Synthesis and Aqueous Solution Behavior of Novel pH Responsive

Downloaded by UNIV MASSACHUSETTS AMHERST on September 24, 2012 | http://pubs.acs.org Publication Date: November 28, 2000 | doi: 10.1021/bk-2001-0780.ch006

Chapter 6

Synthesis and Aqueous Solution Behavior of Novel p H Responsive, Zwitterionic Cyclocopolymers *

David B. Thomas, R. Scott Armentrout, Charles L. McCormick The University of Southern Mississippi, Department of Polymer Science, Hattiesburg, MS 39406

The free radical photopolymerizations of N,N-diallyl-N -methylamine with 3-(N,N-diallyl-N-methylammonio) propane sulfonate, and N,N-diallyl-N,N-dimethylammonium chloride with 4-(N,N-diallyl-N-methylammonio) butanoate have been studied. Reactivity ratios indicate random incorporation of the comonomers. Molecular weights range from 4.13 to 8.42 x 10 g mol for the sulfobetaine-containing polymers, and from 8.9 to 21.8 x 10 g mol for the carboxybetaine-containing polymers. Macroscopic phase separation was observed for the sulfobetaine containing polymers with changes in pH, while the carboxybetaine containing polymer solutions remained homogeneous throughout the pH range investigated. 4

-1

4

-1

Introduction Ion-containing water-soluble polymers are the most diverse class of polymers, ranging from biopolymers such as nucleic acids and proteins that mediate life processes to commercial polymers with applications in water

© 2001 American Chemical Society

In Stimuli-Responsive Water Soluble and Amphiphilic Polymers; McCormick, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

101


102 remediation, drag reduction, and formulation of pharmaceutics, cosmetics, and coatings. Charged polymers can be arbitrarily divided into two classes: polyelectrolytes and polyampholytes. The former have ionizable functional groups, either anionic or cationic, along or pendent to the macromolecular backbone; charges are balanced by small gegenions or counterions. Zwitterionic polymers have both cationic and anionic charges along or pendant to the backbone. Copolymers containing cationic and anionic functionality on different mer units are termed polyampholytes while those having both charges on a single mer unit are called polybetaines. Charge-charge repulsions along the polyion backbone and osmotic effects resulting from counterion mobility are responsible for chain extension and the large hydrodynamic volume of polyelectrolytes in water at low ionic strength. The degree of extension depends greatly on copolymer composition, flexibility, and the effective charge density (/, 2, 3). The solution behavior of polyampholytes is also governed by the charge balance on the polymer chain. A sufficient excess of either charge can cause the polymer to exhibit typical polyelectrolyte behavior (4, 5, 6, 7, 8, 9). Polymers with charge balance, on the other hand, typically exhibit antipolyelectrolyte behavior. This behavior is characterized by a collapsed conformation in deionized water and an expansion in the presence of small electrolytes (5, 9, 10). Studies of the pH responsiveness of polymers (4, 6, 8) containing the carboxylate group or protonated amines confirm this result, as well as reveal potentially useful chemistry for the design of responsive polymer systems in aqueous solution. Charge density is also an important factor affecting the solution properties of polyampholytes near their isoelectric points. Higher charge density polymers are largely insoluble in pure water and require addition of a critical concentration of salt for solubilization (5, 8, 9, 11, 12, 13, 14, 15). Low charge density polyampholytes can be solubilized in water provided comonomers are sufficiently hydrophilic (16). A class of polyampholyte that has received special attention over the last several years is the polybetaine. Zwitterionic monomers have the advantage of providing a well-controlled balance of charge within the polymer. For example, homopolymers synthesized from zwitterionic monomers yield polyampholytes with zero net charge. Both sulfobetaines (17, 18, 19, 20, 21, 22, 23) and carboxybetaines (24, 25, 26, 27, 28) have been reported in the literature. The carboxybetaine polymers offer the possibility of pH responsiveness through protonation of the carboxylate group, converting the zwitterion to a cation (27, 29 ). Cyclopolymerization of a number of diallyl quaternary ammonium salts was first reported by Butler in 1949 (30 ). The solubility of the resulting polymers and lack of residual unsaturation led to the hypothesis of alternating


103 intermolecular and intramolecular propagation steps, resulting in a linear polymer with cyclic groups in the backbone (31). Butler et al. originally proposed a six membered ring based on the prevailing view that the most thermodynamically stable product would form. It was later shown by C NMR, however, that the cyclic structure was, in fact, a five membered ring which is believed to be formed by a kinetically controlled mechanism (32, 33). Since the discovery of cyclopolymerization, there has been a large volume of work in the area including synthesis of many new types of monomers (30, 34) Recent work by McCormick et al. in the area of cyclopolymers includes the synthesis of novel hydrophobically modified polyelectrolytes, (35) water soluble polymers containing zwitterionic groups and hydrophobic backbone segments from the cyclopolymerization of WN-diallyl-N-methylamine (DAMA), (36,37, 38) and the investigation of polymerization in vesicles of diallyl monomers with twin hydrophobic tails (39). In this report we describe two series of pH responsive, zwitterionic cyclocopolymers. The first is a copolymer series consisting of sulfobetaine monomers copolymerized with a tertiary amine monomer; the latter provides the pH response in the form of a hydrophilic to hydrophobic transition upon deprotonation. The second series of copolymers consists of the quaternary amine DADMAC copolymerized with a carboxybetaine monomer. In this case, the pH response involves a transition from polyampholyte to polyelectrolyte upon protonation.


13

Experimental

Monomer Synthesis AÂ^-diallyl-iîV-dimethylammonium chloride (1) was purchased from Aldrich Chemical Company and used as received. Ar,iV-diallyl-iV-methylamine (2), (40) 3-(M/V-diallyl-iV-methylammonio) propane sulfonate (3), (41) and 4(iîST-diallyl-iV-methylammonio) butanoate (4) (42) were prepared as previously reported. J


CH I

2


o=s=o

CH I

2

c=o

Οθ 0Θ 1 2 3 4 Scheme 1. Monomers utilized in cyclocopolymerization

Polymer Synthesis Polymers were prepared in 0.5M NaCl aqueous solution at 35°C using the photoinitiator, 2-hydroxy-1 -[4-(hydroxy-ethoxy)phenyl]-2-methyl-1 -propane (Irgacure 2959) (Ciba). Total Monomer concentration was held constant at 2.5 M. The pH of polymerization solutions containing 2 was adjusted to pH 4 so that the monomer was in the ionized form. A low conversion sample,

Synthesis and Aqueous Solution Behavior of Novel pH Responsive

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