A Novel Surface Functionalization Method for Producing Carboxyl

Chapter 20

A Novel Surface Functionalization Method for Producing Carboxyl-Functional Poly(methyl styrene) Latexes

Downloaded by FUDAN UNIV on March 3, 2017 | http://pubs.acs.org Publication Date: November 6, 2001 | doi: 10.1021/bk-2002-0801.ch020

Pei Li Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, Peoples Republic of China

A facile synthesis of functionalized poly(methyl styrene) latexes containing aldehyde and carboxylic acid groups using anionic and cationic surfactants or an emulsifier-free system is described. The synthesis involves an emulsion polymerization of methyl styrene, followed by a copper (II)-catalyzed aqueous oxidation with f-butyl hydroperoxide under air. Investigation of the effect of metal catalyst, oxidant, surfactant and reaction temperature in these systems are discussed. Properties of the resulting functional latexes including their chemical structure, concentration of functional groups in the surface layer, particle size, size distribution, morphology, and stability are also studied.

Polymer latexes containing reactive surface functionalities have received much interest in the last two decades because of their wide range of applications. For example, they have been extensively used as binders in paints, adhesives, coatings, etc., and as carriers for covalently-immobilized catalyst, antibodies, enzymes, drugs, and other biomolecules (1,2). The reactive functionalities include hydroxyl, aldehyde, carboxylic acid, amino, epoxy, sulfhydryl, succinimide, benzotriazole and chloromethyl groups (3). The selection of these

© 2002 American Chemical Society

Daniels et al.; Polymer Colloids ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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294 functional latexes depends on the intended application. For example, aldehyde groups are especially useful for the direct immobilization of amino-containing biomolecules because they can react under mild conditions with primary amino groups, forming the corresponding imine (Schiff base) linkages without prior activation. The imine group can be further reduced to a stable - C H - N H - linkage with sodium cyanoborohydride. Latexes containing carboxylic acid surface groups are particularly important in water-borne coatings because they are capable of surface crosslinking via hydrogen and covalent bondings. They can also undergo ionic crosslinking with metal cations such as C a , Z n . Although various approaches have been reported on the synthesis of functional latexes containing aldehyde or carboxylic acid surface groups, homo- and copolymerization of acrolein or different methacrylate or acrylate acids as the functional monomers have been the dominant approaches. Unfortunately, these reactions have some major drawbacks. For example, significant differences in solubility and reactivity between the functional and matrix monomers in hetergeneous polymerization result in low surface incorporation of functional groups. In addition, a large amount of water-soluble polymers is formed during the polymerization. Thus, an extensive cleaning process is required to purify the latexes. To bypass the problems associated with the use of functional monomers, we have developed a novel appoach via a direct surface oxidation of poly(methyl styrene) (PMS) latexes to the corresponding aldehyde or carboxylic acid groups (4-8). In this article, the synthesis and characterization of the functionalized poly(methyl styrene) latexes, containing aldehyde and carboxylic acid groups in the presence of anionic and cationic surfactants such as sodium dodecyl sulfonate and cetyltrimethylammonium bromide (CTAB) as well as in an emulsifier-free system will be described. The functionalization process involves an emulsion polymerization of methyl styrene forming highly monodispersed poly(methyl styrene) latexes, followed by an in situ surface oxidation under air with copper (II) chloride and teri-butyl hydroperoxide catalysts (Scheme 1). 2


2+

2+

Results and Discussion Preparation of Poly(methyI styrene) Latexes Highly monodispered poly(methyl styrene) latexes were first prepared by the emulsion polymerization of 3(4)-methyl styrene in the presence of either anionic or cationic surfactant, or even in the absence of surfactant. Figure 1 shows an S E M photograph of nearly monodisperse PMS latex (mean diameter = 683 nm) prepared by an emulsifier-free emulsion polymerization using 2,2azobis(2-amidinopropane) dihydrochloride (V-50) as the initiator. The particle size could be easily altered by varying the concentration of the surfactant, the


295


monomer, the initiator, the ionic strength, and the temperature. Thus, monodisperse PMS latexes of a wide size range could be readily prepared.

Figure 1. SEM micrograph of PMS latex prepared by an emulsifier-free emulsion polymerization using V-50 as the initiator.

Catalytic Oxidation of the Poly(methylstyrene) Latexes The stable poly(methyl styrene) latexes prepared in various systems were used directly in the subsequent catalytic oxidation reactions. Scheme 2 shows the reaction mechanism of the catalytic oxidation. The f-BuOOH is first decomposed with the metal catalyst to both f-BuO' and ί-BuOO* radicals. These reactive radical species can readily abstract benzylic hydrogens to generate benzylic radicals, which can be oxidized to the corresponding aldehyde groups under air. Since the aldehyde groups are easily further oxidized to the carboxylic acid groups under the oxidative conditions. Thus, a mixture of aldehyde and carboxylic acid functionalities are generated concurrently. On the other hand,


296


distribution of the two functional groups can be readily controlled by various reaction parameters including the surfactant, the concentration of metal catalyst and oxidant, the reaction temperature and time.

Scheme 2. Catalytic oxidation of poly (methyl styrene) latexes to the corresponding aldehyde and carboxylic acid groups.

Effect of Metal Catalyst A variety of metal catalysts including cobalt(II) acetate, cobalt(II) chloride, manganese(II) chloride, copper(II) chloride and iron(II) chloride were investigated in both anionic and cationic systems. The copper(II) chloride was found to have the highest reactivity among these catalysts. Although C o / C o redox system is known to be one of the most effective autoxidation catalysts for the decomposition of alkyl hydroperoxide because of its comparable stability of the two valence states, it was found to be less effective than C u ions in our systems. This may be due to the deactivation of cobalt ions as a result of the presence of coordinating ligands such as carboxylic acid groups generated during the reaction. In comparison, the transformation between copper(II) and copper(I) can not only occur by the redox reaction as shown in Scheme 2, but also by other mechanistic pathways that are insensitive to redox potential. The n

2 +


ra

297 alternative pathways leading to the regeneration of copper (I) include electron transfer oxidation of alkyl radicals and ligand transfer. The effect of catalyst concentration on the degree of oxidation was also examined. In the anionic system, the concentration of metal catalyst significantly affected the rate of oxidation and the stability of the emulsion. Decreasing the concentration of catalyst resulted in a reduction in the oxidation rate. No oxidation was detected when the concentration of Co(OAc) was lower than 3 m M . However, when the concentration exceeded 0.016 M , the emulsion became unstable. Such an effect may be attributed to the decrease of the effective catalyst concentration as a result of complexation between the metal ion and anionic surfactant. On the contrary, in the cationic systems, the concentration of metal catalyst was found to have little effect on the overall rate of oxidation. This can be explained by the fact that the metal cations in the cationic system are unable to form a complex with the cationic surfactant or latex surface, thus their catalytic activity can be retained. For example, oxidation of cationic P M S latexes in C T A B could achieve 18 to 24% after 4 hours at 60 °C with C u C l concentrations ranging from 0.004 M to 0.02 M . Similar results were also obtained for the emulsifier-free cationic P M S latexes prepared with V-50. Although the concentration of the metal catalyst had very little effect on the rate of oxidation in the cationic systems, it was found to alter the aldehyde and carboxylic acid concentrations. Higher C u C l concentrations facilitated the conversion of aldehyde to the corresponding carboxylic acid groups.


2

2

2

Effect of Oxidant The oxidant played a key role in the metal-catalyzed oxidation reaction. No oxidation was observed when oxygen was bubbled through the anionic or cationic P M S latex in the presence of metal catalyst. When hydrogen peroxide was employed as the oxidant, the reaction proceeded violently and was difficult to control. In contrast, little oxidation was observed with di-i-butyl peroxide. In comparison to all these oxidants, f-BuOOH appeared to be an excellent one for all the systems. The degree of oxidation in both anionic and cationic systems could be controlled readily by varying the amount of ί-BuOOH. A n example of this effect is demonstrated in Figure 2. The degree of oxidation was found to increase linearly as the concentration of f-BuOOH increased. However, when the concentration of ί-BuOOH exceeded 0.5 M, the latex became unstable.


298

Ο

0.2

0.4

0.6

0.8


Oxidant concentration (M) Figure 2. Effect of oxidant concentration on the degree of oxidation for cationic PMS latex in the emulsifier-free system. (Reproduced from reference 8. Copyright 2000 American Chemical Society.)

Effect of Surfactant Three kinds of anionic surfactant were examined including sodium dodecylbenzene sulfonate, sodium dodecyl sulfonate, and sodium dodecyl sulfate. They were all good for the emulsion polymerization of 3(4)-methyl styrene, giving stable latexes. However, the use of sodium dodecylbenzene sulfonate gave an unstable emulsion during the oxidation, while sodium dodecyl sulfate underwent hydrolysis to give the alcohol. In comparison, use of sodium dodecyl sulfonate (SDS) gave a stable emulsion, even under oxidation conditions. When the anionic P M S latexes containing SDS surfactant were subquently oxidized with f-BuOOH and cobalt catalyst, little reaction took place after 24 hours under air. The lack of catalytic reactivity of the metal ions in SDS might be caused by the metal ions complexing to the sulfonate ions of the surfactant. Thus, attempts to reduce such complexation were made by adding a small amount of ί-BuOH. It was found that the rate of oxidation was increased dramatically. The profound effect of the f-BuOH might be attributed to its ability to solvate the metal ions through weak coordination, thus, preventing the interaction between metal ion and sodium dodecyl sulfonate, while still maintaining the reactivity of the metal ion. The rate of oxidation and the stability of the latex were also found to be affected by the quantity of t-BuOH added. When the concentration of ί-BuOH was higher than 25% by volume, the latexes became unstable. However, little oxidation (

A Novel Surface Functionalization Method for Producing Carboxyl

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