Formation of linear polymers with pendant vinyl groups via inclusion complex mediated polymerization of divinyl monomers

Article abstract of DOI:10.1021/ja062568z

Ethylene glycol dimethacrylate (EGDMA) and ethylene glycol methacrylate 4-vinyl benzoate (EGMAVB) were shown to form 1:1 inclusion complexes with cyclodextrin and were characterized by instrumental techniques. Computational analysis showed that the bent conformation of the included divinyl monomer was more stable than its linear conformation. Complexation of the divinyl monomer with the first CD molecule offered substantial stabilization than with the second CD molecule. The vinyl group included in the CD cavity did not participate in polymerization. As a result, solvent soluble, linear polymers with pendant vinyl unsaturation per repeat unit were obtained. This was unequivocally established by the polymerization of a complex comprising CD and EGMAVB. The unreacted vinyl group can be polymerized in the subsequent step to yield cross-linked products. Copyright

Full text of DOI:10.1021/ja062568z

Published on Web 05/25/2006
Formation of Linear Polymers with Pendant Vinyl Groups via Inclusion
Complex Mediated Polymerization of Divinyl Monomers
Sunita S. Satav,† Rohini N. Karmalkar,† Mohan G. Kulkarni,*,† Nagaraju Mulpuri,‡ and
G. Narahari Sastry‡
Polymer Science and Engineering DiVision, National Chemical Laboratory, Pune 411 008, India, and
Molecular Modeling Group, Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500 007, India
Received April 13, 2006; E-mail: mg.kulkarni@ncl.res.in
Cross-linked polymers obtained by simultaneous polymerization/
cross-linking of monomers containing multiple double bonds find
a wide range of applications, such as ion exchange resins,
adsorbents, molecularly imprinted polymers, supports for reagents
in organic synthesis, enzyme immobilization, and drug delivery
systems. A sequential approach wherein a soluble linear polymer
is first synthesized, isolated, condensed with a functional monomer,
and then cross-linked offers significant advantages.1a-c Soluble
Figure 1. The lowest energy conformation of (a) EGDMA and (b)
EGMAVB obtained through molecular dynamics simulations.
linear poly(ethylene glycol dimethacrylate), that is, poly(EGDMA),
containing pendant unsaturation could be obtained by esterifying
poly(methacrylic acid) with 2-hydroxyethyl methacrylate (HEMA).
of intramolecular hydrogen bonding in native â-CD by intermo-
lecular hydrogen bonding between the guest molecule and â-CD.3b-e
The stoichiometry of IC was established by 1H NMR analysis.3a-d
The peak at δ 4.34 corresponds to four protons of EGDMA
(-OCH2CH2O-), and the peaks at δ 4.48 and 4.82 correspond to
seven protons of â-CD for (-CH2OH) and (C1-H), respectively.
The integration shows formation of a 1:1 EGDMA-â-CD complex.
EGMAVB-â-CD and EGDMA-DM-â-CD also formed 1:1 ICs.
The cross polarization and magic angle spinning (CP/MAS) solid-
state 13C NMR spectra of the complexes display well-resolved,
single peaks for each carbon of all glucose units. Also, the peaks
at ∼78.59 and ∼101 ppm, corresponding to C1 and C4, adjacent to
a conformationally strained glycosidic linkage, disappeared. These
results indicate symmetrical conformation of glycosidic linkage due
to inclusion of the EGDMA molecule within the cavity of â-CD.3a-e
The X-ray diffractograms of EGDMA-â-CD and EGMAVB-â-CD
exhibit a cage-type structure.3a-f Mass spectral analysis showed that
both EGMAVB and EGDMA formed a 1:1 complex with â-CD.3g
The conformational analyses of EGDMA and EGMAVB, their
complexation with â-CD, and thermodynamics of oligomerization
were analyzed by computational studies. The force field and density
functional theory, B3LYP/6-31G, calculations indicate that the bent
conformation is more stable than the linear one by 3-5 kJ/mol.
The complexation of the ligands with â-CD was analyzed by
docking, quantum chemical, and molecular dynamics simulations
using Autodock 3.0.5,8 Gaussian 03,9 and AMBER 8.010 programs,
respectively. The lowest energy complex, obtained from the
Autodock calculation, was taken as the initial structure for MD
simulations. TIP3P water models were used as solvent, and
equilibration was performed for 500 ps and MD simulation was
carried for 2 ns. The MD simulations of EGMAVB complexation
with â-CD revealed that the conformation wherein the styrene end
is outside the cavity is stable (Figure 1).
However, it is not clear if complete conversion could be achieved
without cross-linking or homopolymerization of HEMA. There is
a need to devise a one-step methodology since recent approaches
have led to hyperbranched structures.2a-c Cyclodextrins (CD) form
host-guest complexes with a variety of organic compounds and
polymers.3a-g Polyrotaxanes are stabilized by hydrogen bonding
between the hydroxyl groups of neighboring CDs and secondary
valence interactions between the repeat unit of the polymer and
the hydrophobic cavities of CDs.3a-e Complexation of anisole with
R-CD suppressed the reaction at the ortho position and enhanced
the p/o ratio of the chlorinated product from 1.48 to 21.6.4
Polymerization of methacrylate and methacrylamide monomers
could be carried out in aqueous media as the solubility of monomers
was enhanced by complexation with CD.5 1,4-Butane diol diacrylate
completely penetrated the CD cavity when it formed a complex
with dimethyl-â-CD (DM-â-CD), whereas in the case of the 1,4-
butane diol dimethacrylate, the CD ring was located at the terminal
end of the guest molecule.6 Polymerization of the diacrylate-CD
complex in water resulted in cross-linked polymers containing CD.6
In this communication, we report that CDs form a 1:1 inclusion
complex (IC) with divinyl monomers, such as EGDMA and
ethylene glycol methacrylate 4-vinyl benzoate (EGMAVB). Po-
lymerization of the complex in a medium, in which it is stable and
both the complex and the polymer are soluble, leads to a solvent
soluble, linear polymer containing a free methacrylate group per
repeat unit since the methacrylate group included in the CD cavity
does not react with the growing radical chain.
The ICs, EGDMA-â-CD, and EGMAVB-â-CD were obtained
by a precipitation method from aqueous solutions of â-CD.
EGDMA-DM-â-CD complex was isolated by a solvent evaporation
method. The FTIR analysis of the EGDMA-â-CD complex showed
the shift in ester carbonyl stretching vibrations from 1720 to 1726
cm-1, which is comparable to that reported by Jeromin and Ritter.7
The O-H stretching band at 3370 cm-1 of â-CD shifted to 3323
cm-1 and was narrowed. This shift was attributed to replacement
Computations were also carried out to ascertain if the monomers
could bind to a second â-CD. Docking studies and the subsequent
binding energies evaluated at the B3LYP/6-31G level of theory
revealed that the complexation of the ligand with the first â-CD
has substantial stabilization of the order of 50-100 kJ/mol, while
the addition of a second â-CD does not provide substantial
stabilization (see Supporting Information). Thus, the observed 1:1
† National Chemical Laboratory.
‡ Indian Institute of Chemical Technology.
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10.1021/ja062568z CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
are almost equally reactive. This is evident from the fact that, in
the spectrum of EGMAVB polymer prepared at low conversion
and in the absence of CD, the peaks at δ 6.69-6.83 (q), 5.92, 5.83
(d), 5.42, and 5.37 (d) are due to the presence of a styrenic double
bond, and the peaks at δ 5.59 and 6.15 are due to the methacrylate
double bond. The integration shows that 58% of the styrenic double
bond reacted, while 42% of the methacrylate double bonds reacted.
The 1H NMR analysis of poly(EGMAVB) prepared by the
polymerization of the EGMAVB-â-CD complex showed that the
peaks at δ 5.59 and 6.15, characteristic of the methacrylate double
bond, and that the peaks corresponding to the styrenic double bond
disappeared. This proves unequivocally that the methacrylate group
included within the CD cavity does not participate in polymeriza-
tion.
Poly(EGDMA) containing pendant unsaturation was cast into
films and cross-linked by both thermal and UV irradiation. FTIR
analysis showed no unsaturation, and Tg of the polymer was
enhanced from 74 to 93 °C. The polymer solution was suspended
in aqueous medium and cross-linked to yield microparticles. Cross-
linking below critical concentration leads to nanoparticles in the
range of 30-40 nm.
Figure 2. Intrinsic viscosity versus molecular weight for linear and
hyperbranched2a poly(EGDMA).
complexation and the preferential polymerization at the styrene end
of EGMAVB are supported by the computations carried out.
The EGDMA-â-CD complex was polymerized in N,N′-dimeth-
ylformamide (DMF) at 65 °C in the presence of azobisisobuty-
ronitrile (AIBN). FTIR analysis showed that the isolated polymer
was free from CD6 and soluble in solvents, such as DMF, dimethyl
sulfoxide (DMSO), chloroform (CHCl3), and tetrahydrofuran (THF).
1H NMR analysis showed that only one of the two unsaturated sites
of EGDMA participated in polymerization.
Intrinsic viscosity versus molecular weight plots for two poly-
(EGDMA)s are shown in Figure 2. The Mark-Houwink-Sakurada
exponent for the hyperbranched poly(EGDMA) is 0.14, while the
value for the polymer synthesized by us is 0.29. The values of Mw
obtained by multi-angel laser light scattering (2.16 × 105) and by
gel permeation chromatography (2.46 × 105) are comparable. In
contrast, the values differ by an order of magnitude for the branched
polymers.2c The second virial coefficient (3.5 × 10-4 mol mL/g2)
for poly(EGDMA) of Mw 2.16 × 105 obtained by us is closer to
that for linear polystyrene (4.4 × 10-4 mol mL/g2) of Mw 2.9 ×
105 than for the hyperbranched poly(EGDMA) (7.5 × 10-6 mol
ml/g2) of Mw 7.68 × 105.2c These results confirm that the polymer
obtained by us is linear.
In summary, we have demonstrated that, in the polymerization
of divinyl monomers, the double bond included in the CD cavity
does not react with the growing radical chain. This results in soluble,
linear polymers containing pendant vinyl unsaturations. These can
subsequently be cross-linked by UV irradiation to yield films,
micro-, and nanoparticles or grafted with hydrophilic monomers.
Acknowledgment. S.S., R.K., and N.M. thank CSIR, New
Delhi, for financial support. The authors wish to thank Dr. M.
Vairamani, IICT, Hyderabad, for MASS spectral analysis.
Supporting Information Available: Inclusion complexation, po-
lymerization, NMR, FTIR, XRD, MASS, molecular modeling DSC,
and GPC data. Complete refs 9 and 10. This material is available free
of charge via the Internet at http://pubs.acs.org.
The formation of a soluble, linear polymer in DMF was attributed
to the fact that DMF stabilizes the IC.11 Synthesized poly(EGDMA)
is soluble in DMF, and presumably CD encapsulates the meth-
acrylate group even after polymerization. To prove this, the
photoinitiator Irgacure was added to the polymer solution, which
was then cast into films and exposed to UV irradiation. The film
did not cross-link, indicating thereby that the methacrylate group
was still included in the CD cavity, which suppressed cyclization
and cross-linking reactions.12 The complex comprising EGDMA
and DM-â-CD is soluble in CHCl3 and so is poly(EGDMA).
Polymerization of the EGDMA-DM-â-CD complex in CHCl3
resulted in a soluble polymer in which only one double bond of
EGDMA reacted during polymerization, and the unreacted double
bonds in the cavity of CD appeared as pendant unsaturation.
Polymerization of the EGDMA-DM-â-CD complex in water
resulted in cross-linked product, as poly(EGDMA) is insoluble in
water.6 During polymerization, DM-â-CD slips off the growing
polymer chain, exposing the unsaturated group to the growing
radical chain.
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