Polymerization,%20or%20the%20formation%20of%20a%20growing%20chainlike%20polymer%20from%20small%20molecules,%20or%20monomers,%20is%20of%20utmost%20importance%20to%20physical%20science.%20In%20regards%20to%20this%20particular%20project,%20polymers%20were%20synthesized%20by%20free-radical%20polymerization.

1
Hydrogels based upon random copolymers of
2-hydroxy- ethyl acrylate and 2-hydroxyethyl
methacrylate
Amy R. Luce and W. Tandy Grubbs, Department of
Chemistry, Stetson University, Unit 8271,
DeLand, FL 32720.
Experimental Methods
Abstract
Experimental Methods (cont.)
Results (cont.)
While Table 1 outlines the average percent
absorbencies of the five reactions, the standard
deviations are also important in order to assess
the accuracy of the results. The standard
deviations for the reactions are
All three ingredients were combined in a
small bottom flask with a stir bar. The flask
was sealed with a rubber septum and purged for 10
minutes with N2 gas in order to rid the mixture
of any O2. If O2 gas was left in the flask, it
will consume radicals, thus interrupting the
polymerization reaction. After purging, the
flask was submerged into a hot water bath, and
heated until about 70 oC with constant stirring.
The time was recorded as the solution began to
thicken, signifying significant polymer
formation. The purification of the polymer
began once the solution was cooled. The reaction
mixture was transferred to a beaker of
tetrahydrofuran (THF). THF was used to dissolve
any leftover DMF, as well as any unreacted
monomer. The polymer was stirred in THF for
three washes, or until the THF wash displayed a
clear appearance. The polymer was left to dry in
the hood overnight, so that the THF could
evaporate. The polymer was placed in a vacuum
oven at about 50 oC for at least twenty-four
hours. After sufficient drying, the polymer
was removed from the oven, and three small
spatula sized samples were obtained. The weights
of each sample was recorded, submerged in a
beaker containing 2/3 water, and covered with
aluminum foil for at least twenty-four hours.
The swelled hydrogels were ultimately removed,
dabbed dry with a Kimwipe, and the final
masses were recorded. The percent of water
absorption was calculated using the equation
Five polymer reactions were carried out
using two different monomers. For three of these
reactions, a co-polymer was made of the monomers
2-hydroxyethyl acrylate(HEA) and 2-hydroxyethyl
methacrylate(HEMA). These monomers possess a
hydroxyl group located on the end of the
structure, opposite of the double bond.
Poly(2-hydroxyethyl acrylate) and
poly(2-hydroxyethyl methacrylate) are hydrogel
materials. The former is capable of absorbing
over 400 percent its weight in water, whereas the
latter absorbs only 40 percent. Random
copolymers of 2-hydroxyethyl acrylate and
2-hydroxyethyl methacrylate have been synthesized
by radical polymerization with a goal of
controlling the degree of water absorption by
regulating the fraction of each monomer in the
copolymer. Details of the synthetic route and
the characterization of the product copolymers
are presented.
Reaction Standard Deviation
1 74.9
2 12.3
3 4.2
4 4.2
5 1.7
Introduction
Table 2 The five reactions and their respective
standard deviations
2-hydroxyethyl acrylate
2-hydroxyethyl methacrylate

Polymerization, or the formation of a
growing chainlike polymer from small molecules,
or monomers, is of utmost importance to physical
science. In regards to this particular project,
polymers were synthesized by free-radical
polymerization. Free-radical polymerization
involves three distinct reactions initiation,
propagation, and termination. The initiation
step consists of two individual radicals formed
from a broken bond these radicals react with the
double bond of the monomer to form a new radical.
Due to the fact that these hydroxyl groups
are water-solubilizing, polymer made from these
individual monomers (shown below) will be very
hydrophilic. Meanwhile, the backbone of the
polymer chain is insoluble in water. The
combination of hydrophilic and hydrophobic
regions on the same chain results in a polymer
that tends to adsorb water, yet the bulk sample
remains insoluble. Such polymers are called
hydrogels.
Results
A graphical interpretation of the average
percentage of water absorption for each reaction.
The standard deviation is represented with an
error bar.
Reaction Composition Reaction Time Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Average Abs.
1 100 Acrylate 14 min 478.9 473.7 324.4 393.2 329.2 399.9
2 100 Methacrylate 49 min 42.9 48.3 24.8 ------- ------- 38.7
3 50, 50 68 min 100.0 108.1 102.4 ------- ------- 103.3
4 75 Acrylate, 25 Methacrylate 25 min 279.5 281.7 287.7 ------- ------- 283.0
5 75 Methacrylate, 25 Acrylate 41 min 59.7 60.2 57.1 ------- ------- 59.0
poly(2-hydroxyethyl acrylate)
poly(2-hydroxyethyl methacrylate)
Conclusions
The continuation of this growing chain is
marked by the propagation step of the
free-radical mechanism, where one radical reacts
with another double bond to form yet another
radical.
Hydrogels that are capable of absorbing
large amounts of water are often known as
superwater adsorbents. In this experiment, the
pure 2-hydroxyethyl acrylate was
expected to absorb more water than the
2-hydroxyethyl methacrylate, as the latter
hydrogels are restricted due to its limited water
intake, the hard and brittle nature in its dry
state, and its poor mechanical strength.
Therefore, by copolymerizing the two monomers,
the water-absorption properties of the resultant
hydrogel can be controlled between the two
extremes of polyHEMA and poly-HEA.
Upon completion of this experiment, it was
obvious that the 100 acrylate was the best
absorbent, while the 100 methacrylate retained
the least amount of water. The various
combinations of monomers in between these varied
as expected as indicated in Figure 1. Therefore,
it can be concluded with confidence that the
amount of water absorbed based on the composition
of the two monomers used can be controlled.
References
The termination step occurs when two
radicals combine to form a molecule with no
unpaired electrons.
The three main ingredients that contributed
to the synthesis of the polymers were as
follows -20 mg of 2,2-azobisisobutyronitri
le, or AIBN (initiator)
Sperling, L.H. Introduction to Physical Polymer
Science. John Wiley Sons, Inc. 2001, p.9,
407. Bhattacharya, A. Prog. Polym. Science.
2000, 25, p.376. Bajpai, A.K. and Mudita
Shrivastava. Journal of Biomaterials
Science-Polymer Edition. 2002, Vol. 13, Issue 3,
p.238. Bae, S. Chakrabarty, K. Seery, T.
Weiss, R. Journal of Macromolecular Science
Pure Applied Chemistry. 1999, Vol. 36, Issue
7/8, p.932.
When more than one monomer is involved in a
polymerization reaction, each new radical reacts
randomly, displaying no preference for which
monomer it will react with. Therefore, the final
product chain can be composed of a random mixture
of monomer types.
-15 mL monomer -20 mL dimethyl
formamide (DMF) The DMF is present simply
as a solvent.
Acknowledgements
Table 1 Results of all five reactions, including
composition, reaction time, and the water
absorption results for each trial (with average).
This project was supported by the National
Science Foundation (DMR-0215407)