Effect of Chain End Functional and Chain Architecture On

EFFECT OF CHAIN END FUNCTIONAL AND CHAIN ARCHITECTURE ON

SURFACE SEGREGATION

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Zimo Zhang

June, 2017

EFFECT OF CHAIN END FUNCTIONAL

AND CHAIN ARCHITECTURE ON SURFACE SEGREGATION

Zimo Zhang

Thesis

Approved: Accepted:

______Advisor Dean of the College

Dr. Mark D. Foster Dr. Eric J. Amis

______Faculty Reader Dean of the Graduate School

Dr. Li Jia Dr. Chand Midha

______Department Chair Date

Dr. Coleen Pugh

ABSTRACT

The objective of the research was to study the effects on surface segregation in binary polymer blends of both chain end functionalization of linear chains, and changes in architecture. An important question for the formation and application of a polymer thin film is the degree to which end group functionalization can influence the segregation of a chain to the air/polymer and polymer/substrate interfaces. For the first part of this study, well-defined polystyrene and hydroxyethylated functionalized polystyrene of exactly the same molecular weight (Mn = 6000 g/mol) were synthesized using anionic polymerization in order to minimize the impact of factors other than end group functionalization in the study of the segregation driven by the functionalization. Thin (90 nm) films of blends of these two chains spun cast on silicon substrates were investigated. Key to the study was use of a new method called Surface Layer Matrix Assisted Laser Desorption Ionization

Time-of-Flight Mass Spectrometry (SL-MALDI-TOF-MS) which determines the composition at the surface (< 2 nm depth) of entire polymer chains, rather than the segment or chain end composition measured with other techniques. This technique requires no isotopic labeling. The most striking finding is that the surface region is not only depleted in the high energy chain end functionality, but, in fact, depleted in chains containing the functional group. Thus, for the first time, depletion of the entire chain, driven by only a single functionalized end group, was observed directly. The depletion of the surface in

functionalized chains varies with composition and is more pronounced for blends of near- symmetric composition.

For the study of the effect of architecture on surface segregation, star-branched polymers with two different architectures were synthesized. Well-defined 5.5k 4-arm star was successfully synthesized using a combination of anionic polymerization and silane linking chemistry. The structure of the product was characterized using Size Exclusion

Chromatography (SEC) and MALDI TOF MS. The results show controlled molecular weight, a well-defined structure, and very high purity. “H-shaped” polymers have been much less commonly studied than star polymers and are more challenging to synthesize.

Well-defined H-shaped polystyrene was synthesized using end linking of living arms to a polymeric linking agent. The α,ω-functionalized polymeric linking agent was made using a combination of anionic polymerization with a difunctional initiator and a silicon chloride functionalization reaction with inverse addition. The final product was characterized using multi-detector SEC with MALDI quantified parameters of the structure. The overall molecular weight matches well those of a series of well-defined branched chains already studied for their blend surface segregation and pure melt surface fluctuations behavior, allowing for insightful comparisons.

iii

ACKNOWLEDGEMENTS

I would like to thank my advisor Dr. Mark D. Foster for his guidance in this research.

I would like to thank my co-advisor Dr. Roderic P. Quirk for his helping in anionic polymerization. I would like to thank Dr. Li Jia as a reader and a committee member for this thesis. I would like to thank Dr. Chrys Wesdemiotis for the help in MALDI-TOF characterization.

I would like to thank my senior students Dr. Qiming He and Mr. Fan Zhang for their helping in the synthetic work. I would like to Thank Mr. Jake Hill and Mr. Kevin Endres for the help in the research for surface segregation. I would like to thank Mr. Selim

Gerislioglu for the MALDI measurements. I would also like to thank Dr. Foster’s research group for the support in my research progress.

I would like to thank all my friends especially, Ji-Song for the encouragement during my two-year study in U.S.

Finally I would like to thank my father and my mother to give me this great chance to study in The University of Akron.

TABLE OF CONTENTS

CHAPTER Page

ABSTRACT ...... ii

ACKNOWLEDGEMENTS ...... iv

TABLE OF CONTENTS ...... v

LIST OF FIGURES ...... ix

INTRODUCTION ...... 1

1.1 Anionic Chain End Functionalization ...... 1

1.2 Anionic Synthesis of Star-Branched Polymers ...... 4

1.2.1 Arm-First Method ...... 4

1.2.2 Core-First Method ...... 6

1.3 Surface Segregation ...... 7

1.4 Surface Layer Matrix-Assisted Laser Desorption Ionization- Time of Flight

Mass Spectrometry (SL-MALDI-TOF MS) ...... 10

1.5 Statement of the Problem ...... 11

EXPERIMENTAL ...... 13

2.1 Inert atmosphere techniques ...... 13

2.1.1 High vacuum techniques ...... 13

2.1.2 Dry Box Manipulation ...... 15

2.2 Purification of Reagents ...... 16

2.2.1 Solvents...... 16

2.2.2 Monomer ...... 18

2.2.3 Terminating Agent and Linking agent ...... 19

2.3 Synthesis of 6k hydroxyethylated functionalized polystyrene ...... 21

2.4 Synthesis of Difunctional Initiation system ...... 24

2.4.1 Purification of 1,3-bis(1-phenylethenyl)benzene (DDPE) ...... 24

2.4.2 DDPE\sec-BuLi system ...... 25

2.5 Synthesis of Branched Chains ...... 26

2.5.1 Synthesis of 4-Arm-Star polystyrene ...... 26

2.5.2 Synthesis of H-shaped polystyrene ...... 30

2.6 Molecular and Blend Surface Characterization ...... 37

2.6.1 Size Exclusion Chromatography ...... 37

2.6.2 1H NMR and 13C NMR spectroscopy ...... 37

2.6.3 MALDI-TOF mass spectrometry ...... 37

2.7 Characterization of Surface Composition ...... 38

2.7.2 SL-MALDI-TOF MS Measurement ...... 39

CHAPTER III ...... 40

3.1 Molecular weight and distribution determination...... 40

3.2 Chain end functional group characterization ...... 41

3.3 MALDI-TOF mass spectrometry ...... 44

3.4 Surface segregation due to the chain end functionalization ...... 46

3.4.1 Calibration using Bulk MALDI-TOF mass spectrometry measurement .. 46

3.4.2 SL- MALDI-TOF mass spectrometry for polymer blends ...... 48

3.4.3 Comparison Between Surface and Bulk Measurements ...... 53

SUMMARY ...... 56

CHAPTER IV ...... 57

4.1 Preparation of Hydrocarbon Soluble Difunctional initiator ...... 57

4.1.1 Purification of DDPE ...... 57

4.1.2 Synthesis of the DDPE/sec-butyllithium difunctional initiator ...... 60

4.2 Synthesis of Well-defined 5.5k 4-arm Star Polystyrene ...... 65

4.3 Synthesis of Well-defined 38k H-shaped Polystyrene ...... 67

4.3.1 Synthesis of the 2-arm Precursor ...... 69

4.3.2 Synthesis of the Polymeric Linking Agent ...... 72

4.3.3 Synthesis of H-shaped Polystyrene ...... 77

SUMMARY ...... 82

vii

REFERENCES ...... 83

viii

LIST OF FIGURES

Figure Page

Figure 1.1 Basic concept for chain end functionalization...... 2

Figure 1.2 Chain end functionalization using an alkyl halide...... 2

Figure 1.3 Chain end functionalization using substituted 1,1-diphenylethylenes ...... 2

...... 2

Figure 1.4 Chain end functionalization using functional chlorosilane...... 3

Figure 1.5 Ethoxy chain end functionalization using ethylene oxide...... 3

Figure 1.6 Synthetic route of star shaped polystyrene using DVB as “living linking agent”...... 6

Figure 1.7 Schematic of surface segregation and interfacial segregation in a binary blend film on a substrate. In this particular case the same component is favored at both interfaces. From reference 38, used with permission...... 8

Figure 2.1 Illustration of the high vacuum line apparatus. (From Quirk, R. P.; Ocampo,

M. Material Matters, 1, 10, 2006. used with permission) ...... 14

Figure 2.2 General view of the dry box system (From Quirk, R. P.; Ocampo, M.

Material Matters, 1, 10, 2006. used with permission) ...... 16

Scheme 2.3 Reaction scheme for the synthesis of polystyrene and polystyrene with ethoxyl end group...... 21

Figure 2.4 Illustration of the apparatus used to synthesize polystyrene and hydroxyethylated polystyrene...... 22

Scheme 2.5 Synthetic route for DDPE\sec-BuLi difunctional initiator ...... 25

Scheme 2.6 Reaction scheme for the synthesis of 4-arm star polystyrene...... 27

Figure 2.7 Illustration of the reactor used to synthesize four-arm polystyrene...... 28

Scheme 2.8 Reaction scheme for the synthesis of H-shaped polystyrene...... 31

Figure 2.9.1 Illustration of the reactor used to synthesize 2-arm precursor...... 32

Figure 2.9.2 Illustration of the reactor used to synthesize polymeric linking agent. 34

Figure 2.9.3 Illustration of the reactor used to synthesize H-shaped polystyrene. .. 35

Figure 3.1 SEC trace for PS-H and PS-etOH...... 40

Figure 3.2 NMR result for 6K PS-H...... 42

Figure 3.3 NMR result for 6K PS-etOH...... 42

Scheme 3.4 Possible side reaction from the fomaldehyde impurity in the methanol during termination for PS-H (upper) and Ps-etOH (lower)...... 43

Figure 3.5a MALDI-TOF MS result for 6K PS-H...... 44

Figure 3.5b MALDI-TOF MS result for 6K PS-etOH...... 45

Figure 3.6 Detail of the bulk MALDI-TOF MS spectrum for the PS-etOH(90) – PS-

H(10) blend, with the entire spectrum shown in the inset...... 47

Figure 3.7a SL-MALDI-TOF result for the PS-H film, with the entire spectrum shown in the inset...... 49

Figure 3.7b Enlargement of a portion of the SL-MALDI-TOF spectrum for the PS-H film, showing the small peaks between peaks of the major distributions...... 50

Figure 3.8a SL-MALDI-TOF result for a PS-etOH film, with the entire spectrum shown in the inset...... 50

Figure 3.8b Enlargement of a portion of the SL-MALDI-TOF spectrum for a PS-etOH film, showing the details of the region between peaks of the major distributions...... 51

Figure 3.9a SL-MALDI-TOF result for a PS-etOH(90) – PS-H(10) blend film, with the entire spectrum shown in the inset...... 52

Figure 3.9b Enlargement of a portion of the SL-MALDI-TOF spectrum for a PS- etOH(90) – PS-H(10) blend film, showing details of the region between peaks of the major distributions...... 52

Figure 3.10 Comparison of apparent compositions measured using the conventional approach for “bulk” and SL-MALDI-TOF MS for the surface. Ideal behavior corresponding to the apparent bulk composition equaling the design composition is shown with a red line...... 54

Figure 4.1.1 13C NMR result for DDPE...... 58

Figure 4.1.2 FT-IR result for DDPE, with chemical structures shown in the inset. The arrow marks where a peak for the carbonyl group would appear if some of the carbonyl has not been reacted...... 58

Figure 4.1.3 Chromatograms for DPPE using SEC with UV detection after chromatography (black) and later after four recrystallizations...... 59

Scheme 4.1.4 Preparation of dilithium initiator...... 60

Figure 4.1.5 SEC-UV results of the methanolysis products after each stepwise addition of butyllithium to the DPPE...... 61

Figure 4.1.6 1H NMR result for the methanolysis difunctional initiator. The orange square indicates where peaks would be expected if the product were not pure diadduct. 63

Figure 4.1.7 MALDI result for (upper) the difunctional initiator with salt and (lower) for matrix and salt alone measured as a background for comparison...... 64

Figure 4.2.1 SEC results for precursor arm (blue), crude product (red dashed), and purified 4-arm star (black)...... 66

Figure 4.2.2 Enlarged portion of the MALDI result for the 4-arm star polystyrene, with the chemical structure and whole spectrum shown as insets...... 67

Figure 4.3 Reaction scheme for the synthesis of H-shaped polystyrene...... 68

Figure 4.3.1 SEC result for the methanolysis α,ω-lithium living chain with the chemical structure shown in theas inset...... 70

Figure 4.3.2 MALDI result for the product of methanolysis of the α,ω-lithium living chains with the whole spectrum and chemical structure shown as insets...... 71

Figure 4.3.3 MALDI result for the product of methanolysis of the polymeric linking agent with the chemical structure and whole spectrum shown as insets...... 73

Figure 4.3.4 SEC result for the product of methanolysis of the polymeric linking agent with the chemical structure as inset. Also shown is an enlargement of that portion of the chromatogram highlighted by the orange square around the shoulder...... 74

Figure 4.3.6 SEC chromatograms for various stages of the synthesis of the 6-end pompom polystyrene: curve (1), precursor polystyrene before end-capping with excess tetrachlorosilane; curve (2), after freeze-drying of the end capped precursor polystyrene with excess tetrachlorosilane; curve (3), arm PS-6-oligoBD after end-capping with BD units; curve (4), after one week linking reaction between α,ω -chlorosily-functionalized polystyrene and PS-6-oligoBDLi; curve (5), after fractionation. From reference62. Used with premission from ACS...... 76 xii

Scheme 4.3.5 Possible side reaction during the termination with methanol...... 76

Figure 4.3.7 SEC results for the, arm (black), crude H-shaped PS product (red), after chromatography (blue), and after three fractionations (pink). The curves are normalized to overlap with each other...... 78

Figure 4.3.8a MALDI spectrum in the low molecular weight region for the crude H- shaped PS product ...... 80

Figure 4.3.8b Zoom in spectra for the crude prodrct on low molecular region with chemical structure as insert...... 81

Figure 4.3.8c MALDI result in the low molecular weight region for the purified product after three fractionations...... 81

xiii

CHAPTER I

INTRODUCTION

In this chapter, the principles of anionic polymerization including methods for chain end functionalization and methods for synthesis of star-shaped branched chains will be reviewed. After this, the phenomenon of surface segregation in polymer blend films will be discussed. In the last part of this chapter, a recently developed method for the determination of surface layer composition of polymer films, called Surface Layer Matrix-

Assisted Laser Desorption Ionization- Time of Flight Mass Spectrometry (SL-MALDI-TOF

MS) will be introduced.

1.1 Anionic Chain End Functionalization

Anionic polymerization, first reported by Karl Ziegler and coworkers1 in 1936, is a chain-growth polymerization using an anion as the active center. The unique feature of living anionic polymerization is the absence of chain transfer and chain termination reactions2,3. The benefits from this unique feature is that one can make polymers with predictable molecular weight and the polydispersity of the product can be very small. Using the the living carbanionic chain end (e.g. in poly(styryl)lithium), several techniques have been developed to make to facile quantitative chain end functionalization4. The basic concept exploited for these functionalization reactions can be seen in Figure 1.1.

Figure 1.1 Basic concept for chain end functionalization.

Figure 1.2 Chain end functionalization using an alkyl halide.

In Figure 1.1 P is the polymer chain, X is the functional group and Y can be one of several groups depending on the method used. One example, shown in Figure 1.2, uses an alkyl halide as Y to react with the living chain end5,6. However, the problems for these reactions are the existence of side reactions and the relatively low yield. Another method shown in Figure 1.3 uses a substituted 1,1-diphenylethylene as the reagent to react with the living chain end to introduce the functional group to the main chain7,8,9–13.

Figure 1.3 Chain end functionalization using substituted 1,1-diphenylethylenes

This is a relatively efficient way to functionalize the chain end. The drawback is that it is hard to synthesize the substituted 1,1-diphenylethylene and the functional groups need to be protected for the relatively acidic functional group14. For the study of interfacial segregation a disadvantage of using these polymers is that the end group is quite bulky, which may affect the dynamics of the main chain as well as the thermodynamics of the surface segregation15–19. The third method of chain end functionalization, shown in Figure

1.4, is to use a functional chlorosilane to react with the living chain end.

Figure 1.4 Chain end functionalization using functional chlorosilane.

This technique has the same problem as the previous technique, which is that the functional group needs to be protected during the reaction. Since chlorosilane is subject to hydrolysis, this agent is not stable in air and needs to be handled carefully. Quirk and Ma20 first reported in 1988 an efficient method to generate an ethoxy chain end functionality using ethylene oxide reacting with a living chain end . The reaction can be seen in Figure 1.5.

Figure 1.5 Ethoxy chain end functionalization using ethylene oxide.

This technique is effective for the chain end functionalization because of its efficiency and the elimination of side reactions. Polymers for this research were synthesized using this method.

1.2 Anionic Synthesis of Star-Branched Polymers

An important route to well-defined branched polymers, specifically star-shaped polystyrene, is anionic polymerization4,21. Studies in the variation in physical properties of polymer with changing chain architecture benefit from advances in synthetic methods22,23.

There are two main approaches to synthesizing star-branched polymers: (1) the “arm-first method” 24,25 and (2) the “core-first method”.26,27

1.2.1 Arm-First Method

The idea for the arm first method is to synthesis the living chain first. Then a linking reaction between the anionic chain ends and a multi-functional electrophile is used to make the star-branched polymer. In 1962 Morton et al.28 used methyltrichlorosilane and tetrachlorosilane as the linking agent to terminate the living poly(styryl)lithium to synthesize three and four arm symmetric stars. The results showed that the product contained two arm impurity for the synthesis of 3-arm star and both two and three arm impurities for the synthesis of the 4-arm star due to incomplete linking. The linking reaction for the first arm could be performed within 10 minutes while it took several hours for the second arm and a few days for the third arm to link, depending on the molecular weight of the arm. This diffusion controlled reaction is strongly affected by steric hindrance. Based on this, several years later Roovers et al. 29 used 1,2-bis(dichloromethylsilyl)ethane instead

of tetrachlorosilane as the linking agent and successfully synthesized a well-defined 4-arm star polystyrene. The reason why 1,2-bis(dichloromethylsilyl)ethane was used as linking agent was that the two silicon atoms are separated by an ethylene unit. This spacer group reduces the steric hindrance. At the same time, the reactivity of the chlorosilyl groups is increased by distributing the four chlorine atoms over two silicon atoms. This method is one of the most efficient ways to synthesize star-branched polymers because the whole reaction is controlled and there is no side reaction. The disadvantage of this method is that there is no living chain end after the linking reaction is done, so no further functionalization or modification of the product is possible.

Another method is to use so-called “living linking agents.” As an example, living chains can react with a small amount of divinylbenzene (DVB) to synthesize star polymers.

This method was first reported by Milkovich et al.30 and later developed by Rempp et al.31,32. The polymerization of this difunctional monomer produces a micro-gel compound which acts as the core of the star. The synthetic route can be seen in Figure 1.6. This method includes three steps: (1) the crossover reaction of the living chains to DVB, (2) the homopolymerization of DVB and (3) the linking reaction of excess living arms with the vinyl groups of the DVB repeating units. The final star-shaped product is still living, which provides the chance for further functionalization. Studies have shown that the rate of the crossover reaction between PSLi and DVB is of the same order of magnitude as the rate of the homopolymerization of DVB31. This means that the DVB block will have a uniform structure. However, this method is very complicated since the agent is hard to purify and

it is hard to minimize the side reaction. The final product always shows broad dispersity with impurities that are highly branched.

Figure 1.6 Synthetic route of star shaped polystyrene using DVB as “living linking agent”.

1.2.2 Core-First Method

For the circumstance that living chain ends on the final product are needed for further functionalization, the core-first method is preferred over the arm-first method using the living linking agent. In the core-first method, the polymerization is initiated by a multifunctional organometallic compound. In this case, the initiator species acts like a core.

This multifunctional initiator needs to fulfill several requirements to produce a well- defined product. First of all, the initiator needs to be soluble in hydrocarbon solvent. 6

Second, the reactivities of all the initiation sites need to be similar. Third, the initiation rate of the active centers must be higher than the propagation rate of the polymer33,34. Few organometallic compound meet these requirements and have been used for the synthesis of star-branched polymers.

The first multifunctional initiator used was sodium naphthanlene35. The two active centers of the naphthalene sodium associate strongly, so that the use of a polar solvent such as tetrahydrofuran at low temperature (-78 °C) is needed. However, these conditions lead to a loss of control in the synthesis of polybutadienes. Another problem is that the initiation rate is relatively low compared with the rates for other butyllithium initiators, so the polydispersity of the final product is large. Ronald and coworkers36 used sec-butyllithium to react with pure m-divinylbenzene at -79°C in the presence of triethylamine. The results showed that the initiator had an overall functionality of 1.98. At the same time, no trifunctional and tetrafunctional impurities were detected. Quirk and Ma37 reported a stable, reproducible, hydrocarbon soluble difunctional initiator using the quantitative addition of

2 moles of sec-butyllithium into a solution of 1,3-bis(1-phenylethenyl)benzene. Narrow dispersity was observed for polystyrene and polybutadiene chains initiated by this difunctional initiator in the presence of THF or lithium butoxide. It is a very efficient to use this difunctional initiator in a core-first method to synthesize an α,ω-living chain and for the further synthesis of complicated branched architectures.

1.3 Surface Segregation

Surface segregation refers to a phenomenon in which the compositions of polymers at the air/polymer interface of a polymer/polymer blend are different from the compositions

in the bulk. The term "interfacial segregation" can be used for both segregation to the air/polymer interface and segregation to the polymer/substrate interface. This interfacial segregation occurs so that the free energy of the whole system is minimized. When a system contains two components with different surface energies, the system distributes the two components so that an overall free energy reduction is achieved by putting one species preferentially at an interface, even though the gradient in composition caused by the segregation has a free energy cost. In that case the component with lower surface energy will be enriched at the surface. An example of the composition profile resulting in the case that the same species if preferred at both interfaces is shown in Figure 1.638.

The amount of the surface enrichment by a component can be quantified using the interfacial excess, Z*. The interfacial excess is an integral measure of the preference for

one species at the interface relative to the composition in the "bulk" of the phase and it is measured once the system reaches thermodynamic equilibrium38:

푍 푍* = 푏푢푙푘[ɸ(푍) − ɸ (푍)]푑푍. （2.1） ∫0 푏푢푙푘

In equation 2.1, ɸ(푍) is the volume fraction of the component of interest at depth Z and

ɸ푏푢푙푘(푍) is the volume fraction of the same component in the bulk. Zbulk is a depth at which the composition becomes uniform with depth. The units of Z* are units of length. When

Z* = 0, it means there is no surface segregation. When Z* > 0 for a certain component, that component enriches the interface. When Z* < 0 that component is depleted from the interface. Several factors have been reported to have an impact on interfacial segregation.

Jones and coworkers39 reported in 1989 that deuterated polystyrene was enriched at the surface of blends of conventional and deuterated polystyrene of the same molecular weight.

Hariharan and coworkers40 reported that the shorter polymer chain was enriched at the surface due to the molecular weight effect. Botelho do Rego and coworkers41 observed that polystyrene with the last two repeat units predeuterated was enriched at the surface of a blend with chains not having this end labeling. This represents a special case of the isotopic effect in which the isotopic driving force for segregation is localized to just the end of the chain. The chain architecture can also have an impact on surface segregation42,43. Long- branched chains are generally preferred at the surface over linear analogs44. Cyclic chains are also preferred at the interface over linear analogs for sufficiently high molecular weight38,45. Differences in configurational statistics between polymers of otherwise very similar chemistry (e.g. polyolefins) can also lead one or the other of two polymers to be preferred at the surface43.

1.4 Surface Layer Matrix-Assisted Laser Desorption Ionization- Time of Flight Mass

Spectrometry (SL-MALDI-TOF MS)

In general, the compositions of surfaces are difficult to study. This is due to the small amount of material that is present at the surface and the fact that the composition varies with depth only over a molecular scale. So the measurements need to be done with great depth resolution and high sensitivity. Previous studies of surface segregation have been done with contact angle measurements46,47, x-ray photoelectron spectroscopy (XPS)48,49, secondary ion mass spectroscopy (SIMS)49–51, sum frequency generation vibrational spectroscopy (SFG)52,53, x-ray and neutron reflectivity36,54,55, and surface-enhanced Raman scattering56. However, each of these measurements faces limitations in probing the surface composition directly or quantitatively. SL-MALDI-TOF MS is a new method for surface analysis45. This technique is based on the concept behind MALDI-TOF MS and can be done on the same instrument. The basic idea of MALDI-TOF MS is to use ionization of entire chains and Time of Flight analysis to determine the mass to charge ratio, m/z, for the chain. A laser striking a sample containing an ionizing salt and “matrix” is used to generate a chain with a charge. The wavelength of the laser is carefully chosen to fit the wavelength region in which the matrix has a strong absorption. The matrix is introduced into the analyte at a ratio from 1:1000 to 1:10000. When the sample is heated by the laser beam, the solid mixture of analyte and matrix can be ionized and then vaporized57. The unique and important aspect of SL-MALDI-TOF MS is to limit the probing depth using a special sample preparation. Since the objective of the measurement is to characterize the surface

composition, the sample preparation technique must avoid any changes to the surface composition. Wang and coworkers45 have proven that the SL-MALDI-TOF MS technique can detect the composition of a cyclic/linear blend at the surface (a depth of less than 2 nm).

1.5 Statement of the Problem

Questions remain regarding the roles of chain end functionalization and star-like chain branching in the behavior of polymer surfaces. Well-defined chains are needed to address each scientific challenge and therefore improvements of synthetic strategies are key to advances in understanding both the dependence of surface segregation on chain end functionalization and on chain branching. In the case of studying the chain end effect an additional issue remaining to be clarified is how the chain end functionality affects not only the enrichment of the surface by chain ends themselves, but by the entire chains that include those functionalized ends. To see the composition of entire chains at the surface will require the new capabilities of SL-MALDI-ToF MS.

For studying the effect of chain branching on surface behavior two types of well- defined chains are sought. The first is small star-branched chains for which the high concentration of chain ends should magnify the effects of chain branching. The second type is an H-shaped chain of the same molecular weight as that of several branched architectures studied by Lee et al.36. Matching the molecular weight will make close comparisons easier to make. These molecules with architectures of interest for studying

surface segregation will also be of interest for study in the effect of chain architecture on surface fluctuation dynamics.

CHAPTER II

EXPERIMENTAL

Three aspects of the experimental work will be described here. The synthesis of well- defined end functionalized polystyrene, branched polystyrene including 4-arm star and H- shaped are discussed in sections 2.3-2.5. In sections 2.6 methods of molecular characterization are discussion. Finally, in section 2.7 the determination of surface composition using SL-MALDI ToF MS is described.

2.1 Inert atmosphere techniques

The organolithium compounds and living chain ends are sensitive to air and moisture.

These impurities are capable of deactivating the reaction. To exclude side reactions during the polymerization mixture, techniques including high vacuum techniques and dry box manipulations are used.

2.1.1 High vacuum techniques

Reactions for anionic polymerization were all performed in glassware. Chemicals were kept in glass ampoules and flasks, and high vacuum techniques were used to exclude impurities from the glassware. The layout of the high vacuum system is shown in Figure

2.1. The main components of the high vacuum line are the mechanical pump (Edwards

RV-8), the silicone oil diffusion pump (ChemGlass), the liquid nitrogen trap, the upper glass tube rig, and the lower glass tube rig containing stopcocks and glass joints.

Figure 2.1 Illustration of the high vacuum line apparatus. (From Quirk, R. P.; Ocampo, M. Material Matters, 1, 10, 2006. used with permission)

Ampoules and reactors were attached to the glass joints directly by a flame sealing technique, and round bottom flasks were connected to the grease traps. A Tesla coil

(Electro-technic BD-10A), which can produce a high voltage up to 45,000 V, was used for detection of the high vacuum condition. That the line was under high vacuum was confirmed when there was no noise and no discharge when the Tesla coil was bought near the walls of the vacuum system.

2.1.2 Dry Box Manipulation

The dry box (Vacuum Atmospheres, Model #HE-43-2) was used to provide an inert atmosphere for transferring and weighing air-sensitive compounds and making the difunctional initiator. A column packed with copper catalyst and molecular sieves was used as the purification system to remove oxygen and water from the atmosphere in the box.

The regeneration (with solvent removal) process using 5% hydrogen in nitrogen gas was done every two months and when the atmosphere in the dry box was not good. An antechamber was used to transfer glassware and chemicals into the dry box. Once an item was placed in the antechamber the atmosphere in the antechamber was pumped out and replaced with argon gas three times before the door between the box and the antechamber was opened and the item was transferred into the dry box. A dark green solution of

58 (Cp2TiCl)2ZnCl2 prepared following Sekutowski and Stueky’s method was stored in a cramp-cap bottle in the dry box to be used as an indicator to check the oxygen level in the dry box. When the solution of (Cp2TiCl)2ZnCl2 was exposed to the dry box atmosphere, a remaining green color indicated that a 5 ppm or lower oxygen concentration in the dry box.

If the indicator turned yellow a regeneration process was needed before any further use of the box.

Figure 2.2 General view of the dry box system (From Quirk, R. P.; Ocampo, M. Material Matters, 1, 10, 2006. used with permission)

2.2 Purification of Reagents

Any impurities in the solvents, monomers and terminating agents can lead to loss of control of the anionic polymerization. In this section, methods to purify reagents will be discussed.

2.2.1 Solvents

Solvents, mainly benzene, were used to dilute the reaction mixture to obtain the appropriate monomer concentration for the anionic polymerization. All of the solvents were purified according to the procedures given below.

2.2.1.1 Benzene Benzene (ACS grade, EMD; 99%) was stirred over freshly-crushed calcium hydride

(Sigma-Aldrich, 95%) in a round-bottom flask on the vacuum line overnight. During purification, the benzene in the flask was frozen using a dry ice/isopropyl alcohol bath, degassed until there was no discharge when the Tesla coil approached the vacuum line, and then warmed to room temperature using a water bath. This procedure is called a freeze- pump-thaw cycle. The cycle was first done after benzene was added into the round-bottom flask, and was repeated two more times later. Then, a vacuum distillation technique was used to transfer benzene into another round-bottom flask containing sodium metal dispersion. The benzene was stirred with the sodium dispersion overnight and then subjected to three additional freeze-pump-thaw cycles. The purified solvent was finally vacuum distilled into a storage round-bottom flask equipped with a Rotoflo® stopcock and containing oligomeric poly(styryl)lithium. The orange color in the solution showed that solvent was free of reactive impurities.

2.2.1.2 Hexane and Cyclohexane Alkanes including hexane (ACS grade, Sigma) and cyclohexane (ACS grade，Fisher scientific) were purified in a manner similar to that described for benzene.

2.2.1.3 Tetrahydrofuran (THF) THF (1.2 L, ACS grade, Fisher scientific, >95%) was stirred over freshly-crushed calcium hydride (Sigma-Aldrich, 95%) in a round-bottom flask on the vacuum line overnight. The mixture was then degassed using a freeze-pump-thaw cycle and a liquid nitrogen bath. After the cycle was performed three times, the THF was vacuum distilled 17

into another flask with sodium dispersion in it. The THF was then stirred overnight and subjected to three (four if needed) additional freeze-pump-thaw cycles. The purified THF was finally vacuum distilled onto a sodium mirror in a storage flask with a Rotoflo® stopcock.

2.2.2 Monomer

In addition to solvents, monomers were purified to remove any impurities and inhibitor.

2.2.2.1 Styrene Styrene (Aldrich, 99%) was stirred over freshly-ground calcium hydride overnight in a long necked, round-bottom flask. At least three freeze-pump-thaw cycles were used to degas the monomer. Another round-bottom flask with a Rotoflo® stopcock containing

1,10-phonanthridine (indicator) was then connected to the high vacuum and the non- discharge state for the Tesla coil test was reached. The lower rig of the vacuum line and the flask were purged under argon flow. The Rotoflo® stopcock was removed and dibutylmagnesium solution (ca. 3 mL) (FMC Lithium Division, 17% in Heptane) was injected into the flask. Heptane was removed by vacuum, and styrene was vacuum distilled into the second flask using a liquid nitrogen bath. A dark red color was obtained. After stirring with dibutylmagnesium for two hours, styrene was finally vacuum distilled into ampoules and the ampoules flame-sealed.

2.2.2.2 Ethylene oxide Ethylene oxide (Aldrich, >99.5%) was condensed from the vacuum line into a long- necked round-bottom flask with ground calcium hydride in it by placing a dry ice/isopropyl alcohol bath around the flask. After four hours of stirring in an ice water bath and three degassing cycles using a liquid nitrogen bath, ethylene oxide was vacuum distilled into another round-bottom flask with a Rotoflo® stopcock containing dibutylmagnesium. Since ethylene oxide is an extremely dangerous chemical and it is in the gas phase at room temperature, the author suggests that one needs to check and refill the ice water bath or liquid nitrogen bath every 15 minutes during these procedures. The mixture was stirred for an additional two hours in an ice water bath until the mixture showed a light yellow color.

Purified ethylene oxide was finally transferred into ampoules and diluted with benzene.

2.2.3 Terminating Agent and Linking agent

2.2.3.1 Methanol Methanol (Aldrich, ACS reagent, 99.8%) was degassed using three freeze-pump-thaw cycles and a liquid nitrogen bath, and was vacuum distilled into ampoules and the ampoules sealed off using flame sealing.

2.2.3.2 1,2-Bis(dichloromethylsilyl)ethane In the dry box 1,2-bis(dichloromethylsilyl)ethane (99%, Gelest,Inc.) was placed in a round bottom flask equipped with a Rotoflo® stopcock and a side ampoule containing calcium hydride. The mixture was stirred overnight in a warm water bath (35 °C). After three freeze-pump-thaw cycles using a liquid nitrogen bath (-196 °C), the purified 1,2- bis(dichloromethylsilyl)ethane was transferred into the side ampoule using short path 19

distillation technique. Only the middle fraction was collected to be used. The ampoule was then flame sealed under vacuum and transferred into the dry box for storage. The exact amount of 1,2-bis(dichloromethylsilyl)ethane for the reaction was weighed and dissolved in benzene in the dry box. The solution was then transferred into an ampoule. The ampoule was flamed sealed under vacuum.

2.2.3.3 Methyltrichlorosilane In the dry box, methyltrichlorosilane (Sigma-Aldrich, 99%) was distributed into a round bottom flask equipped with a Rotoflo® stopcock and containing freshly crushed calcium hydride (Sigma-Aldrich, 97%). The mixture was stirred overnight under vacuum and was degassed three times using freeze-pump-thaw cycles with a liquid nitrogen bath (-

196 °C). The reagent was transferred into another flask containing calcium hydride using vacuum distillation and stirred overnight. The purified methyltrichlorosilane was distilled into ampoules with breakseals. Only the middle fraction was collected to be used. The ampoules were then removed from the vacuum line using flame-sealing. The ampoules were stored in the freezer.

2.3 Synthesis of 6k hydroxyethylated functionalized polystyrene

In this reaction sec-butyllithium was used as initiator, styrene was used as monomer, and methanol and ethylene oxide were used to terminate the reaction. The synthesis route can be seen in Scheme 2.3.

Scheme 2.3 Reaction scheme for the synthesis of polystyrene and polystyrene with ethoxyl end group.

The glass reactor used for the synthesis of polystyrene and of polystyrene with ethoxyl end group is shown in Figure 2.4. The labeled parts of the reactor are referenced in the following description of procedures.

Figure 2.4 Illustration of the apparatus used to synthesize polystyrene and hydroxyethylated polystyrene.

Styrene (ampoule A, 7.7 mL, 6.99 g, 0.067 mol), ethylene oxide (ampoule B, 1.5 mL,

0.03 mol), and methanol (ampoules D,E, 5 mL total) were purified and sealed in ampoules using the methods described above. F is a thin tube used for the injection of the initiator and was carefully sealed by a septum. The reactor equipped with these four ampoules was connected to the high vacuum line through C by flame sealing. After the reactor was fully

evacuated, the septum was removed and the sec-butyllithium initiator (0.85 mL, 1.165 mmol) was injected into the reactor through the thin glass tube at F using a 5 mL, gas-tight syringe under argon gas protection. Benzene (80 mL) was then distilled into the reactor using a dry ice/isopropyl alcohol bath after the thin glass tube F was flame sealed. The reactor was then separated from the vacuum line at C and warmed to room temperature in the fume hood using a room temperature bath. The breakseal of the styrene ampoule was smashed and then the solution immediately turned to a red color, which showed that the polymerization had been initiated. The whole reactor was kept in a 30o C water bath for 3 hours. Half of the solution (45 mL) was then transferred into a sampling ampoule. The sampling ampoule was separated from the main reactor by flame sealing with the reactor and the sampling ampoule with each held in a dry ice/isopropyl alcohol bath. After the glass that had been heated to melting cooled down to room temperature, the reactor and the sampling ampoule were warmed to room temperature in the fume hood using a room temperature bath. The breakseal of the ethylene oxide ampoule was then smashed and the ethylene oxide reacted with the living chain ends for two minutes to achieve functionalization. Finally, the ampoules of methanol were broken to terminate the reaction.

Polystyrene and polystyrene with the ethoxy end group were isolated by precipitation in methanol.

2.4 Synthesis of Difunctional Initiation system

2.4.1 Purification of 1,3-bis(1-phenylethenyl)benzene (DDPE)

1,3-bis(1-phenylethenyl)benzene (DDPE) was synthesized using a reaction between

1,3-bis(benzoyl)benzene and Witting reagent59 following the steps analogous to that reported by Schulz and coworkers60. The crude product was fully dissolved in hexane and passed through a column filled with silica gel (100 mesh, Aldrich) to remove the unreacted ketone and other polar impurities resulting from oxidation. The product was yellow when entering the column and the purified product that came out of the column was white. Thin

Layer Chromatography (TLC), 13C NMR spectroscopy and FTIR were used the check the purity of the product after chromatography. The presence of only one spot at Rf =0.6 showed that polar impurities were fully removed by the chromatography. Recrystallization of the product in methanol in a 50°C water bath was performed multiple times. The purity of the product was checked using SEC with UV detection and measurement of the melting temperature with a capillary melting temperature apparatus after each recrystallization.

Once a sharp, mono-modal peak was shown in SEC and the temperature of a sharp melting point around 46 °C did not change between two successive recrystallizations, a small amount of the product was taken as cell for one more recrystallization to get ultra-pure

DDPE crystals. The needle-shaped, white crystals of pure DDPE give a sharp melting point of 46.6 °C (c.f. literature value of m.p. 46°C 60).

2.4.2 DDPE\sec-BuLi system

Scheme 2.5 Synthetic route for DDPE\sec-BuLi difunctional initiator

Difunctional initiator was synthesized by the addition of sec-butyllithium to DDPE37.

The difunctional initiator is extremely sensitive to the air because of the difference in reactivity of two reactive centers, Impurities like oxygen will terminate one reactive center of the two generating one arm chain impurities in the following synthesis. In order to obtain high performance difunctional initiator, ultra-pure DDPE was used and the reaction was performed in an argon atmosphere dry box. In the dry box DDPE (0.8691 g, 3.08 mmol) was placed in a reactor equipped with a Rotoflo® stopcock and a stir bar. Purified benzene

(60 mL) was then added into the reactor and the solution was stirred until all of the DDPE crystals were fully dissolved. An aliquot of diluted and freshly double-titrated sec- butyllithium solution61 (9 mL, 5.4 mmol, 0.6M in cyclohexane) was added into the reactor dropwise using a syringe while stirring. The color of the solution turned from colorless to dark red in a few seconds due to the presence of the diphenylalkylithium active center. The reaction solution was stirred for 30 minutes at room temperature. A small amount of the solution (0.1 mL) was taken out from the reactor and was terminated with purified methanol. SEC with UV detection was used to check the efficiency of the reaction. Two thirds of the reaction solution (46 mL) was taken out from the reactor and was separated into two crimp-cap bottles, with 23 mL in each bottle. The crimp-cap bottles were then 25

sealed and stored in the freezer. For the remaining 23 mL of reaction mixture in the reactor,

0.3 mL (0.18 mmol, to achieve 96% yield), 0.12 mL (0.072 mmol, to achieve 99% yield) and 0.03 mL (0.018 mmol, to achieve 100% yield) of sec-butyllithium was added sequentially to complete the reaction. The reactants were stirred for 30 minutes after each addition and 0.1 mL of the solution was taken out and quenched with purified methanol for

SEC-UV characterization. 1H and 13C NMR were also used for the characterization of the final product. The initiator solution was split into calibrated ampoules which were then flame sealed under high vacuum conditions.

2.5 Synthesis of Branched Chains

Four arm stars and H-shaped polystyrene were synthesized using anionic polymerization and silane linking chemistry.

2.5.1 Synthesis of 4-Arm-Star polystyrene

In this reaction sec-butyllithium was used as initiator and styrene was used as monomer for the arms. Methyltrichlorosilane was used as linking agent. Methanol and ethylene oxide were used to terminate the reaction. The synthetic route can be seen in

Figure 2.6. The glass reactor used for the synthesis of 4-arm star polystyrene can be seen in Scheme 2.7. The labeled parts of the reactor are referenced in the following description of procedures.

Scheme 2.6 Reaction scheme for the synthesis of 4-arm star polystyrene.

Figure 2.7 Illustration of the reactor used to synthesize four-arm polystyrene.

Styrene (ampoule A, 6.2 mL, 5.64 g), ethylene oxide (ampoule B, 1.5 mL, 0.03 mol), methanol (ampoules D,F, 2 mL total), and 1,2-bis(dichloromethylsilyl)ethane (ampoule E,

0.138 g, 0.538 mmol) were purified and sealed in ampoules using the methods described above. The reactor was connected to the high vacuum line through C by flame sealing.

After the reactor was evacuated overnight on the vacuum line, the septum on G was removed and the sec-butyllithium initiator (3.13 mL, 3.75 mmol, 1.20 mmol/mL in cyclohexane) was injected into the reactor through the side tube using a 5 mL, gas-tight syringe under argon gas protection. The side tube was then sealed by flame. Benzene (120 mL) was distilled into the reactor through the vacuum line using a dry ice/isopropyl alcohol bath on the reactor. The reactor was then separated from the vacuum line at C and warmed to room temperature using a room temperature bath. Styrene was introduced to the system 28

by smashing the breakseal of the styrene ampoule. The orange color of the poly(styryl)lithium appeared immediately. The whole reactor was kept in a 30o C oil bath for 2 hours. A small amount of the reaction solution (5 mL) was then transferred into the sampling ampoule. The sampling ampoule was separated from the main reactor by flame sealing with both the reactor and the sampling ampoule held in a dry ice/isopropyl alcohol bath. After the glass that had been melted cooled down to room temperature, the reactor and the sampling ampoule were warmed to room temperature in the fume hood using a room temperature bath. The breakseal of the methanol ampoule on the sampling arm of the reactor was smashed for the termination. A sample of the arm polymer was then obtained by precipitation in methanol and was characterized. The breakseal of the linking agent was smashed and the reactor was kept in a 30o C oil bath to complete the linking reaction. The ethylene oxide was introduced to the system and stirred for 10 minutes to functionalize the excess arm, followed by the termination with methanol. The crude product was recovered by percipitation in methanol. Arm polymer with a hydroxy end group was removed from the desire star product by silica gel chromotography (100 mesh, Sigma, activated), and the purity was tested by thin lay chromotography. The 4-arm star was characterized using SEC with multiple detectors and MALDI ToF MS.

2.5.2 Synthesis of H-shaped polystyrene

Polymeric linking agent for the H-shaped polystyrene was synthesized using anionic polymerization with the difunctional initiator and inverse addition of silicon chloride linking agent. The H-shaped polystyrene was then synthesized by the linking reaction between the living arm and polymeric linking agent25, 24,62. The synthetic route can be seen in Scheme 2.8. The glass reactor used for the synthesis of H-shaped polystyrene can be seen in Figures 2.9.1, 2.9.2, and 2.9.3. The labeled parts of the reactor are referenced in the following description of procedures.

Scheme 2.8 Reaction scheme for the synthesis of H-shaped polystyrene.

Figure 2.9.1 Illustration of the reactor used to synthesize 2-arm precursor.

Styrene (ampoule A, 4.8 mL, 4.36 g), methanol (ampoules C, 2 mL), and THF

(ampoule E, 1 mL, 10 mmol, THF : Li = 20) were purified and sealed in ampoules. F is a storage ampoule with a stopcock and a breakseal. The reactor was connected to the high vacuum line through B. After the reactor was evacuated overnight on the vacuum line, the septum on D was removed and the DDPE/sec-butyllithium difunctional initiator (5.46 mL,

0.26 mmol) was injected into the reactor through the side tube using a 5 mL, gas-tight syringe under argon gas protection. The side tube was then sealed by flame. Benzene (150 mL) was distilled into the reactor through the vacuum line using a dry ice/isopropyl alcohol bath. The reactor was then separated from the vacuum line at B and warmed to room 32

temperature using a room temperature bath. Styrene was introduced to the system by smashing the breakseal of the styrene ampoule. THF was then added into the reaction solution. The solution turned from dark red into a red/orange color after the addition of

THF. The reaction was carried out in an ice water bath while stirring for 3 hours. UV measurement was done after the addition of styrene, the addition of THF, and every 30 minutes during the reaction. The living chains (45 mL, 0.005 mmol/mL) were transferred into the storage ampoule F followed by flame sealing with both the reactor and the storage ampoule held in a dry ice/isopropyl alcohol bath. After the glass that had been melted cooled down to room temperature, the reactor and the ampoule were warmed to room temperature in the fume hood using a room temperature bath. Methanol was used to terminate the reamining living chains in the reactor for further characterization.

Storage ampoule F was connected to reactor II by flame sealing. Benzene for redissolving (25 mL, ampoule J) and methyltrichlorosilane as linking agent (6.34 mL,

8.51g, silane : Li > 200, ampoule K) were purified using the methods described above and their ampoules attached to the reactor by flame sealing. H was a sampling ampoule for the characterization of the polymeric linking agent. Tube I was used to reconnect the reactor to the vacuum line. L was a storage ampoule. The reactor was connected to the vacuum line at G. After the reactor was evacuated overnight, benzene (130 mL) was distilled into the reactor through the vacuum line using a dry ice/isopropyl alcohol bath. The reactor was isolated from G, and warmed to room temperature in a water bath. Methyltrichlorosilane was added into the benzene, and the solution was stirred for 15 minutes in an ice/salt water bath (-5 °C) to fully dissolve the linking agent. The

Figure 2.9.2 Illustration of the reactor used to synthesize polymeric linking agent.

breakseal on F was smashed and the living chain solution was added into the silane dilute solution dropwise (1 drop/sec) through a stopcock and two capillaries, while stirring in the ice/salt water bath. The momentary orange/red color of the living chain disappeared immediately after each drop was added, and the whole solution remained colorless through the addition process. The reactor was reconnected to the vacuum through I. Freeze drying with a dry ice/isopropyl alcohol bath and refilling with benzene was done two times to 34

remove the excess methyltrichlorosilane. The reactor was removed from the vacuum line again, and the polymer was dissolved in 25 mL benzene from ampoule J. Most of the solution (23 mL, 0.15 mmol) was transferd into the storage ampoule L, A small amout of the solution (2 mL) was transferred into the sampling ampoule H. Both L and H were removed from the reactor by flame sealing. Ampoule H was transferred and opened in the dry box. The polymeric linking agent was quenched by slowly adding the solution in H into methanol (100 mL) in the presence of triethylamine (TEA, 3 mL) in the dry box for further characterization.

Figure 2.9.3 Illustration of the reactor used to synthesize H-shaped polystyrene.

Storage ampoule L was connected to reactor III by flame sealing. Styrene (5.94 mL,

5.4 g, ampoule M), methanol (2 mL, ampoule N), ethylene oxide (1.5 mL, ampoule P) were purified using the methods described above. The reactor was connected to the high vacuum line through O. After the reactor was evacuated overnight on the vacuum line, the septum on R was removed and the sec-butyllithium initiator (1.5 mL, 0.9 mmol, 0.6 mmol/mL in cyclohexane) was injected into the reactor through the side tube using a 5 mL, gas-tight syringe under argon gas protection. The side tube was then sealed by flame. Benzene (100 mL) was distilled into the reactor through the vacuum line using a dry ice/isopropyl alcohol bath. The reactor was then separated from the vacuum line under vacuum and warmed to room temperature using a water bath. Styrene was introduced to the system by smashing the breakseal of the styrene ampoule. The whole reactor was kept in a 30 °C water bath for

2 hours. A small amount of the reaction solution (5 mL) was transferred into the sampling ampoule Q. The sampling ampoule was separated from the main reactor by flame sealing with both the reactor and the sampling ampoule held in a dry ice/isopropyl alcohol bath.

The breakseal of the methanol ampoule on the sampling arm of the reactor was smashed for the termination. The arm polymer was then obtained by precipitation in methanol and characterized. The polymeric linking agent was added to start the linking reaction with living arm. The linking reaction was carried out in a 40 °C oil bath for 10 days. Ethylene oxide and methanol were used to functionalize and terminate the excess arm using the method described above. Silica gel chromatography with toluene as eluent and several fractionations using toluene/methanol as the solvent/nonsolvent pair were used to remove the linear impurities. The final product was characterized using SEC with multiple detectors and MALDI ToF MS. 36

2.6 Molecular and Blend Surface Characterization

The polymers were characterized using SEC, 1H NMR spectroscopy, and MALDI-

ToF mass spectrometry. The surface segregation was detected using Surface Layer

MALDI-ToF Mass Spectrometry.

2.6.1 Size Exclusion Chromatography

Size exclusion chromatography characterization for the linear chain was carried out on a HLC-8320 GPC from TOSOH equipped with RI and UV detectors using PMMA or

PS standards. The eluent for this experiment was chloroform at 30 °C. Size exclusion chromatography characterizations for the difunctional initiator and branch chains were carried out on a HLC-83200 GPC TOSOH equipped with RI, UV, differential pressure, and multi-angle light scattering detectors using PS standards. The eluent was THF at 40 °C.

2.6.2 1H NMR and 13C NMR spectroscopy

Samples were dissolved using CDCl3 (99.8% Cambridge Isotopes), and the experiment was performed using a Varian Mercury 500 instrument (500 MHz).

2.6.3 MALDI-TOF mass spectrometry

The MALDI-ToF mass spectrometry was done on a Bruker Ultraflex-III MALDI-ToF mass spectrometer (Bruker Daltonics, Bullerica, MA) with a Nd:YAG laser (355 nm).

Samples for this characterization were made using the methods described below. Solutions of 20 mg/mL of the matrix molecule {2-[(2E)-3-(4-t-butylphenyl)-2-methylprop-2- enylidene]malononitrile} (DCTB, Alfa Aesar, 99+%) and 10 mg/mL of the cationizing salt

(silver trifluoro acetate (AgTFA, Aldrich, 98%)) were made using THF as the solvent. A solution of polymer sample (10 mg/mL) was made using THF as solvent. The matrix

solution and cationizing salt solution were mixed together at a ratio of 10:1. Drops of the mixture were then placed onto the MALDI sample target. After the mixture was dried, drops of the polymer solution were placed onto the dried mixture and allowed to dry.

Finally, drops of the matrix solution were placed onto the dried polymer to make the sample like a sandwich, with layers rich in matrix and cationizing salt above and below a layer rich in polymer. The intensity of the laser beam was optimized and the mass-to-charge ratio was calibrated using a polystyrene standard with a molecular weight close to that expected for the polymer sample.

For the bulk MALDI measurements for the study of surface segregation. A solution of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile (DCTB) matrix (20 mg/mL), a solution of AgTFA ionization agent (10 mg/mL) and PS sample solutions all in

THF were spotted using the dried droplet method onto a standard MALDI plate with a volumetric ratio of 100:10:1, respectively. Since the PS sample solutions were spotted in pure THF, it can be assumed that the formation of hot spots can occur. We took precautions to circumvent these features by averaging a large number of shots (>2000 shots) from regions over the entire sample.

2.7 Characterization of Surface Composition

2.7.1 Sample Preparation Each silicon wafer (El Cat Inc.) was cleaned by placing it in 70o C Piranha solution, which is a 3:1 volume ratio mixture of sulfuric acid and hydrogen peroxide, for 20 minutes*.

* Piranha solution is corrosive, so acid-resistant gloves, protective goggles, and lab coats are required when handling the piranha solution. 38

A 20-25 nm silver layer was deposited onto the surface of the silicon wafer using physical vapor deposition at a pressure below 5 × 10-5 Torr. The deposition rate of 0.5 Å/s was detected by a sensor. The deposition rate did not change much during the whole procedure.

About 1 mL of a 2 wt% polymer blend solution in toluene was spun cast onto the silver surface using 2000 rpm and 120 s spin time. Ellippsometrically determined film thicknesses were 90 ± 10 nm. Each sample was annealed in vacuum ( < 10-5 Torr ) at 150 oC, which is about 50 oC above the glass transition temperature of the 6k polystyrene, for

12 hours before the further measurement. Dry matrix powder was ground up with vortexer and spread on the film, without the use of solvent63.

2.7.2 SL-MALDI-TOF MS Measurement

MALDI-ToF-MS experiments were performed in positive linear mode using a Bruker

Ultraflex-III MALDI-ToF/ToF mass spectrometer (Bruker Daltonics Inc., Billerica, MA) equipped with a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (355 nm).

Linear mode was chosen due to the low intensity produced in the SL-MALDI-ToF-MS technique. This lower intensity (relative to conventional MALDI-ToF-MS) limits the molecular weight range over which chains can be analyzed. The spectra collected were analyzed using Bruker flex Analysis software.

CHAPTER III

HYDROXYETHYLATED POLYSTYRENE AND ITS INTERFACIAL

SEGREGATION

In this chapter detailed characterization of the hydroxyethylated polystyrene (PS-etOH) and its surface segregation behavior in blends with conventional PS presented and discussed.

3.1 Molecular weight and distribution determination

SEC provides information about the molecular weight and polydispersity. The SEC traces from the RI detector for both the polystyrene (PS) and hydroxyethylated functionalized polystyrene (PS-etOH) are shown in Figure 3.1.

Figure 3.1 SEC trace for PS-H and PS-etOH. 40

The SEC chromatogram for each sample showed a single narrow peak and the retention volumes of the peaks from the two samples were very close to each other (19.2 mL and 19.3 mL). The number average molecular weights were calculated to be 5900

(±10%) and 5600 (±10%) for the PS and PS-etOH, respectively, and both two samples were found to have a polydispersity of 1.02. The molecular weights of the two samples should be close, with the molecular weight for the functionalized PS-etOH slightly higher.

However, considering the uncertainty of SEC measurements, in general, is around 10%, the result is still acceptable. The MALDI-TOF MS spectra discussed in section 3.3 provide more details about the two polymers.

3.2 Chain end functional group characterization

NMR was used for the detection of chain end functionalization. Results for PS-H and

PS-etOH are shown in Figures 3.3 and 3.4, respectively.

Figure 3.2 NMR result for 6K PS-H.

Figure 3.3 NMR result for 6K PS-etOH.

Peaks at δ 1.3-2.2 ppm and peaks at δ 6.4-7.4 ppm are responses for the main chain structure of polystyrene. The butyl initiator fragment at one chain end is evidenced by peaks at δ 0.75 ppm for both products. Notice that the sharp peak at chemical shift 2.4 ppm is due to the presence of toluene used for the precipitation procedure. It was hard to remove the small amount of residual toluene (less than 1 wt% bass on the integration) in the product because of its high boiling point. A small peak appears at δ 3.35 ppm for PS-etOH, while for PS-H there is no peak at this chemical shift, which indicates a successful ethoxy chain end functionalization. The ratio of the peak area for the functional group to the peak area for the initiator group should be 2:6. The ratio observed experimentally is slightly larger than the theoretical value, which may be due to a tiny amount of formaldehyde in the methanol29. As it shown in Scheme 3.4 (lower) the formaldehyde impurity can generate a double functionalized chain end. H1 and H2 are under similar condition and show peak at same chemical shift.

Scheme 3.4 Possible side reaction from the fomaldehyde impurity in the methanol during termination for PS-H (upper) and Ps-etOH (lower).

3.3 MALDI-TOF mass spectrometry

MALDI-TOF is a powerful characterization technique that can elucidate several details of the chains in the sample. This technique was used here for the further determination of the molecular weight and molecular weight distribution, and it was used to prove the sample is pure enough for further study and also can be used for the calibration curve. Results for PS-H and PS-etOH are shown below in Figures 3.5a and 3.5b, respectively.

Figure 3.5a MALDI-TOF MS result for 6K PS-H.

Figure 3.5b MALDI-TOF MS result for 6K PS-etOH.

Analyses of the two spectra, summarized in the table above each spectrum, indicate both synthetic polymers are extremely clean with less than 0.4 mole% impurities. The number average molecular weight is close to the target molecular weight and the molecular weight dispersity is very narrow, which indicates a successful polymerization. The impurity species appears to be due to the tiny amount of formaldehyde present in the methanol used for terminating the reaction. Since the effect of chain end functionalization on surface segregation is the main focus of this work, the presence of this functionalized impurity is troublesome, but since MALDI-TOF MS is being used to analyze the species at the blend surface, the impurity species can always be identified separately from the species that is the target of the study. Both the PS-etOH chains and the impurity chains should have higher surface energies than PS-H has. Thus both species we expect to be depleted from the surface.

3.4 Surface segregation due to the chain end functionalization

SL-MALDI-TOF mass spectrometry was used to quantify the surface segregation of the blend of PS and PS-etOH. The bulk MALDI measurement for the same polymer blend was used to create a calibration matching observed signal ratio to composition.

3.4.1 Calibration using Bulk MALDI-TOF mass spectrometry measurement

In order to assign a surface composition from a SL-MALDI-TOF MS spectrum, the ratio of peaks for two species corresponding to a particular composition has to be determined by a calibration. The two species will not necessarily have the same ionization efficiency. Thus, bulk blend samples of known composition were measured with conventional MALDI-TOF MS and the assumption made that the ratio of peak intensities for the two species observed in the bulk spectrum would be the same as that seen in a surface layer spectrum for a sample with that composition in the surface layer. A polymer blend containing 10 w% PS-H and 90 w% PS-etOH was measured with MALDI-TOF MS and the bulk result is shown in Figure 3.6. Results from the analysis of the data are shown in Table 3.1.

Figure 3.6 Detail of the bulk MALDI-TOF MS spectrum for the PS-etOH(90) – PS- H(10) blend, with the entire spectrum shown in the inset.

Table 3.1 Results from Bulk MALDI-TOF MS Analysis for PS-etOH(90) – PS-

H(10) blend.

Component Mn Standard deviation Composition (%)

(Da) in Mn (Da)

C4H9 [C8H8]n H + Ag 6270 14 3.45

C4H9 [C8H8]n C2H4OH + Ag 6166 28 93.90

C4H9 [C8H8]n C3H7O2 + Ag 6374 31 2.65

Table 3.1 presents compositions for three species identified from the bulk data for the blend. The standard deviation reported is calculated from three parallel spectra from the same spotting. The chain with a C3H7O2 end group is the difunctional chain responsible for the secondary distribution which can be seen in Figure 3.5. As expected, the hydroxyethylated chain appears as the most abundant. However, there are some surprises here. First, the percentage of PS-etOH observed, 93.9%, is larger than expected from the manner in which the sample was made. Also the percentage calculated for PS-H is substantially smaller than expected from the sample preparation – 3.45% rather than 10.0%.

Finally, the percentage of the impurity chain with the C3H7O2 end group is substantially larger than expected. The percentage expected would be 0.9 × 0.29 = 0.26. How can the ten times larger percentage observed in the bulk measurement of the blend, 2.65%, be explained? Currently we do not have an explanation. However, since the difunctional impurity contains even more oxygen than does the hydroxyethylated chain, we expect it to be more depleted from the surface than is the hydroxyethylated chain.

3.4.2 SL- MALDI-TOF mass spectrometry for polymer blends

SL-MALDI-TOF was done for three PS-H films, three PS-OH films, and three PS- etOH(90) – PS-H(10) blend films, and the results are shown below. From Figure 3.7a and

Figure 3.8a we can see that the average molecular weights of the polymer chains on the surface are 6012 and 5948 for the PS-H and PS-etOH, respectively. When comparing these results with results from the bulk measurements (Figures 3.5a and 3.5b), for which the molecular weights of the PS-H and PS-etOH are 6177 and 6268, respectively, one can see

that the average molecular weights of the chains on the surface are smaller than those in the bulk. The molecular weight driven surface segregation appears to be stronger for the

PS-etOH chain. This is interesting since the density of high energy end groups is larger for the smaller molecular weight chains and that effect would tend to offset the entropic effect favoring shorter chains at the surface.

Figure 3.7a SL-MALDI-TOF result for the PS-H film, with the entire spectrum shown in the inset.

400

Intens. [a.u.]

300

200

100

0 6130 6140 6150 6160 6170 6180 6190 m/z

Figure 3.7b Enlargement of a portion of the SL-MALDI-TOF spectrum for the PS-H film, showing the small peaks between peaks of the major distributions.

Figure 3.8a SL-MALDI-TOF result for a PS-etOH film, with the entire spectrum shown in the inset.

Figure 3.8b Enlargement of a portion of the SL-MALDI-TOF spectrum for a PS-etOH film, showing the details of the region between peaks of the major distributions.

Figure 3.9a SL-MALDI-TOF result for a PS-etOH(90) – PS-H(10) blend film, with the entire spectrum shown in the inset.

1200

Intens. [a.u.] 1000

800

600

400

200

0 6120 6130 6140 6150 6160 6170 6180 6190 m/z

Figure 3.9b Enlargement of a portion of the SL-MALDI-TOF spectrum for a PS-etOH(90) – PS-H(10) blend film, showing details of the region between peaks of the major distributions.

The surface layer distribution for the PS-H film seen in Figure 3.7a is consistent with

that seen for the PS-H bulk in terms of the percentage of impurity seen. That percentage

is less than 0.5% for the surface layer spectrum. The spectrum from the PS-etOH film is

even cleaner which can be seen in Figures 3.8a and 3.8b. From Figures 3.7b and 3.8b we

can see that no impurity signal can be distinguished from the background at the surface.

This suggests that the PS-etOH species is preferred at the surface over the impurity species

with two oxygens, as we would expect. The impurity is depleted from the surface. The

spectrum from the blend surface (Figures 3.9a & 3.9b) is also quite clean with respect to the presence of impurity species. Certainly the impurity is depleted from the surface as compared to the bulk spectrum for the blend shown in Figure 3.6, where an apparent impurity concentration of 2.65% was seen. Neglecting then the impurity in further consideration of the blend surface, we concentrate on the compositions of PS-H and PS- etOH at the surface. From analysis of the data in Figure 3.9a, the apparent composition of the PS-H chain is 5.96 %. This is substantially higher than the apparent percentage of PS-

H of 3.45% derived from the bulk measurement summarized in Table 3.1. Thus we see a modest depletion of PS-etOH chains from the surface. Or put another way, it shows that

PS-H chains are enriched at the surface.

3.4.3 Comparison Between Surface and Bulk Measurements

The comparison of apparent compositions measured in the bulk and at the surface was expanded to include the entire range of compositions to see if the enrichment seen for 10% was seen for other compositions.

The comparison between the apparent compositions for the bulk and surface are is shown for the entire range of composition in Figure 3.10. The bulk measurements were done using MALDI-TOF MS and the data were analyzed using the method described in section 3.4.1. The surface measurements were done using SL-MALDI-TOF MS and the data were analyzed using the method described in section 3.4.2. The first observation to make is that the apparent concentrations for the bulk can differ slightly from the composition intended from the way the sample was made. For compositions below 50% the apparent composition is somewhat lower than the design compositions. For compositions above 50% the apparent composition exceeds somewhat the design composition. There is good agreement for the 50% blend. The “S-shape” of the apparent bulk composition curve about the ideal behavior line with agreement at 50% is similar to

the shape seen for the same types of measurements made for isotopic blends of d-PS and h-PS reported by Shih-fan et al.38,45 However, the effect with these blends of end- functionalized chains is more subtle than for the case of the isotopic blends. No explanation has yet been offered for this kind of deviation of apparent composition from design composition in conventional MALDI-TOF MS measurements of blends.

The second important result is the manner in which the apparent degree of surface segregation varies with blend composition. The difference between the curves shows the degree of depletion of the entire functionalized chains from the surface. For the compositions at nearly symmetric blends (40-70 mol %), the depletion is most pronounced

(greater than 10 mol percentage points). No surface depletion can be resolved for 90 mol% design composition.

The third important result is depletion of the whole chain from the surface. Because the salt is attached to the phenyl ring on the backbone, not the chain end during the MALDI measurements. Meaning the entire chains have moved out of contact with the surface rather than reorientation of some parts of the chain. The results are surprising, because the ethoxy end group makes up only 0.7 wt % of a chain based on the molecular weight and have an effect on whole chains. This has not been observed with other techniques like neutron reflectivity or x-ray photonelectron spectroscopy. It was therefore proposed in the past that chain confirmations rearrange to take functional groups away from surface, but not entire chains. This phenomenon may change with high molecular weight polymers while the contribution of the chain end become lower. It will be interesting to see how this changed with molecular weight for the further study.

SUMMARY

Well-defined 6K polystyrene and hydroxyethylated functionalized polystyrene were successfully synthesized using anionic polymerization. The structures of the products were proven using SEC and NMR. These results show that everything in the reaction was well controlled. A MALDI-TOF MS measurement proves that the structure of the major product is exactly what we want. The result from MALDI-TOF MS also proves that the samples are clean enough for the further study in surface segregation, although there is a small composition of a contaminant chain with surface energy even higher than that of PS-etOH.

SL-MALDI-TOF MS was used to detect the composition of the polymer on the surface.

For both the conventional PS and the hydroxyethylated chains, the smaller chains were enriched at the film surface. The higher surface energy impurity was depleted from the surface of both the PS-etOH film and the PS-etOH(90) – PS-H(10) blend film. The PS- etOH chains are modestly depleted from the surface of the blend. The depletion is more pronounced for the symmetric blends. For the first time, depletion of the entire chain, driven by only single functionalized end group, was observed directly. This observation is an important improvement in the study of surface segregation. Further study can be done with different molecular weight to see how the depletion changing with the contribution of the end group.

CHAPTER IV

WELL-DEFINED BRANCHED POLYSTYRENES FOR STUDYING THE

EFFECT OF BRANCHING ON SURFACE FLUCTUATIONS

In this chapter two well-defined branched chains synthesized for the study of the effect of branching on surface fluctuation dynamics are presented and discussed.

4.1 Preparation of Hydrocarbon Soluble Difunctional initiator

4.1.1 Purification of DDPE

The purity of the DDPE crystal is very important for the synthesis of the difunctional initiator and the following synthetic work. A high purity of DDPE was reached using silica gel chromatography and several recrystallizations. The purity was verified primarily with

TLC and a capillary melting point measurement. Further confirmation of the structure and purity came from measurements with 13C NMR, infrared spectroscopy, and SEC-UV.

The 13C NMR spectrum of DDPE is shown in Figure 4.1.1. The appearance of a peak for vinyl carbon (C2) at δ 114 ppm is in good agreement with the chemical shifts reported

64,65 previously . The resonances of C1 and C3 appearing at δ 150 and 141 ppm match with what can be expected from the structure. The peak at around δ 76 is for the solvent used which was CDCl3. The ratio of integration of the peak for the phenyl carbons to the integration of the peak for vinyl carbons is consistent with the calculated ratio (14 : 2).

Thus, we can make a quantitative argument with the NMR measurement for the purity of the DDPE.

Figure 4.1.1 13C NMR result for DDPE.

Figure 4.1.2 FT-IR result for DDPE, with chemical structures shown in the inset. The arrow marks where a peak for the carbonyl group would appear if some of the carbonyl has not been reacted.

The FT-IR spectrum of purified DDPE is shown in Figure 4.1.2. Peaks appearing at wavenumbers from 600 to 1000 cm-1 represent the vibrations of the aromatic vinyl group65.

FT-IR is very sensitive and observing these peaks provides a strong argument for the detection of the functional group. The total absence of a peak at 1650 cm-1 66 indicates that no carbonyl groups from the bis-phenylmethanone (right chemical structure in the inset) remain due to insufficient reaction or from oxidation during storage.

Figure 4.1.3 Chromatograms for DPPE using SEC with UV detection after chromatography (black) and later after four recrystallizations.

The SEC-UV result for DDPE is shown in Figure 4.1.3. Silica gel chromatography is only good for removing the impurities with polar groups. The shoulder and a peak at higher

molecular weight after chromatography indicates that there are oligomers that may come from thermally initiated radical reaction with the vinyl group. Recrystallization with the presence of pure crystal as seed was used to generate ultra-pure crystals. A very slow crystallization rate was used to prevent impurities being trapped in the crystal during crystallization. The SEC trace after four recrystallizations shows a monomodal peak without any shoulder or peak for the impurities, indicating very high purity for the DDPE.

4.1.2 Synthesis of the DDPE/sec-butyllithium difunctional initiator

The synthesis of a useful dilithium initiator by the addition reaction of two equivalents of sec-butyllithium with DDPE requires high quality stoichiometric control (Scheme 4.1.4).

Previous studies of this addition reaction7 indicated that if an excess of organolithium is used, one

Scheme 4.1.4 Preparation of dilithium initiator. obtains from SEC analysis either multimodal distributions or broad molecular weight distributions with low molecular weight tails. On the other hand, if excess DDPE is present, the remaining DDPE acts like a linking agent during the reaction. In this case, the SEC chromatogram of the initiator will be multimodal with either a shoulder or a peak at higher molecular weight. Thus, it is essential to prepare pure dilithium initiator with precise stoichiometric control. The key to attaining precise stoichiometric control is to minimize

the last addition of the sec-butyllithium. In this case, a stepwise addition process as well as

a diluted sec-butyllithium solution (0.6 mmol/mL in cyclohexane) was used.

Figure 4.1.5 SEC-UV results of the methanolysis products after each stepwise addition of butyllithium to the DPPE.

After each addition a small amount of the reactants (0.2 mL) was taken out of the

reactor and terminated using methanol. Figure 4.1.5 shows the SEC-UV chromatograms of

the methanolysis products after four different additions. When less than the stoichiometric

amount of sec-butyllithium had been added, SEC results clearly showed the presence of

unreacted DDPE (peak at retention volume 19.3 mL), and the methanolysis products from

the monoadduct (peak at retention volume 18.9 mL), and the diadduct (peak at retention

volume 18.4 mL). During the stepwise addition, the intensities of the DDPE and monoadduct peaks dropped, and the intensity of the diadduct peak became stronger. In the trace for 99 mol% a tiny amount of remaining DDPE and monoadduct are evidenced by a small peak and a shoulder, respectively. The last addition was minimized, only 0.03 mL of

0.6 mmol/mL solution, or 0.018 mmol total of the sec-butyllithium was added in that step.

A clear SEC result with only a sharp diadduct peak can be seen in the 100 mol% trace, which means that stoichiometric control was reached. 1H NMR and MALDI characterization were also done to make a stronger case for the difunctional initiator being well-defined.

Figure 4.1.6 1H NMR result for the methanolysis difunctional initiator. The orange square indicates where peaks would be expected if the product were not pure diadduct.

Figure 4.1.6 shows the 1H NMR result for the 100 mol% difunctional initiator with the diadduct and monoadduct structures shown as insets. The absence of peaks at around δ 5.5-

6.5 chemical shift indicates the absence of vinyl group from the monoadduct or DDPE.

The ratio of integration of the peaks for hydrogen on the phenyl ring to the integration of the peaks for hydrogens on the methyl group (14.02:11.14) is consistent with the expected structure (14:12).

MALDI results for the 100 mol% difunctional initiator and background are shown in

Figure 4.1.7. MALDI is actually not designed to look at species in this range of molecular weight. However, because of the good energy absorption of the phenyl structure and the precise measurements with reflectron mode, the initiator species can be seen by comparing the results with a background measurement. As it shown in the plot, the intensity of the peak for the difunctional initiator at 505.430 m/z is much larger for the sample than for the background, while no peak corresponding to the DDPE or monoadduct appears in the sample spectrum.

Figure 4.1.7 MALDI result for (upper) the difunctional initiator with salt and (lower) for matrix and salt alone measured as a background for comparison.

4.2 Synthesis of Well-defined 5.5k 4-arm Star Polystyrene

The structure of well-defined 4-arm star polystyrene was confirmed by the combination of SEC results for the arm and final product and MALDI TOF MS result for the purified star.

The SEC results can be seen in Figure 4.2.1. The sec-butyllithium initiated base polystyrene used as arm shows a single narrow peak in SEC. The number average molecular weight determined by SEC is 1500 ±150 g/mol, which is consistent with the calculated value of 1500 g/mol. The polydispersity of the arm is 1.02. The consistency with calculation and low PDI indicate that there is no chain transfer or chain termination during the living anionic polymerization. After a five day linking reaction between the living arm and the 1,2-bis(dichloromethylsilyl)ethane linking agent in 30 °C oil bath, the excess arms were functionalized using ethylene oxide and terminated using methanol. In the SEC trace for the crude product, the presence of functionalized excess arm results in a small peak at the retention volume for the arm precursor. The functionalized arm was then removed using silica gel chromatography. No peak at the retention volume for the arm can be seen in the

SEC trace for purified 4-arm polystyrene, which indicates the complete removal of the arm.

The apparent number average molecular weight of the 4-arm star determined by SEC is

4600 ±460 g/mol and the PDI is 1.02. However, the SEC was calibrated using a linear polystyrene that has a larger hydrodynamic volume than the star that has the same molecular weight. Therefore, the apparent molecular weight when using this calibration is smaller than the actual value. The accurate molecular weight was determined using

MALDI.

Figure 4.2.1 SEC results for precursor arm (blue), crude product (red dashed), and purified 4- arm star (black).

The MALDI result for the 4-arm star polystyrene obtained using reflectron mode is shown in Figure 4.2.2. The whole spectrum has a Gaussian shape and the distribution is narrow. The number average molecular weight from MALDI is 5445.7 g/mol which is around 500 g/mol lower than the value calculated for the whole star and around 125 g/mol lower than calculated for each arm. The error in molecular weight may come from the error in the volume of initiator injected. No signal is detected at 1000-2000 m/z where the signal from any remaining arm would appear, which indicates that the excess arm was fully removed by the chromatography. The m/z value of 4-arm star with 47, 48, and 49 mers can be seen in the enlarged view of the spectrum. The m/z value is marked for each

monoisotopic peak. The m/z difference between two monoisotopic peaks is around 104.7, which is the mass for one styrene repeating unit. The SEC and MALDI results support a strong argument that clean, well-defined, 4-arm star polystyrene with narrow PDI was successfully synthesized.

Figure 4.2.2 Enlarged portion of the MALDI result for the 4-arm star polystyrene, with the chemical structure and whole spectrum shown as insets.

4.3 Synthesis of Well-defined 38k H-shaped Polystyrene

Well-defined H-shaped polystyrene was synthesized using end linking of living arms to a polymeric linking agent. The α,ω-functionalized polymeric linking agent was made

using a combination of anionic polymerization with the difunctional initiator and a silicon chloride functionalization reaction with inverse addition.

Figure 4.3 Reaction scheme for the synthesis of H-shaped polystyrene.

4.3.1 Synthesis of the 2-arm Precursor

The synthesis of the difunctional initiator was described previously in section 4.1. The

α,ω-lithium living middle chain (2-arm precursor) was synthesized using anionic polymerization of styrene initiated by the difunctional initiator in the presence of THF

(THF:Li = 20:1). The reactivity, or the initiation rate, of the second alkyl lithium on the dilithium initiator is relatively low compared with the lithium that initiates first. Therefore, three special conditions were used to ensure the full initiation of both lithium active centers.

(1) THF was added to the reaction mixture as a Lewis base (THF:Li = 20:1). This is helpful for breaking the aggregation of the lithium species in the solution67,4. (2) A solution dilute in active centers (1.4 mmol in 150 mL benzene) favored the presence of free ions. (3) A relatively high precursor molecular weight (12000 g/mol) was targeted, based on a previous study68. This allows more time for both centers in each precursor to initiate.

The SEC result for the 2-arm precursor is shown in Figure 4.3.1. A narrow, monomodal peak at retention volume 15.6 mL indicates the complete initiation of both lithium centers. The number average molecular weight is determined as 14000 ±1400 g/mol, and PDI is determined as 1.08. The polydispersity is higher than linear chains initiated with sec-butyllithium chains because of the relatively slow initiation of the dilithium initiator. The number average molecular weight from SEC is slightly higher than the calculated value (12000 g/mol) and the actual value (determined by MALDI as shown below). This may be due to error in the SEC calibration curve that resulted from insufficient polymer standards for the RI detector at this specific molecular weight.

Figure 4.3.1 SEC result for the methanolysis α,ω-lithium living chain with the chemical structure shown in theas inset.

The precursor molecular weight of 14000 g/mol is relatively high for MALDI measurement characterization, requiring the use of large laser power. Also the difunctional initiator species acts as a weak part of the chain under laser illumination which makes the generation of fragments more likely. Therefore, the MALDI measurement was done in linear mode with high laser power to get enough intensity, and the fragment signals generated by the high laser power ( in the range of m/z <1400) were ignored. Separately, a spectrum for just the low molecular weight region (<5000 m/z) was collected for the sample using low laser power to make sure that no signal from the sample itself comes in that region. An enlarged portion of the MALDI spectrum can be seen in Figure 4.3.2 with the whole spectrum shown in the inset. The whole spectrum shows a monomodal Gaussian distribution which again indicates complete initiation of the difunctional initiator. However,

a secondary distribution with peaks at m/z values around 26-30 m/z higher (uncertainty due to the low resolution) than the peaks of the main distribution appears in the zoomed-in plot.

This may come from the formaldehyde impurity in the methanol used for termination of the precursor to allow for characterization. This kind of impurity can be seen consistently in the MALDI spectra for methanol terminated products in this work and is discussed above in section 3.4. Since these impurities were generated during the sampling termination process, the remainder of the living chains, which are used for the next step of the synthesis, will not have this problem.

Figure 4.3.2 MALDI result for the product of methanolysis of the α,ω-lithium living chains with the whole spectrum and chemical structure shown as insets.

4.3.2 Synthesis of the Polymeric Linking Agent

The polymeric linking agent was made by the inverse addition of the α,ω-lithium living chains into a solution with a large excess of trichlorosilane. The sample was prepared by adding the polymeric linking agent solution into a large amount of methanol in the presence of TEA using the procedure developed by Jaesik Lee and coworkers62,34 to create a methoxy-terminated polystyrene chain. MALDI was used to monitor the end capping reaction. The result can be seen in Figure 4.3.3. The resolution of the MALDI result is low due to the high molecular weight and the facts that the methoxysilyl group and the difunctional initiator species provide points in the chain where fragmentation can easily occur. Though the shape of the peak in the zoomed-in plot is not perfect and the background is noisy, the result is good enough to determine the peak position. The peak shows up at a value consistent with the calculated mass for the two-arm functionalized chain. There is no evidence of a distribution corresponding to singly functionalized chains. This indicates a successful end capping reaction. The second distribution with peaks at m/z values around

28 m/z units higher than those for the peaks in the main distribution is from the formaldehyde impurities in the methanol used for quenching.

Figure 4.3.3 MALDI result for the product of methanolysis of the polymeric linking agent with the chemical structure and whole spectrum shown as insets.

The SEC result can be seen in Figure 4.3.4. The peak is broader than the peak for the

2-arm precursor in Figure 4.3.1, and a shoulder has shown up on the higher molecular weight side. This may be due to the side reaction that is shown in Scheme 4.3.5. The acid generated from the termination reaction can readily catalyze the condensation of the methoxysilyl group.

Even though TEA was added into the methanol to react with the hydrogen chloride generated during the termination, the side reaction can still occur and evidence of the product of such a reaction can be seen in the zoomed-in portion of the SEC trace. In the paper by Lee et al.62 that presented the procedure used here, this problem has already been noted62. However, no solution for the problem was given there. Those authors used the polymeric linking with methoxy groups at the chain ends for further synthetic steps without purification62. It can be easily seen in Figure 4.3.634,62, reproduced from their publication, that there is a peak on the high molecular weight side in curve 2. Curves 4 and 5 show a monomodal peak for the final product, but the retention volume for the final product actually overlaps with that for the peak from the impurity in curve 2. So the argument for

the purity of the final product is relatively weak. Here, a new method was developed and used to prevent the polymeric linking agent from suffering from this side linking reaction.

The excess trichlorosilane was removed from the end capped precursor using two freeze- pump-thaw cycles. The reactor was then removed from the vacuum line after being fully evacuated. Fresh benzene was introduced into the reactor from the attached ampoule to redissolve the polymeric linking agent, followed by flame sealing the redissolved linking agent in a storage ampoule. In this way, the silane chloride at the chain end could be reacted directly with the living chains in the following step, avoiding the problem with the side reaction.

Scheme 4.3.5 Possible side reaction during the termination with methanol.

4.3.3 Synthesis of H-shaped Polystyrene

Well defined H-shaped polystyrene was synthesized by the linking reaction between the living arms and polymeric linking agent. The structure and purity of the final product was characterized using the combination of multi-detector SEC and MALDI. The SEC results for the crude and purified product are shown in Figure 4.3.7. The SEC trace for the sec-butyllithium initiated arm shows a single sharp peak at retention volume 17.9 mL. The number average molecular weight is determined as 6000 ± 600 g/mol, and the PDI is 1.02.

The trace for the crude product shows two peaks. The peak at retention volume 15.9 mL corresponds to the final product. The peak for the final product is broader than the peak for the arm precursor. This is because of the larger PDI of the polymeric linking agent due to the slow initiation of the dilithium initiator. The crude product was run through a silica gel chromatography to remove the functionalized excess arm. Surprisingly, half of the arm still remained in the product even after the chromatography, while testing with TLC showed all the chains with polar end groups were removed by the chromatography. Several fractionation were done for the further purification of the product. Due to the large molecular weight difference between the H-shaped polystyrene (38000 g/mol) and the arm

(6000 g/mol), fractionation is an efficient method for the separation of these two materials.

The SEC trace for the product after three fractionations shows no peak or shoulder at the retention volume corresponding to the arm precursor, which indicates that all the arm impurities have been removed. The molecular weight of the H-shaped polystyrene was determined using a multi-angle light scattering detector. A value of dn/dc of 0.191 for polystyrene in THF at 40°C from the literature69,70 was used. The number average molecular weight was determined to be 39,500 ± 6000 g/mol using the Zimm plot with 77

PDI = 1.10. The experimental value is consistent with the molecular weight calculated by

adding the MALDI results for the polymeric linking agent and arms together (38,000

g/mol). From this we can make a strong argument that all four arms have been successfully

linked to the polymeric linking agent to make an H-shaped PS.

The molecular weight of the final H-shaped product is much too high for MALDI

measurement. However, an argument for the well-defined architecture can be made using

a combination of MALDI results for both the precursor materials (polymeric linking agent

and arm) with SEC and light scattering results for the H-shaped product. Here, MALDI

was done in reflectron mode with low laser power to investigate the low molecular weight region specifically to check the purity of the final product. Results are shown in Figure

4.3.8. Surprisingly, three distributions are shown in the result. These correspond to (a) hydrogen terminated linear chain, (b) ethoxy terminated linear chain, and (c) linear chain with more than one ethoxy unit at the terminus. The hydrogen terminated chain may come from insufficient functionalization or termination during a long time for the linking reaction (10 days at 40 °C). These hydrogen terminated chains cannot be removed by chromatography since there is no polar group. In previous studies71,72 no oligomerization of ethylene oxide was detected for reaction with poly(styryl)lithium in benzene for reaction time less than 12 hours. Oligomerization was only detected as the concentration of ethylene oxide (EO:Li) was increased to 10 equivalents and as the reaction time increased to 4 weeks72. However, a large amount of lithium chloride is generated during the linking reaction and this remains in the system during functionalization which favors breaking the aggregation of the active center and making the chain ends more reactive73. This might explain why the oligomerization occurred here, but further study is needed for complete understanding. Figure 4.3.8c shows the MALDI result in the low molecular region for the purified product after three fractionations. The spectrum only shows background noise and no signal from polymer chains is detected, which shows that all the linear arm has been removed and that the well-defined H-shaped polystyrene is of high purity as regard arm contaminant.

170505_H -Crude_170264_RM 0:D19 MS, BaselineSubtracted, Sm oothed 6563.338 6459.297 6663.370

Intens. [a.u.] Intens. 6247.146 8000 6767.429

6143.089 6871.477

5998.027 6975.530

5934.965

6000 7079.576

5830.912 7184.608

7288.642 5686.838

7392.686 5625.795 4000

7496.709 5521.740

5414.644 7600.749 7704.791 5310.573 2000 5206.522 7808.804 5102.452

5000 5500 6000 6500 7000 7500 8000 8500 m /z

Figure 4.3.8a MALDI spectrum in the low molecular weight region for the crude H-shaped PS product .

Figure 4.3.8b Zoom in spectra for the crude prodrct on low molecular region with chemical structure as insert.

Figure 4.3.8c MALDI result in the low molecular weight region for the purified product after three fractionations.

SUMMARY

Well-defined 5.5k 4-arm star was successfully synthesized using a combination of anionic polymerization and silane linking chemistry. The structure of the product was characterized using SEC and MALDI. The results show controlled molecular weight, a well-defined structure, and very high purity. Difunctional initiator for synthesis of an H- shaped PS was successfully synthesized by the addition of sec-butyllithium to an ultra-pure

DDPE solution under stoichiometric control. The reaction was monitored using SEC-UV and the final product was characterized using 13C NMR and MALDI. The α,ω- functionalized polymeric linking agent was made using a combination of anionic polymerization with the difunctional initiator and a silicon chloride functionalization reaction with inverse addition. A previously noted side reaction62 was avoided using a modification of the synthetic approach. Well-defined H-shaped polystyrene was synthesized by the linking reaction of living arm with the polymeric linking agent.

Characterization by multi-detector SEC with MALDI quantified parameters of the structure. It would be very interesting to study the architecture effect on surface segregation in blends with linear chains and to compare with the results of Lee et al.36 for other branched chains of this molecular weight but different topologies. Also it would be very interesting to study the dynamic properties of these novel chains, e.g. the diffusion of chains in the pure melt and surface fluctuations of pure melt films. It is currently thought that the interpenetrability of neighboring branched chains is key to determining confinement effects for the surface fluctuations74. This architecture should allow more interpenetration than does a six-arm star, and perhaps more than a four arm star.

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