15
Complex MacromolecularChimeras
Hermis Iatrou, Marinos Pitsikalis, Georgios Sakellariouand Nikos HadjichristidisDepartment of Chemistry, University of Athens, Panepistimiopolis, Zografou, 15771
Athens, Greece
15.1 Introduction
The synthetic polypeptides, although far from being strictly monodisperse or constructed from
a precise sequence and composition of a-amino acid residues, possess the ability, as their
natural relative – proteins, to form a-helix and b-sheet motifs. In order to approach the
perfection found in nature, these synthetic polypeptides must possess the lowest possible
molecular weight and compositional dispersities. Thus, their self-assembly into precisely
defined nanostructures, a requirement for appropriate functionality, will be favored. Such
nanostructures can then serve in the construction of nanomaterials and devices for biomedical
and pharmaceutical applications (Duncan, 2003).
For many years, synthetic homo/copolypeptides have been obtained by the ring-opening
polymerization (ROP) of a-amino acid N-carboxyanhydrides (NCAs) (Leuchs, 1906),
(Leuchs and Manasse, 1907), (Leuchs and Geiger, 1908), (Szwarc, 1965), (Sekiguchi, 1981)
generally using primary amines (e.g. n-hexylamine) as initiators, under an inert atmosphere.
However, attempts to synthesize well-defined living polypeptides (Scheme 15.1) with amino
initiators have been plagued, for more than 50 years, by unwanted polymerization mecha-
nisms and termination reactions (Kricheldorf, 2006). This problem has been overcome by
replacing the amino-initiators with organonickel (Deming, 1997) or ammonium chloride
(Dimitrov and Schlaad, 2003) and very recently by hexamethyldisilazane (Lu and Cheng,
2007) initiators.
Complex Macromolecular Architectures: Synthesis, Characterization, and Self-Assembly, First Edition.
Edited by Nikos Hadjichristidis, Akira Hirao, Yasuyuki Tezuka and Filip Du Prez.
© 2011 John Wiley & Sons (Asia) Pte Ltd. Published 2011 by John Wiley & Sons (Asia) Pte Ltd. ISBN: 978-0-470-82513-6
Alternatively, high-vacuum techniques (HVT) have been employed to create living con-
ditions for the amino-initiated ROP of NCAs. Apparently with this method, the unwanted
polymerization mechanism (activated monomer) is either avoided or is insignificant, thus
favoring the desired normal amine mechanism (Aliferis et al., 2004).
All these living systems lead to well-defined homo/copolypeptides with high molecular
weight and structural homogeneity (Aliferis et al., 2005; Papadopoulos et al., 2005; Hanski
et al., 2006; Iatrou et al., 2007). Furthermore, these systems can be employed for the synthesis
of polymer/polypeptide hybrid block copolymers, molecular chimeras (Schlaad and
Antonietti, 2003), a term borrowed from the Greek mythology. The Chimera (in Greek
Xı́maira) was an awesome fire-breathing she-monster (Homer, C8th BC) usually represented
as an hybrid of a lion, goat and serpent (Figure 15.1). Biosciences started using the term
chimera long ago to describe an organism composed of tissues that are genetically different.
Polymer–polypeptide hybrid block copolymers are able to self-assemble either in bulk
(microphase separation) or in selective solvents (micellization), like conventional block
copolymers, and in addition are organized within the microphases into a-helix or b-sheet
Scheme 15.1
Figure 15.1 The Chimera, a fire-breathing she-monster of the Greek mythology (Homer, Iliad) usually
represented as a hybrid of a lion, goat, and serpent.
462 Complex Macromolecular Architectures
motifs (Klok and Lecommandoux, 2006) thus leading to a rich variety of nanostructures
(Hadjichristidis et al., 2003).
The variety of the nanostructures could become even richer andmore complex inmultiblock
copolymers, multiblock multicomponent (e.g. terpolymers) or in nonlinear (e.g. miktoarm
stars) (Lodge, 2003) chimeras, approaching that found in nature. Therefore, the synthesis of
well defined complex chimeras adds another dimension to the suprastructural hierarchy of the
block copolymers with potential biomedical applications (Deming, 2007).
Although, there are many reviews on linear diblock chimeras, only a very few have
addressed the synthesis of polypeptide-based materials with different macromolecular archi-
tectures (Hadjichristidis et al., 2009; Deming, 2006; Deming, 2000). In this chapter our
discussion on the synthesis of well-defined complex chimeras, including linear multiblock co/
terpolymer, as well as nonlinear copolymer chimeras (e.g. star, graft, block-graft, dendritic),
will hopefully enrich the current literature on macromolecular chimeras. This chapter will not
address the self-assembly of these complex chimeras since this issue is discussed in another
chapter of this volume.
15.2 Linear Multiblock Chimeras
Research on the synthesis of polypeptide hybrid diblock copolymers began in the mid-1970s
(Yamashita et al., 1975). Advances in polymer synthesis, culminating in the development of
controlled/living polymerization techniques and click chemistry (Sumerlin and Vogt, 2010), in
combination with the living ROP of NCAs, allowed for the preparation not only of simple
diblock but also of a wide variety of multiblock Chimeras. In this section we will describe the
materials synthesized according to the macroinitiators used for the ROP of the NCAs: amino-
and transition-metal complexes.
15.2.1 Primary Amine Macroinitiators
o-Functionalized polymers carrying amines on both ends can be used asmacroinitiators for the
ROP of NCAs. This remains the most widely applied method for the synthesis of a plethora of
linear multicomponent hybrid block co/terpolymers. Themajor advantage of thismethod is the
fact that several living/controlled polymerization techniques are able to prepare a variety of
well-defined polymers with a high degree of amine functionalization.When combinedwith the
recently developed ROP of NCAs under high vacuum, these methods lead to well-defined
poplypeptide hybrids.
Most of these hybrid copolymers involve the use of telechelic poly(ethylene oxide), PEO,
carrying amine end groups. This interest can be attributed to the fact that the amino-PEOs are
commercially available in a variety of molecular weights with narrow molecular-weight
distributions, are water soluble and biocompatible. Furthermore, these PEO polypeptide
hybrids can form amphiphilic, double-hydrophilic and rod–coil–rod block copolymers,
leading to materials with very interesting properties in solution and in bulk.
a,o-Diamino-PEOswere employed as difunctionalmacroinitiators for the synthesis ofABA
triblock copolymers, where A the polypeptide and B is PEO. Using this difunctional initiator,
PBLG-b-PEO-b-PBLG (BLG: g-benzyl-L-glutamate)(Cho, Kim and Komoto, 1990; Floudas
et al., 2003), PZLL-b-PEO-b-PZLL (ZLL: e-carbobenzyloxy L-lysine) (Cho et al., 1994;
Complex Macromolecular Chimeras 463
Yang et al., 2005) and poly[(DL-Val)-co-(DL-Leu)]-b-PEO-b-poly[(DL-Val)-co-(DL-Leu)]
(Cho et al., 2003) have been prepared. Earlier studies resulted in poorly defined products, since
diblocks and homopolymers were present in the final reaction products.
The better purification of the reagents (NCA, initiator, solvent) was found to offer a higher
level of structural control. Thus, using a PEO-NH2macroinitiator and employing high-vacuum
techniques, the synthesis of the PEO-b-PZLL-b-PBLG triblock terpolymers was achieved
(Karatzas et al., 2009) by sequential polymerization. Size-exclusion chromatography (SEC)
analysis revealed that the macroinitiator was quantitatively consumed and that the final
products displayed monomodal peaks of relatively narrow molecular-weight distribution. In
addition, the molecular weights were very close to the stoichiometric values, indicating that
when using the high-vacuum techniques the polymerization was free of termination reactions.
Hybrid block copolymers with aliphatic polyesters, such as poly(e-caprolactone), PCL and
polylactide, PLA, represent a very interesting class of polymeric materials. Due to the low
immunogenicity, biocompatibility, biodegradability and excellent mechanical properties of
aliphatic polyesters, these hybrids have found applications in tissue engineering and drug
delivery (Langer, 2000; Tomson et al., 1995;Gombotz andPetit, 1995).Moreover, the presence
of the polyesters modifies the stability of the hybrids, since it reduces the enzymatic
degradation of the polypeptides. Finally, the formation of crystalline polyester domains greatly
influences the self-assembly behavior of the hybrid structures.
The general synthetic scheme for the polyester hybrids starts with the synthesis of the
diamino polyester (A) macroinitiators, followed by the ROP of the desired NCA to give the
corresponding BAB hybrids (B are the polypeptide chains).
The seven-membered cyclic initiator 2,2-dibutyl-2-stanna-1,3-dioxepane was used by
Kricheldorf et al. (1998) to produce macrocyclic PCL (Scheme 15.2). Subsequent reaction
of the two OHs with 4-nitrobenzoyl chloride yielded the corresponding a,o-dinitrobenzoylPCL, which was converted to the a,o-diamine PCL by catalytic hydrogenation. These
polymers were then employed as macroinitiators for the polymerization of the Gly-, Ala-,
Phe- andBLG-NCAs to afford the respective triblock chimeras (Kricheldorf andHauser, 2001).
The low nucleophilicity of the 4-aminobenzoyl end-groups prevented the aminolytic cleavage
of the PCL chains during synthesis and storage. The sampleswere characterized by viscometry,
IR and NMR spectroscopy. The yields of the NCA polymerization were high but not
quantitative (between 60 and 80%). The poor solubility of the polypeptides in organic solvents
rendered impossible the determination of the molecular-weight distribution of the copolymers
by SEC. However, there was evidence (NMR) that during the BLG-NCA polymerization, a
mixture of di- and triblocks was obtained, due to the low initiation rate compared to that of
propagation for this specific monomer.
Hybrid terpolymers have also been prepared by Deng et al. (2005) using poly(ethylene
oxide)-b-poly(L-lactic acid) (PEG-b-PLLA) macroinitiators (Scheme 15.3). PEG-b-PLLA-
OH was prepared by the ROP of LLA using monomethoxy PEG-OH in the presence
of Sn(Oct)2. The end-hydroxyl group was transformed to amino-group by reaction with
tert-butoxycarbonyl-L-phenylalanine in the presence of dicyclohexylcarbodiimide, DCC. The
deprotection of the amino group was performed under anhydrous conditions with trifluor-
oacetic acid at 0 �C, affording the PEG-b-PLLA-NH2 macroinitiator, which was further used
for the ROP of BLG-NCA leading to the synthesis of the triblock terpolymer PEG-b-PLA-b-
PBLG. Characterization of the products revealed that the macroinitiators were efficiently
prepared andwere quantitatively consumed during the polymerization reactions. SEC analysis
464 Complex Macromolecular Architectures
and NMR spectroscopy confirmed that rather well-defined products of relatively narrow
molecular-weight distribution were prepared.
The procedure developed by Chen et al. was adopted by Xia, Whittaker et al. for the
synthesis of PEG-b-PLLA-b-PLL triblock terpolymers (Peng et al., 2009), as shown in
Scheme 15.4. The amine end groups of the PEG-b-PLA-NH2 block copolymers served as
macroinitiators for the polymerization of ZLL-NCA leading to the preparation of the desired
terpolymers. The samples were characterized only by NMR and IR spectroscopy.
Scheme 15.2
Complex Macromolecular Chimeras 465
Linear pentablock terpolymers PBLL-b-PBLG-b-PS-b-PBLG-b-PBLL (BLL: tert-butylox-
ycarbonyl-L-lysine, PS: polystyrene) were recently synthesized by Karatzas et al.
(2008) using a,o-diamino-PS. This polymer was prepared by anionic polymerization of
styrene with sodium/naphthalene (difunctional initiator) and termination with 1-(3-bromo-
propyl)-2,2,5,5-tetramethyl-aza-2,5-disilacyclopentane, and of deprotection by successive
Scheme 15.3
466 Complex Macromolecular Architectures
precipitations of the polymer in methanol. The a,o-diamino-PS was then used for the
sequential polymerization of BLG-NCA and BLL-NCA under high vacuum (Scheme 15.5).
The molecular weights were in very close agreement with the stoichiometric values and the
molecular-weight distributions very narrow, indicating that well-defined samples were pre-
pared with this methodology.
Step-growth polymerization is another technique leading to telechelic amine macro-
initiators. The Yamamoto coupling polymerization of 2,7-dibromo-9,9-dihexylfluorene
was performed by Kong et al. (2004) followed by the end-capping reaction with N-(p-
bromobenzyl)phthalimide. Deprotection with hydrazine afforded the telechelic amine
macroinitiators, which were subsequently used for the polymerization of BLG-NCA.
Due to the nature of the step-growth polymerization the molecular-weight distribution,
of the triblock copolymers was relatively broad (Mw/Mn values higher than 2.0)
(Scheme 15.6).
An amine-functionalized TEMPO radical was employed by Karatzas et al. (2009) for
the synthesis of poly(N-vinyl pyrrolidone), PNVP, copolymers with PBLG and PZLL
blocks, PNVP-b-PBLG-b-PZLL (Scheme 15.7). The polypeptide blocks were initially
prepared using the amino group as initiator of the ROP of NCAs. The TEMPO
moiety did not interfere with the ROP and polypeptides with very narrow molecular-weight
Scheme 15.4
Complex Macromolecular Chimeras 467
distributions were obtained. The final step involved the nitroxide-mediated polymerization
of NVP leading to chimeras with relatively low polydispersity (1.16–1.22) and controlled
molecular weights (22.6–96.5� 103 gmol�1).
15.2.2 Transition-Metal Complex Macroinitiators
Zero-valent metal complexes cannot be used directly for the synthesis of polypeptide hybrid
block copolymers. However, Na-allyloxycarbonyl-amino acid allyl amides can be used as
universal precursors for the amido-amidate nickelacycle initiators (Scheme 15.8). As shown in
Scheme 15.8, the Na-allyloxycarbonyl-amino acid derivatives may undergo tandem oxidative
additions to nickel(0) giving the nickelacycle initiators. Thismethodwas initially employed for
the synthesis of block copolypeptides and was then expanded by Deming and coworkers to a
variety of hybrid structures.
This methodology was applied for the synthesis of PBLG-b-polyoctenamer-b-PBLG and
PBLG-b-polyethylene-b-PBLG triblock chimeras (Brzezinska and Deming, 2001)
(Scheme 15.9). At the beginning, acyclic diene metathesis, ADMET, polymerization was
employed for the synthesis of a,o-diamino-functionalized polyoctenamer using Grubbs’
catalyst RuCl2-(¼CHPh)(PCy3)2. This product was converted to the bisallyloxycarbonyl-L-
leucine -terminated polyoctenamer, followed by reaction with [1,2-bis(diethylphosphino)
ethane]Ni(COD), depeNi(COD), to produce the active sites for the ROP of BLG-NCA.
After the NCA polymerization, the Wilkinson catalyst was employed to hydrogenate the
polyoctenamer block, affording the corresponding polyethylene block. This methodology
was also applied for the synthesis of triblock copolymers where the middle block was
either poly(ethylene glycol), PEG, or poly(dimethyl siloxane), PDMS (Brzezinska
et al., 2002).
Scheme 15.5
468 Complex Macromolecular Architectures
It is well known that amido-metallacycle end groups are the active species for the synthesis
of polypeptides by transition-metal complexes. Therefore, electrophilic reagents, such as
isocyanates, can react with these end groups through formation of stable urea linkages. Based
on this concept, excess isocyanate end-capped PEG was reacted with living PBLG to produce
PBLG-b-PEG (Brzezinska et al., 2002) (Scheme 15.10). SEC and NMR measurements
revealed that the coupling reaction was near quantitative. This work was further expanded
to the synthesis of CABAC pentablock terpolymers (C: PEG, A: PBLG and B: polyoctenamer,
PEG or PDMS). The synthesis was achieved through the coupling reaction of the living ABA
triblock copolymers with isocyanate end-capped PEG. Repeated precipitation from THF
solutions into methanol was employed for the purification of the pentablock chimeras. SEC
Scheme 15.6
Complex Macromolecular Chimeras 469
analysis confirmed that monomodal traces of relatively narrow molecular-weight distribution
were obtained.
15.3 Nonlinear Chimeras
Controlled/living polymerization techniques in combination with living ROP of NCAs can
lead not only to linear but also to nonlinear chimeras. In this section we will describe the
materials synthesized according to their structure: star, comb, brush-block, and dendritic-like
chimeras.
15.3.1 Star Chimeras
Star polymers are branched polymers consisting of several linear chains emanating from a
central core. Among the general synthetic routes (Mishra andKobayashi, 1999; Hadjichristidis
etal., 2001;2006;2007) for starpolymers, theuseofmultifunctional initiatorshaspreferentially
been used for the synthesis of polypeptide-based stars.
Scheme 15.7
Scheme 15.8
470 Complex Macromolecular Architectures
This method is referred to as the “core-first” or “arm-out” or divergent approach. According
to this procedure, multifunctional compounds capable of simultaneously initiating the
polymerization of several arms are used. There are several requirements a multifunctional
initiator has to fulfill in order to produce star polymers with controllable molecular weights,
uniform arm lengths, and lowmolecular-weight distribution.All initiating sitesmust be equally
Scheme 15.9
Complex Macromolecular Chimeras 471
reactive and the initiation rate must be higher than the propagation rate. The characterization of
the star polymers produced by this method is difficult, since the molecular weight of the arms
cannot be measured directly. The number of arms can be defined indirectly by several methods,
such as end-group analysis, determination of the branching parameters, which are the ratios of
either the mean square radius of gyration, or intrinsic viscosity or hydrodynamic radius of the
star to the corresponding linearwith the samemolecularweight. Finally, the determinationof the
functionality can be achieved by isolation of the arms after cleavage (i.e. hydrolysis), if possible,
and subsequent analysis. This approach is the most widely used for the synthesis of polypeptide
stars. Using this methodology and a combination of different polymerization techniques several
polypeptide-based star hybrid polymers have been prepared.
o-Amino 4-arm PEO stars were efficiently used by Karatzas et al. (2009) for the ROP of
BLG-NCA leading to (PEO-b-PBLG)4 star-block copolymers (Scheme 15.11). The multi-
functional macroinitiator was quantitatively consumed and thus giving a product of
relatively narrow molecular-weight distribution and molecular weight very close to the
stoichiometric value.
A combination of ATRP and ROP was reported by Abraham et al. (2006) for the synthesis of
(PS-b-PBLG)3 3-arm star-block chimeras. ATRP techniques were initially adopted for the
synthesis of 3-arm PS star using a suitable cynurate-based initiator. The end-bromine groups
were transformed to amines by a three-step azidation route followed by conversion to nickel-
amine complex macroinitiators suitable for the polymerization of BLG-NCA (Scheme 15.12).
The functionality of the precursor PS star was confirmed by the measurement of the molecular
weights of the stars and the corresponding arms. The armswere isolated by the hydrolysis of the
ester groups at the core. The final products were obtained in moderate yields and possessed
relatively narrow molecular-weight distributions (1.1<Mw/Mn< 1.4).
The polyamidoamine with four terminal amine groups (G0-PAMAM) was employed by
Dong et al. as tetrafunctional initiator for the polymerization of BLG-NCA leading to the four-
arm (PBLG)4 stars (Qiu et al., 2009). The terminal amine groupswere further functionalized by
reaction with 2-bromo-2-methylpropionyl bromide to introduce initiation sites for ATRP.
Scheme 15.11
Scheme 15.10
472 Complex Macromolecular Architectures
These sites were then employed for the polymerization of D-gluconamidoethyl methacrylate
providing the corresponding star-block copolymers (Scheme 15.13). The reaction sequence
was monitored by NMR and IR spectroscopy and SEC revealing that rather well-defined
structures having relatively low polydispersities (1.18<Mw/Mn< 1.45) were obtained.
Miktoarm star polymers, consisting of chemically different arms, including polypeptide
chains have also been synthesized. A combination of ATRP and ROP were employed by
Babin et al. (2005; 2008) for the synthesis of PS(PBLG)2 miktoarm star chimeras
(Scheme 15.14). Styrene was first polymerized by ATRP, followed by reaction with a
Scheme 15.12
Complex Macromolecular Chimeras 473
large excess of 1-aminotriethylenetriamine for the nucleophilic substitution of the end-
bromine group, leading to PS chains bearing two amine end groups. Due to the nonquantitative
nature of this reaction, the unfunctionalized PS chains were removed by selective precipitation
in hexane after treatment of the PS(NH2)2 with aqueous HCl. The o-amine groups were
subsequently used for the polymerization of BLG-NCA, to afford the desired structures.
Hydrolysis with an excess of hydrogen bromide in trifluoroacetic acid led to the cleavage of the
benzyloxycarbonyl protective groups to afford the corresponding amphiphilic miktoarm star
chimeras. SEC analysis revealed that the PS(NH2)2 macroinitiators were quantitatively
consumed and that the molecular-weight distributions of the final products were fairly narrow
(1.09<Mw/Mn< 1.22).
(PLLA)2PBLG 3-miktoarm stars were prepared by Sun et al. (2009) using the trifunctional
initiator 2-benzyloxycarbonylamino-1,3-propanediol (Scheme 15.15). The two available hy-
droxyl groups were employed for the polymerization of L-lactide in the presence of Sn(Oct)2.
Subsequent cleavage of the benzyloxycarbonyl protective group, with hydrogen bromide in
acetic acid, afforded PLLA chains with a central amine group, (PLLA)2NH2, which served as
macroinitiator for the polymerization ofBLG-NCA togive the desiredmiktoarm.NMRanalysis
revealed that the (PLLA)2NH2 macroinitiator was efficiently prepared. SEC experiments
confirmed the complete consumption of themacroinitiator and the absence of any homopolymer
trace. However, the molecular-weight distributions of the final products were not measured due
to the self-assembly of the miktoarm star copolymers in the solvent used for SEC.
A commercially available PEO bearing two central- and two end-amine groups was
employed by Cho et al. (2000) as a macroinitiator for the polymerization of BLG-NCA,
leading to the synthesis (PEO-b-PBLG)2(PBLG)2 4-miktoarm star copolymers,
(Scheme 15.16). The molecular weights were determined by NMR analysis. However,
details concerning the efficiency of this synthetic approach and the characterization of the
products were not provided.
Taking advantage of the unique features of anionic polymerization to produce end- or in-
chain amino-functionalized polymers, as well as of the living nature of the ROP of NCAs under
Scheme 15.13
474 Complex Macromolecular Architectures
high-vacuum conditions, a variety of well-defined miktoarm star hybrids was synthesized
and fully characterized by Karatzas et al. (2008). These structures include: (PS)2(PBLG),
(PS)2(PBLL), (PS)(PI)(PBLG), (PS)(PI)(PBLL), (PS)2[P(a-MeS)](PBLG), (PS)2[P(a-MeS)]
(PBLL), (PS)2(PBLG)2 and (PS)2(PBLL)2 (Scheme 15.17). The synthetic strategy involved the
preparation of diphenyl ethylene, DPE, functionalized polymers [DPE-chain-end-functiona-
lized PI (PI-D), DPE-in-chain-functionalized polystyrene (PS-D-PS) and DPE-in-chain-
difunctionalized polystyrene], shown in Scheme 15.18. These structures were subsequently
activated by reaction with a living polymer chain or s-BuLi followed by reaction with 1-(3-
bromopropyl)-2,2,5,5-tetramethyl-aza-2,5-disilacyclopentane. The silyl-protected group was
cleaved by treatment with HCL or p-toluenesulfonic acid, leading to the synthesis of the
corresponding amine-functionalized polymers. These products were employed as macroini-
tiators for the polymerization of BLG- and BLL-NCAs to afford the desired miktoarm stars.
Combined characterization by NMR, SEC and light-scattering measurements confirmed the
Scheme 15.14
Complex Macromolecular Chimeras 475
efficiency of this synthetic scheme and the homogeneity of prepared products. The only
restriction of this procedure was the steric hindrance of the in-chain functionalized macro-
initiators. This became obvious from the fact that (PS)2(NH2)2 macroinitiators led tomiktoarm
stars with broader molecular-weight distributions, whereas the more sterically hindered
(PS)4(NH2)2 macroinitiators afforded multimodal distributions.
15.3.2 Comb, Brush-Block, Dendritic-Like Chimeras
The limitations in achieving true living polymerization conditions of NCAs have, in the past,
rendered difficult the synthesis ofmore complex structures containing polypeptide chains, such
Scheme 15.15
Scheme 15.16
476 Complex Macromolecular Architectures
as comb-shaped chimeras. Three general methods have been developed for the synthesis
of randomly branched graft copolymers: (a) the “grafting-onto”, (b) the “grafting-from”
and (c) the macromonomer method (or “grafting-through” method) (Pitsikalis et al., 1998;
Hadjichristidis et al., 2001; 2003b; 2006).
In the “grafting-from” method active sites are generated randomly along the backbone.
These sites are capable of initiating the polymerization of a second monomer, leading to the
synthesis of graft copolymers. This method is widely used for the synthesis of graft hybrids,
especially with polypeptide side chains. Amine groups were generated along the backbone and
then served as initiation sites for theROPofNCAs.An early example includes the conventional
radical copolymerization of N-benzylacrylamide with N-(3-aminopropyl)methacrylamide
hydrochloride. The copolymer obtained was then dehydrochlorinated to afford free primary
amine groups, subsequently functioning as initiation sites for the polymerization of Ala-NCA
(Boulahia et al., 1989). (Scheme 15.19) In another study, N-methyl-N-(4-vinylphenethyl)
ethylenediamine was copolymerized with 2-hydroxyethyl methacrylate. The side amine
groups were subsequently used to copolymerize the NCAs of b-benzyl L-aspartate and
b-(4-phenylazobenzyl) L-aspartate to give the graft copolymer chimera (Aoyama
et al., 1990a; 1990b (Scheme 15.20) These structures were not characterized thoroughly in
terms of the molecular and structural characteristics.
The “grafting-from” approach was also employed in more recent studies. N-tert-butox-
ycarbonyl-N0-(2-methacryloyl)-1,3-diaminopropane was prepared by Zhang et al. (2005)
and polymerized by free-radical polymerization. The quantitative deprotection of the
amine group was performed in a mixture of dichloromethane and trifluoroacetic acid,
Scheme 15.17
Complex Macromolecular Chimeras 477
followed by oligomerization of the BLG- or ZLL-NCAs affording the corresponding molecu-
lar brushes (Scheme 15.21). The samples were characterized by NMR and AFM techniques.
A similar methodology was applied by Xiang et al. (2009) for the synthesis of chitosan
grafted with poly(L-tryptophan) chains (Scheme 15.22). Deacetylated chitosan (deacetylation
degree of amino groups 75%) was employed as a multifunctional macroinitiator for the
polymerization of the L-tryptophan-NCA in ethyl acetate solutions leading to the synthesis of
chitosan-g-poly(L-tryptophan) graft copolymers. The products were analyzed by NMR and
IR spectroscopy.
Using the grafting-from methodology amphiphilic block-brush copolymers were prepared
by Cai et al. (2009). Amine-terminated PEO was employed as macroinitiator for the
polymerization of BLG-NCA leading to the synthesis of PEO-b-PBLG block copolymers.
Subsequent aminolysis of the benzyl ester groups of the PBLG block using 1,2-diaminoethane
led to the synthesis of the corresponding PEO-b-poly[(2-aminoethyl)-L-glutamate)], PEO-b-P
(ELG-NH2), copolymers. The amine moieties of the P(ELG-NH2) blocks served as initiators
for the polymerization of BLG-NCA leading to the synthesis of the desired block-
brush copolymer, PEO-b-[PELG-g-PBLG] (Scheme 15.23). The reaction sequence was
monitored by SEC. It was found that the molecular-weight distribution of the initial PEO-
b-PBLG block copolymer (Mw/Mn¼ 1.2) was substantially broadened after the grafting from
Scheme 15.18
478 Complex Macromolecular Architectures
procedure (1.6<Mw/Mn< 2.0). The molecular weights of the samples were obtained by
NMR spectroscopy.
The thirdmethod for the synthesis of graft copolymers is based onmacromonomers,which is
an oligomeric or polymeric chain bearing a polymerizable end group.Macromonomers having
two polymerizable end groups have also been reported. Copolymerization of preformed
macromonomers with another monomer yields graft copolymers. In an early report,
m,p-vinylbenzylamine was used as initiator for the oligomerization of DL-phenylalanine-
NCA, leading to narrow molecular-weight distribution macromonomers of low molecular
weight (Scheme 15.24). Thesemacromonomers were then copolymerizedwithMMAand S, to
give PMMA-g-PPhe and PS-g-PPhe graft copolymers, respectively (Takaki et al., 1987).
Polystyrenes dendronized with L-Lysine dendrons of different generations were prepared by
the procedure given in Scheme 15.25 (Lubbert et al., 2005). 4-Vinylbenzylamine was linked
with the appropriate protected L-Lysine dendrons under standard peptide coupling conditions,
affording the corresponding dendronized macromonomers. The macromonomers were then
subjected to conventional radical polymerization to give productswith broadmolecular-weight
distributions.Monomer conversion and polymermolecular weight were found to increasewith
increasing dendron size.
A polyamidoamine dendron (PAMAM) carrying a Boc-protected amine group was
also synthesized by Harada et al. (2006; 2009). The protective group was then removed
Scheme 15.19
Complex Macromolecular Chimeras 479
and the primary amine group was employed as initiator for the polymerization of ZLL-NCA
leading to linear-dendritic block copolymer (Scheme 15.26).
Linear/dendron-like polypeptide hybrid structures were prepared by Dong et al. by
combination of ROP and click chemistry (Hua et al., 2009). The synthesis of linear PCL/
dendron-like PBLG, PCL-b-(Dm-PBLG), hybrid copolymers having 2m (m¼ 0,1,2,3) PBLG
branches is described in Scheme 15.27. Benzyl alcoholwas used as initiator and Sn(Oct)2 as the
catalyst for the ROP of e-CL. The end-hydroxyl group was then quantitatively tosylated and
was further transformed into azide-terminated PCL, PCL-N3. Propargyl focal point PAMAM-
typed dendrons with 2m (m¼ 0,1,2,3) primary amine groups, Dm, were synthesized and then
click conjugated with the PCL-N3 to produce amine-terminated PCL dendrons. The available
amine groups were employed as initiation sites for the polymerization of BLG-NCA leading to
the synthesis of the desired structure. SEC analysis revealed that the products had moderate
polydispersities (1.13<Mw/Mn< 1.36). Detailed characterization by SEC, MALDI, NMR
spectroscopy, DSC and WAXD verified the synthesis of the target structures.
Using similarmethodology the synthesis of asymmetric PEO-b-(Dm-PBLG) and symmetric
(Dm-PBLG)-b-PEO-b-(Dm-PBLG) hybrid copolymers having 2m (m¼ 0,1,2,3) PBLG
Scheme 15.20
480 Complex Macromolecular Architectures
branches was reported by Peng et al. (2009) (Scheme 15.28). Both the “arm-first” and
“core-first” strategies were employed for the synthesis of the desired products. According
to the “arm-first” approach, the propargyl focal point dendrons, Dm having 2m (m¼ 0,1,2,3)
primary amine groups were first used for the ROP of BLG-NCA, leading to the formation of
“clickable” dendron-like PBLGpolymers having 2m branches. These structureswere then click
conjugated with azide-terminated PEO at the one or both chain ends (PEO-N3, N3-PEO-N3)
producing the asymmetric and the symmetric block hybrids, respectively. Alternatively,
according to the “core-first” approach the propargyl focal point dendrons were first click
conjugated with PEO-N3 or N3-PEO-N3 generating primary amine-terminated PEO dendrons,
which were subsequently used for the ROP of BLG-NCA leading to the synthesis of the target
structures. SEC analysis, NMR and FT-IR spectroscopy revealed that the products were rather
well defined and had relatively narrow molecular-weight distributions.
Hyperbranched amphiphilic multiarm polyethyleneimine-PBLG copolymers were
prepared by Tian et al. (2006) The synthetic approach involved the ROP of BLG-NCA,
Scheme 15.25
Complex Macromolecular Chimeras 483
initiated by a hyperbranched polyethyleneimine (PEI) macroinitiator. No data concerning
the polydispersity indices of the PEI precursors or the final copolymers were provided. In
this work, the micellar properties in aqueous solutions of the multiarm copolymers
synthesized were examined.
15.4 Concluding Remarks
It is clear that the new developments in living/controlled polymerizations, as well as in the
living ROP of NCAs, hold tremendous promise for the synthesis of well-defined complex
chimerasmaterials with controllablemolecular weight, sequence, composition andmolecular-
weight distribution. Although much development is still required, these complex polypeptide-
based materials begin to rival their natural counterparts in terms of complexity and accuracy.
Such well-defined materials can self-assemble into precisely defined nanostructures, a
requirement for the development of new biomedical and pharmaceutical tools with a wide
range of tunable properties. Furthermore, studies on these novel chimeraswill shed light on the
natural phenomena associated with protein function and functionality.
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