Bridging adhesion and barrier properties with functional dispersions : towards waterborne anti-corrosion coatings

Bridging adhesion and barrier properties with functional

dispersions : towards waterborne anti-corrosion coatings

Soer, W. J. (2008). Bridging adhesion and barrier properties with functional dispersions : towards waterborne anti-corrosion coatings. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR632237

DOI:

10.6100/IR632237

Document status and date: Published: 01/01/2008

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Bridging Adhesion and Barrier Properties with Functional

Dispersions

Towards Waterborne Anti-Corrosion Coatings

Bridging Adhesion and Barrier Properties with Functional Dispersions: Towards Waterborne Anti-Corrosion Coatings/ by Willem Jan Soer

Eindhoven: Technische Universiteit Eindhoven, 2007 Proefschrift. – ISBN 978-90-386-1197-6

Printed by PrintPartners Ipskamp, Enschede, 2007 Cover design by Willem Jan Soer

© 2007, Willem Jan Soer

The research described in this thesis was financially supported by innovation-oriented research program (IOP) on surface technology (IOT03001), sponsored by the Dutch Ministry of Economic Affairs.

A catalogue record is available from the Eindhoven University of Technology Library (http://w3.tue.nl/en/services/library).

Bridging Adhesion and Barrier Properties with Functional Dispersions

Towards Waterborne Anti-Corrosion Coatings

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof.dr.ir. C.J. van Duijn, voor een

commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op dinsdag 22 januari 2008 om 16.00 uur

door

Willem Jan Soer

prof.dr. R.A.T.M. van Benthem en

prof.dr. C.E. Koning Copromotor: dr. W. Ming

1.1 Introduction 2

1.2 Anti-corrosion coatings 2

1.3 Requirements for anti-corrosion coatings 3

1.4 Latexes and dispersions 6

1.5 “One stone, multiple birds“ strategy 8

1.6 Objectives and thesis outline 8

1.7 References 10

CHAPTER 2. SYNTHESIS AND MODIFICATION OF MALEIC ANHYDRIDE

CONTAINING COPOLYMERS 15

2.1 Introduction 16

2.2 Experimental 20

2.3 Results and discussion 25

2.3.1 Free radical copolymerization with maleic anhydride and α-olefins 25

2.3.2 RAFT-mediated polymerization 27

2.3.3 Imidization of PSMA copolymers 38

2.4 Conclusions 44

2.5 References 44

CHAPTER 3. SURFACTANT-FREE ARTIFICIAL LATEXES FROM MALEIC

ANHYDRIDE CONTAINING COPOLYMERS 49

3.1 Introduction 50

3.2 Experimental 51

3.3 Results and discussion 52

3.3.1. Used polymers 52

3.3.2. Preparation of artificial latexes 53 3.3.3 Latexes obtained from other polymers 60

3.4 Conclusions 63

4.1 Introduction 66

4.2 Experimental 69

4.3 Results and Discussion 72

4.3.1 Properties of the latexes in the presence of crosslinker 72 4.3.2 pH stability of the latexes in the presence of crosslinker 75 4.3.3 Interaction between crosslinker and latexes 78 4.3.4 Model studies on crosslink chemistry 81

4.3.5 Crosslinking kinetics 86

4.3.6 Fate of the amic acid moieties originating from ammonolysis 91

4.3.7 Film formation 92

4.4 Conclusions 101

4.5 References 102

CHAPTER 5. MECHANICAL AND ANTI-CORROSIVE PROPERTIES OF

LATEX-BASED COATINGS 105

5.1 Introduction 106

5.2 Experimental 110

5.3 Results and discussion 115

5.3.1 Coating properties 116 5.3.2 Adhesion 121 5.3.3 Barrier properties 125 5.3.4 Immersion tests 141 5.4 Conclusions 156 5.5 References 157 CHAPTER 6. EPILOGUE 159 SUMMARY 163 SAMENVATTING 165 CURRICULUM VITAE 169 DANKWOORD 171

Abstract

In the introduction of this thesis the strategy to obtain anti-corrosion coatings from water-borne systems will be explained. A short introduction to coatings will be given, followed by a more detailed look at the properties of typical anti-corrosion coatings, such as barrier properties and adhesion. The approach of using water-borne latexes to obtain homogeneous films with a combination of different properties, referred to as “one stone, multiple birds” approach, will be explained. Finally, an overview of the contents of the thesis will be given.

1.1 Introduction

Coatings play a crucial role in the applicability and the lifetime of a large group of products in modern society. They are used for both esthetic and protective purposes, and can be evaluated by a large variety of properties, such as color, gloss, durability and mechanical properties. Protective coatings prolong the lifetime of a wide variety of products, such as wood and metal objects, by creating a barrier between the actual product and its surroundings. By doing so, the product does not suffer from degradation as it would while being in direct contact with the environment. Wood is painted to prevent rotting, while metals are coated to prevent corrosion (or “rust”) of the metal. Coatings can be roughly divided in four groups; organic, inorganic, conversion and metallic coatings 1. In this thesis, the focus will be on organic coatings.

Almost all commercial organic coatings consist of a large number of different “ingredients”, each of which serves a specific purpose. Polymers form the matrix of the final coating, while other compounds can be added to obtain the desired processing or final product properties. To obtain an appealing color pigments are added, improved scratch resistance can be obtained by adding small silica, clay or titanium dioxide particles, and good chemical properties are obtained by crosslinking the polymers providing a dense network. Furthermore, additives are frequently used to improve film formation, viscosity and formulation stability among other properties 2_{. Every coating is a balanced system consisting of different ingredients, leading to the} specified desired properties.

The main goal of this thesis is to prepare a polymer matrix that can protect metallic substrates against corrosion by combining as many of the desired properties as possible in one polymer system. Furthermore, to obtain an environmentally friendly system, this coating will be applied from a water-borne system. The obtained coating will be studied as a possible candidate for use in anti-corrosion coatings.

1.2 Anti-corrosion coatings

Corrosion is a process of a metal reacting with oxygen or water, leading to the formation of metal oxides 1_{. One of the most commonly observed examples of}

porous iron oxide layer. Due to the porosity of the layer, the corrosion products (such as Fe2+_{-ions) can be freely transported from the substrate to its surroundings. This} leads to a progressive corrosion of the metal object which finally leads to decreased mechanical properties and therewith failure of the object. Other compounds, such as pure aluminum, form a closed oxide layer, which forms a barrier against the transport of water and oxygen, and therewith preventing it from corrosion.

A proper coating applied on the substrates prevents the corrosion reactions to take place, and therewith increases the lifetime of the product. The use of magnesium, which is one third lighter than aluminum, in weight saving applications such as automotive, aerospace and electronic industries is severely limited by the lack of corrosion resistance. Therefore, a large number of different techniques to protect the metal from corrosion has been studied in recent years 3, 4_{. A wide variety of protective} coatings, such as those consisting of ceramics 5, 6, (rare earth7-9)-metals 10-14, phosphates15, ionic liquids16-18, diamond like carbon19, silanes 20, 21 or anodic pretreated magnesium 22 _{have been reported. Most of these techniques suffer from} drawbacks like environmental issues, inhomogeneities, high costs or poor mechanical properties of the final coating 3_.

The use of organic (or polymeric) coatings reduces these drawbacks significantly. Most organic anti-corrosion coatings that are currently used are applied from systems with a high content of volatile organic compounds (VOCs)23. Due to stringent regulations concerning environmental impact and human safety, the use of VOCs in coating formulations is strictly limited by law 24_{. Therefore, since the late 1980’s,} water-borne anti-corrosion coatings were developed. Water-borne coatings that are used for this purpose usually rely on the addition of anti-corrosive compounds, such as zinc-phosphates 25-27 _{or iron oxides}28 _{or anticorrosive surfactants}29_{. The aim of this} work is to develop a coating that protects metallic substrates without the use of any of these compounds. To meet the requirements for anti-corrosion coatings, multiple aspects should be considered.

1.3 Requirements for anti-corrosion coatings

Anti-corrosion properties can be obtained by applying a coating with good barrier properties to the substrate that needs to be protected. Coatings with good barrier

through the coating, and therewith ions, a sufficiently densely crosslinked polymer should be applied 33, 34_{. The polarity of the polymers also plays a role in the barrier} properties of coatings against water transport35_{. With increasing polarity, the barrier} properties towards liquid water generally decrease 34_{. Therefore, an anti-corrosion} coating is preferably hydrophobic after it has been applied. Besides, water transport at the interface layer between polymer and substrate should be prevented as well, since this will enhance corrosion processes that occur underneath the coating (or filiform corrosion). This process starts in most cases at the edges of the coated substrates, as well as at defects in the coating 36, 37_{. This will cause the coating to delaminate from} the substrate, which in turn enhances the corrosion processes 38, 39_.

Apart from excellent barrier properties, also good chemical and mechanical properties should be obtained. The chemical properties should prevent the coating from dissolving in organic solvents. A good chemical resistance can be obtained by forming a highly crosslinked network. The mechanical properties should prevent the coating from failure after impact. The coating should be both hard and flexible, resulting in high toughness. If the coating is very hard, but not flexible, damage can occur easily upon impact. If the coatings are soft and flexible, the coating is also destroyed easily, for instance by scratching.

As a third requirement, a good adhesion of the coating to the substrate is essential to increase the (filiform) corrosion resistance 31, 36, 37. In most cases metallic substrates, such as aluminum or magnesium alloys, contain a lot of oxides and hydroxide groups on the surface (Figure 1.1), due to pretreatments or reactions with oxygen or water from the air. These metal hydroxides or oxides allow for good interaction with functional groups in polymers. Chemical bonds of the coating to the substrate can be obtained by choosing materials that have strong interaction or undergo reaction with the metal oxides that are present on the surface of metals 40-43_{. Carboxylic acids and} amines are known to give strong ionic bonds with aluminum oxide, while other functional groups such as alcohols give weaker interactions by dipole interactions 44. These weak bonds are easily destroyed when water accumulates at the interface 45_{. To} optimize the interactions between coating and substrate, the substrates need to be thoroughly cleaned from oil or lubricants before a coating is applied. Furthermore, an appropriate surface pretreatment can increase the number of metal oxides, and therewith provide stronger bonding between the coating and the substrate 37, 46, 47.

Figure 1.1. Hydrophilic metal hydroxyls on an aluminum oxide surface 41.

To cover all these requirements, multiple layers are frequently used (Figure 1.2) 3, 28, 32_{. This allows the adhesive layer to interact with the substrate surface, while the} protective layer can function as a barrier. Finally a top layer can be applied to increase the hydrophobicity of the surface, or for esthetic purposes. A significant drawback of these types of systems is that several application steps are necessary, involving several application cycles, which are time and energy consuming. Furthermore, the adhesion between the different layers should be provided, to maintain the properties of the system as a whole.

Figure 1.2. Multiple layers of an anti-corrosion system on a metallic substrate. In this thesis, the anti-corrosion properties of a new type of water-borne polymer coatings are examined. The use of multiple layers is omitted by combining the different functionalities into one single layer. This is a relatively low-cost alternative, which should allow for easy application and minimal environmental drawbacks. Furthermore, this coating is not only developed for magnesium alloys, but also for

Substrate

Adhesion layer

Top layer

1.4 Latexes and dispersions

Water-borne coatings can exist either as homogeneous (solutions) or heterogeneous (dispersions (solid in liquid) or emulsions (liquid in liquid)) systems 48-51_{. Because} water soluble polymers are highly water sensitive, the focus of this thesis will be on polymers that are applied from heterogeneous systems. A dispersion (or in the case of polymers a latex) consists of two phases, the dispersed phase and the dispersion medium. The dispersed phase consists of a material that is not soluble in the dispersion medium. One very well known example of an emulsion is milk. The main content of milk, apart from water, is fat, which is distributed throughout the continuous water phase as small spherical droplets, with a size in the range of 50 nm to ~1 μm (Figure 1.3).

Figure 1.3. Transmission electron microscopy (TEM) image of milk 52 _{(a), and a} cryo-TEM image of a latex with insoluble polymer particles in water (b) 53_.

Because fat is not soluble in water, these fat particles are surrounded by a shell of proteins, or emulsifiers, that partially dissolve in the water phase and keep the particles stable in the water phase. This same principle can be used for the production of water-borne coatings. The water-insoluble polymer particles can be dispersed in the water phase by using emulsifiers or surfactants (Figure 1.3b). These surfactants stabilize the particles by either electrostatic or entropic interactions 54, 55_{, (Figure 1.4),} and a so-called latex is obtained 56_{. Emulsifiers are usually amphiphilic, meaning they} have one part that has affinity for the water phase, and the other part has affinity for the dispersed phase 56. This can be a small molecule, such as a soap with an

One frequently used surfactant is sodium dodecyl sulfate (SDS) (Scheme 1.1). This compound has a hydrophobic tail, and a hydrophilic head, which gives a negative charge, leading to electrostatic stabilization. Latexes that are stabilized by electrostatic interactions repel each other when the distance of two approaching particles reduces, due to the similar charges. Apart from anionic surfactants, also cationic surfactants can be used for electrostatic stabilization. Steric (or entropic) stabilization can be obtained by non-ionic surfactants and is based on the thermodynamically unfavorable ordering of the long polymer chains when two particles approach each other, when compared to the free movement the chains have in the dispersed phase56_.

In this thesis, the focus will be on particles that are stabilized by electrostatic interactions, which will be built into the main chain of the polymer. By following this approach, there is no need to use any surfactants.

Na+

S

O O

O O

-Scheme 1.1 Structure of sodium dodecyl sulfate in water, an ionic surfactant that is frequently used in latex stabilization.

Figure 1.4. Schematic representation of the stabilization of polymer particles in water by (a) electrostatic interactions and (b) entropic interactions.

1.5 “One stone, multiple birds“ strategy

As already mentioned, current systems for anti-corrosion purposes often rely on multiple layers. In this work the focus is on a polymer system that combines the different required properties into a single polymer chain. The coating must therefore show good adhesion to the substrate, good barrier properties to prevent ionic species to travel through the coating and therewith allow for corrosion processes, as well as good mechanical properties. Furthermore, from an environmentally friendly point of view the system should be applied from a water-borne system, and therefore hydrophilic groups should be present. These hydrophilic groups should provide the stabilization of the latexes, as well as strong interaction with the metal oxides that are present at the substrate surface to which the coating is applied. After film formation these groups should have reacted either with the substrate surface to give adhesion, or with a crosslinker to obtain hydrophobic moieties. This will increase the barrier properties of the final film. This so called “one stone, multiple birds” strategy should lead to a system that is capable of protecting different metallic surfaces against corrosion.

1.6 Objectives and thesis outline

In this work the preparation of anti-corrosion coatings based on surfactant-free latexes will be described. When these latexes are heated above their respective minimum film formation temperature, homogeneous films will be obtained 58-60_{. After} film formation and the evaporation of water, these coatings should crosslink and therewith display good barrier properties against water or ion transport. Furthermore, in this process a good adhesion to the metallic substrate should be obtained as well (Figure 1.5).

Heat -H2O ↑

Figure 1.5. Schematic representation of the formation of a barrier type coating from a latex upon heating and film formation. Water is evaporated during this process.

The “one stone, multiple birds” strategy can be realized by utilizing the reactivity of maleic anhydride. These groups are known to react with a wide range of different functionalities 61-65, allowing for different modifications on one polymer chain. The maleic anhydride units are relatively easily built into polymer chains, either in the backbone66, 67_{, or by grafting to the polymer backbone} 68-70_{. Once these monomers are} built in, for instance by free radical copolymerization, the maleic anhydride monomer unit is used to connect to other chemical groups to obtain desired properties.

In Chapter 2, the synthesis, as well as the modification, of different polymers containing maleic anhydride will be described. It will be shown how the different functionalities can be obtained for a number of different polymers. Chapter 3 deals with the preparation and analysis of latexes based on the polymers described in Chapter 2. Different commercially available polymers containing maleic anhydride will be compared to self-synthesized polymers. The stabilization as well as the properties of these latexes will be described in detail. In Chapter 4 the interaction of these latexes with two crosslinkers will be studied in terms of reactivity and interaction with the latex before applying the films. The film formation of the latexes and the interaction of the polymers with the crosslinkers will be described in this chapter as well. The chemical and mechanical properties of the cured coatings

films as well as the anti-corrosion properties will be addressed as well. Also the adhesion of the polymer to different substrates and the influence on the anti-corrosive behavior are discussed in this chapter. In Chapter 6 some conclusions will be drawn, and the applicability of the described concept to other substrates will be briefly addressed.

1.7 References

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anhydride containing copolymers

Abstract

In this chapter the synthesis of different maleic anhydride containing copolymers is described. First, the successful synthesis of alternating copolymers of maleic anhydride with α-olefins by free radical polymerization is described. To control the molecular weight of the synthesized polymers, RAFT-mediated polymerization was performed to obtain alternating copolymers from styrene and maleic anhydride (PSMA). Block copolymers were successfully prepared via this technique as well. Two different approaches were used to obtain polymers with one hydrophobic and one hydrophilic block. The first route was by converting the maleic anhydride monomers into hydrophobic maleimides with n-heptane, followed by a RAFT polymerization step to obtain poly(styrene-alt-maleimide) (PSMI) macro-RAFT agent. A second polymerization step with styrene and maleic anhydride resulted in a PSMI-PSMA block copolymer. The second approach consisted of modifying a PSMA macro-RAFT agent, by reacting the anhydrides with n-butanol or 3,3,4,4,5,5,6,6,6-nonafluorohexanol. These esterified polymers were further chain extended with styrene and maleic anhydride to obtain block copolymers.

It was shown that after synthesis of the polymers, the RAFT moieties that are incorporated into the polymer chains can be removed by treatment with tri-n-butyltin hydride (TBTH). To prevent full hydrolysis of PSMA in water during emulsification, it was stabilized by reacting part of the anhydride with n-heptylamine to maleimides. This imidization of the anhydrides affected the Tg of the polymer, depending on the degree of imidization. With both commercially available PSMA and PSMA obtained with RAFT-mediated polymerization, it was shown that the Tg was decreased from about 150 °C for unmodified PSMA to less than 100 °C for a fully imidized PSMA. This decrease in Tg is expected to lower the minimum film formation temperature of the polymers, as will be shown later in this thesis.

2.1 Introduction

Maleic anhydride containing polymers display very interesting properties in the field of coatings, since the reactivity of the anhydride allows for a large number of different reactions. These polymers should therefore be able to fulfill the demands that are required for the “one stone, multiple bird” strategy, as described in the previous chapter.

Hydrophobic groups can be introduced by partial imidization of the anhydride groups with primary amines 1-5 _{(route a in Scheme 2.1), whereas partial ammonolysis} of the anhydride with ammonia leads to an amide and a carboxylic acid group 4 (so called amic acid), which in aqueous medium will introduce ionic charges. Furthermore, crosslinking of anhydride containing polymers can be performed with, for instance, diamines 6, 7 _{as is schematically represented in Scheme 2.1.}

N O O O O O OH O O OH NH R _{hydrophobicity} ionic charges crosslinking O _O O n NH₄+ n n n n (a) n-heptylamine (b) NH₃/H₂O (c) diamine (a) (b) (c)

Scheme 2.1. Schematic representation of the different possible reactions with cyclic anhydride units in the PSMA polymer. This scheme is also applicable for other copolymers containing maleic anhydride.

Maleic anhydride containing polymers are mainly produced by free radical polymerizations. One of the most studied systems containing maleic anhydride is the copolymerization of styrene and maleic anhydride (PSMA) 8, 9_{. Potentially PSMA can} be applied as adhesion promoter for polymers on substrates such as glass and metal surfaces 10-12 _{or for enhancing the compatibility of immiscible polymers} 13-16_{. The use} of these polymers as actual coating however, is still very limited 17.

Copolymerization of maleic anhydride with monomers that have an opposite polarization on the double bond due to the difference in polarity leads to a strongly alternating character of the copolymers. Monomers such as styrene or 1-octene are electron rich at the double bond, while maleic anhydride has an electropositive double bond. The alternating character can be roughly predicted by using the Q-e scheme 18-21_{, as was first described by Alfrey and Price} 22_{. The rate constant for} cross-propagation is given as:

) ( 2 1 12

exp

Q

P

k

₌

where P1 is the reactivity parameter of the propagating radical M and Q1• 2 is the reactivity parameter of the propagating radical •

2

M . The e1 and e2 parameters relate to the polarity or charge density on the double bond of the respective monomers. This Q-e schQ-emQ-e can bQ-e usQ-ed to prQ-edict monomQ-er rQ-eactivity ratios r1 and r2 by formulating expressions for the propagation constants k11, k22 and k21 analogues to equation 2-1 18:

(

)

{

}

(

)

{

}

More generally r1 and r2 represent the ratio of the reactivity of the propagating species with its own monomer to the reactivity of the propagating species with the other monomer 18_{. The values of r}

1 and r2 can be empirically obtained 23, and allow for an accurate prediction of the alternating tendency of the comonomer pairs by multiplying r1 with r2 21:

(

)

{

}

The more the product derives from unity and approaches zero, the stronger the alternating tendency. From this equation it can be seen that cross-propagation is more

anhydride the values are -0.8 and 3.69 respectively 21,23, leading to a value of r1r2 close to zero. This high value of the e parameter for maleic anhydride implies that it does not homopolymerize due to the too large electron deficiency of the propagating species, repelling other maleic anhydride molecules. Monomers with an electron rich double bond, such as styrene or α-olefins, should copolymerize with maleic anhydride in an alternating way.

This alternation leads to polymers consisting of units of one styrene and one maleic anhydride moiety, which can be considered as repeating unit.

One special type of radical polymerization is the controlled or living polymerization. Living polymerization was first described by Szwarc for anionic polymerizations 24, 25_{. Living radical polymerization was first shown by Otsu} 26 _{in the} early 1980’s, while earlier reports 27 _{on emulsion polymerization also describe a} system with a living character. The living polymerization is generally characterized by a relatively narrow molecular weight distribution (MWD), a continuous increase in molecular weight with conversion and the lack of termination reactions 24, 25, 27_{. The} lack of termination reactions allows further growth of the polymer as soon as new monomer is added to the mixture, making it very powerful in the synthesis of block copolymers 7, 10, 28-30_.

The design of block copolymers is useful if a combination of different functional groups present in one single polymer molecule is desired, such as a hydrophobic block that leads to hydrophobic properties after application, and a hydrophilic block that enables good interaction with the predominantly hydrophilic substrate to which the final coating is applied. The advantage of block copolymers over random copolymers can be clearly seen in Figure 2.1, where a polymer that consists of 50 mol% hydrophobic units, and 50 mol% of hydrophilic units, which interact with the metal oxides as explained in Chapter 1. In the case of a random copolymer the different properties of both groups are not fully utilized. Some hydrophobic groups will be forced to be close to the substrate due to strong interactions between the hydrophilic groups and the metal oxides present on the metal surface. This will lead to a decreased overall adhesion due to lack of attractive interactions between the hydrophobic groups and the substrate.

When block copolymers are used, one block will give the layer its adhesion to the substrate, whereas the second block will lead to an optimized hydrophobicity of the

the hydrophobic surface properties of the final coating. Although the main focus of this research is on randomly modified copolymers to meet the different demands as mentioned in Chapter 1, block copolymers may be a useful approach to optimize the use of different functional groups, and therewith improve the final properties of the coatings.

Figure 2.1. Schematic representation of random copolymers (a) vs. block copolymers (b) and the interaction with a hydrophilic substrate.

Reversible addition fragmentation chain transfer (RAFT) polymerization is a special form of living radical polymerization that is based on reversible deactivation 31, 32_{. After its discovery a wide variety of RAFT agents were studied in controlled} polymerizations. RAFT mediated polymerizations have the characteristics of a typical living radical polymerization; a good control of the number average molecular weight of the polymer (Mn) as well as the molecular weight distribution (MWD) 33. The theoretical Mn of the polymer after full conversion can be calculated with equation 2-5 31_:

[ ]

[

]

(a)

(b)

In which the product of the initial monomer concentration and its molecular weight (

[ ]

]

One of the drawbacks of the use of RAFT agents is that they give a strong color to the polymers. Therefore it is necessary to remove the RAFT agent after the polymerization. Postma 34 _{described the removal of RAFT end groups by means of} thermolysis. This technique uses high temperatures (>200 °C) to cleave the C-S bonds, resulting in colorless and odorless polymers. However, its use is limited to those polymers that are stable under these conditions. For alternating PSMA copolymers these temperatures cause the polymer to degrade by means of decarboxylation 35 _{and therefore an alternative for this approach should be used.} Primary (and even secondary) amines are also known to cleave RAFT agents from the polymer chains 36-38, but they are very likely to react with the anhydrides in the polymer as well. Chen et al. 39 describe the removal of end-groups originating from RAFT-agents based on thiocarbonylthio (S=C(Z)-SR) moieties with tri-n-butyltin hydride (TBTH). TBHT has a high affinity for the sulfur atoms, and gives a selective cleavage of the C-S bond to remove the RAFT moiety 40_{. This approach will also be} followed in this work.

The emphasis of this chapter will be on the PSMA based systems. These systems will be studied in most detail and are compared to other (alternating) copolymers containing maleic anhydride, such as α-olefins and polybutadiene.

2.2 Experimental

Materials

All materials were purchased from Aldrich and used as received, unless noted otherwise. Solvents were purchased from Biosolve and used without further purification. Styrene and α-olefins are disposed of inhibitor by aluminum oxide (basic, activated, obtained from Acros prior to use. α,α’-Azobisisobutyronitrile (AIBN) was purchased from Merck and recrystallized from ethanol. Maleic anhydride (Aldrich, 99%, briquettes) was ground and used without any further purification.

n-SMA1000F, poly(styrene-alt-maleic anhydride) (referred to as PSMA-FR) synthesized with standard free radical techniques was kindly provided by Sartomer. 1,2-Cyclohexanedicarboxylic anhydride (CDA) was purchased from Acros, and dried at 120 ºC prior to use. Fluowet EA 400 (3,3,4,4,5,5,6,6,6-nonafluorohexanol) was kindly provided by Clariant GmbH, Germany. Deuterated solvents were obtained from Cambridge Isotope Laboratories, Inc.

RAFT synthesis

Three different RAFT agents were used to produce PSMA, i.e S-1-dodecyl-S’-(α,α’-dimethyl- α’’-acetic acid)trithiocarbonate (1) and S,S’-bis(α, α’-dimethyl- α”-acetic acid)-trithiocarbonate (2) and 2-phenylprop-2-yl dithiobenzoate (3) (Scheme 2.2), were synthesized as described in literature 31-33_{, and purified by liquid} chromatography on a silica column using a mixture of equal volumes of pentane and heptane. S S S C12H25 O HO (1) S S S O HO (2) O OH S S (3)

Scheme 2.2. Chemical structures of the different RAFT compounds that were used for the synthesis of PSMA; S-1-dodecyl-S’-(α,α’-dimethyl- α’’-acetic acid)trithiocarbonate (1) and S,S’-bis(α, α’-dimethyl- α”-acetic acid)-trithiocarbonate (2) and 2-phenylprop-2-yl dithiobenzoate (3).

Monomer synthesis

A hydrophobic maleimide was synthesized by reacting maleic anhydride with n-heptylamine, with p-toluene sulfonic acid as catalyst, as reported in literature41_. Maleic anhydride and catalyst were added to toluene and heated to 60 °C, followed by the dropwise addition of the amine, forming amic acid, which gave a yellow color to the mixture. After complete addition of the amine the temperature of the mixture was increased to reflux and left for 6 h to form the final imide. The imidized end-product was found to be a brownish liquid and was purified by dissolving the intermediate

product in heptane. The end-product was dissolved in pentane, while the half product (the amic acid) did not dissolve and could be filtered off.

1H,-1H-perfluorooctylamine was reacted with maleic anhydride in a similar way, resulting in a white powder after evaporation of the solvent. This product contained mainly the amic acid form of the monomer (as indicated with NMR), and could not be purified by dissolving in pentane due to the poor solubility of the product. Therefore, we tried to purify the product with column chromatography. This, however, appeared to be unsuccessful as well. No sufficient amount of fluorinated maleimide to perform any polymerizations was obtained.

Polymer synthesis and modification

Free radical polymerization with α-olefins and maleic anhydride. Synthesis of poly(octadecene-alt-maleic anhydride) was performed in bulk as described by Verbrugge et al.42. Maleic anhydride and AIBN initiator were dissolved in an excess of 1-octadecene and heated to 120 °C for 3 hours, followed by precipitation in isopropanol and drying overnight under vacuum at 70 °C. Polymerization of 1-octene and maleic anhydride was performed in MEK solution at 80 °C for 72 hours, with AIBN as initiator.

RAFT-mediated synthesis of poly(styrene-alt-maleic anhydride) (PSMA) was performed as described by others31. Equimolar amounts of styrene and maleic anhydride, α,α’-azobisisobutyronitril (AIBN) (0.002 eq.) and RAFT agent (0.04 eq.) were dissolved in methyl ethyl ketone (MEK). The reaction mixture was then heated to 90 °C and kept stirring under an argon atmosphere overnight, followed by precipitation in isopropanol and drying overnight under vacuum at 70 °C.

Block copolymers via RAFT-mediated polymerization.

Block copolymers were obtained via two different routes. For the first route, the maleimides, that were synthesized as described above, were, together with an equimolar amount of styrene, AIBN (0.015 eq.) and RAFT agent (2) (0.07 eq.), dissolved in MEK and heated to 90 °C. After 6 h 1 eq. of styrene and 1 eq. of maleic anhydride were added to the reaction mixture and left overnight, followed by precipitation in isopropanol.

For the second route, PSMA was synthesized as described above, followed by precipitation in isopropanol. After drying the polymer it was redissolved in THF, to

4-dimethylaminopyridine (DMAP) (0.04 eq.) were added to obtain a half-ester of PSMA. This mixture was heated to 60 °C and left for 72 hours. After precipitation in pentane and drying the polymer, it was redissolved, together with equimolar amounts of styrene and maleic anhydride as well as AIBN (0.01 eq. compared to monomer), in MEK and heated to 90 °C. After 24 h the block copolymer was precipitated in isopropanol. For the fluorinated alcohol the same approach was followed, however the excess of alcohol compared to anhydride was decreased to 1.2.

Cleaving of the RAFT moiety

The removal of RAFT moieties from polymer chains was done with 500 mg (2.3 x 10-4 _{mol) PSMA as synthesized in the presence of S-1-dodecyl-S’-(α,α’-dimethyl-} α’’-acetic acid)trithiocarbonate. Tri-n-butyltinhydride (0,270g, 0,9 x 10-3 _{mol), AIBN} (70.4 mg, 0.43 x 10-3 mol), and toluene (10 ml) were added and the mixture was degassed by three freeze-pump-thaw cycles. During heating at 70 °C for 3 h the color changed from yellow to colorless. Precipitation in heptane yielded the polymer without the RAFT moiety. Tri-n-butyltinhydride was removed by soxhlet extraction in pentane.

Imidization of anhydride groups in the polymer

A 40 ml acetone solution containing 3.4 g n-heptylamine was dropwise added to a solution of 20 g PSMA in acetone. This was left to stir for 30 min at room temperature, followed by removal of solvent at 40 ºC with a rotary evaporator. The dry material was then put in a vacuum oven at 150 ºC for 20 h to ring-close the amic acid, yielding 23.0 g of partially imidized polymer.

Analysis

Gel Permeation Chromatography (GPC) was used to determine both molecular weight and molecular weight distribution of the polymers. A Waters GPC with a Waters model 510 pump and a model 410 differential refractometer were used at room temperature with THF, containing 5 wt% of acetic acid, as eluent. Two mixed bed columns (Mixed-C, Polymer Laboratories) which were calibrated with polystyrene standards ranging from 600 to 7×106 g/mol were used to perform the

Gas Chromatographs – Mass Spectroscopy (GC-MS) was performed on a Shimadzu GCMS-QP5000.

Matrix-Assisted-Laser-Desorption-Ionization Time of Flight Mass-Spectrometry (MALDI-ToF-MS) measurements were performed on a Voyager-DE STR (Applied Biosystems). As cationization agent potassium trifluoroacetate (PTFA, Aldrich >99%), and as matrix trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB, Fluka) were used. A 40 mg/ml solution of matrix in THF was mixed with a 5 mg/ml of PTFA and a 1 mg/ml polymer solution in a 5:1:5 ratio. These solutions were hand spotted on the target and dried under atmospheric conditions.

1_{H and} 13_{C-NMR analyses were performed on a Varian Gemini 300 or Varian}

Mercury 400 spectrometer, in deuterated DMSO.

Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) measurements were performed on a Bio-Rad Excalibur FTS3000MX infrared spectrometer (4 scans per spectrum, resolution 4 cm-1_{) with an ATR diamond unit (Golden Gate). The} monitoring of the imidization kinetics was performed by adding solutions of PSMA and n-heptylamine in DMSO on the ATR diamond followed by a heating step. A full spectrum was taken every 5 s. The heights of the carbonyl peaks at 1700 cm-1 (originating from the carbonyl signals of the imide) were taken as a measure for the extent of the reaction. The signal arising from the aromatic ring of the styrenic unit at 700 cm-1 was used as internal standard. The spectra of the different stages in the block copolymerization were taken by applying the polymer in powder form on top of the ATR crystal.

Differential scanning calorimetry (DSC) was carried out on a Perkin Elmer Pyris 1 differential scanning calorimeter. Poly(styrene-alt-maleic anhydride) (PSMA) and poly(octadecene-alt-maleic anhydride) (POMA) samples were heated from 20 to 180 ºC at a rate of 10 ºC/min followed by an isothermal period of 5 min. A cooling cycle to 25 ºC with 10 ºC/min was performed prior to a second heating run to 180 ºC at 10 ºC/min. Polybutadiene with 17 wt% grafted maleic anhydride (PBDMA) samples were cooled from 30 to -110 °C at 50 °C/min, followed by an isothermal period of 10 min. A heating step from -110 to 50 °C was performed at 20 °C/min followed by an isothermal period of 1 min. Second cooling and heating steps at 30 and 20 °C/min, respectively, were performed, with an isothermal period of 10 min at -110 °C. The

2.3 Results and discussion

Anhydride containing polymers were synthesized by either RAFT mediated polymerizations or by free radical polymerizations. First the synthesis of copolymers consisting of α-olefins and maleic will be described. After that, RAFT polymerizations of styrene with maleic anhydride will be discussed. Besides the synthesis of poly(styrene-alt-maleic anhydride) (PSMA) also two different routes to obtain block copolymers with RAFT polymerization will be explored. Finally, the PSMA copolymers, both the PSMA synthesized via RAFT as well as a commercially available grade of PSMA will be partly imidized to increase the hydrophobicity of the polymers.

2.3.1 Free radical copolymerization with maleic anhydride and α-olefins

Alternating co-polymers of α-olefins and maleic anhydride were synthesized 42-47. Although attempts have been made to synthesize these polymers via RAFT-mediated polymerization, no successful synthesis via this route could be obtained. Successful RAFT-mediated polymerizations of these monomers with acrylates and methacrylates have been reported by others, but no alternating copolymers were obtained for these monomers 48_{. Furthermore, the allylic monomers are prone to transfer to monomer} during the radical polymerization 48_{, leading to lack of control of the polymerization.} Synthesis of the poly(α-olefin-alt-maleic anhydride)s was therefore performed both in bulk 42 as well as solution 49 by standard free radical polymerizations with AIBN as initiator.

Table 2.1. Properties of the alternating copolymers containing maleic anhydride and α-olefin

α-olefin Solvent Temperature (°C) Mn* (g/mol) MWD* _T g (°C) conversion 1-octene MEK 70 3200 1.60 105 38% 1-octadecene Bulk 140 9500 2.33 90 42%

For the 1-octadecene and maleic anhydride copolymer (POMA) a conversion of 42% was obtained, while for the poly(octene-alt-maleic anhydride) (POCMA) a conversion of 38% was found. For POMA a melting temperature of the side chains was observed at -10 °C, which is in line with values reported in literature (-15 °C) 44, 45, 50_{. The T}

g of this polymer is ca. 90 °C, while the Tg for the POCMA is, as also reported by others 45, at a higher temperature, i.e. 105 °C.

1600

1800

2000

2200

2400

0

10

20

30

40

50

60

70

80

90

100

110

B

B

A

A

Intensity (%)

m/z

A

A

B

B

_B

B

B

B

B

B

Figure 2.2. MALDI-ToF-MS spectrum of an alternating copolymer of POCMA. The alternating character can be seen by the stepwise increase of 210 g/mol (matching the sum of the individual masses of both monomer residues) between the major peaks (A) consisting of equal amounts of octene and maleic anhydride. The peaks labeled B1 and B2 have respectively one maleic anhydride and one octene in excess.

The expected alternating character of the α-olefin containing polymers (POCMA) was confirmed with MALDI-ToF-MS (Figure 2.2). The molar mass of the repeating unit, in this case one monomer unit originating from maleic anhydride and one from

210

98 112

the MALDI-ToF-MS spectrum shown in Figure 2.2. The secondary distribution consists of an excess of one maleic anhydride residue (B1), as indicated by a mass increase of 98, or one octene residue (B2), as indicated by a mass increase of 112 with respect to a fully alternating and equimolar copolymer.

2.3.2 RAFT-mediated polymerization PSMA homopolymers

To allow for a controlled polymerization, different RAFT agents were studied in the polymerization of styrene with maleic anhydride. To study the molecular architecture of the polymers obtained with RAFT-mediated polymerizations, a low molecular weight polymer (obtained with RAFT agent 2) with a Mn of 1140 g/mol and a PDI of 1.10 (as determined by GPC) was studied with MALDI-ToF-MS. This allowed for the determination of the amount of anhydride groups relative to the styrene groups, as well as the alternating character of the copolymer (Figure 2.3). The alternation of the styrene and maleic anhydride in the polymer is obviously shown by the difference in molecular weight of the main peaks (202 g/mol), which matches the masses of the repeating units of one styrene (104 g/mol) and one maleic anhydride (98 g/mol).

The peak at 1523 g/mol can be attributed to a chain consisting of 6 repeating styrene-maleic anhydride units, together with the end groups that are present due to the attached RAFT-agent residue, and an additional 39 g/mol due to potassium originating from the ionization agent. The other peaks labeled A can be attributed to the other chains consisting of equal amounts of styrene and maleic anhydride units. The masses of the peaks forming the second distribution consist of double peaks, i.e. the lower peaks B1 and B2. One of these is formed by a chain which has one styrene in excess (e.g. 1425 g/mol, B1), i.e. 5 maleic anhydride and 6 styrene units; the other is formed by chains having one extra maleic anhydride built in (e.g. 1419 g/mol, B2). This shows that a strictly alternating copolymer is obtained.

The alternating character of the polymer allows for easy calculation of the number of maleic anhydride units in a polymer chain. In the remainder of this thesis, the PSMA chains will be considered to consist of repeating units of styrene and maleic anhydride, with a cumulative molecular weight of 202 g/mol.

1000 1200 1400 1600 1800 2000 2200 0 20 40 60 80 100 1119.35 1321.46 1523.49 1725.53 B₂ B₂ B₂ B₂ B1 B₁ B₁ B₁ Inten sity (%) Mass (m/z) A A A A A 1300 1350 1400 1450 1500 1550 0 20 40 60 80 100 1321.46 1419.41 1523.49 A B₂ B₁ Intensity (%) Mass (m/z) 1425.52 A

Figure 2.3. MALDI-ToF-MS spectrum of low molecular weight PSMA showing the alternating character of the polymer (a). The peaks labeled A represent peaks of equal amounts of styrene and maleic anhydride, B1 has an excess of 1 repeating unit of maleic anhydride, and B2 an excess of 1 repeating unit of styrene. An enlargement (b) shows the two peaks with one unit of maleic anhydride (B1) or styrene (B2) in excess more clearly. 202 98 104

(a)

(b)

Block copolymers

To obtain polymers with a hydrophobic and a hydrophilic block, RAFT-mediated polymerizations were used. Two approaches were explored in this thesis. In the first approach, the maleic anhydride monomer is modified to obtain the desired hydrophobic groups. With these modified monomers the first copolymer block is formed, followed by the polymerization of the second block, consisting of PSMA. The second approach consists of modification of the anhydrides that are present in the first block, before the second block is built in.

Approach one Monomer synthesis

The first approach consisted of polymerization of n-heptyl maleimide with styrene, followed by the synthesis of a SMA block. The synthesis of this monomer was performed in two steps, viz. the reaction between maleic anhydride and the amine at 60 °C, followed by imidization in refluxing toluene (Figure 2.4). After purification by dissolving the end product in pentane, in which the amic acids were insoluble and the maleimide did dissolve. The n-heptyl maleimide was analyzed by GC-MS and 1H- and 13C-NMR. A single peak was observed with GC-MS, with a mass that corresponds to the theoretical mass (i.e. 195 g/mol). With NMR it was confirmed that the desired product was obtained (Figure 2.5, for n-heptylmaleimide). The ring-opened amic acid was not observed since only one peak is observed for the carbonyl signal in the 13_{C-NMR spectrum, showing that the purification by dissolving the} product in pentane was successful.

O O N O O O H2N + O O NH OH 60°C Toluene reflux

200 150 100 50 0

ppm

10 8 6 4 2 0

ppm

Figure 2.5. (a) 1_{H- and (b)} 13_{C-NMR spectra of n-heptylmaleimide.}

It was also attempted to synthesize maleimides based on 1H,-1H-perfluorooctylamine via the same approach. This, unfortunately, could not be performed with good yields. The majority of the final product did not ring-close to the imide, but remained in the amic acid form. Purification by dissolving the product in pentane appeared not feasible, and also purification with column chromatography failed to yield enough material to be used in polymerizations. This monomer was therefore not considered for the remainder of this work. A second approach was used to incorporate fluorinated groups to the polymer, as described later on in this chapter.

O O N a b c d e f g h i CDCl3 a-g h i O O N a b c d e f g h a g f b-e h Toluene

(a)

(b)

Block copolymerization

The synthesis of the poly(styrene- alt-n-heptylmaleimide)(PSMI) was performed with (2) as RAFT-agent, and AIBN as initiator. The alternating character of the maleimide block was indeed observed with MALDI-ToF-MS. The peaks labeled with “A” show the alternating character of the copolymer, the peaks are 300 g/mol apart, the mass of one styrene and one heptylmaleimide unit (Figure 2.6).

2000 2250 2500 2750 3000 20 40 60 80 100 2047 2151 2242 2347 2451 2542 2647 2751 2842 2947

C

C

B

B

B

B

B

Intensity (%)

Mass (m/z)

A

A

A

A

B

C

Figure 2.6. Enlargement of the MALDI-ToF-MS spectrum of poly(styrene- heptylmaleimide) (PSMI), showing the alternating character of the polymer. The peaks labeled with A represent peaks of equal amounts of styrene and heptylmaleimide, B1 has one repeating unit of styrene in excess, B2 has one heptylmaleimide unit in excess and C has two styrene units in excess.

The peaks labeled with “B1” and “B2” have respectively one styrene and one heptylmaleimide unit in excess. The numbers at the peaks of B2 are one lower than expected, due to rounding off of the numbers given in the figure. The peaks labeled

that the alternating character for styrene and heptyl maleimides is less pronounced than for styrene and maleic anhydride. An excess of three styrene units was not observed. No peaks could be addressed to an excess of two heptyl maleimide units, indicating that this monomer does not homopolymerize.

After synthesis of the first macro-RAFT block, the second block consisting of PSMA was synthesized by adding styrene and maleic anhydride monomer to the reaction mixture (Figure 2.7), resulting in a final conversion of ~80 %. With GPC a single peak with Mn at 7100 g/mol and a polydispersity index (PDI) of 1.13 was obtained, indicating the successful synthesis of a block copolymer (PSMA-b-PSMI) via RAFT-mediated polymerization. MALDI-ToF-MS could not be performed successfully, due to the relatively high molecular weight of the polymer.

C₁₂H₂₅S S S N O OH O O n O _N O + RAFT (2) AIBN C12H25S S S O O O m O OH O O n N Styrene Maleic anhydride

Figure 2.7. The first approach in the block copolymer synthesis consists of the copolymerization of heptyl maleimide and styrene, followed by the synthesis of the second block, consisting of styrene and maleic anhydride.

Approach two

The second approach in the block copolymer synthesis was performed by modification of the first block, followed by the synthesis of the second block.

A first block, consisting of alternating styrene and maleic anhydride, was synthesized with (1) as RAFT moiety as described above. The most straightforward approach of modification would be a reaction between the anhydrides and primary

However, the amine is known to be able to cleave of the RAFT moiety 36-38, which is needed to retain the possibility of block copolymer synthesis. The anhydride of this polymer was therefore ring-opened with 1-butanol to obtain the half-ester, with DMAP as catalyst 43, 51-53 _{(Figure 2.8).}

C12H25S S S O O OH O O n O OH O O n n-butanol OH O DMAP Styrene Maleic anhydride C12H25S S S C12H25S S S O O O m O OH O O n OH O

Figure 2.8. Block copolymerization as followed in approach 2. First a PSMA copolymer is synthesized, followed by esterification of the anhydride to the half-ester. This esterified polymer is then used as the macro-RAFT agent for the synthesis of the new PSMA block

With ATR-FTIR it was shown that all anhydride groups reacted within 72 h at 60 °C (Figure 2.9). A second polymerization step of styrene and maleic anhydride was performed with the esterified polymer as “macro-RAFT” agent. ATR-FTIR analysis further showed that the anhydride signal at 1780 cm-1 _{reappeared, indicating that the} maleic anhydride was built into the polymer chain. The intensity of this peak, relative to the signal arising from the styrene moiety (not shown, 700 cm-1), was found to decrease by a factor of 2 compared to the initial PSMA.

1500 1550 1600 1650 1700 1750 1800 1850 1900 0,00 0,01 0,02 0,03 0,04 0,05 0,06 Ab sorban ce Wavenumber (cm-1₎ PSMA Esterified PSMA Block-copolymer

Figure 2.9. Formation of the block copolymer as followed by ATR-FTIR. For PSMA a clear signal originating from the anhydride is present at 1780 and 1850 cm-1_{. After} esterification the signal is no longer found, but the signal originating from the half-ester arises in the 1670-1750 cm-1 _{region. After a second polymerization step, the} anhydride signal is observed again, due to the formation of the new PSMA block.

The IR spectra do not show a clear difference between a mix of two different polymer chains and a block copolymer. Therefore, GPC was used to check the development of the molecular weight during the reactions.

Table 2.2. Mass of the polymers (in g/mol) in the different stages of block copolymer synthesis, as measured by GPC with respect to polystyrene standards.

Starting polymer Esterified polymer Block copolymer

Mw (g/mol) 3300 3900 7800

Mn (g/mol) 2700 3350 4800

MWD 1.23 1.17 1.62

The GPC measurements showed an increase in Mn from 2700 g/mol for the starting polymer to 3350 g/mol for the esterified polymer (Table 2.2). The chemical composition and the chain stiffness of the esterified polymer and the polymer without esterification differ, and therefore the values of the GPC measurement can only be

FTIR it can be concluded that most of the initial anhydrides have been ring-opened to the half-ester by this procedure.

The second step, the use of this esterified polymer as macro-RAFT agent to form a block copolymer, indeed yields a single peak at higher Mn (4800 g/mol). Although the MWD increased slightly, the formation of a single peak, entirely shifted to higher molecular weight, indicates that a block copolymer has indeed been formed.

The same approach of esterification was followed to introduce a more hydrophobic fluorinated alcohol on the polymer backbone. The fluorinated alcohol (3,3,4,4,5,5,6,6,6-nonafluorohexanol) was found to be built in less efficiently compared to the butanol, based on mass increase of the total amount of polymer that was obtained after precipitation (4 g before esterification, 7.5 g after). The molecular weight of the alcohol is 264 g/mol, which should result in a mass increase of the polymer (repeating unit of styrene-maleic anhydride = 202 g/mol) with a factor 466/202 = 2.3. The mass increase of 7.5/4 = 1.9, indicates that ~80% of the anhydrides had reacted with the fluorinated alcohol. This may be due to the small excess of alcohol that was added to the polymer during the first step (i.e. 1.2 eq.) compared to the 1-butanol that was added in large excess (i.e. 6 eq.). After the preparation of the first block, a second PSMA block was synthesized as described above. The fluorine signal of the fluorinated side-groups present in the final block copolymer was clearly found by 19F-NMR, indicating that the fluorine was successfully built in into the polymer main chain (Figure 2.10). No MALDI-ToF-MS analysis could be performed on this polymer due to the high amount of fluorine containing groups. Longer fluorinated alcohols are expected to give less efficient incorporation54_{, since the insolubility problems increase with increasing length of the} perfluoroalkyl chain, and are therefore not considered in this work.

-80 -90 -100 -110 -120 -130 -140

ppm

Figure 2.10. 19_{F-NMR of the block copolymer, indicating the presence of the} fluorinated side-groups in the block copolymer.

RAFT-agent removal

To be able to use the synthesized polymer for coating applications, the RAFT-agent moiety from the chain ends needs to be cleaved off, since these groups give a strong coloration to the polymers. For the ease of analysis a monofunctional RAFT agent (1) was used, since the removal of a difunctional moiety implies breaking of the whole polymer chain into two shorter chains.

As can be seen in the MALDI-ToF mass spectrum (Figure 2.11a), the peak of a polymer with eight repeating styrene and maleic anhydride units, is located at 2319.9 g/mol. After reaction with tri-n-butyltinhydride (TBTH) 39 the C-S bond of the RAFT moiety and the polymer chain was broken (Scheme 2.3). This led to a decrease of 276 g/mol, in this case to 2043.5 g/mol, as can be seen in Figure 2.11b. This mass difference corresponds to the mass of the RAFT agent moiety that is attached via the C-S bond. For the other peaks, a decrease in the molecular weight of 276 g/mol is observed as well. Also, more importantly, no significant peaks of the original polymer can be observed. This proves that the cleavage of the RAFT moiety was complete. The removal of the endgroups indeed led to the discoloration of the polymer sample in solution after soxhlet extraction in pentane.

R R O O OH O CF2 F2C CF2 F3C a b c d a d c b

1400 1600 1800 2000 2200 2400 2600 0 20 40 60 80 100 1915.73 2117.80 2215.81 2319.89 2522.94 B₂ B₁ A A A A

Intensity (%)

Mass (m/z)

A

A

A

Intensity (%)

Mass (m/z)

A

A

Figure 2.11. MALDI-ToF-MS spectra before (a) and after (b) reaction with tri-n-butyltin hydride. The peaks labeled with A represent peaks of equal amounts of styrene and maleic anhydride, B1 has an excess of 1 repeating unit of maleic anhydride, and B2 an excess of 1 repeating unit of styrene.

(a)