Block Copolymers Via Reverse Addition-Fragmentation Chain Transfer Polymerization As a Viable Resin for Packaging Coatings

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Block Copolymers via Reverse Addition-Fragmentation Chain Transfer Polymerization as a Viable Resin for Packaging Coatings

A thesis submitted to the

Graduate School of the

University of Cincinnati, in partial fulfillment

of the requirements for the degree of

Master of Science

in the Department of Chemistry

of the College or Arts and Sciences

Claudia Lascu

University of Cincinnati

February 2015

Committee Chair: Neil Ayres, Ph.D., James Mack, PhD. and Hairong Guan, PhD.

ABSTRACT

Synthesis of diblock copolymers of acrylic acid/ ethyl acrylate and of acrylic acid/ methyl methacrylate were performed using reversible addition-fragmentation chain- transfer (RAFT) polymerization with 1-dodecyl-(dimethyl acetic acid) trithiocarbonate

(DDMAT) chain transfer agent (CTA). Synthesis of poly(acrylic acid) macro-RAFT agent resulted in a lower degree of polymerization (DP) of 22 compared to the theoretical DP of 100 at 95% conversion. Poly(acrylic acid-b-ethyl acrylate) block copolymer showed poor conversion as well therefore the synthesis of diblock copolymers from acrylic acid was aborted. The main cause for lack of monomer conversion was cited to be from impurities.

Synthesis of poly(tert-butyl acrylate) macro-RAFT agent achieved higher degree of polymerization, and provided excellent molecular weight control and dispersity (Ð) for the polymerization of ethyl acrylate and methyl methacrylate. The tert-butyl groups were removed using dilute trifluoroacetic acid affording hydrophilic acrylic acid groups. Both poly(acrylic acid-b-ethyl acrylate) and poly(acrylic acid-b-methyl methacrylate) copolymers showed poor cure response and poor adhesion over aluminum substrate in

1% Joy detergent and 3% acetic acid solutions when compared to an bisphenol A type epoxy resin and bisphenol A free resin control. The adhesion to aluminum substrate was improved of a bisphenol A free polymer when poly(acrylic acid-b-ethyl acrylate) was incorporated rather than poly(acrylic acid-b-methyl methacrylate). Tert-butyl acrylate groups of poly(acrylic acid-b-methyl methacrylate) diblock copolymer were

ii found present via FTIR confirming that not all acrylic acid groups were available to aid adhesion to aluminum substrate. Additionally, poly(acrylic acid-b-ethyl acrylate) improved flavor scalping of a bisphenol A free polymer by 35% and poly(acrylic acid-b- methyl methacrylate) by 2%. This is due to the increase of polarity of the coating when incorporating poly(acrylic acid-b-ethyl acrylate) hence showing less affinity for non-polar components present in beverages.

iii

TABLE of CONTENTS

LIST OF TABLES or FIGURES……………………………………………………………..vi INTRODUCTION……………………………………………………………………………1-8 EXPERIMENTAL………………………………………………………………………….9-16 RESULTS AND DISCUSSION…………………………………………………………17-23 CONCLUSIONS………………………………………………………………………….24-25 REFERENCES…………………………………………………………………………...26-27

LIST of TABLES and/or FIGURES

Figure 1: Structures of different classes of reagents currently used as RAFT agents………………………………………………………………………………………………………………4 Figure 2: General RAFT polymerization mechanism………………………………………6 Figure 3: Schematic representation of various block copolymer architectures……...... 7 Figure 4. IR Spectra of poly(tert-butyl acrylate-b-ethyl acrylate) and poly(acrylic acid-b- ethyl acrylate)……………………………………………………………………………….....20

Figure 5. IR Spectra of poly(tert-butyl acrylate-b-methyl methacrylate ) and poly(acrylic acid-b-methyl methacrylate)…………………………………………………………….…...20

Figure 6. Flavor Scalping………………………………………………………………..…..22

Table 1: Experimental conditions for acrylic acid polymerization at various [DDMAT] / [Vazo 67] ratios…………………………………………………………………………………………...13 Table 2. Molecular Weight, polydispersity and % conversion of poly(acrylic acid)…………………………………………………………………………………………....17

Table 3. Mn measured versus theoretical per % conversion of poly(acrylic acid)……………………………………………………………………………………..……..18

Table 4. Size Exclusion Chromatography Data of the Polymers……………………….19

Table 5. - Resin dry film testing……………………………………………………………..21

Table 6. - Block polymers as additives………………………………………………….....22

INTRODUCTION

Bisphenol A (BPA) is the key building block of epoxy resins used to make plastics including materials that come into contact with food such as refillable drink bottles and food storage containers. Bisphenol A containing epoxies are also used to make protective coatings and linings for food and drinks cans. Metal food and beverage cans have a thin layer of coating on the interior surface, which is essential to prevent corrosion of the can and contamination of food and beverages with dissolved metals.1 In addition, the coating helps prevent canned foods from becoming tainted or spoiled by bacterial contamination. Bisphenol A containing epoxy resins have achieved wide acceptance for use as protective coatings because of their exceptional combination of toughness, adhesion, formability, chemical resistance and have been used safely for over 40 years. In addition to protecting contents from spoilage, these coatings make it possible for food products to maintain their quality and taste, while extending shelf life.

In 1995, it was reported that BPA could migrate from can coatings into food during the food canning process.2 Later that same year, the Society of the Plastics Industry, Inc.

(SPI), initiated a study to quantify the migration of bisphenol A from can coatings.3 In the first phase of this study, high performance liquid chromatography (HPLC) and gas chromatography with mass spectroscopy (GC/MS) were used to detect bisphenol A.

During the analysis of both HPLC and GC/MS spectra’s it was concluded that not all of the material detected was really BPA. Regardless of the inconclusive results of the study, bisphenol A initiated a concern among scientists.

The Food and Drug Administration (FDA) approved bisphenol A under FDA’s food additive regulations in the 1960s. Substances that may migrate from material packaging into food (e.g. BPA) are subject to premarket approval by the FDA. The FDA classifies these substances as indirect food additives or food contact substances.4 The

Food and Drug Administration released a document in 2008 titled “Draft Assessment of

Bisphenol A for Use in Food Contact Applications”.4 In July 2012, the FDA decided to no longer allow the use of BPA-based polycarbonate resins in baby bottles, sippy cups and the use of BPA-based epoxy resins as coatings in packaging for infant formula. Food and beverage companies have begun to follow suit and are eager to move away from packaging based on BPA. Similarly, coating manufacturers and their suppliers are working to find replacements for epoxides. Synthetic candidates to replace BPA- containing epoxies in can coatings fall largely into two main chemical categories: acrylics and polyester resins. Neither of these alternatives have the performance such as chemical and corrosion resistance. For example, acrylics used in food-contact applications are brittle, and the commonly used acrylic monomer ethyl acrylate has a noticeable odor even at low quantities. Polyesters can fail when attacked by acidic foods due to hydrolytic attack of the ester bond and has poor corrosion resistance.

However, polyester resins are flexible and does not impart taste or odor to foods and beverage but does have ‘scalping’ properties, and absorbs flavors from a small number of foods and beverages.4, 5

The coating industry has decided to focus their research in developing an acrylic based coating that is equal in performance as a bisphenol A containing epoxy, despite the acrylic resin’s shortcomings. Currently the most widely used process for 2 synthesizing acrylic resins is via free radical polymerization.6 The main factors responsible for radical polymerization’s popularity are as followed; it can be used with a large variety of monomers, tolerant to a wide range of functional groups and reaction conditions and is simple to implement and inexpensive in relation to competitive technologies. Even though free radical polymerization is widely used, some notable limitations are present. Initiator must be continuously added throughout the polymerization due to the high rate of termination. Quantitative effects of repeated initiator addition are difficult to determine, making chain growth almost uncontrollable.

As a result, polymers created by free radical polymerization exhibit relatively broad polydispersity indexes (Ð =Mw/Mn > 2, where Mw is the weight average molecular weight and Mn is the number average molecular weight). Furthermore, the synthesis of block copolymers is not possible with free radical polymerization. Traditionally, block copolymers were made using living anionic polymerization.7 However in the last few decades, several new techniques were developed which afforded many of the benefits of living polymerization using free radical chemistry. There are three main techniques for controlled/“living” free radical polymerization. These are nitroxide-mediated polymerization (NMP) 8, atom transfer radical polymerization (ATRP) 9, 10, and reversible addition-fragmentation chain transfer (RAFT) polymerization.11 RAFT polymerization has proven to be the most versatile. It is compatible with a broad range of monomers12 and can be carried out under “normal” free radical polymerization conditions.13 RAFT polymerizations are carried out in the same conditions as free radical polymerization, except for the addition of a chain transfer agent (CTA), which is also referred to as the

RAFT agent. All RAFT agents are thiocarbonylthio compounds, for example, 3 dithioesters, dithiocarbamates, trithiocarbonates, and xanthates (Figure 1). The effectiveness of the RAFT agent depends on the monomer being polymerized and the properties of the Z and R groups. The Z group should activate the C=S double bond towards radical addition, whereas the R group should be a good free radical leaving group and a good reinitiating group.

Figure 1: Structures of different classes of reagents currently used as RAFT agents.13

The RAFT polymerization mechanism can be broken down into 5 steps (Figure 2):

1) Initiation. In the early stages of the polymerization, an initiator generates radicals

that combine with monomer units to form polymer chains. Since new chains are

constantly created by means of initiator decomposition, the total amount of

initiator should be low in comparison with the amount of RAFT agent (1).

2) Pre-Equilibrium. A propagating chain (Pn·) becomes capped when the radical

attacks the carbon-sulfur double bond of the RAFT agent (1). The resulting

radical intermediate (2) fragments into a dormant polymeric thiocarbonylthio

compound (3) and a reinitiating radical (R·). The transformation of RAFT agent

(1) into the dormant polymeric thiocarbonylthio compound (3) should be rapid to

ensure that all polymer chains start growing at the same time. That is why the

leaving group of the original RAFT agent (i.e., the R group) needs to be chosen

in such a way that it is a better homolytic leaving group than the polymer chain

(Pn). Furthermore, the expelled radical (R·) should be a good reinitiating group.

3) Chain Propagation and Reinitiation. Reaction of the new radical (R·) with

monomer forms a new propagating radical (Pm·).

4) Main Equilibrium. The new propagating radical (Pm·) reacts with the dormant

polymeric thiocarbonylthio compound (3), forming a new dormant polymeric

thiocarbonylthio compound (5) and another propagating radical (Pn·). Rapid

equilibrium between the propagating radicals (Pn· and Pm·) and the dormant

polymeric thiocarbonylthio compound provides equal probability for all chains to

grow at the same rate, leading to a linear evolution of the molecular weight with

conversion and very narrow polydispersity. The efficiency of this step determines

the living character of the polymerization.

5) Termination. Throughout the polymerization some “dead” polymer chains will be

formed. Chain termination may occur via coupling reactions of two active centers

(referred to as combination), or atomic transfer between active chains (termed

disproportionation). However, this step should be minimized in order to achieve

living behavior.

Figure 2: General RAFT polymerization mechanism.14

Block copolymers are linear copolymers consisting of long sequences or blocks of constant composition linked by covalent bonds.15 As seen in Figure 3, the simplest arrangement is the diblock structure, commonly referred to as an A-B block copolymer, which is composed of one segment of “A” repeat units and one segment of “B” repeat units. Another type is the triblock, or A-B-A, block copolymer structure, consisting of a single segment of B repeat units located between two segments of A repeat units.

A-A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B-B A-B block copolymer

A-A-A-A-A-A-B-B-B-B-B-B-A-A-A-A-A-A A-B-A block copolymer

Figure 3: Schematic representation of various block copolymer architectures

As mentioned before, it is generally impossible to produce block copolymers by conventional free radical polymerization. Therefore, living radical polymerization is used to polymerize block copolymers of desired chemical composition and molar mass, with a narrow PDI.14 Designing block copolymers for drug deliveries are known through the pharmaceutical field17 with a high rate of success however, the use of block polymers in packaging coating industry was never explored. Exploiting the architectural ability of block copolymers to resolve the growing concerns of flavor scalping in the food and beverage industry has become somewhat attractive. Flavor scalping is a term used in the packaging industry to describe the loss of quality of a packaged item due to either its volatile flavors being absorbed by the package or the food/ beverage absorbing undesirable flavors from the packaging material.18 Flavor scalping can be influenced by many factors however in this paper only few will be discussed such as: polarity- flavors are absorbed more easily in a polymeric film of similar polarity19, glass transition temperature- below the Tg the polymer molecules are still (glassy state) the chance for flavor molecule to find sufficient void to penetrate through is limited20 and polymer density- the higher the density the lower the amount of aroma compounds absorbed in

7 polymers.21 As mentioned earlier, bisphenol A type epoxies have been used for over 40 inside of beverage and food cans with little or no flavor scalping due to its chemical composition, glass transition temperature, polymer density and corrosion.20, 21

Polymerization of acrylic resins via RAFT polymerization may provide the packaging coatings the opportunity to engineer a resin that has similar properties as the bisphenol

A epoxies. The objectives of this thesis is to study simple diblock copolymers of acrylic acid/ ethyl acrylate and that of acrylic acid and methyl methacrylate as potential resin for packaging coatings and compare the diblock copolymers against bisphenol A epoxy and bisphenol A free epoxy control for chemical resistance, adhesion to aluminum under alkaline and acidic conditions and most importantly flavor scalping.

EXPERIMENTAL

Materials

All monomers were purchased from Sigma-Aldrich Chemicals at the highest available purity and used as received unless otherwise stated. 1- Dodecyl-(dimethyl acetic acid) trithiocarbonate was synthesized according to literature protocols.16 AIBN-

2, 2’-azobis (isobutyronitrile) was recrystallized in methanol prior to use.

Aluminum body substrate (Ball, Findley), Aluminum Clean End Stock (Alcoa), #10 wire wound bar (film applicator rod from Gardco), Oven (Despatch Electric Oven), Methyl

Ethyl Ketone (Nexeon Solutions), 2 lb Hammer, Non-sterile cotton gauze sponge (4”x4”

Fisherbrand), Hot plates with temperature controller (Corning PC-420D), 500 mL beakers ( Fisher Scientific), Ultra Joy Dishwashing Liquid (Kroger product of Procter &

Gamble), Scotch 610 tape (3M Company), Glacial Acetic Acid (Fisher Scientific), Butyl cellosolve (Nexeon Solutions), Biphenol A Epoxy and Biphenol A free epoxy coatings provided from PPG Industries as testing controls.

Methods

1 13 H and C NMR measurements were performed in CDCl3, with Si (CH3)4 standard, using a 400 MHz Bruker Ultrashield (100 MHz for 13C). 1H NMR and 13C NMR spectra were analyzed with MestReNova software. Fourier transform infrared (FTIR) spectra were collected on a Nicolet 6700 spectrometer and analyzed with OMIC 32 software. Molecular weight and molecular weight dispersity were determined by gel 9 permeation chromatography (GPC). The samples were characterized with an Agilent

1200 series HPLC equipped with a PSS Gram (10 µm) guard column and 2 PSS Gram columns (10 µm) (linear range of MW=100-1x106 g/mol). A mobile phase of dimethyl formamide (DMF) with 0.1% LiBr w/v was used at a flow rate of 0.5 mL/min at 60°C. An

Optilab rEX differential refractometer (light source=658nm) detector was used and calibrated against poly (methyl methacrylate) standards (850 Da-2,000,000 Da). ASTRA software v.6.1.0 was used to determine molecular weights and dispersity values.

Solvent extraction of flavor compounds were analyzed by 7890/5975 Agilent GC/MS.

Blush Resistance: Blush resistance measures the ability of a coating to resist attack by various testing solutions. When the coated film absorbs a test solution it generally becomes cloudy or looks white. Blush is measured visually using a scale of 1-10 where a rating of “10” indicates no blush and a rating of “0” indicates complete whitening of the film. Blush ratings of at least 7 are typically desired for commercially viable coatings.

The coated panel tested is 2 x 4 inches (5 x 10 cm) and the testing solution covers half of the panel being tested.

Adhesion: Adhesion testing was performed to assess whether the coating adheres to the substrate. The adhesion test was performed according to ASTM D 3359-Test

Method B using Scotch 610 tape. Adhesion is generally rated on a scale of 0-10 where a rating of “10” indicates no adhesion failure, a rating of “9” indicates 90% of the coating remains adhered, a rating of “8” indicates 80% of the coating remains adhered, and so on.

Joy Detergent Test: The “Joy” test was designed to measure the resistance of a coating to a hot 180 ºF (82 ºC) Joy detergent solution. The solution was prepared by mixing 30 grams of Ultra Joy Dishwashing Liquid (product of Procter & Gamble) into

3000 grams of deionized water. Coated strips were immersed into the 180 ºF (82 ºC)

Joy solution for 10 min. The strips were then rinsed and cooled in deionized water, dried, and immediately rated for blush and adhesion as described previously.

Acetic Acid Test: The “Acetic Acid” test was designed to measure the resistance of a coating to a boiling 3% acetic acid solution. The solution was prepared by mixing 90 grams of Glacial Acetic Acid into 3000 grams of deionized water. Coated strips were immersed into the boiling Acetic Acid solution for 30 min. The strips were then rinsed and cooled in deionized water, dried, and immediately rated for blush and adhesion as described previously.

Cure Resistance: Coatings were evaluated for their resistance to methyl ethyl ketone solvent by dousing a non-sterile cotton gauze sponge (4”x4”) with the solvent, attached to a 2 pound hammer that is used to rub across the coating surface until the gauze broke through the coating to the metal surface. The gauze was re-doused with methyl ethyl ketone every twenty five double rubs across the coating surface. The number of double rubs to break through the coating was recorded for a maximum of 100 double rubs.

Flavor Scalping: Flavor scalping was evaluated by a PPG propriety method.

Synthesis of S-1-Dodecyl-Sˊ-(α, αˊ-dimethyl-αˊˊ-acetic acid) trithiocarbonate

1-Dodecanethiol (19.2 mL, 0.0802 mol), acetone (49 mL, 0.666 mol), and Aliquot

336 (tricaprylylmethylammonium chloride (1.5 mL, 0.00422 mol) were mixed in a 500 mL round bottom flask that has been purged with nitrogen and kept under a positive nitrogen pressure via a nitrogen balloon. The mixture was stirred for 20 minutes and cooled to 10 °C under a nitrogen atmosphere. Sodium hydroxide solution (50%) (7.0 mL, 0.058 mol) was added over 15 min and the reaction stirred for an additional 15 min.

To minimize air exposure carbon disulfide (5.0 mL, 0.083 mol) was mixed in acetone

(10 mL, 0.22 mol) and added to the flask drop wise over 20 min. Ten minutes later, chloroform (10 mL, 0.056 mol) was added in one portion, followed by drop wise addition of 50% sodium hydroxide solution (18 mL, 0.15 mol) over 15 min. The reaction was stirred overnight. After this time, 100 mL of water was added, followed by 24 mL of concentrated HCl (9.81 M) (caution! gas, odor) to acidify the aqueous solution. A clear solution with large orange particles was produced. The flask was air purged 5 hours to evaporate the acetone. The orange particles were collected with a Buchner funnel leaving behind a light yellow transparent aqueous solution. The orange solid was stirred in 2-propanol then rotary evaporated the 2-propanol leaving a dark orange/red oil. The undissolved solid was filtered off and was identified as S, Sˊ-bis(1-dodecyl) trithiocarbonate. The orange/red oil was concentrated to dryness in a small amount of boiling hexane and left in the fridge overnight to crystalize the product. The afforded yellow crystalline solids were harvested and dry under vacuum overnight. The light yellow transparent aqueous solution was also rotary evaporated and recrystallized from

12 hexane to afford a total of 15.2 g of yellow crystalline solid. Total yield = 52% NMR:

0.86-0.90 (t, 3H), 1.25-1.68 (m, 20H), 1.72 (s, 6H), 3.26-3.30 (t, 2H), 13.05 (s, 1H).

Synthesis of Macro–RAFT Poly(acrylic acid)

A 100-mL round-flask was charged with acrylic acid (14.76 mL, 0.21 mmol),

DDMAT RAFT agent (0.78 g, 2.13 mmol trithiocarbonate), and AIBN (25 mg, 0.15 mmol) in 20 mL of tert-butanol. The reaction flask was sealed with a rubber septum and purged with N2 in an ice bath for 30 min. The flask was then heated for 4 h at 80 °C before being cooled by exposure to air (O2) and rapid cooling. The macro-RAFT polymer was precipitated from methanol to afford 13.99 g of pale yellow material. Yield

= 88% NMR: 1.4 (t, 1H), 3.42 (m, 3H),

Additional studies

The effects of DDMAT on acrylic acid polymerization were determined by varying the molar ratio of DDMAT to Vazo 67 from 5 to 10 while keeping the molar ratio of acrylic acid to Vazo 67 constant. The experimental conditions are outlined in Table 1.

Temperature [AA] [DDMAT] [Vazo67] [AA]/[Vazo Experiment [DDMAT]/[Vazo 67] (°C) (mol L-1) (mmol L-1) (mmol L-1) 67]

1 75 0.215 2.13 0.426 5 504

2 75 0.215 2.13 0.213 10 1009

Table 1: Experimental conditions for poly(acrylic acid) polymerization at various [DDMAT] /

[Vazo 67] ratios.

Synthesis of Poly(acrylic acid-b-ethyl acrylate) Copolymer

A 100-mL round-flask was charged with poly(acrylic acid) macro-RAFT homopolymer (10.00 g, 3.5 mmol) ethyl acrylate (35.36 g, 353.6 mmol) and AIBN (19.35 mg, 0.12 mmol) in 20 mL of tert-butanol. The reaction flask was sealed with a rubber septum and purged with N2 in an ice bath for 30 min; the flask was then heated for 3 hr. at 80°C before being quenched by exposure to air (O2) and rapid cooling. The block copolymer of poly(acrylic acid-b-ethyl acrylate) was precipitated twice from hexane and dried in vacuum to yield 28.97 g of pale yellow viscous material. Isolated yield- 64%

NMR: 1.3 (t, 3H), 2.4 (m, 2H), 4.1 (t, 2H)

Synthesis of Macro-RAFT Poly(tert-butyl acrylate) 22

A 100-mL round-flask was charged with tert-butyl acrylate (13.6 mL, 93.6 mmol),

DDMAT RAFT agent (0.341 g, 0.936 mmol trithiocarbonate), and AIBN (51 mg, 0.312 mmol) in 20 mL of anhydrous anisole. The reaction flask was sealed with a rubber septum and purged with N2 in an ice bath for 30 min; the flask was then heated for 2 hr. at 65°C before being quenched by exposure to air (O2) and rapid cooling. The macro-

RAFT polymer was precipitated from water through the ice chiller overnight to afford

12.29 g of pale yellow viscous material. Isolated yield- 99%

Synthesis of Poly(tert-butyl acrylate-b-ethyl acrylate) Copolymer

A 100-mL round-flask was charged with poly(tert-butyl acrylate) macro-RAFT homopolymer (6.145 g, 0.047 mmol) ethyl acrylate (4.73 g, 0.0473 mmol) and AIBN (25 mg, 0.156 mmol) in 24 mL of anhydrous anisole. The reaction flask was sealed with a 14 rubber septum and purged with N2 in an ice bath for 30 min; the flask was then heated for 3 hr. at 80°C before being quenched by exposure to air (O2) and rapid cooling. Air was purged overnight to remove anisole solvent. The block copolymer of poly(tert-butyl acrylate-b-ethyl acrylate) was precipitated twice from hexane and dried in vacuum to yield 6.466 g of pale yellow viscous material. Isolated yield- 59% NMR: 1.24 (t, 3H),

1.43 (m, 9H), 1.64-2.28 (s, 2H), 4.09 (t, 2H)

A 2000 g beaker was charged with above polymer (6.466 g, 0.17 mmol) and diluted in

150 mL of DCM. 150 mL of trifluoroacetic acid (1:1 ratio to DCM) was added to the beaker and covered with a glass stopper and stirred for 2 hr. at ambient temperature.

The yellow pale solution was dried in air overnight and a pale yellow solid of 4.17 g was obtained. Isolated yield- 88% NMR: 1.24 (t, 3H), 1.43 (m, 2H), 1.64-2.28 (s, 2H), 4.09 (t,

3H)

Synthesis of Poly(tert-butyl acrylate-b-methyl methacrylate) Copolymer

A 100-mL round-flask was charged with poly(tert-butyl acrylate) macro-RAFT homopolymer (6.145 g, 0.047 mmol) methyl methacrylate (4.73 g, 0.0473 mmol) and

AIBN (25 mg, 0.156 mmol) in 24 mL of anhydrous anisole. The reaction flask was sealed with a rubber septum and purged with N2 in an ice bath for 30 min; the flask was then heated for 3 hr. at 80°C before being quenched by exposure to air (O2) and rapid cooling. Air was purged overnight to remove anisole solvent. The block copolymer of poly(tert-butyl acrylate-b-methyl methacrylate) was precipitated twice from hexane and

15 dried in vacuum to yield 3.82 g of white powder material. Isolated yield- 35% NMR:

0.85 (t, 27H), 1.02 (m, 14H), 1.44 (t, 9H), 1.82-1.94 (m, 22H), 3.6(t, 40H)

A 2000 g beaker was charged with above polymer (3.82 g, 0.17 mmol) and diluted in

150 mL of DCM. 150 mL of trifluoroacetic acid (1:1 ratio to DCM) was added to the beaker and covered with a glass stopper and stirred for 2 hr. at ambient temperature.

The yellow pale solution was dried in air overnight and a clear hard film of 2.55 g was obtained. Isolated yield: 66% NMR: 0.85 (t, 24H), 1.02 (m, 14H), 1.44 (t, 3H), 1.82-1.94

(m, 21H), 3.6(t, 39H)

RESULTS and DISCUSSION

Characterization of Poly(acrylic acid-b-ethyl acrylate) Copolymer

Acrylic acid polymerization, in the presence of DDMAT, was investigated at different parameters:

 Ratio of [DDMAT]/ [Vazo 67]

Percent conversion was determined using GC/ FID. Molecular weight and polydispersity were obtained by size exclusion chromatography (SEC). To investigate the influence of this parameter, two experiments (run 1 and 2, respectively, in Table 1) were compared differing only by the ratio of [DDMAT] to [Vazo 67], while keeping the ratio of [AA] to

[Vazo 67] constant. Number-average molecular weight Mn (SEC) and polydispersity values decreases with increasing Vazo 67 (Table 2) concentration in good agreement with the theory of living polymerization. During the beginning of a RAFT polymerization, polydispersity is dictated by conventional free radical polymerization, giving rise to a high polydispersity. As the polymerization proceeds, RAFT-mediated chain equilibration is established, control over molecular weight occurs and a decrease in polydispersity is expected.

[DDMAT]/[Vazo 67] Mn (g/mol) MW (g/mol) PDI % Conversion

5 1584 1964 1.33 21.8

10 1598 2561 1.60 22

Table 2. Molecular Weight, polydispersity and % conversion of poly(acrylic acid)

Although samples were not withdrawn to determine the polymerization kinetics data at

[DBTTC] to [Vazo 67] ratio of 5 and 10, Mn (SEC) values displayed an increased linearly with monomer conversion and were close to Mn (theoretical) (Table 3). The linearity of Mn (SEC) vs. conversion indicates a constant number of propagating chains and hence the lack of termination reactions.

[DDMAT]/[Vazo 67] % Conversion Mn (g/mol)(SEC) Mn (g/mol)(Theor)

5 21.8 1584 1606

10 22 1598 1622

10 95 / 7003

Table 3. Mn measured versus theoretical per % conversion of poly(acrylic acid)

A 95% conversion of acrylic acid was not achieved in this experiment regardless of conditions described above and very low conversion was achieved overall for diblock copolymer poly(acrylic acid-b-ethyl acrylate) as seen below on Table 4. The probable cause is due to impurities.

Characterization of Poly(acrylic acid-b-ethyl acrylate) and Poly(acrylic acid-b-methyl methacrylate) copolymers from tert-butyl acrylate

Diblock copolymers from tert-butyl acrylate were successfully synthesized as described below in Table 4. The tert-butyl acrylate was cleaved by stirring with trifluoroacetic acid for two hours at ambient temperature to generate the corresponding acrylic acid segments, and hence formed water soluble resins. Molecular weight and

18 polydispersity obtained by size exclusion chromatography (SEC) demonstrated that the polymerization was controlled. The PDI for poly(tert-butyl acrylate-b-methyl methacrylate) is broader than of poly(tert-butyl acrylate-b-ethyl acrylate) revealing that the polymerization behavior for extending the polymer was different when using poly(tert-butyl acrylate) as the macro-CTA for the synthesis of poly(tert-butyl acrylate-b- methyl methacrylate) block copolymer. The final product yield of 35% was also a good indication that the synthesis was not as favored as of poly(tert-butyl acrylate-b-methyl methacrylate). Poly(tert-butyl acrylate) as the macro-CTA is not active for methyl methacrylate polymerization because is not a good homolytic leaving group. Generally, one requirement for the formation of a narrow polydispersity block copolymer is that the first formed macro-CTA (A block) should have a high transfer coefficient in order to give another block (B block). The homolytic leaving group requires the ability of propagating radical A. to be comparable to or greater than of the propagating radical B. under reaction condition.

Polymer Mn (g/mol) MW (g/mol) PDI DP

Poly(AA)-Raft 1598 2561 1.60 22

Poly(AA-b-EA) 1184 1584 1.34 6.8

Poly(tert-BuA)-Raft 13,000 15,000 1.16 101

Poly(tert-BuA-b-EA) 31,000 38,000 1.23 136

Poly(tert-BuA-b-MMA) 26,100 59,800 2.30 129

Table 4. Size Exclusion Chromatography Data of the Polymers

The tert-butyl cleavage was confirmed by FTIR spectra providing a carboxyl functional group signal at 2971cm-1. The removal of tert-butyl group to achieve a diblock copolymer of poly(acrylic acid-b-ethyl acrylate) was more successful than of poly(acrylic acid-b-methyl methacrylate) (Figure 4 and 5).

Figure 4. IR Spectra of poly(tert-butyl acrylate-b-ethyl acrylate) and poly(acrylic acid-b- ethyl acrylate)

Figure 5. IR Spectra of poly(tert-butyl acrylate-b-methyl methacrylate) and poly(acrylic acid-b-methyl methacrylate) 20

Characterization of coated dry film samples Poly(acrylic acid-b-ethyl acrylate) was dissolved in ethylene glycol monobutyl ether and water to yield a 25% solution. Poly(acrylic acid-b-methyl methacrylate) was dissolved in methyl ethyl ketone to yield a 25% solution. Both solutions were applied over aluminum substrate metal and baked for 3 minutes at 380°F. Poly(acrylic acid-b- ethyl acrylate) resin solution compared to the both epoxy and bisphenol A free epoxy controls exhibited poor adhesion, solvent and chemical resistance under alkaline and acidic conditions due to lower Tg characteristics. However, poly(acrylic acid-b-methyl methacrylate) resin solution displayed better solvent resistance and chemical resistance was improved a good indication that Tg does have an effect on the robustness of resin

(Table 5).

1% Joy Pasteurization 3% Acetic Acid

MEK Rubs Adhesion/ Blush Adhesion/ Blush

Epoxy Control 90 10/8 10/8

BPA-NI Control 3 10/8 10/8

Poly(AA-b-EA) 1 0/5 0/4

Poly(AA-b-MMA) 10 0/7 0/7

Table 5. - Resin dry film testing

Although both diblock copolymers displayed poor film properties compared to epoxy and bisphenol A free epoxy resin controls, both poly(acrylic acid-b-ethyl acrylate) and poly(acrylic acid-b-methyl methacrylate) were mixed with a bisphenol A free epoxy resin

(B) known to have poor adhesion to aluminum substrate under acidic conditions. 21

Sample C, (Table 6) is correspondent to poly(acrylic acid-b-ethyl acrylate) mixed in with bisphenol A free epoxy resin (B) at ratio of 10:90 and sample D is poly(acrylic acid-b- methyl methacrylate) at same ratio.

1% Joy Pasteurization 3% Acetic Acid

MEK Rubs Adhesion/ Blush Adhesion/ Blush

Epoxy Control 90 10/8 10/8

BPA-NI Control 3 10/8 10/8

B 1 10/6 0/2

C 3 10/7 10/7

D 12 10/7 5/7

Table 6.- Block polymers as additives

40 BPA-free Coating 35 D

25 C

10 Epoxy Coating 5

0 1 2 3 4

Figure 6. Flavor Scalping

Both diblock copolymers, improved the film properties of resin B significantly with the exception of poly(acrylic acid-b-methyl methacrylate) under the acidic conditions. A probable cause due to lower acrylic acid groups available as the cleavage of the tert- butyl group was not fully successful. Sample C demonstrated a 35% improvement in flavor scalping compared to sample D with a 2% improvement against epoxy and bisphenol A free epoxy control coatings (Figure 6). The addition of poly(acrylic acid-b- ethyl acrylate) diblock copolymer proves that polarity had a positive effect on flavor scalping more so than Tg in this study.

CONCLUSIONS

The synthesis of diblock copolymers via RAFT polymerization was studied to determine its feasibility as a viable resin for packaging coatings. The first attempt to prepare a poly(acrylic acid) macro-RAFT was not successful due to impurities and possible side reactions that lead to formation of products that retarded the polymerization. The monomer conversion was poor leading to low molecular weight polymer. Poly(tert-butyl acrylate) macro-RAFT provided better polymerization of finished diblock copolymer of poly(tert-butyl acrylate-b-ethyl acrylate) and hydrolysis of the tert- butyl group generated acrylic acid functional groups readily available as crosslinking sites. Molecular weight and polydispersity of poly(acrylic acid-b-ethyl acrylate) was very well controlled. Poly(acrylic acid-b-ethyl acrylate) copolymer alone didn’t provide adequate film properties as compared to epoxy and bisphenol A free epoxy control resins. However, in combination with a known poor performer bisphenol A free epoxy resin, the poly(acrylic acid-b-ethyl acrylate) enhanced the film properties of that resin under acidic conditions as well improving the flavor scalping performance. Introducing a polar resin to a coating reduces the ability for non-polar beverage components to be absorbed or migrate to or from beverage/ coating. Exploring further a triblock from same monomer composition may open the window to additional improvements in coating film properties.

Synthesis of poly(acrylic acid-b-methyl methacrylate) from poly(tert-butyl acrylate) macro-RAFT was not completely successful. Polydispersity was broader pointing to the lack of transfer coefficient of the poly(tert-butyl acrylate) macro-RAFT to methyl 24 methacrylate. Also the lack of available acrylic acid groups due to poor cleavage of tert- butyl group. Overall, poly(acrylic acid-b-methyl methacrylate) copolymer didn’t significantly exhibited improvements in film properties or flavor scalping. The synthesis of a poly(acrylic acid-b-methyl methacrylate) will require the change in order of addition starting with a poly(methyl methacrylate) macro-RAFT followed by the completion of diblock copolymer with tert-butyl acrylate and hydrolysis of the tert-butyl group to expose the acrylic acid groups. Based on the data provided in this study, diblock copolymers via RAFT polymerization are potentially viable resins for packaging coatings. Although the study was very crude the learnings are very important and provide a good baseline for future exploration.

REFERENCES

1 UK FSA (United Kingdom Food Standards Agency), 2002, “Food Safety Information Sheet: Tin in Canned Fruits and Vegetables”, August 2002.

2 Brotons, J.A., M.F. Olea-Serrano, M. Villalobos, V. Pedraza and N. Olea, 1995, Environmental Health Perspectives, vol. 103, pages 608-612.

3 Howe, S.R., L. Borodinsky and R.S. Lyon, 1998, “Potential Exposure to Bisphenol A from Food-Contact Use of Epoxy Can Coatings,” Journal of Coatings Technology, vol. 70, no. 877, pages 69-74.

4 U.S. Food and Drug Administration, Draft Assessment of Bisphenol A for Use in Food Contact Applications, 14 August 2008.

5 Oldring, P.K.T. and Nehring, U. (2007) ‘Metal packaging for foodstuffs’, Packaging Materials, ILSI (International Life Sciences Institute), Belgium, pp.1–40.

6 Brendley, W.H. and Haag, T.H. (1973) ‘Physical characterization of water dispersed and soluble acrylic polymers’, in Gardon, J.L. and Prane, J.W. (Eds): Nonpolluting Coating and Coating Processes, Plenum Press, New York, pp.5–24. 7 Szwarc, M.; Levy, M.; Milkovich, R. J. Am. Chem. Soc. 1956, 78, 2656-2657

8 Solomon, D.H.; Rizzardo, E.; Cacioli, P. Eur. Pat. Appl. 1985, EP 135280 A2.

9 Wang, J.S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614-5615.

10 Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1995, 28, 1721-1723.

11 Chiefari, J.; Chong, Y.K.; Ecrole, F.; Krstina, J.; Jeffrey, J.; Le, T.P.; Mayadunne, R.T.; Meijs, G.; Moad, C.; Moad, G.; Rizzardo, E.; Thang, S.H. Macromolecules 1998, 31, 5559-5562

12 Moad, G.; Rizzardo, E.; Thang, S.H. Polymer 2008, 49, 1079-1131.

13 Moad, G.; Chiefari, J.; Chong, Y.K.; Kristina, J.; Mayadunne, R.T.; Postma, A.; Rizzardo, E.; Thang, S.H. Polym. Int. 2000, 49, 993-1001. 14 Noshay, A,; McGrath, J. Block Copolymers: Overview and Critical Survey. New York, New York: Academic Press, Inc.; 1977.

15 Moad, G.; Chong, Y.K.; Postma, A.; Rizzardo, E.; Thang, S.H. Polymer 2005, 46, 8458-8468.

16 Lai, John T.; Filla, Debby; Shea, Ronald Macromolecules 2002, 35, 6754-6756 17 Gaucher, G.; Dufresne, M.; Sant, V.; Kang, N.; Maysinger, D.; Leroux, J. Block copolimer micelles: Preparation, characterization, and application in drug delivery. J. Controll. Rel. 2005, 109, 169-188. 18 Roland AM, Hotchkiss JH. 1991. Determination of flavor-polymer interactions by vacumm-microgravimetric method. In: Risch SJ, Hotchkiss, editors. Food and packaging interactions II. Washington, D.C.: American Chemical Society. p 149-160. 19 Brody AL. 2005. Packaging. Food Tech 59(1):74-77 20 Gallo JAQ, Debeaufort F, Voilley A. 1999. Interactions between aroma and edible films. 1. Permeability of methylcellulose and low-density polyethylene films to methyl ketones. J Agric Food Chem 47: 108-113. 21 Gremli Hh.1996. Flavor changes in plastic containers. Perfum Flav 21:1-8 22 Chen, Xiaoping; Ayres, Neil Synthesis of Molecular Brush, Journal of Polymers Science Part A: Polymer Chemistry, Vol 49, 3033-3037 Wiley Online Library 2011