesicle-type assemblies from
amphiphilic polymers. Polymeric vesicles are formed by bolamphiphilic block
