Thiol−ene Coupling of Renewable Monomers: at the forefront of bio-based polymeric materials

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Thiol–ene Coupling of Renewable Monomers:

at the forefront of bio-based polymeric materials

Mauro Claudino

Licentiate Thesis

Kungliga Tekniska Högskolan, Stockholm 2011

AKADEMISK AVHANDLING

Akademisk avhandling som med tillstånd av Kungliga Tekniska högskolan i Stock- holm, framlägges till offentlig granskning för avläggande av teknologie licentiatexa- men fredagen den 30 september 2011, kl.14.00 i sal K1, Teknikringen 56, KTH.

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Paper II accepted 2011, Journal of Polymer Science Part A: Polymer Chemistry TRITA-CHE Report 2011:49

ISSN 1654-1081

ISBN 978-91-7501-094-6

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To My Family

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Abstract

Plant derived oils bear intrinsic double-bond functionality that can be utilized directly for the thiol–ene reaction. Although terminal unsaturations are far more reactive than internal ones, studies on the reversible addition of thiyl radicals to 1,2- disubstituted alkenes show that this is an important reaction. To investigate the thiol–ene coupling reaction involving these enes, stoichiometric mixtures of a tri- functional propionate thiol with monounsaturated fatty acid methyl esters (methyl oleate or methyl elaidate) supplemented with 2.0 wt.% Irgacure 184 were subjected to 365-nm UV-irradiation and the chemical changes monitored. Continuous (RT–

FTIR) and discontinuous (NMR and FT–Raman) techniques were used to follow the progress of the reaction and reveal details of the products formed. Experimental results supported by numerical kinetic simulations of the system confirm the reac- tion mechanism showing a very fast cis/trans-isomerization of the alkene monomers (<1.0 min) when compared to the total disappearance of double-bonds, indicating that the rate-limiting step controlling the overall reaction is the hydrogen transfer from the thiol involved in the formation of final product. The loss of total unsaturations equals thiol consumption throughout the entire reaction; although product formation is strongly favoured directly from the trans-ene. This indicates that initial cis/trans-isomer structures affect the kinetics. High thiol–ene conversions could be easily obtained at reasonable rates without major influence of side-reactions demon- strating the suitability of this reaction for network forming purposes from 1,2- disubstituted alkenes. To further illustrate the validity of this concept in the formation of cross-linked thiol–ene films a series of globalide/caprolactone based copolyesters differing in degree of unsaturations along the backbone were photopol- ymerized in the melt with the same trithiol giving amorphous elastomeric materials with different thermal and viscoelastic properties. High thiol–ene conversions (>80%) were easily attained for all cases at reasonable reaction rates, while maintain- ing the cure behaviour and independent of functionality. Parallel chain-growth ene- homopolymerization was considered negligible when compared with the main coupling route. However, the comonomer feed ratio had impact on the thermoset properties with high ene-density copolymers giving networks with higher glass tran- sition temperature values (Tg) and a narrower distribution of cross-links than films with lower ene composition. The thiol–ene systems evaluated in this study serve as model example for the sustainable use of naturally-occurring 1,2-disubstituted alkenes at making semi-synthetic polymeric materials in high conversions with a range of properties in an environment-friendly way.

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Sammanfattning

Vegetabiliska oljor som innehåller dubbelbindningar kan användas direkt för thiol- ene reaktioner. Trots att terminala dubbelbindningar är mycket mer reaktiva än interna visar dessa studier att den reversibla additionen av thiyl radikaler till 1,2- disubstituerade alkener är en viktig reaktion. För att undersöka tiol–ene reaktionerna, som ivolverar dessa alkener förbereddes stökiometriska blandningar av en tri- funktionell propionat tiol och enkelomättade fettsyrametylestrar (metyloleat eller metyl elaidat) samt 2.0 vikt.% Irgacure 184. Dessa blandningar utsattes för 365-nm UV-strålning och de kemiska förändringarna studerades. De kemiska förändringar- na analyserades med olika kemiska analysmetoder; realtid RT–FTIR, NMR och FT–Raman. Dessa användes för att analysera de kemiska reaktionerna i realtid och följa bildandet av produkterna. Reaktionsmekanismen bekräftades med hjälp av experimentella data och beräkningar av numeriska och kinetiska simuleringar för systemet. Resultaten visar en mycket snabb cis/trans-isomerisering av alkenmonome- ren (<1.0 min) jämfört med den totala förbrukningen av dubbelbindningarna, vilket indikerar att det hastighetsbegränsande steget kontrolleras av väteförflyttningen från tiolen till slutprodukten. Förbrukningen av den totala omättade kolkedjan är lika med tiolförbrukningen under hela reaktionen, även om bildandet av produkten gynnas från trans-enen. Detta indikerar att den första cis/trans-isomerstrukturen påverkar kinetiken. Höga tiol-ene utbyten kan enkelt erhållas relativt snabbt utan inverkan av sidoreaktioner. Detta innebär att denna reaktion kan användas som nätverksbildande reaktion för flerfunktionella 1,2-disubstituted alkenmonomerer.

Vidare användes fotopolymerisation i smälta på en serie globalid/kaprolakton- baserade sampolyestrar med varierad grad av omättnad med samma tritiol vilket resulterade i bildandet av amorfa elastomeriska material med olika termiska och viskoelastiska egenskaper. Hög omsättning (>80%) uppnåddes relativt enkelt för samtliga blandningar oberoende av den initiala funktionaliteten. Homopolymerisat- ion av alkenen var försumbar i jämförelse med den tiol–en-reaktionen. Mängden alkengrupper har inverkan på härdplastsegenskaperna där en hög andel alken ger en nätstruktur med högre glastransitionstemperatur (Tg). Tiol–ene reaktionen utvärde- rades i modellsystem baserade på naturlig förekommande 1,2-disubstituterade alkener för att demonstrera konceptet med tiol-förnätade halvsyntetiska material.

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List of Publications

This thesis is an extensive summary of the following articles, which are referred to in the text by Roman numerals and appended at the end of the thesis:

Paper I (published)

Thiol–ene Coupling of 1,2-Disubstituted Alkenes: the kinetic effect of cis/trans-isomer structures. Claudino, M.; Johansson, M.; and Jonsson, M. Euro- pean Polymer Journal, 2010 (46), p. 2321–2332.^*

Paper II (accepted manuscript)

Photoinduced Thiol–ene Crosslinking of Globalide/ε-caprolactone Copoly- mers: curing performance and resulting thermoset properties. Claudino, M.; van der Meulen, I.; Trey, S.; Jonsson, M.; Heise, A.; and Johansson, M. Journal of Pol- ymer Science Part A: Polymer Chemistry, 2011.

* For corrections to Paper I see page 49.

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Contribution to the Papers

The contribution of the author to the appended papers goes as follows:

Paper I. All of the experimental work and measurements, all numerical kinetic simulations (modelling) and chemical data analysis, and most of the preparation of the manuscript.

Paper II. Most of the experimental work and measurements, a large part of the data analysis and writing of the manuscript.

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List of Abbreviations and Terms

UV Ultraviolet

BP Benzophenone

BDE Bond Dissociation Energy

RSH Thiol Group

PI Photoinitiator

RSSR Dissulfide Product

Tg Glass Transition Temperature

PGI Polyglobalide

ROP Ring-opening (co)Polymerization

THF Tetrahydrofurane

TMP 2-ethyl-(hydroxymethyl)-1,3-propanediol

NMR Nuclear Magnetic Ressonance

CDCl3/d-solvent Deuterated Chloroform

TMS Tetramethylsilane

RT–FTIR Real-Time Fourier Transform Infrared Spectroscopy MCT Mercury Cadmium Telluride (photoconductive detector) FTIR Fourier Transform Infrared Spectroscopy

ATR Attenuated Total Reflectance

TGS Triglycine Sulfate (piroelectric detector)

FT Fourier Transform

NIR Near Infrared

MO Methyl Oleate

ME Methyl Elaidate

FAME Fatty Acid Methyl Ester

P(GI-co-CL) Poly(globalide-caprolactone) copolymer

PCL Poly(ε-caprolactone)

LSODA “Livermore Solver for Ordinary Differential Equations”

ODE Ordinary Differential Equation

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DSC Dynamic Scanning Calorimetry

Tm Melting Point Temperature

DMTA Dynamic Mechanical Thermal Analysis RTIR Real-Time Infrared Spectroscopy

TE Thiol–ene

CL (or ε-CL) ε-caprolactone

PAm Polyambrettolide

DCP Dicumyl Peroxide

GI Globalide

Am Ambrettolide

DXO 1,5-dioxepan-2-one

4MeCL 4-methyl caprolactone

CALB Candida Antarctica Lipase B

PS Polystyrene

fene Ene Functionality

fthiol Thiol Functionality

α Critical Fractional Conversion (gel-point^¶)

r Thiol–ene Molar Ratio (based on functional groups)

FA Fatty Acid

SEC Size Exclusion Chromatography (analogous to GPC)

In Initiator

I^• Primary Initiating Free-Radical / or Iodide Radical

SD Standard Deviation

TBD Triazabicyclodecene (catalyst)

AIBN Azobisisobutyronitrile (thermal radical initiator)

DMPA 2,2-dimethoxy-2-phenylacetophenone (Irgacure 651, photoinitiator)

([1, 2]).

¶ Defined as the point in the polymerization reaction at which an infinite network first ap- pears in the system (onset of gelation) accompanied with fluidity loss and the viscosity be- comes so large that an air bubble cannot rise through it [1, 2].

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Nomenclature of the polymers and films

Composition Polymer

P(GI-co-CL) Homopolymerized film Thiol–ene film G:C 100:0 P(G:C) 100:0 f-H(G:C) 100:0 f-TE(G:C) 100:0 G:C 50:50 P(G:C) 50:50 f-H(G:C) 50:50 f-TE(G:C) 50:50 G:C 40:60 P(G:C) 40:60 f-H(G:C) 40:60 f-TE(G:C) 40:60 G:C 30:70 P(G:C) 30:70 f-H(G:C) 30:70 f-TE(G:C) 30:70 G:C 20:80 P(G:C) 20:80 f-H(G:C) 20:80 f-TE(G:C) 20:80 G:C 10:90 P(G:C) 10:90 f-H(G:C) 10:90 f-TE(G:C) 10:90

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1. PURPOSE OF THE STUDY

ree-radical induced photopolymerization of thiol–ene systems have obtained a renewed interest during the last decades thanks to several attractive key features differing from conventional free-radical polymerization reactions such as fast reaction rates with reduced influence of oxygen inhibition, uniform cross-linking density, a step-wise growth chain mechanism leading to a late gel point, improved curing control, low shrinkage at high monomer conversions, and the ability to initiate polymerization without addition of photoinitiator [3]. Another area with increased attention concerns materials based on renewable resources to obtain a greener sustainable material production in the future. One group of bio-based alkenes is fatty acids and derived compounds containing internal main-chain double-bonds. De- spite the relatively slow rates of reaction when compared to olefins with external unsaturations, studies on the effect of thiol structure and different unsaturations within the aliphatic chain demonstrate potential applicability in the creation of cross-linked thiol–ene networks [4].

The main aim of this thesis is to provide a better understanding of the free-radical thiol–ene coupling of 1,2-disubstituted alkenes in bulk with special focus on the kinetic effect of initial cis/trans-configurations promoted by photogenerated thiyl radicals. The purpose is furthermore to determine the main reaction routes at high conversions relevant for end-use applications such as organic coatings and how initial ene compositions of closely-related unsaturation systems, such as those based on polyglobalide copolymers, affects the cross-linking kinetics and final film properties as consequence of possible diffusion and/or mobility restriction(s) developed within the network upon the cure process.

F

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2. INTRODUCTION

2.1. Free-radical Induced Thiol–ene Reaction 2.1.1. Short Historical Overview

The thermal cross-linking of natural rubber [poly(cis-isoprene)] with elemental sul- fur was discovered in 1839 by Charles Goodyear [5]. This process later became known as vulcanization, and is still to this date extensively used by the tire industry.

The thiol–ene reaction was already observed in 1905 by Postner showing that enes and thiols could react spontaneously with each other or in the presence of an acid [6]. In 1926 it was presented for the first time as a polymer forming reaction where it was discovered that allyl mercaptan ‘spontaneously gelled’ upon heating [7].

However, its basic mechanistic formulation as a free-radical ‘mediated’ polymeriza- tion, including the individual reaction steps, was accomplished later by Kharasch et al. just before the Second World War [8]. The early work involving the coupling of thiols to olefins was concisely described in 1970 by Griesbaum [9] and in 1993 Jacobine reviewed extensively all the aspects of thiol–ene photopolymerizations [10]. Since then, this unique reaction has attracted significant attention especially in organic [11] and polymer syntheses [12, 13].

In the past, the main large-scale applications of free-radical thiol–ene chemistry included the manufacture of relief printing plates (also known as the Latterflex process), wear layers for floor tiles (based on UV-curable resins) and coatings for elec- tronics [14]. However, the use of thiol–ene systems had been restricted to some extent due to issues of bad odor and difficulties in stabilizing the systems leading to short pot-life of formulated monomer mixtures. Moreover, the erroneous impres- sion that all thiol–ene coatings were subject to rapid yellowing (caused by residues of photoinitiator) and discoloration upon weathering, in part due to the large usage of benzophenone (BP) as photoinitiator, as well as the introduction of cheap, readily-available acrylate monomers, made the popularity of thiol–ene photopolymerization decrease severely and gave way to acrylate-based photocurable systems. The revival of this chemistry was attributed mainly to the development of cleavage-type photoinitiators to initiate the thiol–ene photopolymerization (eliminating the prob- lem of yellowing) and the incorporation of thiols into acrylate formulations to decrease oxygen sensitivity and improve the final network properties. In the last 10 years most of the research in thiol–ene chemistry has been focused on the develop-

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ment of new materials and applications, inclusively substrate surface modifications, formation of networks with unique properties, polymer functionalization and pho- tocuring, high-impact energy absorbing materials, among very many others [14, 15]. Due to its high efficiency, the term (‘click’) was recently coined to this reaction (2008) [16-18] and there are already a vast number of excellent reviews on the sub- ject covering a broad range of scientific fields and applications [10, 12, 14, 15, 19].

2.1.2. Basic Chemistry and Reaction Mechanisms

The lability of thiol hydrogens differentiates thiol–ene polymerizations from conventional free-radical polymerizations. The thiol–ene reaction takes advantage of the easily abstractable hydrogen atom of the thiol group due to the relatively weak sulphur-hydrogen bond (BDE, MeS–H=368.44 kJ⋅mol^–1) [20]. This happens because the electron-poor hydrogen is bonded to the less electronegative sulphur atom (RS^δ^– –H^δ⁺) if compared with a more electronegative oxygen from an alcohol group where this is more difficult (BDE, MeO–H=435.43 kJ⋅mol^–1). The cleavage of S–H bonds can be promoted either by direct photolysis (or simply by thermolysis) or indirectly from heat- or light- generated nucleophilic alkyl radicals obtained from the cleavage of initiators. The resulting electrophilic thiyl radicals (RS^•) are extremely reactive and can add to a wide variety of unsaturated compounds (both electron-rich/poor double-bonds) to form new carbon–carbon linkages. This addition reaction (also termed hydrothiolation of C=C bonds) is exothermic [15] and energetically favoured as a new strong σ C–C bond (∼370 kJ⋅mol^–1) is formed at the expense of a weaker alkene π-bond (∼235 kJ⋅mol^–1) [21]. However, the overall rate of addition is strongly dependent on the chemical structure of the thiol and ene (internal, isolated, conjugated, non-conjugated, and substituted), with thiyl radicals adding faster to electron-rich double-bonds (terminal and monosubstituted) than to electron- deficient ones (e.g. acrylates). An accurate summary of the general trends in reactivi- ty involving thiols and enes is given by Hoyle et al. [12, 14].

The thiol–ene reaction proceeds as a typical radical chain process with initiation, propagation and termination steps. The characteristic two-step mechanism for the hydrothiolation of an isolated unsaturation is represented in Scheme 1. At first, the reaction starts via initiation (often UV-induced) which promotes hydrogen transfer from the thiol to one of the initiating free-radicals generated, for instance, via the cleavage of a photoinitiator. The resulting thiyl radical then adds across the C=C double-bond (propagation step 1) yielding an intermediate β-thioether carbon- centered radical followed by chain transfer to a second thiol group (propagation step

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2) to give the final thiol–ene addition product with anti-Markovnikov orientation.

The mechanism regenerates the RS^• radical, thus there is no net consumption of thiol groups, allowing the polymerization to be continued in a cyclic sequence.

Termination reactions are frequently considered unimportant if compared with the rates of propagation and usually involve bimolecular combination of the intervening radical species (β-carbon or thiyl radicals), although these processes still remain obscure.

One of the most prominent competing reactions to thiol–ene additions is the pro- pensity of the ene to homopolymerize via a pure chain-wise radical growth mechanism. In this case the choice of the ene will affect the progress and outcome of the polymerization as one route is favoured over the other thus leading to different structure build-up patterns in multifunctional monomer systems. For example, enes such as acrylates, prone to rapid homopropagation, will to some extent homopolymerize even in the presence of thiol whereas monomers such as allyl ethers to a sig- nificantly less extent will do this (Scheme 2) [15]. This feature has allowed the ad- justment of the overall reaction sequence in systems based on more than two monomers to create novel polymeric structures with exclusive properties as described by Bowman et al. [22-24]. A thiol–ene system is therefore to a larger extent affected by the reaction kinetics for the different reactions than conventional systems.

propagation cycle

R1 S initiator (if used)

+

thiol–ene addition product

i.

ii.

Scheme 1. Step-wise growth mechanism of the free-radical thiol–ene coupling involving a terminal ene with alternating propagation (i.) and chain transfer (ii.). In ideal stoichiometry and absence of competing reactions, such as homopolymerization of the alkene monomer (i.e., chain growth), a single thiol group couples with an ene functionality to yield the final thioether (C–S linkage) product.

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PI + h kd 2 I

I + R₂

I R₂ kadd

I R₂

+ R2

kpr1

I

R2 R2 n

R2

R1 S +

R₂

R₁ S

R2 R2 n k_pr2

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Acrylate Vinyl ether Allyl ether Methacrylate

no homo‐

polymerization

very slow homo‐

polymerization slow homo‐

polymerization

fast homo‐

polymerization

< < <

Scheme 2. Chain-wise radical growth mechanism showing the degrees in homopolymeriza- tion of the ene-monomer.

Although internal olefins show generally much lower reactivity towards the hydrothiolation of unsaturations than singly substituted ones, this is not uniquely due to an increased degree of substitution of the double-bonds (causing steric hindrance effects) but also because of isomerization. One exception to this is the double-bond of norbornene which exhibits an exceptionally high reactivity toward thiol addition attributed to bond angle distortion in association with ring strain relief [14]. When 1,2-disubstituted enes are involved, for instance those present in fatty acids, isomerization further reduces the rate at which this occurs due to interchangeability be- tween cis/trans configurations (cf. Scheme 3). Additionally, the intermediate alkyl radical formed between the two isomeric forms has a rather short lifetime and low resonance stability which further hampers its reaction with the thiol. This reduces the rate of reaction of the second hydrogen-transfer step due to the inherently low hydrogen-abstraction rate constant [9]. Two common ways used to speed-up this process is by increasing the concentration of thiol in the reaction system or by low- ering the reaction temperature; even though sometimes these approaches are not feasible in practical terms [9, 25]. When equimolar thiol–ene ratios are required, for example in network formation, thiol concentration cannot be increased and the reaction temperature should be elevated to prevent crystallization of the polymer.

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Additionally, higher amounts of thiol groups promotes the occurrence of secondary reactions such as disulfide formation [3]. The contribution of all these factors has made internal main-chain alkenes less accountable for the thiol–ene reaction, in particular to what concerns polymer synthesis.

Initiation

Isomerization – propagation

Termination

Thiol–ene coupled product

Scheme 3. General mechanism proposed for the free-radical thiol–ene reaction involving an internal (isomerizable) ene. The species (A^•) denotes the equilibrium radical structure of the intermediary fragmentation adduct.

From a chemical perspective, the cis-to-trans conversion of unsaturations is a ther- modynamically driven process, which can also be induced by reversible addition with thiyl radicals (RS^•) using the thiol–ene reaction [26, 27]. The mechanism con- ceived involves the abstraction of hydrogen from the thiol group, the insertion of the generated thiyl radical to the cis unsaturation of an oleate moiety to form the radical adduct ( A^•_Z) followed by half-rotation about the C9–C10 bond to give ( A^•_E) and subsequent ejection of the thiyl radical by β-fragmentation (regeneration) (cf.

Scheme 4). Mechanistically, the reaction scheme shows essentially the same elementary processes as for terminal enes except the existence of the isomerization step.

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The formation of either cis- or trans-isomers depends on the conformational state of the intermediary fragmentation adduct at the instant of thiyl radical loss [28]. This event changes the double-bond geometry leading to the thermodynamically more stable trans-isomer. The energy difference between the two geometrical isoforms based on the catalysis of 2-butene by HOCH2CH2S^• is 1.0 kcal⋅mol^–1 at 20°C [26].

Thiyl radicals are amongst the most effective agents known to catalyze the cis/trans- isomerization because even a small amount of radical species is capable of making the reaction to proceed [27, 29]. Constitutional isomers cannot be obtained as reaction products since the mechanism does not allow positional shift of the double- bond [29]. Also, the location of the unsaturation along the aliphatic chain, together with tail length, has proven to have no relevant effect on the isomerization itself [28] although it affects the reactive character of the C=C bond as a result of steric hindrance effects. It was shown, for instance, that 1-hexene is 8-times more reactive than trans-2-hexene and 18-times more reactive than trans-3-hexene based on equal C=C/RSH mole ratios [14]. Many other free-radicals (e.g., RSO2•, R3Sn^•, RSe^•, NO2•, or (Me3Si)3Si^•) and elementary radicals (such as Br^• or I^•) are known to in- duce cis/trans-isomerization through an insertion–elimination sequence, although with different efficiency than with thiyl radicals [27, 29-31]. There is also support- ing evidence that oxygen (<0.3 mM) does not seem to play a strong influential role in the effectiveness of cis/trans-isomerization if internal cis-alkenes are employed [32, 33].

a

kZ k_f^Z k_f^E k_a^E

A^•_E A^•_Z

Scheme 4. Proposed reversible addition mechanism of the thiyl radical to a 1,2- disubstituted ene at position C9–C10 (adapted partially from [32]). The conformers (A^•_Zand

A_E^•) denote intermediary fragmentation states of the carbon-centered radical adduct.

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2.1.3. Advantages of the Thiol–ene Reaction

Some distinct benefits of the thiol–ene coupling reaction have already been mentioned in the above sections. One of the key advantages offered by this chemistry over classical free-radical polymerizations is the high ability to overcome oxygen inhibition. In the case of free-radical polymerization using (meth)acrylates, if oxygen adds to a polymeric propagating chain terminus, that chain will end immediately because the formed alkyl peroxy radical will have insufficient reactivity to add to a new ene monomer. This results in short chain fragments, a loose network structure and reduced cross-link density [34]. For the thiol–ene polymerization this does not happen since the peroxy radical is still able to abstract hydrogen from a thiol monomer producing a new thiyl radical that propagates the polymerization with only minimal impact on the main reaction route (Scheme 5). Since thiols act as strong hydrogen donors, they can be added (even in small amounts) to acrylic formulations to suppress the effect of oxygen through trapping peroxy radicals [35]. Importantly, the thiol–ene reaction is also considered an environmentally benign tool that proceeds without the need of solvents and under clean and mild reaction conditions (if induced photochemically), and that rejects potentially toxic metal catalysts so com- monly employed in other ‘click’ reactions [12, 19].

final thiol–ene product

Scheme 5. Hydrogen abstraction vs. oxygen-scavenging routes for the free-radical-mediated thiol–ene reaction involving terminal enes.

Concerning thin-film formation as thermosets for organic coating applications, individual attributes of the thiol–ene reaction are summarized as follows:

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• Self-initiation without the need of photoinitiator. This allows the polymeriza- tion of very thick geometries since UV-light is able to penetrate throughout the thickness of the film without being absorbed by the photoinitiator resulting in homogeneous cure. Therefore, the photopolymers formed are less prone to degradation/yellowing, without generation of any coloured or vola- tile by-products, and fast aging during prolonged exposure to sunlight [36].

If a photoinitiator is to be used, then it must be ensured that the main emission peak of the UV-light source overlaps with the absorption peaks of the initiator for maximum cure efficiency.

• High uniformity in the network cross-link density. This results in materials with homogeneous properties across all dimensions as a consequence of free- radical two step-growth mechanism which ensures optimal mechanical per- formances and tendency for extensive sub-Tg relaxation processes related with physical aging (enthalpy and volume relaxation) [15, 37].

2.2. Renewable Resources for the Thiol–ene Reaction

One remarkable feature inherent to the thiol–ene reaction is that virtually any alkene functional group can participate [9]. As mentioned already, the chemical nature of the double-bond affects, to a large or small extent, the efficiency of the reaction and this much often dictates which ene monomers can be selected for a particular application. So far external enes and norbornenes have had preferential choice due to their highly reactive character. They are, however, derived mostly from pet- rochemical feedstocks and a continual search for sustainable ‘green’ alternatives is of paramount importance in order to curtail their dependence on evermore depleting fossil oil reserves. Moreover, by using bio-based alkenes the nature’s synthetic potential is fully exploited directly in a very efficient way. Many good examples of natural ene-compounds are reported in the literature as building-blocks in polymer science using a wide variety of chemistries [38-41], but only ‘a handful’ are mentioned in particular for the thiol–ene reaction.

For instance, Johansson et al. (2003) functionalized metallic aluminium sheets with mercapto silanes and then reacted the pendant thiol groups with linseed oil under photochemical conditions to yield thin-film vegetable coatings (∼25Å) that exhibited reduced surface friction when subjected to heavy loads [42]. Bantchev and co- workers (2009) investigated the formation of sulphide-modified vegetable and cano- la oils bearing internal cis-unsaturations by butanethiol under UV-irradiation, with

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the aim of producing new lubricants from natural renewables to improve wear and friction properties. The results showed that, working under optimized reaction conditions, it was possible to attain double-bond conversions up to 97% with an isolat- ed yield of 61% [43]. In another study, Türünç et al. (2010) reported the addition of mono- and di-functional thiols to methyl 10-undecenoate, a castor oil derived ene monomer, and successfully polymerized the resulting monomers using TBD as a catalyst, to linear as well as hyperbranched aliphatic polyesters bearing thioether linkages that exhibited good thermal properties [44]. An indirect approach involves the grafting of the double-bonds via an intermediate step with thiol-terminated precursors to make the polymers amenable for further chemical modifications or even post-curing purposes. In this case, Ates et al. (2011) successfully functionalized PGI (a linear C=C unsaturated polyester produced by enzymatic ROP from the macrocyclic lactone globalide) with 6-mercapto-1-hexanol (MH), butyl-3-mercapto propionate (BMP) and N-acetylcysteamine (nACA) as pendant side-chain linkers [45]. Following the same approach, Desroches et al. (2011) successfully synthesized bio-based oligo-polyols by free-radical photo-addition of 2-mercaptoethanol onto rapeseed oil and then the functionalized triglyceride was used in the synthesis of polyurethanes [3]. The combination of all these different methods has culminated recently (2011) in the development of a new vegetable-oil based polyamine issued from grapeseed oil and cysteamine chloride by use of the thiol–ene photoreaction which was then employed in a second step to thermally create a cross-linked material from epoxidized linseed-oil [46]. Limonene, a readily available monoterpenic fragrance, has also been studied in the synthesis of monomers and polymers using methyl thioglycolate as model compound [47]. According to the authors, optimal reaction conditions were developed for the selective functionalization of the terminal vinyl-bond allowing the synthesis of a diversity of different monomers. Direct polymerization of limonene with dithiols was also reported in this proceeding. A few more examples exist in the literature [48-50] but concerning the formation of cross-linked thiol–ene networks the references on bio-based enes are even more scarce. For example, several allyl-, acrylate- and vinyl- ether derivatives of ricinolein (the chief unsaturated triglyceride constituent of castor oil) were synthesized and used together with multifunctional thiols in the preparation of UV-curable systems.

Testing on the films immediately after UV-exposure and one week later indicated increased cross-linking and superior physical properties upon aging [41].

Based on the works developed by Samuelsson et al. (2004) [4] and van der Meulen et al. (2008, 2011) [51, 52], herein it is highlighted the use of monounsaturated oils (Paper I) and globalide based (co)polymers (Paper II) together with a propionate

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ester trithiol to evaluate the reactivity of these 1,2-disubstituted alkenes in bulk and aiming at the production of cross-linked thiol–ene networks.

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3. EXPERIMENTAL

Bellow follows a comprehensive summary of the methods and experimental proce- dures for the presented results. Complete details can be found in Paper I and Paper II.

3.1. Chemicals

The tri-functional thiol cross-linker trimethylolpropane tris(3-mercaptopropionate) (TMP-trimercapto propionate, 398.56 g⋅mol^–1) was kindly supplied by Bruno Bock (Marschacht, Germany). Methyl oleate (99%), methyl elaidate (99%), ε- caprolactone and THF were purchased from Sigma-Aldrich (Stockholm, Sweden) and the photoinitiator 1-hydroxy-cyclohexyl-phenyl-ketone (Irgacure 184^®) was obtained from Ciba Specialty Chemicals. Globalide was a kindly gift of Symrise. All chemicals were used as received without further purification (Figure 3.1).

TMP–trimercapto propionate

Irgacure 184

Methyl oleate Methyl elaidate

Globalide ε-caprolactone

Figure 3.1List of chemical compounds used in this study.

3.2. Techniques and Instrumentation 3.2.1. NMR Spectroscopy

1H and ¹³C NMR spectra of the samples were recorded on a 400 MHz Bruker As- pect NMR spectrophotometer (Karlsruhe, Germany). CDCl3 containing 0.05 vol.% of tetramethylsilane (TMS) was used as d-solvent. Chemical shifts (δ) were

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reported in parts per million (ppm) relative to the tetramethylsilane internal standard (TMS, δ=0.00 ppm). Spectral analysis was made using the Mestrec^® Software.

3.2.2. FTIR Spectroscopy

RT–FTIR spectra were recorded in the mid-region with a Perkin-Elmer Spectrum 2000 (Norwalk, CT) using an MCT detector cooled with liquid nitrogen. The FTIR instrument was equipped with a heat-controlled Golden Gate single reflection ATR-accessory from Graseby Specac Ltd. (Kent, England). The horizontal ATR- sampling unit was modified in order to accommodate a vertical UV-light cable. All FTIR measurements were performed in the reflection mode via the single-bounce diamond ATR crystal. Conventional ATR-FTIR measurements were performed on a Perkin-Elmer Spectrum 2000 equipped with a TGS detector using the Golden Gate setup. Each spectrum collected was based on 32 scans averaged at 4.0 cm^–1 resolution in the range of 600–4000 cm^–1. Data were acquired and processed using the software Spectrum from Perkin-Elmer.

3.2.3. FT–Raman Spectroscopy

FT–Raman measurements were performed with a Perkin-Elmer Spectrum 2000 NIR–Raman equipment with Spectrum software to determine the residual unsaturation and pendant thiol groups remaining in the thiol–ene mixtures and cross- linked films. Each spectrum collected was based on 16-scans using a laser power of 800–1000 mW.

3.2.4. UV/Vis–Spectrometry

Ultraviolet–visible spectroscopy was conducted on a double-bean Cary E1 UV–Vis spectrophotometer. The scan resolution of the UV–Vis spectrometer was 1.0 nm.

3.2.5. UV–light Sources

A Hamamatsu L5662 equipped with a standard medium-pressure 200W L6722-01 Hg–Xe lamp and provided with optical fibers was used as the UV-source for the photo-RTIR measurements. A condenser lens adapter, model A4093 from Hama- matsu, was employed to focus the UV-beam. Neutral density filters with optical densities of 0.2, 0.4, 0.6, 1.0 and 2.0 from CVI Laser Corp. LCC were used to obtain a constant irradiance of 5.0 mW⋅cm^–2. The UV-intensity was measured using

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a Hamamatsu UV-light power meter (model C6080-03) calibrated for the main emission line centred at 365 nm. An UV Fusion Conveyor MC6R equipped with Fusion electrodless bulbs standard type BF9 (UV-fusion lamp) was employed for network formation purposes. The UV-light intensity was determined with a UVICURE^®Plus from ET, Sterling, VA.

3.3. Procedures

3.3.1. Sample Preparation

Two different reaction mixtures containing TMP-trimercapto propionate (89.6 mg) and 1,2-disubstituted alkene (MO or ME, 200 mg) in a molar ratio of 1:1 with respect to thiol–ene functionalities were prepared. Irgacure 184 (2.0 wt.%, ∼2.8 mg) was added to each mixture. The approximate molarities of trithiol, FAMEs and PI in the samples were 0.74, 2.21 and 0.045 M, respectively. The initial bulk mixtures had to be heated slightly prior to the analysis in order to mix all the compo- nents completely.

Thiol–ene mixtures of the different unsaturated P(GI-co-CL) copolymers were pre- pared by dissolving the copolymer in 5.0 ml of THF solvent and then mixing equimolar amounts of trithiol with respect to thiol and ene functionalities so that all groups could react theoretically with each other at the same stoichiometry. The samples were supplemented with a small amount of the UV-initiator Irgacure 184 (∼2.0 wt.%) and then kept at 4°C protected from light until further use. A typical formulation is as follows: 525 mg of P(GI-co-CL) 47/53, 190 mg of TMP- trimercapto propionate and 14.3 mg of Irgacure 184. The same recipe and amounts were used, with omission of the thiol reactant, for the homopolymerized samples.

For the thiol–ene films, the mass ratio necessary for equimolar reaction of thiol–ene functional groups was calculated using the expression:

⋅ ⋅

= ⋅

trithiol

polym. n

t

3

m f DP M

m M (1)

where f is the mole fraction of ene in the copolymer, DP is the degree of polymeri- zation, Mt is the molecular weight of the trithiol and Mn the number-average molecular weight of the copolymer. The number of ene functionalities per copolymer chain is given byf DP⋅ .

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3.3.2. Initiator Photolysis

To obtain a rough estimate for the production rate of primary free-radicals, a dilut- ed solution of Irgacure 184 in methyl oleate was prepared (0.02 wt.%, ∼0.86 mM).

The sample was sealed inside a 1-mm thick quartz cell and then irradiated intermit- tently in the presence of air at 20°C with an UV-intensity of 5.0 mW⋅cm⁻². The absorbance was measured in the range 190–600 nm. The decomposition of photoinitiator with time was monitored by the decrease of the main absorption peak at 247 nm until there was no significant change in the absorption spectrum. All spectra were subtracted from a blank sample containing only methyl oleate in order to eliminate spectral contribution of the solvent (background spectra). Concentrations over time were calculated from the Lambert–Beer law.

3.3.3. UV–induced Reactions

For RT–FTIR measurements of photoinitiated reactions, the heat-controller was set to 60°C and the system left to equilibrate for 10 min. Two drops of the monomers/photoinitiator mixture (∼75 μl) were then applied onto the surface of the ATR diamond probe connected in-situ to a circular heater plate and the measurements were started immediately. The photoreaction and photopolymerizations were initi- ated by vertically irradiating UV-light from the Hamamatsu lamp. Duplicate IR- runs were conducted in the presence and absence of air (by covering the liquid samples with a thin quartz lamella) over a period of 30 min using independent samples.

RTIR continuously recorded the chemical changes over the range of 4000–600 cm^–

1. Spectroscopic data were collected at an optimized scanning rate of 1 scan per 1.67 seconds with a spectral resolution of 4.0 cm^–1 using the TimeBase^® software from Perkin-Elmer. The course of the reaction was followed by monitoring the peaks corresponding to the cis- and trans-unsaturation carbon-carbon double-bonds oc- curring at ∼3010 and 968 cm^–1, respectively. Figure 3.2 illustrates the custom-made RTIR setup utilized in the experiments. Aliquot samples for ¹H NMR analysis were taken before and after the reaction in order to estimate the final conversion of double-bonds (photoreaction system only).

3.3.4. Discontinuous Conversion Studies

Both ¹H NMR and FT–Raman spectroscopies were used for the discrete evaluation of thiol–ene conversions (photoreaction system).The procedure involved the pour-

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ing of a small volume of the thiol–ene sample on top of a glass slide set in contact with the ATR crystal and thermostatted to 60°C. The liquid films were irradiated with a certain dose (mJ⋅cm^–2) and the main portion analysed by FT–Raman to as- sess the level of intensity of the –SH peak at ∼2576 cm^–1 after spectral normalization with respect to the ester carbonyl peak at 1735 cm^–1. The residual fraction attached to the glass was immersed in cold CDCl3 and then analysed by NMR. Proton integration signals of the C=C bonds (vinylic: 5.05–5.54 ppm and allylic: 2.0 ppm) were used to estimate the extent of the reaction using the integral areas of two con- served signals: (i) the methyl ester protons at 3.66 ppm, and (ii) the aliphatic protons at 2.29 ppm next to the ester group. The degree of conversion into C–S bonds was expressed as the cross-average of the values obtained from both unchanged signals.

Figure 3.2 Experimental RT–FTIR setup equipped with an ATR-accessory for single reflec- tion with a fixed incidence angle of 45°.

For the UV-cured films based on PGI/PCL the degree of conversion was estimated via FT–Raman spectroscopy by first normalizing all spectra with respect to the ester carbonyl peak and then taking the ratio of the band between 2600–2560 cm^–1 resulting from the thiol functional group and/or the band at 1660–1680 cm^–1 result- ing from the trans-unsaturation in the unreacted and reacted spectra. Conversions for the homopolymerized ene films and thiol–ene films were calculated by using equation (2), where A denotes the band area before and after cure.

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final

start

Conversion (%) 1 A 100

=⎛ − A ⎞×

⎜ ⎟

⎝ ⎠ (2)

3.3.5. Product Identification

Identification of reaction products and characterization of the chemical structures was accomplished via proton and carbon NMR.

3.3.6. Simulation Software and Modelling

Dynamic kinetic simulations of the reaction system (Scheme 3) were performed using the application software GEPASI version 3.30 [53]. A deterministic routine algorithm called LSODA was used by GEPASI to compute the numerical solution of a set of ODEs. LSODA is a very robust adaptive step-size solver that calculates the stiffness of equations and dynamically switches the method of integration according to this measure [54-57]. The mechanistic model was entered in the software according to the elements listed in Table 1. The kinetic curves generated were plotted against the experimental profiles obtained from RT–FTIR data. To check the agreement between the generated output profiles and experimental conversion data, the simulated concentrations for the Z- and E-isomers were combined and plotted against time conversions of total unsaturations (cis + trans) estimated by ¹H NMR.

A similar procedure was used to verify the simulated-experimental agreement for thiol conversion evaluated by FT–Raman.

3.3.7. Film-formation and UV-curing

For the photo cross-linking of the different (co)polymers, thiol–ene mixtures were spread on glass substrates that were previously cleaned and rinsed with acetone.

Liquid films were applied sequentially up to two layers by letting the solvent evapo- rate between applications and then melted in the oven pre-heated to 85°C for 2–3 min until transparent films were observed. The coated slides were immediately placed in the conveyor belt while still in the molten-state (by use of a 0.4 cm thick glass plate to help the sample retain the molten temperature) and passed four times under the UV-fusion lamp with a line speed of 6.52 m⋅min^–1 to give an overall exposure dose of 0.14 J⋅cm^–2. The samples were then left at room temperature to cool.

Smooth, non-tacky films of 30–40 μm thickness resulted.

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3.3.8. Dynamic Scanning Calorimetry (DSC)

The thermal properties of the cross-linked films were analyzed by DSC. The experiments were conducted on a DSC 820 equipped with a sample robot and a cry- ocooler (Mettler Toledo). The DSC runs were carried out in closed sample pans sealed in air, using the following temperature program: heating from 25 to 120°C (50°C⋅min^–1), cooling from 100°C to –65°C (50°C⋅min^–1), then heating up to 120°C (5°C⋅min^–1). Isothermal segments of 5 min were performed at the conclusion of each dynamic segment. The melt enthalpy was determined from the integration of the Tm peak of the second heat.

3.3.9. Dynamic Mechanical Thermal Analysis (DMTA)

To examine the physical properties of the thiol–ene networks, DMTA was performed on a Q800 DMTA (TA-instruments), equipped with a film fixture for ten- sile testing. Film tension DMTA measurements were performed on rectangular dried film samples (5 × 0.05 mm, width × thickness) were performed between –30 and 140°C, with a heating rate of 3°C⋅min^–1. The tests were performed in controlled strain mode with a frequency of 1 Hz, oscillating amplitude of 0.12 μm, and force- track of 125%.

3.3.10. Sol-content Determination

All cross-linked thiol–ene films were cut into small rectangular sections with approximate dimensions of 1.0 cm × 2.0 cm, dried in the vacuum oven at 50°C for 1 hour, weighed and then soaked overnight in 5.0 ml of THF under gentle stirring conditions. The films were subsequently washed (2×) with THF solvent and placed again in the vacuum oven until all the residual solvent was evaporated. Sol-fractions for the thin-film specimens were determined from mass losses relative to initial dry mass according to the following equation:

f

s

Sol content (%) 1 W 100

=⎛ −W ⎞×

⎜ ⎟

⎝ ⎠ (3) where Ws is the initial dry weight of the film samples and Wf is the dry weight of the same film specimens after sol-extraction. The analyses were performed in triplicate from two independent UV-cured films and the results averaged.

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Table 1. Model definition used to generate the simulations with GEPASI. Sequence ReactionChemical equations a Rate constantsRef. Initiation 1a d In2Ik • ⎯⎯→

31 d1410sk.−− ×this work 1bRSH IRSHIHRSk •• ++⎯⎯⎯→RSH(1)=1.010Msk−− ×⋅711 [25] Isomerization 2a

a fRSA

Z Zk kZ•• +ZZZXYZZZ

aMsZ k.−− =×⋅511 1610 [26] 71 f2010s

Z k.− =× [25 ]

2bf aARS

E E

k kE•• +ZZZXYZZZ

511 a2910MsE k.−− =×⋅ [26]

81 f1610s

E k.− =× [25, 26] Propagation 3 RSH ARSHPRSk •• ++⎯⎯⎯→RSH(2)=1.010Msk−− ×⋅711 [25, 26] Termination 4 t 2RSRSSRk • ⎯⎯→

911 t3010Msk.−− =×⋅ [26] a Initial concentrations: [In]0=4.48×10–2 and [RSH]0=[Z]0=[E]0=2.21 M.

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Thiol−ene Coupling of Renewable Monomers: at the forefront of bio-based polymeric materials