telechelic polymers derived from natural resources as building blocks for polymer...

Telechelic polymers derived

from natural resources as

building blocks for polymer

thermosets

Susana Torrón Timhagen

Licenciate thesis

KTH Royal Institute of Technology, Stockholm 2015 Department of Fiber and Polymer Technology

Copyright © 2015 Susana Torron All rights reserved Paper 1 © 2014 WILEY-VCH

TRITA-CHE Report 2015:13

ISSN 1654-1081 ISBN 978-91-7595-469-1

To my family

Abstract

The growing interest for replacing fossil-based materials with other

greener alternatives, leads to the search of feed stocks from sources

traditionally used for other purposes. Good examples of this are corn,

seeds or beans from which is possible to obtain diverse compounds

extensively used nowadays for the preparation of different polymeric

materials. One important source of raw materials is wood. Its main

component, cellulose, has been used for decades for the preparation of

materials with various applications. Another component of wood that

has gained a lot of interest due to its complex polyester structure is the

suberin present in the bark. Through an hydrolysis it is possible to cleave

those ester bonds and retrieve a large variety of epoxy fatty acids2.

In the present work a procedure for the isolation and purification of an

epoxy fatty acid proceeding from outer Birch bark3, the 9,10-epoxy-18-

hydroxydecanoic acid, is presented. The inbuilt epoxy-functionality of

this compound opens up a broad range of possibilities due to the

multiple chemistries it can be subjected to. Moreover, when functional

groups are introduced at the end of the polymer chain4 we create a large

“tool box” with different options for network formation.

The polymerization of this monomer has been done using enzymatic

catalysis. With this not only is possible to preserve the epoxy group

intact during the whole process5, 6, but also to control the degree of

polymerization and introduce the end-functionalities in a “one-pot

synthesis”.

With an increased number of functional groups in the main chain, the

possibilities for network formation also increases. The different ways

these groups react with each other will then determine the properties of

the network. A control in the reactivity will then lead in the possibility of

material design which in terms of applications adds an additional value

to these biobased materials.

Sammanfattning

Det ökande intresset för att ersätta fossilbaserade material med andra

mer miljövänliga alternativ leder sökandet till nya råmaterial som

traditionellt sett används för andra ändamål. Några råmaterial som

redan används för framställning av olika polymera material är majs, frön

och bönor.

En viktig källa till råmaterial är träd. Dess huvudkomponent, cellulosa,

har använts i sekler för framställning av material för olika applikationer.

En annan komponent av träd som har fått ökad uppmärksamhet på

grund av dess komplexa polyesterstruktur är suberin som utvinns från

bark. Genom hydrolys är det möjligt att klyva esterbindningarna i

Suberin och separera ut olika fettsyror2. Bland dem är 9,10-epoxi-18-

hydroxyoktadekansyra, som kan extraheras från björknäver3, av särskilt

intresse. Denna monomer med en inbyggd epoxifunktionalitet

representerar ett perfekt substrat för olika bindemedelstillämpningar.

Ett sätt att öka reaktiviteten hos 9,10-epoxi-18-hydroxyoktadekansyra är

att tillföra olika funktionella grupper i dess ändar, vilket resulterar i

"telekeliska polymerer". Dock är syntesen av telekeliska polymerer ofta

komplicerad med flera olika reaktionssteg där man ofta använder olika

skyddsgruppskemier. Ett sätt att kringgå detta problem är att använda

sig av en selektiv katalysator, som exempelvis ett enzym. Med hjälp av

dessa naturliga katalysatorer är det möjligt att införa olika

funktionaliteter och samtidigt bevara närvarande reaktiva grupper5, dvs

epoxigruppen, samtidigt som polymerisationsgraden kan kontrolleras

genom reglering av de stökiometriska förhållandena.

Med ett ökat antal funktionella grupper i huvudkedjan, ökar också

möjligheterna till att bilda nätverk. De olika sätten dessa grupper

reagerar med varandra kommer då bestämma egenskaperna hos den

slutliga härdplasten. Alltså, genom att kontrollera reaktiviteten ges

möjligheten att skräddarsy den slutgiltiga produkten, vilket tillför ett

mervärde för dessa biobaserade material.

List of appended papers

This thesis is a summary of the following papers:

Paper I

“Polymer thermosets from multifunctional polyester resins based

on renewable monomers”, S. Torron, S. Semlitsch, M. Martinelle,

M. Johansson, Macromol. Chem. Phys. 2014, 215, 2198-2206.

Paper II

“Oxetane terminated telechelic epoxyfunctional-polyesters as

cationically polymerizable thermoset resins – tuning the reactivity

with structural design”, S. Torron and M. Johansson, Submitted.

Other publications that do not appear in this thesis:

“Telechelic polyesters and polycarbonates using enzyme

catalysis”, Susana Torron, Mats K G Johansson, Eva Malmström,

Linda Fogelström, Karl Hult, Mats Martinelle, Chapter 2 on

“Handbook on Telechelic Polyesters and Polycarbonates”, edited

by Pan Stanford Publishing, Submitted.

Abbreviations

ATR-FTIR Attenuated Total reflectance- Fourier Transform Infrared

CALB Lipase B from Candida Antarctica

CDCl3 Deuterated Chloroform

DA Dimethyl Adipate

DHB 2,5-dihydroxy benzoic acid

DMA Dynamic Mechanical Analysis

DP Degree of Polymerization

EGDMA Ethylene glycol Dimethacrylate

eROP Enzymatic Ring Opening Polymerization

HCl Hydrochloric acid

1H NMR Proton- Nuclear Magnetic Resonance

H2SO4 Sulphuric acid

LiBr Lithium Bromide

MALDI-ToF Matrix-Assisted Laser Desorption/Ionization- Time of Flight

MS Mass Spectrometry

Mw Molecular Weight

NaOH Sodium Hydroxide

PI Photoiniator

PMA Poly(methyl acrylate)

PMMA Poly(methyl methacrylate)

ROP Ring Opening Polymerization

RT-FTIR Real Time- Fourier Transform Infrared

SEC Size Exclusion Chromatography

Tg Glass Transition temperature

THF Tetrahydrofuran

TMPO Trimethylpropyl oxetane

UV Ultraviolet

Sample Code:

EFA Epoxy Fatty Acid (9,10-epoxy-18-hydroxydecanoic acid)

EFA-EGMA Epoxy Fatty Acid end-capped with Ethylene Glycol Methacrylate

EDP3 Epoxy-functional oligomer with Degree of Polymerization 3

ODP3 Oxetane-functional oligomer with Degree of Polymerization 3

EODP3 Epoxy/oxetane oligomer with Degree of Polymerization 3

EODP6 Epoxy/oxetane oligomer with Degree of Polymerization 6

Table of content

1. Purpose of the study………………………………..............…1

2. Introduction……………………………………………………….…2

2.1. Background…………………………………………….………..2

2.2. Monomers from renewable resources………..…..…2

2.2.1. Suberin……………………………………………………….…3

2.3. Polymers by enzymatic catalysis………………….....…5

2.3.1. Telechelic polymers by enzymatic catalysis………….…….7

2.4. Thermoset formation…………………………...………..…8

2.4.1. Photopolymerization………………………..…………..……9

Free-radical polymerization………………………..………9 Cationic ring opening polymerization/ Cationic UV-

curing…………………………………………................…11

3. Experimental………………………………………..………….…12

3.1. Materials…………………………………………………………12

3.2. Extraction of 9,10-epoxy-18-hydroxyoctadecanoic

acid from outer birch bark……………………….……..12

3.3. Synthesis of telechelic oligomers by enzyme

catalyzed esterification …………………………...........13

3.3.1. Synthesis of ,ω-methacrylate end-capped

epoxyfunctional oligomers (EFA-EGMA)…………….…..13

3.3.2. Synthesis of ,ω-oxetane end-capped epoxyfunctional

polyester DP3 (EODP3) and DP6 (EODP6)…………..….13

3.3.3. Synthesis of ,ω-hexane end-capped epoxyfunctional

polyester DP3 (EDP3)……………………………………....14

3.3.4. Synthesis of ,ω-oxetane end-capped aliphatic polyester

DP3 (ODP3)……………………………………………….….14

3.3.5. Synthesis of Dimethyl hexadecanedioate………….…..…14

3.4. Thermoset formation…………………………….......…..15

3.4.1. Radical polymerization of methacrylates…………………15

3.4.2. Cationic polymerization……………………………….…... 15

3.4.3. Dual curing……………………………………………………15

3.5. Analytical methods…………………………………...…....16

3.5.1. 1H NMR……………………………………..………………..16

3.5.2. MALDI-TOF………………………………………….…….…16

3.5.3. ATR-FTIR………………………………………………….….16

3.5.4. RT-FTIR…………………………………………………....…16

3.5.5. SEC…………………………………………………………..…17

3.5.6. DMA……………………………………………………………17

3.5.7. UV-Light sources…………………………………….…….…17

4. Results and discussion………………………………….……18

4.1. Extraction of 9,10-epoxy-18-hydroxyoctadecanoic

acid (EFA) from outer birch bark…………………....18

4.2. Synthesis of telechelic oligomers by enzyme

catalyzed esterification………………………………...…18

4.2.1. 1H NMR ……………………………………..…………….…21

4.2.2. MALDI-TOF MS…………………………………….…….…22

4.2.3. SEC………………………………………………………….…23

4.3. Thermoset formation………………………..………….…23

4.3.1. Polymerization performance. Fourier Transform Infrared

and Photo-Real Time Infrared Analysis…….…25

Epoxide/ methacrylate systems. Cationic polymerization

vs radical polymerization………………………………..…25 Epoxide/ oxetane systems. Homopolymerization vs

copolymerization in cationic polymerization……….….26

4.3.2. Mechanical properties of the formed networks……….…29

5. Conclusions…………………………………………………………32

6. Future work……………………………………………..…………34

7. Acknowledgements……………………………………….……35

8. References……………………………………………………..……36

Purpose of study

1

1. Purpose of the study

The search of natural based raw materials as mile stones for material

formation together with the examination of new greener chemical routes

for their preparation, center nowadays the research of many material

scientists. In a county like Sweden where around 23 million hectares are

covered by forest, that trend is inevitably conducted to the search of raw

materials derived from wood.

The overall purpose of this work is to utilize a monomer derived from

outer birch bark, the 9,10-epoxy-18-hydroxydecanoic acid, to prepare

polymeric materials.

This thesis covers some especific goals as:

extraction and purification of a monomer proceeding from outer birch bark,

definition of a synthetic route that will allow the formation of various oligomers end-capped with different functional groups. Focus on enzyme catalyzed esterifications,

preparation of polymer thermosets using different strategies,

analysis of the structural properties of the networks by mean of their mechanical properties

Introduction

2

2. Introduction

2.1. Background

Traditionally, synthetic polymeric materials have been prepared using

raw materials obtained from fossil sources. However, their poor

degradability, high cost and inevitable extinction, have pushed the trend

towards the search of more environmentally friendly alternatives. In

these terms, the use of compounds obtained from e.g. annual crops,

wood or natural oils are seen as promising alternatives7,8, 9.

The production of materials derived from natural resources has been in

continued development for many centuries. The first documented uses

of natural based materials are dated in 1600 BC, when the Mayan

civilization in Central America is assumed to be the first to use rubber

proceeding from the rubber tree to fabricate balls for their kids. Later in

1839, the discovery of vulcanization of rubber by Charles Goodyear

increased the uses of this raw material as the durability highly increases.

Contemporary to Goodyear, Braconnot, along with Christian Schönbein

and others, developed derivatives of the natural polymer cellulose,

producing new, semi-synthetic materials, such as celluloid and cellulose

acetate. The introduction of fossil resources during the 1900s took over

as a resource for synthetic polymers. The foreseen depletion of these

resources has however during the last decades reawakened the interest

for biobased resources. Nowadays, it is possible to see how the so called

“bioplastics” derived e.g. from corn starch, are implemented in the

market and produced in large scale10.

2.2. Monomers and polymers from renewable resources

Depending on the natural source they proceed from, monomers and

polymers are generally divided into two groups: those proceeding from

vegetal biomass, as i.e. cellulose, terpenes, plant oils or sugars and those

proceeding from animal biomass, as i.e. chitin/ chitosan or casein.

Among the vegetal sources, wood is considered to be one of the most

important sources of raw materials due to its rich and variable

Introduction

3

composition. Wood comprises mainly three components: cellulose,

hemicellulose and lignin. These components have been exploited for

decades for the preparation of diverse biomaterials. One of the

components of wood that have not been exploited as much as the others

is the bark. However, it has been shown that the bark of some trees as

e.g. birch tree, is also an important source of raw materials11. Moreover,

it should be taken into consideration that outer birch bark has

traditionally been used for the only purpose of energy recovery12 and

therefore finding an alternative use to it is of great interest.

2.2.1. Suberin

Suberin comprises 40 wt% of outer bark. Suberin acts as a hydrophobic

barrier to control the movement of water, gases, and solutes as well as

an antimicrobial barrier. Suberin consists of a crosslinked polymer

comprising aromatic and aliphatic components12, 13 (respectively

primary cell wall and suberin lamellae on Figure 1). The main

components of the aliphatic domain are ω-hydroxy fatty acids, ,ω-

dicarboxylic acids and mid-chain di-hydroxy and epoxy homologous.

The primary cell wall also comprises of several triterpenoids and

aromatic structures.

Introduction

4

Figure 1. Model for the macromolecular structure of suberin13. The spheres

represent the points of union between the fatty acids and rest of the suberin.

Through a mild alkaline hydrolysis it is possible to depolymerize the

suberin to extract a large variety of long carbonated chain fatty acids. It

has been shown that outer bark of the birch tree is capable to undergo

this process yielding in a large variety of fatty acids2, 3, 14 (see Figure 2).

Introduction

5

Figure 2. Fatty acids present in majority in birch bark suberin

Among those fatty acids, the 9,10-epoxy-18-hydroxydecanoic acid is the

most abundant (100g per kg of bark). The presence of a carboxylic acid

together with an alcohol as end functional groups in the main chain,

make this monomer a suitable candidate for synthesis of polyesters. At

the same time, its intrinsic epoxy group opens a broad range of

possibilities for network formation or post-functionalization. However,

the reaction conditions usually needed for the synthesis of polyesters

might not always be suitable for the prevalence of the epoxide and

therefore new synthetic routes might be taken into consideration. In this

context, enzyme catalysed polyesterification is seen as a good alternative

to the “traditional” synthetic routes.

2.3. Polymers by enzymatic catalysis

At the same time as the interest for finding greener alternatives to the

traditional fossil based raw materials increases, increases the awareness

for trying to find greener solutions to the traditional catalytic methods.

The search for more environmentally friendly systems that would reduce

side products and avoid the use of harsh reaction conditions is, in this

context, of great interest15. In these terms, enzymes are anticipated as

Introduction

6

suitable candidates for this purpose16-21. Due to their high selectivity,

tedious protective/ deprotective steps are usually avoided. This

selectivity allows e.g. to form epoxyfunctional polyesters from epoxy-

functional ω-hydroxy fatty acids keeping intact the epoxy group through

all the process1, 6, 22. This selectivity is consequence of the specificity of

the enzymes; every enzyme is responsible to catalyze a specific reaction.

One type of enzymes that have been widely used in polymer synthesis

are hydrolases. Hydrolases are responsible to catalyze the cleavage of

bonds by hydrolysis reactions, which then will allow the formation of

polyesters by condensation reactions23-26. One specific group of

hydrolases that has been of great importance in polymer synthesis are

lipases25, 27, 28. In order to facilitate the removal of the enzyme and

terminate the reaction, commercial enzymes are usually immobilized in

polymeric resins. Lipase B from Candida antarctica (CALB) is the

lipase that has been most extensively used for polyester synthesis and

commercial preparations are available, e.g. Novozyme 435.

In general terms, the use of enzymes as catalysts in polyester

synthesis offers many advantages:

- In a first term, as all catalysts, their use help to lower the activation

energy required to reach a certain transition state and thus increase

the reaction rate.

- Secondly, their specificity allows milder the reaction conditions

since less energy is required to break or form certain chemical

bonds. Milder reaction conditions also help to avoid the formation of

side products and backbone scission.

- Possibility to perform reactions in bulk, minimizing the use of

solvents.

- Possibility of tailoring the size of the polymer by introducing an end-

capping group. This point will be further discussed in point 2.3.1.

Telechelic polymers by enzymatic catalysis.

However, the use of lipases in polyesterification reactions is not always

trivial and some factors need to be controlled:

- In order to push the equilibrium towards product formation, the

water formed as biproduct needs to be removed. However, enzymes

require a small amount of water to retain their catalytic activity,

therefore is importance to balance the water content.

Introduction

7

- Lipase B from Candida antarctica (CALB) is thermally stable and

retain its activity for hours, however, it exhibit an upper temperature

limit29 around 90oC and therefore its use is limited to certain

monomers.

- Although preferable, the use of bulk conditions increases

dramatically the viscosity of the system. High viscosity limits the

diffusion of the reactants to the active site of the enzyme and the

evaporation of smaller molecular weight compounds. On the other

hand, the use of solvents favors intramolecular cyclations. These

factors restrict the obtainment of high molecular weight polyesters

through enzymatic catalysis. However it is possible to aim for high

molecular weight polyesters through the synthesis of telechelic

polymers.

2.3.1. Telechelic polymers by enzymatic catalysis

By definition, telechelic polymers are “prepolymers capable of entering

into further polymerization or other reactions through its reactive end-

groups”30. The possibility of further reactivity allows the obtainment of

high molecular weight linear polymers and polymer thermosets. The

introduction of an end-functional group allows also to tailor the length

of the polymer chain since the desired functional group will act as a

“stopper” in the propagation of the chain.

The synthesis of ,ω-functionalized telechelic polymers by enzyme

catalyzed reactions31, 32 is usually achieved by: Ring opening

polymerization (eROP)33-38 and polycondensation20, 39, 40. In eROP, the

-functionality is usually introduced by a functional initiator and the ω-

functionality by the terminator (see Scheme 1).

Scheme 1. Synthesis of ,ω-telechelic polyesters through enzymatic ring opening polymerization.

Introduction

8

In polycondensation, the ,ω-functionalities are introduced by a

compound called “end-capper” that stops the condensation reaction by

reacting with the ends of the polymer chain (see Scheme 2).

Scheme 2. Synthesis of ,ω-telechelic polyesters through polycondensation

The key feature of enzyme catalyzed synthesis of telechelic polymers is

mild reaction conditions and selectivity that allow a large variety of

different end-groups to be incorporated without severe effects of side

reactions. These conditions render it possible to incorporate e.g.,

hydroxyl- or carboxylic acids as end-cappers in polyester backbones as

well as other functional groups e.g. acrylates/methacrylates32, 34, thiols34,

35 5, epoxides38, or to conveniently produce epoxy-functional polyesters

directly from epoxy-functional ω-hydroxy fatty acids1, 22.

2.4. Thermoset formation

Through the formation of polymer thermosets it is possible to create

materials with higher mechanical and thermal properties than the

precursor polymer. Contrary to thermoplastics or gels, where the

networks are formed by physical entanglement of the polymer chains, in

thermosets the polymer chains are crosslinked, i.e. linked by covalent

bonds. This difference in their formation is also translated in a

difference in their properties, while thermosets cannot be dissolved or

melted, thermoplastics can41.

In order to form polymer thermosets it is necessary that the prepolymer

possess two or more functional groups capable to chemically react with

each other or with another compound called “crosslinker”. The more

reactive sites present in the polymer, the more possibilities for network

Introduction

9

formation, at the same time as high crosslinked networks will lead to

stronger materials. In this context, the election of the functional groups

present in the polymer and the way they react with each other will be

determining factors in terms of material design.

In general terms, polymerization reactions are triggered by heat or UV-

light. However, polyesters can undergo degradation at certain

temperatures42, therefore only photopolymerization reactions have been

considered in this work. Moreover photopolymerizations are considered

to be the easiest and fastest way to transform a liquid monomer or melt

into a crosslinked polymer network at room temperature43, 44, which

increases the possibilities for industrial applications.

2.4.1. Photopolymerization

To the greatest extend, three components are needed to carry out a

photopolymerization45, 46 reaction: photoinitiator, UV-light and a

reactive monomer or oligomer. Photopolymerizations involves the

propagation of an active center by interaction with the monomer

through a chain wise mechanism. Among these, the photoinitiator plays

a crucial role since it will determine how the polymerization is

conducted. Upon excitation triggered by UV-light, the photoinitiator

dissociates to form either radicals or ions. However, it is the activated

monomer after reaction with the photoinitiator who truly initiates the

polymerization.

Attending to the scope of this thesis, only radical and cationic ring

opening polymerization will be considered.

Free-radical photopolymerization:

In general terms, radical polymerization follows a typical mechanism:

Introduction

10

In the first step (1), the photoinitiator (PI) forms a radical by

dissociation triggered by UV-light. This radical will then react with the

monomer to form another radical (2) that will then initiate the

propagation reaction that will yield in the formation of the polymer (3).

The termination of the polymerization is usually caused by combination

of two radicals or chain transfer.

Radical polymerizations are usually fast and yield in high molecular

weight polymers or highly crosslinked networks, however, except from

thiol-ene reactions47, they exhibit oxygen inhibition and inert

atmospheres are commonly required48, 49.

Acrylates and methacrylates are very reactive species towards radical

polymerization. Both species are vinyl esters which results in the

formation of very stable radicals due to conjugation with the ester

group50. This radical is more stable in methacrylates than in acrylates,

since it is formed in a tertiary carbon (see Scheme 3). This causes that

the propagation reaction in methacrylates is slower than in acrylates51.

Scheme 3. Radical polymerization of methyl methacrylate and methyl acrylate.

The extra methyl group present in methacrylates also confers rigidity to

the polymer and while poly(methyl acrylate) (PMA) is soft and rubbery,

poly(methyl methcrylate) (PMMA) is a hard plastic.

Introduction

11

Cationic ring opening polymerization/ Cationic UV-curing:

Cationic UV-curing is one of the polymerization processes most

extended in industry52. The term “curing” is usually used in material

engineering to refer to the toughening or hardening of a polymeric

material by cross-linking of polymer chains53.

Some of the main advantages of cationic UV-curing are the lack of

oxygen inhibition and the excellent adhesion properties that some of the

compounds present when cured, e.g. epoxy resins.

As in the free radical photopolymerization, in cationic ring opening

photopolymerization, the reaction starts with the cleavage of the

initiator to form a protonic acid. Depending on how strong the formed

acid is, the ring opening will be caused by nucleophilic attack of the

oxerane to the acid or vice versa. The most common photoinitiators

used in cationic ring opening of cyclic ethers consist on diarylidonium54

or triarylsulfonium55 salts. These compounds generate very strong

protonic acids by photolysis which will then triggered the formation of

an oxiranium ion that will then initiate the cationic polymerization. In

Scheme 4 is depicted the general reaction process for the cationic ring

opening of epoxides.

Scheme 4. Cationic ring opening polymerization of epoxides.

When epoxy-functional polymers are used as starting materials of the

cationic polymerization, the reaction yields in polymer networks.

http://en.wikipedia.org/wiki/Polymer

http://en.wikipedia.org/wiki/Cross-link

Experimental

12

3. Experimental

3.1. Materials

All chemicals used are shown in Figure 3. 9,10-epoxy-18-

hydroxyoctadecanoic acid (EFA) was extracted from outer birch bark

(Betula Pendula) supplied by Holmen AB (c/o Holmen Energi, SE-

89810). The outer bark was used as given and finely ground using a ZM

200 (Retsch) grinder. Ethylenglycol methacrylate, dimethyl adipate,

hexanol, hexadecanedioic acid (99%), ethylene glycol and immobilized

CalB (Novozyme 435) were purchased from Sigma-Aldrich.

Trimethylpropyl oxetane (TMPO) was used as received from Perstorp

AB. Photoinitiators Uvacure 1600 and Irgacure 651 were supplied by

CYTEC and BASF respectively.

Figure 3. List of chemical compounds used in this thesis.

3.2. Extraction of 9,10-epoxy-18-hydroxyoctadecanoic acid from outer birch bark

100 mL of a solution 0.8M NaOH were poured into a round bottom flask

containing 10g of grinded, dried, outer birch bark. In order to

impregnate the bark with the solution, the flask was connected to a

water pump, under stirring for 10 min. After that, the mixture was

Experimental

13

refluxed for 1 hour, cooled to room temperature and the solid residue

was removed with the aid of a centrifuge. The resulting solution was

acidified to pH~6 with a solution 5% v/v H2SO4 and left in the fridge

overnight. The resulting precipitate was isolated by centrifugation and

recrystallized from toluene to yield a light-yellow powder (Yield: 60%).

3.3. Synthesis of telechelic oligomers by enzyme catalyzed esterification

3.3.1. Synthesis of ,ω-methacrylate end-capped epoxyfunctional

oligomers (EFA-EGMA)

9,10-epoxy-18-hydroxyoctadecanoic acid (EFA) and ethylene glycol

dimethacrylate (EGDMA) were introduced in a round bottom flask using

a 3:1 (EFA monomer: dimethacrylate) ratio. A small amount of toluene

(500 wt% relative to the monomer) was used to dissolve the reactants.

Molecular sieves 4Å (25 wt% of the total amount of monomers) and

enzyme (20 wt% of the total amount of monomers) were then added to

the solution and the mixture was heated up to 60oC for 24h. The catalyst

was then removed by filtration and the product used without further

purification.

3.3.2. Synthesis of ,ω-oxetane end-capped epoxyfunctional

polyester DP3 (EODP3) and DP6 (EODP6)

EFA monomer, dimethyl adipate (DA) and TMPO were put into a 25 mL

round bottom flask in a, respectively, 3:1:2 ratio for DP3 and 6:1:2 for

DP6, and heated up until they had reached the molten state at around

85oC. When the mixture had reached that temperature, CALB was added

(10% of the total amount of monomers) and the reaction was allowed to

proceed at open atmosphere for 2h and afterwards allowed to run under

vacuum overnight to remove low molecular weight compounds.

The reaction was stopped by dissolving the product in chloroform and

filtering off the immobilized enzyme. The product was characterized and

used without further purification (Yield EODP3: 94%, yield EODP6:

65%).

Experimental

14

3.3.3. Synthesis of ,ω-hexane end-capped epoxyfunctional

polyester DP3 (EDP3)

EFA, dimethyl adipate (DA) and hexanol were put into a round bottom

flask in a 3:1:2 ratio and heated up to 85oC. After the reaction mixture

had reached that temperature the enzyme was added to the flask (10% of

the total amount of monomers) and the reaction was allowed to proceed

at open atmosphere for 2h and afterwards under vacuum overnight.

The reaction was stopped by filtering out the enzyme and the product

was characterized and used without further purification (Yield: 65%).

3.3.4. Synthesis of ,ω-oxetane end-capped aliphatic polyester

DP3 (ODP3)

Dimethyl hexadecanedioate, Ethylene glycol and TMPO were put into a

round bottom flask in a 3:2:2 ratio and heated up to 85oC. After the

reaction mixture had reached that temperature the enzyme was added to

the flask (10% of the total amount of monomers) and the reaction was

allowed to proceed at open atmosphere for 2h and afterwards under

vacuum overnight. The reaction was stopped by filtering out the enzyme.

The product was purified by column chromatography using silica as

stationary phase and a gradient of eluents, starting with a mixture of

Ethyl acetate: Heptane, 30:70 and increasing to 100% Ethyl acetate.

(Yield: 11%).

3.3.5. Synthesis of Dimethyl hexadecanedioate

Hexadecanedioic acid (1eq.) was suspended in methanol (20mL).

Afterwards HCl (2.2eq.) was added dropwise at room temperature. After

4 hours the formed precipitate was filtered out and dried under vacuum

to yield a white powder (yield: 69%).

Experimental

15


3.4.1. Radical polymerization of methacrylates

Mixtures containing 25mg EFA-EGMA, Irgacure 651 (4 wt% of the

monomer) and 100mg chloroform were homogenized and casted on a

glass substrate. The solvent was allowed to evaporate before covering

with a quartz microscope slide. Free-standing films were obtained by

radical polymerization after 2min of light exposure with an intensity of

21mW/cm2. (Dose: 2.52 J/cm2)

3.4.2. Cationic polymerization

All oligomers subjected to cationic polymerization (EFA-EGMA, EODP3,

EODP6, EDP3, ODP3) were prepared by charging vials with 25mg of

each oligomer Uvacure 1600 (4 wt% of the monomer) and 100mg

chloroform. After homogenization of the mixture, the sample was casted

on a glass substrate and the solvent was allowed to evaporate before

proceeding with the curing. In order to obtain homogeneously cured

films, a quartz microscope slide was put on top of the mixture. Polymer

thermosets using cationic polymerization were obtained after irradiating

the sample with an intensity of ~21mW/cm2 during 2.5min. (Dose: 3.15

J/cm2)

3.4.3. Dual curing

Vials were charged with 25mg EFA-EGMA , Irgacure 651 (4 wt%),

Uvacure 1600 (4 wt%) and 100mg chloroform. After homogenization of

the mixture, the sample was casted on a glass substrate and the solvent

was allowed to evaporate before covering with a quartz microscope slide.

Cured resins were obtained after 2.5 min of light exposure with an

intensity of 21mW/cm2. (Dose: 3.15 J/cm2)

Experimental

16

3.5. Analytical methods

3.5.1. 1H NMR

The spectra were recorded at 400 MHz on a Bruker AM 400 using

deuterated chloroform (CDCl3) as solvent. The solvent signal was used

as reference.

3.5.2. MALDI-TOF

MS spectrum acquisitions were conducted on a Bruker UltraFlex

MALDI-TOF MS with SCOUT-MTP Ion Source (Bruker

Daltonics,Bremen) equipped with a N2-laser (337nm), a gridless ion

source and reflector design. All spectra were acquired using a reflector-

positive method with an acceleration voltage of 25 kV and a reflector

voltage of 26,3 kV. The detector mass range was set to 200-2500 m/z. A

THF solution of DHB was used as matrix. The obtained spectra were

analyzed with FlexAnalysis broker Daltonics, Bremen, version 2.

3.5.3. ATR-FTIR

Analysis was carried out using a Perkin-Elmer Spectrum 2000 FT-IR

instrument (Norwalk, CT) equipped with a single reflection (ATR:

attenuated total reflection) accessory unit (Golden Gate) from Graseby

Specac LTD (Kent, England) and a TGS detector using the Golden Gate

setup. Each spectrum collected was based on 16 scans averaged at 4.0

cm-1 resolution range of 600-4000 cm-1. Data were recorded and

processed using the software Spectrum from Perkin-Elmer.

3.5.4. RT-FTIR

The RTIR analysis were made using a Perkin-Elmer Spectrum 2000 FT-

IR instrument (Norwalk, CT) equipped with a single reflection (ATR:

attenuated total reflection) accessory unit (Golden Gate) from Graseby

Specac LTD (Kent, England). RT-FTIR continuously recorded the

chemical changes over the range 4000-600 cm-1. Spectroscopic data

Experimental

17

were collected at an optimized scanning rate of 1 scan per 1.67 seconds

with a spectral resolution of 4.0 cm-1 using Time Base® software from

Perkin-Elmer.

3.5.5. SEC

Using DMF ( 0.2 mL min-1) with 0.01M LiBr as the mobile phase was

performed at 50oC using a TOSOH EcoSEC HLC-8320GPC system

equipped with an EcoSEC RI detector and three columns (PSS PFG 5μm;

microguard, 100Å, and 300Å) (Mw resolving range: 100-300000 g/mol)

from PSS GmbH. A calibration method was created using narrow linear

polystyrenes standards. Corrections for the flow rate fluctuations were

made using toluene as an internal standard.

3.5.6. DMA

The study of the physical properties of the polymerized resins was

performed on a Q800 DMTA (TA instruments), equipped with a film

fixture for tensile testing. The measurements were done between -60°C

and 160°C, with heating rate of 5°C/min. The tests were performed in

controlled strain mode with a frequency of 1Hz, oscillating amplitude of

10μm and forcetrack of 125%.

3.5.7. UV-Light sources

The light source used for the crosslinking of the films was a Black Ray B-

100AP (100 W, λ=365 nm) Hg UV-lamp with an irradiance of

~21mW/cm2 as determined with an UVICURE Plus High Energy UV

integrating radiometer (EIT, USA), measuring UVA at 320 ≤ λ ≤390 nm.

The light source used for the photo-RTIR measurements was a

Hamamatsu L5662 equipped with a standard medium-pressure 200-W

L6722-01 Hg-Xe lamp and provided with optical fibers. The UV intensity

was measured using a Hamamatsu UV-light power meter (model

C6080-03) calibrated for the main emission line centered at 365nm. The

intensity of the lamp was 7mW/cm2.

Results and discussion

18

4. Results and discussion

4.1. Extraction of 9,10-epoxy-18-hydroxyoctadecanoic acid (EFA) from outer birch bark

The extraction of 9,10-epoxy-18-hydroxyoctadecanoic acid (EFA) from

outer birch bark can easily be accomplish through an alkaline hydrolysis

of the suberin. By lowering carefully the pH to 6 it is possible to isolate

this monomer by a selective precipitation and then purify by

recrystallization from toluene. The EFA monomer accounts 10% of the

total weight of outer birch bark. In the present work an average yield of

60% of pure EFA monomer was obtained after extraction. The purity of

the monomer was evaluated by 1H NMR were it is also possible to see

that the epoxide remains intact during the whole process (see Figure 4).

Figure 4. 1H NMR spectrum of 9,10-epoxy-18-hydroxyoctadecanoic acid (EFA)1.

4.2. Synthesis of telechelic oligomers by enzyme catalyzed esterification

One of the main advantages of enzyme catalyzed esterification is the

possibility of tuning the degree of polymerization by adjusting the


19

stoichiometric ratios of the reactants and introducing an end-capper.

The reactants, usually diacids or diesters, react with each other in an

alternated way until they are consumed. In that moment the end-capper,

usually an alcohol, finishes the reaction by reacting at the end of the

chain. In a general manner, the synthesis of the telechelic oligomers

follows the reaction scheme shown in Scheme 5.

Scheme 5. Synthesis of epoxy-derived ,ω-telechelic polyesters by enzyme catalyzed esterification. R-OH represents an alcohol used as end-capper.

However, it is also possible to perform the reaction using a methyl ester

that will act both as reactant and end capper through a

transesterification reaction (see Scheme 6).

Scheme 6. Enzyme catalyzed synthesis of epoxy-derived ,ω-telechelic polyesters through transesterification reaction.

Two approaches have been considered for the synthesis of

multifunctional telechelic oligomers derived from the EFA monomer:

synthesis of oligomers with two functional groups with different

reactivities or with similar reactivities.

For the synthesis of oligomers containing functional groups with

differentiated reactivity, a methacrylate was introduced as end group of

the epoxy-functional polyester. Epoxides exhibit high reactivity towards

ring opening and methacrylates are known for their ease to polymerize

radically. Both reactions can be triggered by UV-light, nevertheless the

activated species are different, i.e. a cation or a radical, and therefore is

possible to tune the reactivity of the oligomer.

In the case of the multifunctional oligomers containing two groups with

similar reactivities, it was necessary to introduce as end functionality

another group capable to undergo cationic polymerization in a similar


20

way as the epoxide does. It was also necessary to consider that the

intrinsic epoxide of the EFA oligomer is a secondary epoxide and its

reactivity differs to the reactivity of primary epoxides. In these terms, an

oxetane was introduced as suitable end groups. Although the epoxide is

slightly more ring strained than the oxetane and will ring open faster

when attacked by a nucleophile, the oxetane is slightly more basic which

will make it more reactive towards cationic polymerization and therefore

these two groups are considered equivalent. In Figure 5 are depicted all

the oligomers used in this work.

Figure 5. List of telechelic oligomers used in this thesis

All oligomers were characterized by 1H NMR, MALDI-TOF MS and SEC.


21

4.2.1. 1H NMR

In order to evaluate the success of the polymerization some factors were

taken into consideration: permanence of the epoxy group during the

whole process and the appearance/ disappearance of new peaks

corresponding to the formation of new ester bonds. In Figure 6 are

represented the 1H NMR spectrum of two of the prepared oligomers

together with the spectrum of the oligomer.

Figure 6. 1H NMR spectra of the EFA monomer (lower spectrum), EODP3 oligomer (mid spectrum) and EFA-EGMA oligomer (upper spectrum). 1H NMR could also be used to estimate the degrees of polymerization of

the oligomers. Oligomer DPNMR

EODP3 2.2 EODP6 5.1 EDP3 2.7 ODP3 2.3

EFA-EGMA 4.4

Table 1. Degrees of polymerization of all oligomers calculated from their 1H NMR spectrum. More information about the 1H NMR spectra and the calculations of the DP can be found in Paper II.


22

Due to volatility issues, all the epoxide/ oxetane oligomers possess a DP

slightly lower than the one aimed for. All products followed the aimed

trend with respect to size although some deviations were found. This is

proposed to be due to factors such as slight errors in stoichiometry as

well as possible evaporation of reactants.

4.2.2. MALDI-TOF MS

The structure of the oligomers was also determined by MALDI-TOF

mass spectroscopy. As exemplified in Figure 7 for the EODP3, an

average degree of polymerization close to the targeted was obtained in

majority. As can be seen, a small fraction of monosubstituted oligomer is

also obtained. This does not significantly interfere with the controlled

formation of the final polymer networks although it must be considered

if it appears to a larger extent.

Figure 7. MALDI-TOF mass spectrum of the EODP3 oligomer. “n” represents the degrees of polymerization of the disubstituted oligomer and “m” the degrees of polymerization of the monosubstitued56.


23

4.2.3. SEC

As can be seen in Figure 8 for the cases of the epoxy-derived oligomers

with DP3 end-capped with methacrylate (EFA-EGMA) and with oxetane

(EODP3), a broad distribution of molecular weights was obtained for

these enzymatic polymerizations. This type of distributions is common

in synthesis of all oligomers in the present study.

Figure 8. SEC traces of EFA-EGMA oligomer (left) and the EODP3 oligomer (right).


The presence of two or more functional groups in the same polymer

chain, gives us the possibility of tailoring the final properties of the

network by controlling the way this groups react with each other. The

possibilities of network formation will increase with the number of

different functional groups. For the type of oligomers used in this work,

i.e. with two different functional groups, four different networks can be

formed. As shown schematically in Scheme 7, this networks can be

formed by homopolymerization reaction of the groups with themselves

separately (Networks A and B) or at the same time (Network C), or by

copolymerization reaction of the two groups with each other (Network

D).


24

Scheme 7. Schematic representation of the possible networks formed starting

from a ,ω- end functional oligomer with 3 functional groups in the backbone

The different ways the networks are formed will have direct

repercussions on their mechanical properties as will be explained in

point 4.3.2. Mechanical properties of the formed networks.

In order to study the reactivity

of the different functional

groups towards network

formation, FTIR and RT-FTIR

studies were performed.

The real time measurements

were performed by recording

spectrum continuously while

irradiating the sample put on

the ATR crystal. A schematic

representation of the

experimental setup can be

found in Scheme 8.

Scheme 8. RT-FTIR setup for the in

situ photopolymerization analysis.


25

4.3.1. Polymerization performance. Fourier Transformed Infrared

and Photo-Real Time Infrared Analysis

Epoxide/ methacrylate systems. Cationic polymerization vs

radical polymerization.

For the epoxide/ methacrylate

oligomers, two possibilities of

network formation yielding in

three different types of networks

were contemplated: either by

reaction of the groups

independently or in parallel by

using one of the photoiniators at

the time or together.

The success of the radical

polymerization could be

monitored by complete

disappearance of the peak at

1638 cm-1 as well as the

conjugated ester carbonyl at 1710

cm-1. The polymerization proceeds rapidly with a full conversion

obtained after 2.5 min (see Figure 9).

In the case of the thermosets crosslinked by cationic polymerization, it is

possible to observe a variation of the peaks in the region between 600

and 1100 cm-1 (see Figure 10). However, due to peak overlapping the

assignment of the peaks corresponding to the vibrations of the oxirane

ring was challenging. Those vibrations appeared in the same region as

peaks originating from the methacrylate moiety (around 845 cm-1).

Nevertheless, the obtainment of free standing films in an “open air

system” together with the variations in the mechanical properties of the

network were considered as sufficient evidences of the cationic

polymerization. Similar behaviors were observed for the dual cured

networks. Whilst the peaks used to assess the reaction between

methacrylates can clearly be differentiated, the ones corresponding to

the epoxides appear overlapping in the aforementioned region. As will

be explained in point 4.3.2.Mechanical properties of the formed

Figure 9. FTIR spectrum of the

methacrylate/epoxy resin measured by

RT-FTIR during the course of the

radical polymerization1.


26

networks, the formation of the network by reaction of both groups will

have a direct repercussion in the properties of the network as a much

“tighter” network is formed when both groups react. The last was taken

as trustworthy evidence of the dual curing procedure

Figure 10. FTIR spectra of the methacrylate/ epoxide oligomer (lower) and the three networks formed from it through radical polymerization, cationic polymerization and dual curing (from bottom to top).

Epoxide/ oxetane systems. Homopolymerization vs copolymerization

in cationic polymerization

When two functional groups with similar reactivity are present in the

oligomer, the possibilities for network formation increase, since these

two groups may also be able to copolymerize with each other. This is the

case of the oligomers containing an epoxide and an oxetane; both

compounds are cyclic ethers and therefore suitable monomers for

cationic polymerization57.

The oligomers were then subjected to cationic polymerization monitored

by FTIR-RT In order to be able to differentiate the peaks corresponding

to each group, analogous oligomers only containing epoxides or only

containing oxetanes were synthesized (EDP3 and ODP3).

It was observed, that for the oligomers only containing one of the

functional groups, the cationic ring opening was faster for epoxides than


27

for oxetanes. This was proven by monitoring the appearance of new

peaks in the region between 1120-1000 cm-1 and tracking the changes

every 10 minutes during one hour (see Figure 11). Homopolymerization

of secondary epoxides gives rise to a new absorption band at 1073 cm-1

due to the formation of a substituted polyethylene glycol structure

(Figure 10 left), while homopolymerization of the oxetane yields in a new

band at 1062 cm-1 corresponding to a polypropylene glycol structure

(Figure 10 right).

Figure 11. Spectra of EDP3 (left) and ODP3 (right) oligomers taken every 10 min

with focus in the area between 1120-1000 cm-1 56

In order to understand the reactivity towards cationic polymerization of

these two groups when present in the same oligomer, similar oligomers

with different degrees of polymerization, 3 and 6, were synthesized.

Different degrees of polymerization will also mean different oxetane/

epoxy ratios and so, for the oligomer with DP=3 the oxetane/ epoxy ratio

will be 2/3 and for DP=6, 1/3.

As can be seen in Figure 12, already after 10 minutes of exposure to the

UV-light of the epoxy/ oxetane oligomer with DP3 (EODP3), there is a

large increase in absorbance in the region between 1120-1000 cm-1 as

consequence of the formation of new ether bonds. When the absorbance

of the peaks corresponding to the epoxide (1073 cm-1) and the oxetane

(1062 cm-1) are plotted against time (Figure 13), it is possible to see that

the absorbance increase faster for the EODP3 oligomer than for the

EDP3 or ODP3 oligomers. This is due to the preferred copolymerization

reaction between epoxide and oxetane. As already proven by Sasaki58

and Crivello59, 60, when a small amount of epoxide is present during the


28

cationic polymerization of a specie containing oxetane, the reaction rate

increases dramatically in comparison with the polymerization without

epoxide. This is due to the fast generation of an oxonium ion from the

epoxy ring that gives a “kick-start” to the cationic polymerization of the

oxetane61. This and other studies mainly dealing with primary epoxides

and oxetane monomers clearly shown that epoxy and oxirane monomers

readily copolymerize with each other to give polyether polymers62, 63. In

the present study we have extended the concept of copolymerization

between the two groups present in the same polymer chain, to its use as

tool for tailoring the final properties of the thermoset.

Figure 12. Spectra of EODP3 (left) and EODP6 (right) oligomers taken every 10

min with focus in the area between 1120-1000 cm-1 56

Figure 13. Absorbance vs time of the peak resulting from the ring opening of

epoxides (1073 cm-1, left graph) and the peak resulting from the ring opening of

oxetanes (1062 cm-1, right graph)56

As can be seen in Figure 12, the increase in absorbance is not as

remarkable for the epoxy/ oxetane oligomers with DP6 as for the


29

oligomers with DP3. This is because the oxetane/ epoxy ratio is smaller

for the oligomers with DP6 and the copolymerization is less probable.

4.3.2. Mechanical properties of the formed networks

The main factors affecting the mechanical performance of a thermoset

are: rigidity of the backbone, polarity, crosslink density and the presence

of loose chain ends. All the present resins are aliphatic polyesters and

therefore possess similar polarity and rigidity of the backbone. The two

distinctive parameters will therefore be the crosslink density and the

presence of dangling ends.

As can be seen in Figure 14 and summarized in Table 2, the lowest glass

transition temperature was obtained for the epoxy/methacrylate resin

only crosslinked radically. In this case, the network is formed by

covalent binding of the end groups yielding in a rather loose network

(network Type B, according to Scheme 7). On the other hand, the highest

Tg value is obtained for the dual cured network (network type C). This

network is formed by covalent reaction of two functional groups

simultaneously, the epoxide and the methacrylate, which produces a

tighter network.

Although similar, the networks formed by cationic polymerization of the

epoxy/ methacrylate resin and the EDP3 resin present different Tg

values: 24 and 5, respectively. In both cases the initial oligomer present

an average of 3 epoxy groups capable to react through cationic

polymerization (network Type A). However, the EDP3 oligomer possess

long carbon chains as end groups which confers more flexibility to the

network and therefore decreases the Tg.

The networks formed starting from oligomers EODP3 and EODP6

present Tg values different to those mentioned before. This is due to the

fact that the networks are built through copolymerization reaction

between both groups (network Type D). The difference in Tg values

between these two are related to the difference in oxetane/ epoxy ratio.


30

Figure 14. Tan vs temperature representations of the cured resins

The homogeneity of the network is furthermore reflected in the width of

the Tg transition64. In this work, the two types of polymerization used,

radical polymerization and cationic polymerization, follow a chain wise

mechanism. Chain wise mechanisms usually present broader glass

transition intervals, since they are less homogeneous processes than the

step wise mechanisms. The last is evident in all cases except of the

networks formed by copolymerization reaction, EODP3 and EODP6. The

narrow transitions are seen as prove of the high homogeneity of this

networks, especially evident in the EODP3 network, where the higher

oxetane to epoxide ratio enhances the possibilities of copolymerization

reaction and therefore the homogeneity of the network. Exact details on

the reaction mechanism for the copolymerization, is not included in the

present study but the results indicate that this is an area worth to further

study. An increased homogeneity with increased crosslink density

indicates a change in mechanism.

The increase in crosslink density is also seen as an increase in the

rubbery modulus (see Figure 15). This parameter should only be taken

into consideration for the epoxide/ oxetane systems. In the case of the

epoxide/ methacrylate oligomers the films were too fragile to be able of

making conclusions in this region.


31

Figure 15. Storage modulus vs temperature representations of the cured resins.

Code DP Type of network E’

(GPa) E’ rp

(GPa) Tg

(oC) Tan

(E’/E’’)

Cationic polym.

3

3 - 24 0.26

Radical polym.

3

3 - 3 0.53

Cationic +

Radical 3

3 - 40 0.31

EDP3 3

2 0.3 5 0.51

EODP3 3

2 1 15 1.03

EODP6 6 2 0.2 12 0.79

Table 2. Summary of the mechanical properties of the cured resins (E’: storage

modulus at -60oC; E’rp: storage modulus in the rubbery plateau).

Conclusions

32

5. Conclusions

In the continuous search of natural based raw materials the utilization of

a side product from the wood industry to prepare polymer networks is

not only of interest from an academic perspective but also from an

industrial point of view. At the same time, the possibility of tuning the

mechanical properties of the polymer network opens up a broad range of

applications in terms of material design. In the present work a series of

polymer networks based on a renewable monomer have been prepared.

In order to be able of tuning the properties of the network, different end

groups and different polymerization methods were used. In a first term,

methacrylates were used as end cappers of the epoxy-derived oligomers.

In this case, the approach used for the formation of the networks was to

react the two functional groups either separately or at the same time

using two different polymerization methods, i.e. radical and/or cationic

polymerization. The second approach was to end cap the oligomers with

functional groups with similar reactivity as the epoxides. For that

purpose, oxetanes were selected as end capping groups. The overall aim

was to understand if in this case the two functional groups would again

form networks by reacting with themselves or if they would react with

each other. What we observed in this case was the clear preference of

these two groups to react with each other which not only yielded in new

mechanical properties but also in an increase in the reaction rate.

We therefore conclude that the main achievements in this work have

been: 1 monomer. Extraction and purification of a monomer that

proceeds from a “zero value source” as the outer bark of the birch tree. We herein demonstrate the possibility of obtaining a bio-based compound, pure enough to be polymerized, with a straightforward process.

2 functionalities. The synthetic route presented in this thesis, gives us the possibility of preparing telechelic polymers with a large variety of functional groups in the main chain. The control of the degree of polymerization and end-functionalization while the epoxide stays intact throughout the whole process, are achieved by enzyme catalyzed polymerization.

3 polymerization methods. All oligomers prepared possess 2 different functionalities: the intrinsic of the epoxy monomer and the telechelic functionality. In order to selectively react the different functional groups, 3

Conclusions

33

polymerization methods were used: radical polymerization, cationic polymerization and a combination of both (dual curing).

4 combinations. As shown in Scheme 7, for the oligomers prepared in this work containing 2 different functional groups in the main chain (present in different proportions), 4 different combinations for network formation are possible: reaction of the end groups with themselves, reaction of the epoxides with themselves, reaction of the two groups at the same time and reaction of the two groups with each other. The different ways these groups are “connected” will determine the properties of the networks.

Future work

34

6. Future work

The knowledge gained during the present work encourages us to

continue our research towards other natural based monomers and other

polymerization methods that will allow the formation of polymer

thermosets. The preparation of polymeric materials derived from

natural resources is an emerging field and in continue development and

therefore invite us to challenge ourselves in this search.

In the present work we have demonstrated how polymer networks can

be easily synthesized by combining an epoxy-functional monomer with

different end-functional groups. The properties of the network can then

be tailored by choosing how these groups will react with each other. In

this context, the only variation of the monomer or the end-capper will

result in the formation of new networks with, inevitably, different

properties.

The use of enzymes in the synthesis of polymers is of great interest and

therefore a field we still want to explore. Enzymatic catalysis has been

proven to be a useful tool for the synthesis of polymers that with other

“traditional” routes cannot be prepared, e.g. polyesters prepared by

eROP of macrolactones65-67. This opens many new possibilities also in

terms of material design.

As already proven by us and others, the selectivity of the enzymes allows

the synthesis of polymers while keeping intact other groups intrinsic of

the monomer. This allows e.g. the possibility of post-functionalizing the

surface of the formed network, which will increase its range of

applications.

The polymers and oligomers prepared in this work are rather fragile and

therefore far away of having an industrial application. However, there

are some options for strengthening the networks as for example using

more bulky monomers.

Acknowledgements

35

7. Acknowledgements

In a first term, I would like to thank the People Programme (Marie Curie

Actions) of the European Union’s Seventh Framework Programme

FP7/2007-2013/ under REA grants agreement no [289253] which have

funded the research leading to these results and the REFINE project.

With this, I would also like to thank all the partners of the REFINE

project, ESRs and ERs, for good discussion about science and not

science, collaborations and great meetings around Europe.

My greatest thank is to my main supervisor Mats Johansson for always

being supportive and helpful and for having given me the opportunity of

performing my PhD-studies.

I would also like to thank my main co-author, Stefan Semlitsch and my

co-supervisor, Mats Martinelle, for a great work together and for always

being open for new ideas and projects. In this context I would also like to

thank some of the other members of the BioCat group, Karl Hult and

Maja Finnveden, for great collaborations and meetings.

Eva Malmström is also thanked for having given me the opportunity to

learn more about “Telechelic polymers”.

Sandra Garcia Gallego is thanked for all the “technical” help while

writing this thesis.

All my current and former colleagues (and friends) at the Division of

Coating Technology are thanked for creating the absolute best working

environment possible, because “there are never stupid questions” and

for always being warm and welcoming to new people.

Last, but definitely not least, I would like to thank my family for always

being a great support and for helping me to see the positive side of the

things.

All this people are thanked because in larger or smaller way, more

professional or more personal, have contributed to this first half of my

PhD studies. My greatest thank to all of you!

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36

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