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.

Hydrogels based upon random copolymers of 2-hydroxy-Hydrogels based upon random copolymers of 2-hydroxy-ethyl acrylate and 2-hydroxyethyl methacrylate ethyl acrylate and 2-hydroxyethyl methacrylate

Amy R. LuceAmy R. Luce and W. Tandy Grubbs, Department of Chemistry, and W. Tandy Grubbs, Department of Chemistry,

Stetson University, Unit 8271, DeLand, FL 32720.Stetson University, Unit 8271, DeLand, FL 32720.

Introduction

Experimental Methods

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.

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.

Abstract

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 poly—HEMA and poly-HEA.

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:

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.

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.

The termination step occurs when two radicals combine to form a molecule with no unpaired electrons.

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.

2-hydroxyethyl acrylate 2-hydroxyethyl methacrylate

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.

poly(2-hydroxyethyl acrylate)

poly(2-hydroxyethyl methacrylate)

The three main ingredients that contributed to the synthesis of the polymers were as follows:

-20 mg of 2,2’-azobisisobutyronitrile, or AIBN (initiator)

-15 mL monomer -20 mL dimethyl formamide (DMF)

The DMF is present simply as a solvent.

Table 1: Results of all five reactions, including composition, reaction time, and the water absorption results for each trial (with average).

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:

Reaction Standard Deviation

1 74.9

2 12.3

3 4.2

4 4.2

5 1.7

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%

Table 2: The five reactions and their respective standard deviations

A graphical interpretation of the average percentage of water absorption for each reaction. The standard deviation is represented with an error bar.

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.

This project was supported by the National Science Foundation (DMR-0215407)

Experimental Methods (cont.)

Results

Results (cont.)

Conclusions

References

Acknowledgements

% 100

swelled dry

dry

mass massAbsorption

mass