ANIONIC SYNTHESIS OF FUNCTIONAL POLYMERS USING
MULTIFUNCTIONAL EPOXIDES AS LINKING AGENTS
A Thesis
Presented to
The Graduate Faculty of the University of Akron
In Partial Fulfillment
of the Requirement for the Degree
Master of Science
Asfiya Q. Contractor
December, 2005 ANIONIC SYNTHESIS OF FUNCTIONAL POLYMERS USING
MULTIFUNCTIONAL EPOXIDES AS LINKING AGENTS
Asfiya Q. Contractor
Thesis
Approved: Accepted:
______Advisor Dean of the College Dr. Roderic P. Quirk Dr. Frank N. Kelley
______Faculty Reader Dean of the Graduate School Dr. William J. Brittain Dr. George R. Newkome
______Department Chair Date Dr. Mark D. Foster
ii ABSTRACT
The linking reaction of poly(styryl)lithium with the difunctional epoxide linking
agent, 1,3-butadiene diepoxide, was investigated. Poly(styryl)lithium was prepared by anionic polymerization in benzene at room temperature. The linked polymer was found to
have in-chain hydroxyl groups. It was characterized by NMR spectroscopy, size
exclusion chromatography, MALDI-TOF MS, as well as with thin layer and column
chromatography.
The coupled product containing in-chain alkoxylithium groups was used to
initiate the polymerization of ethylene oxide in the presence of a phosphazene base with
the objective of synthesizing a hetero, four-armed, (PS)2-star-(PEO)2 polymer. Ethylene
oxide was polymerized at 45°C for two weeks. The product obtained was characterized by NMR spectroscopy, size exclusion chromatography and MALDI-TOF MS.
iii ACKNOWLEDGEMENTS
I would like to thank my research advisor, Dr. Roderic P. Quirk, for his patience and persistence with me in the duration of this work. I am also grateful to him for the guidance and financial support he provided throughout my research.
I would also like to thank all my group members and especially Mr. Michael
Olecknowicz, for his countless help with experimental set-up and discussions about my research.
iv TABLE OF CONTENTS
Page
LIST OF TABLES……………………………………………………………………..viii
LIST OF FIGURES…………………………………………………………………….ix
LIST OF SCHEMES…………………………………………………………………...xii
CHAPTER
I. INTRODUCTION……...... ………………………………………………….1
II. HISTORICAL BACKGROUND……………………………………………....3
2.1 Living Anionic Polymerization…………………………………………....3
2.1.1 General Aspects……………………………………………….3
2.1.2 Monomers……………………………………………………..8
2.1.3 Solvents……………………………………………...... 10
2.1.4 Initiation……………………………………………………...11
2.1.5 Propagation…………………………………………………..14
2.2 Reactions of Polymeric Organolithium Compounds with Epoxides……15
2.2.1 Ethylene Oxide…………………………………...... 15
2.2.2 Propylene Oxide……………………………………………..18
2.2.3 1-Butene Oxide…………………………………...... 21
2.2.4 Ring-Opening Chemistry of Multi-functional Epoxides…….23
2.3 Aggregation of Alkoxylithium Chain-ends……………………………..24
v III. EXPERIMENTAL……………………………………………………………...27
3.1 Inert Atmosphere Techniques……………………………………………27
3.1.1 High Vacuum Line Techniques………………………………27
3.1.2 Dry-box Techniques……………………………...... 28
3.2 Purification of Reagents………………………………………………….29
3.2.1 Monomers…………………………………………………….29
3.2.1.1 Styrene…………………………………………...29
3.2.1.2 Ethylene Oxide……………………...... 29
3.2.2 Solvent (Benzene)………………………………………...... 30
3.2.3 Linking Agent: 1,3-Butadiene Diepoxide………………...... 31
3.2.4 Other Reagents……………………………………………….31
3.2.4.1 sec-Butyllithium………………………………...31
3.2.4.2 Methanol………………………………………...32
3.3 General Polymerization Methods………………………………………..32
3.3.1 Synthesis of polystyrene-b-poly(ethylene oxide)……………32
3.3.2 Synthesis of in-chain, hydroxyl-functionalized polystyrene...35
3.3.3 Synthesis of hetero, four-armed, (PS)2-star-(PEO)2 polymer.37
3.4 Characterization………………………………………………………...40
3.4.1 Size Exclusion Chromatography (SEC)……………………..40
3.4.2 Nuclear Magnetic Resonance (NMR) Spectroscopy………...41
3.4.3 MALDI-TOF Mass Spectrometry…………………………...41
3.4.4 Thin Layer Chromatography (TLC)…………………………42
3.4.5 Column Chromatography……………………………………42
vi IV. RESULTS AND DISCUSSION………………………………………………44
4.1 Analysis of polystyrene-b-poly(ethylene oxide)………………………..44
4.1.1 Size Exclusion Chromatography (SEC)……………………..46
4.1.2 NMR Spectroscopy………………………………………….47
4.2 Analysis of in-chain, hydroxyl-functionalized polystyrene…………….49
4.2.1 Size Exclusion Chromatography (SEC)……………………..50
4.2.2 Thin Layer Chromatography (TLC)…………………………51
4.2.3 Column Chromatography……………………………………52
4.2.4 1H NMR Spectroscopy………………………………………53
4.2.5 13C NMR Spectroscopy……………………………………...55
4.2.6 MALDI-TOF Mass Spectrometry…………………………...56
4.3 Analysis of hetero, four-armed, (PS)2-star-(PEO)2 polymer……………61
4.3.1 Size Exclusion Chromatography (SEC)……………………..62
4.3.2 1H NMR Spectroscopy………………………………………64
4.3.3 13C NMR Spectroscopy……………………………………...66
4.3.4 MALDI-TOF Mass Spectrometry…………………………...67
V. SUMMARY……………………………………………………………….80
REFERENCES……………………………………………………………82
vii LIST OF TABLES
Table Page
2.1 Acidities of conjugate acids of carbanions formed from various monomers in DMSO………………………………………………………………………….10
4.1 Observed 13C NMR chemical shifts for the in-chain, hydroxyl-functionalized 3 polystyrene of Mn= 2.8×10 g/mol……………………………………………...56
13 4.2 Observed C NMR chemical shifts for the product from (PS)2-star-(PEO)2 synthesis……………………………………………………...... 67
viii LIST OF FIGURES
Figure Page
2.1 Initiation and propagation steps for living, anionic, chain-growth polymerization…………………………………………………………………4
2.2 Alkyllithium-initiated polymerization scheme………………………………...8
2.3 Reaction of poly(styryl)lithium with oxygen………………………………….8
2.4 Monomer initiation by butyllithium………………………………………….11
2.5 Chain propagation in an alkyllithium-initiated polymerization……………...12
2.6 Cross-association of monomer and initiator……………………...... 13
2.7 Reactivity and degree of aggregation (n) of alkyllithium compounds for styrene and diene polymerizations in hydrocarbon solvents…………………………………………………………...... 13
2.8 Winstein spectrum of ionic species………………………………………….15
2.9 Reaction of poly(styryl)lithium with ethylene oxide………………………..16
2.10 Reaction of polymeric organolithium compounds with propylene oxide: regiochemistry of ring opening…………………………...... 18
2.11 Reaction of polymeric organolithium compounds with propylene oxide: acidic nature of protons of the methyl substituent on the epoxide ring……………………………………………………...... 19
2.12 Reaction of poly(styryl)lithium with propylene oxide…………...... 19
2.13 Chain-end of polystyrene functionalized with propylene oxide…………….20
2.14 Chain-end of polystyrene functionalized with 1-butene oxide……………...21
2.15 Reaction of polymeric organolithium compounds with 1-butene oxide: regiochemistry of addition……………………...... 22
ix 2.16 Structure of 1,3-butadiene diepoxide………………………………………..23
2.17 Coupling reaction of poly(styryl)lithium with 1,3-butadiene diepoxide……………………………………………………..23
2.18 Preparation of (PS)2-star-(PEO)2………………………………...... 24
3.1 Diagram of the reactor used for block copolymerization of styrene and ethylene oxide…………………………………………………...33
3.2 Diagram of the reactor used for the coupling reaction of poly(styryl)lithium with 1,3-butadiene diepoxide…………………………...36
3.3 Diagram of the reactor used for synthesis of (PS)2-star-(PEO)2 heteroarm star polymer………………………………………………………38
t 4.1 Structure of the phosphazene base Bu P4………………………...... 45
4.2 SEC-RI chromatograms of base polystyrene and polystyrene-b- poly(ethylene oxide).………………….……………………………………..47
4.3 1H NMR spectrum of polystyrene-b-poly(ethylene oxide) of molecular weight 20,000g/mol.…………….………………………………..48
4.4 SEC-RI chromatograms of base polystyrene and the crude product from the coupling reaction of poly(styryl)lithium with 1,3-butadiene diepoxide.…………………….…………………...... 50
4.5 SEC-RI chromatogram of in-chain, hydroxyl-functionalized 3 polystyrene of Mn=2.8×10 g/mol………………………………...... 51
4.6 TLC analysis of the product from the coupling reaction of poly(styryl)lithium with 1,3-butadiene diepoxide…………………………...52
4.7 1H NMR spectrum of in-chain, hydroxyl-functionalized 3 polystyrene of Mn=2.8×10 g/mol: (a) overall spectrum…………………………………………………………53 (b) chemical shifts and peak assignments …………………………………..54
4.8 13C NMR spectrum of in-chain, hydroxyl-functionalized polystyrene of 3 Mn=2.8×10 g/mol…...... 55
x 4.9 MALDI-TOF mass spectrum of crude product from the coupling of excess poly(styryl)lithium with 1,3-butadiene diepoxide: (a) overall mass spectrum….……………………………………...... 57 (b) monoisotopic resolution in the range of m/z 1510-1630...... 57 (c) monoisotopic resolution in the range of m/z 2490-2630………………….58
4.10 MALDI-TOF mass spectrum of in-chain, hydroxyl-functionalized 3 polystyrene of Mn= 2.8×10 g/mol: (a) overall mass spectrum……………………………………………………..60 (b) monoisotopic resolution in the range of m/z 2490-2630……………...... 60
4.11 SEC-RI chromatogram of in-chain, hydroxyl-functionalized 3 polystyrene of Mn=3.7×10 g/mol……………………………………………...63
4.12 SEC-RI chromatogram of product from (PS)2-star-(PEO)2 synthesis……………………………………………………………………….64
4.13 1H NMR spectrum of in-chain, hydroxyl-functionalized polystyrene of 3 Mn=3.7x10 g/mol……………………………………………………………..65
4.14 1H NMR spectrum of product from the synthesis of (PS)2-star-(PEO)2……………………………………………………………..66
13 4.15 C NMR spectrum of the product from (PS)2-star-(PEO)2 synthesis………………………………………………………………………66
4.16 MALDI-TOF mass spectrum of in-chain, hydroxyl-functionalized polystyrene 3 of Mn=3.7×10 g/mol: (a) overall mass spectrum……………………………………………………..68 (b) magnified mass spectrum in the range of m/z 0-3000……...... 68 (c) monoisotopic resolution in the range of m/z 0-3000……………………...69 (d) magnified mass spectrum in the range of m/z 3000-6000………………...71 (e) average masses in the range of m/z 3800-4100………...... 73
4.17 MALDI-TOF mass spectrum of the product from (PS)2-star-(PEO)2 synthesis: (a) overall mass spectrum …………………………………………………...74 (b) magnified mass spectrum in the range of m/z 5500-10,000……………...75 (c) average masses in the range of m/z 3800-4300…………………………..75
xi
LIST OF SCHEMES
Scheme Page
4.1 Synthesis of polystyrene-b-poly(ethylene oxide)……………………………..45
4.2 Synthesis of in-chain, hydroxyl-functionalized polystyrene………………….49
4.3 Synthesis of (PS)2-star-(PEO)2 heteroarm star polymer……………………...61
4.4 Alternative route to synthesis of (PS)2-star-(PEO)2………………………….79
xii CHAPTER I
INTRODUCTION
A diblock copolymer of styrene and ethylene oxide was prepared in a two step
process by using the initiator, sec-butyllithium. The first block of styrene was initially polymerized in benzene at room temperature followed by the addition of ethylene oxide
in the presence of a phosphazene base. Polymerization of ethylene oxide was carried out
at 45°C for two weeks. This polymer was examined by size exclusion chromatography
(SEC) and 1H NMR spectroscopy to determine the molecular weight, polydispersity
index and block styrene content.
The coupling reaction of poly(styryl)lithium with 1,3-butadiene diepoxide was
used to synthesize in-chain, hydroxyl-functionalized polystyrene. A slight excess of
poly(styryl)lithium was used for coupling with 1,3-butadiene diepoxide to avoid
formation of any monofunctional polystyrene. The functional polymer was separated
from the non-functional polymer by column chromatography and analyzed using SEC,
1H NMR, 13C NMR spectroscopy and MALDI-TOF mass spectrometry to determine the
molecular weight and polydispersity index.
A four-armed, star polymer of styrene and ethylene oxide was prepared in a three
step process in benzene at room temperature. In the first step, styrene was polymerized
using sec-butyllithium as the initiator. This was followed by coupling of living
poly(styryl)lithium using 1,3-butadiene diepoxide as the linking agent. In the final step,
1 ethylene oxide polymerization was initiated at the in-chain alkoxylithium groups of the coupled product. Polymerization of ethylene oxide was carried out in the presence of a phosphazene base at 45°C for two weeks. The product obtained was analyzed using SEC,
1H NMR, 13C NMR spectroscopy and MALDI-TOF mass spectrometry to determine the product distribution, molecular weight and polydispersity index.
2 CHAPTER II
HISTORICAL BACKGROUND
2.1 Living Anionic Polymerization
2.1.1 General Aspects
A living polymerization is a chain polymerization that proceeds in the absence of
the kinetic steps of termination and chain transfer.1 This implies that the living center
remains active even after all of the monomer has been consumed, so that if more
monomer were added, the chain would continue to grow, thus increasing the degree of
polymerization.1 The initiating species can be a nucleophile, electrophile, co-ordination complex, or radical which initiates the chain reaction by rupturing one of the bonds in the monomer. Accordingly there are four general types of chain reaction polymerizations: anionic, cationic, co-ordination, and free radical.
The general kinetic steps for an anionic polymerization are shown in Figure 2.1,1 where Mt is a metal cation. The reaction involves initiation and propagation, followed by deliberate termination after all of the monomer has been consumed.
3 R" R" ki RMt + RCH2CMt R' R'
R" R" R" R" k p RCHC CH CMt RCH2CMt + n 2 2 R' n R' R' R' Figure 2.1 Initiation and propagation steps for living, anionic, chain-growth polymerization.
Living anionic polymerization provides versatile methodologies for the synthesis
of polymers with well-defined structure and low degrees of compositional heterogeneity.
The control variables and synthetic applications are outlined below1:
(i) Molecular Weight
The molecular weight is controlled by the stoichiometry of the reaction and the
degree of conversion. For a monofunctional initiator, one polymer chain is expected to be formed per initiator molecule.
Mn= grams of monomer/moles of initiator 2-1
For difunctional initiators, such as alkali metal naphthalenide anions, the
expected molecular weight is twice as high.
Mn= grams of monomer/(1/2) moles of initiator 2-2
4
At intermediate degrees of conversion, the molecular weight is related to the
grams of monomer consumed.
Mn= grams of monomer consumed/moles of initiator 2-3
Thus the number average molecular weight (or Xn, the number average degree of
polymerization) is a linear function of conversion for a living polymerization. From a
practical point of view, it is possible to prepare polymers with molecular weights ranging
from ≈103 to > 106 grams/mole.
(ii) Molecular Weight Distribution
In general, it is possible to make polymers with narrow molecular weight distribution when the rate of initiation is competitive with or faster than propagation1.
This condition ensures that all of the chains grow for essentially the same period of time.
2 A narrow molecular weight distribution is operationally defined as Mw/Mn≤1.1.
The relationship between polydispersity and the degree of polymerization for a living polymerization is given as:3
2 Xw/Xn = 1+ [Xn/(Xn + 1) ] ≈ 1 + [1/Xn] 2-4
Thus, it is easier to make higher molecular weight polymers with narrow molecular
weight distribution, from a practical perspective. Synthesis of lower molecular weight
5 polymers with narrow molecular weight distribution requires careful attention to experimental details. Broad molecular weight distribution polymers can be obtained by using less reactive initiators, a mixture of initiators, or with continuous addition of initiator as in a continuous flow, stirred tank reactor.
(iii) Block Copolymers
Since all of the chains retain their active centers after the monomer has been consumed, sequential addition of different monomers can generate diblocks, A-B, triblocks, A-B-C or A-B-A, and more complex multiblock structures. Each of the blocks can be prepared with controlled molecular weight and narrow molecular weight distribution.
(iv) Chain-end Functionalized Polymers
Since all of the chains retain their active centers after the monomer has been consumed, controlled termination reactions can be effected. Living anionic chain-ends can react with an electrophilic reagent to obtain ω-functional polymers. The efficiency of these reactions is often less than 100%.
─ ─ PP + X-Y (electrophilic reagent) Æ P-X + Y 2-5
Alternatively, α-functional polymers can be obtained by using a suitably protected functional group in the initiator molecule. In principle, this method is 100% efficient because there is no chain termination or chain transfer.
6
X-I*(functionalized initiator) + M Æ X-I-M* 2-6
* * X-I-M + nM Æ X-I-[M]n-M 2-7
* X-I-[M]n-M Æ X-P (terminated polymer chain) 2-8
(v) Star-Branched Polymers
Star-branched polymers are prepared by effecting controlled termination
reactions with multifunctional linking agents as shown in equation 2-9, where SiCl4 is a tetrafunctional linking agent. Analogous to the functionalization chemistry, star polymers can also be obtained by using a multifunctional initiator in a living polymerization.
4PLi + SiCl4 Æ (P)4Si + 4LiCl 2-9
(vi) Microstructure
Microstructure in diene polymerizations can be controlled by solvent, counterion, polar additives and chain-end concentration. Hydrocarbon solvents with lithium as the counterion favor high 1,4-diene enchainment. Polar additives increase the amount of vinyl microstructure for dienes.
Alkyllithium-initiated polymerization is the most frequently used form of living
anionic polymerization. Since the active center is a carbanion, only monomers that can
stabilize a negative charge can be polymerized anionically (Figure 2.2)1.
7 + Li - X − δ X δ X - + R-Li+ + R C C RCH2C Li H2 Y Y Y Figure 2.2 Alkyllithium-initiated polymerization scheme.
Inert atmosphere or high vacuum conditions are used for these reactions to avoid
rapid termination of living chains due to reaction with oxygen, moisture, and carbon
dioxide4. Such termination reactions often give a distribution of different products
(Figure 2.3)5.
CH3OH PSLi + O 2 PS-PS + PS-O-O-PS + PS-O-O-H + PS-O-H
Figure 2.3 Reaction of poly(styryl)lithium with oxygen.
Although polymerizations can be performed in inert atmosphere glove boxes, high vacuum lines provide better control over the reaction conditions6. The initiator,
monomer, and solvent must also be selected very carefully to avoid termination or chain
transfer due to nucleophilic attack of the carbanion.
2.1.2 Monomers
Three main classes of monomers can be polymerized anionically: monomers based on carbon-carbon double bond, heterocyclic monomers, and monomers based on the carbon-heteroatom double (or triple) bond.7 The first two of these have been found to be quite susceptible to anionic polymerization (e.g., dienes, cyclic oxides, cyclic 8 sulfides), while the others (e.g., aldehydes, ketones) are generally not as reactive, i.e. less
exothermic polymerizations.
The polymerizability of vinyl monomers cannot be deduced from the
thermodynamics of polymerization. Almost all vinyl monomers exhibit negative free
energies of polymerization.1 But not all of them exhibit a suitable pathway for
polymerization to occur. This is the major limitation to the polymerizability of vinyl
monomers. According to thermodynamics, the free energy of polymerization, ∆G, must
be negative. The general equation for free energy is stated below in equation 2-10.
∆G = ∆H − T∆S 2-10
For chain growth polymerizations, the enthalpy (∆H) and the entropy (∆S) are
8 usually negative. At the ceiling temperature, Tc, the free energy of polymerization
becomes zero. Above this temperature, ∆G becomes positive and polymerization is not
9 possible. Near Tc, propagation is reversible, which produces polymers with broader
molecular weight distributions.
Anionically polymerizable monomers must be capable of stabilizing a negative
charge, which develops in the transition state (Figure 2.2). Substituents that can stabilize
a negative charge make vinyl monomers polymerizable. The only exception to this is
ethylene, which can be polymerized anionically even though it does not have any
electron-withdrawing substituents to delocalize the negative charge. The driving force for
ethylene polymerization is the reduction in free energy (of the product) as a result of
breaking of a pi bond and forming a sigma bond.1 The polymerization of ethylene
requires polar additives such as N,N,N’,N’− tetramethylethylenediamine (TMEDA).10,11
9 There is a general relationship between the reactivity of a monomer and the stability of the anionic species that it forms upon nucleophilic attack. Monomers that form the least stable carbanions (that have the largest conjugate acid pKa values for their conjugate acids) are the least reactive.1 These less reactive monomers in turn require the most reactive organometallic initiators for polymerization. Table 2.1 outlines the pKa values of the conjugate acids of the carbanions formed from some common monomers in
DMSO. In general, the reactivity of the monomer should match that of the propagating species to avoid side reactions or inefficiencies.
Table 2.1 Acidities of conjugate acids of carbanions formed from various monomers in DMSO.1
Monomer pKa (DMSO) Reference
Ethylene 56 1
Dienes 44 1
Styrene 43 1
Oxiranes 29-32 1
2.1.3 Solvents
Solvents must be chosen carefully because of the highly reactive nature of the initiator and propagating chain ends. Benzene, cyclohexane, tetrahydrofuran (THF) and diethyl ether are some of the common solvents used. Alkenes can also be used but they cause chain transfer in the presence of Lewis base or at high temperature.12 Aromatic solvents result in faster rates of initiation and propagation compared to aliphatic solvents
10 because they reduce the degree of aggregation of the initiator and chain ends.13 Aromatic solvents can cause chain transfer if benzylic protons are present.14 The solvent is present
in large molar excess compared to carbanionic chain ends, hence chain transfer to solvent
can play a large role in the stability of the active species. Chain transfer to solvents such
as toluene leads to polymers with broad molecular weight distributions, and a large
number of polymer chains with shorter lengths. Benzene and cyclohexane are the most
commonly used solvents.
Ethers can cause chain termination due to side reactions with organometallic
initiators and carbanionic chain ends.4 In general, polar solvents (THF, diethyl ether)
require the use of sub-zero temperatures (─78°C) to avoid side reactions.
2.1.4 Initiation
Polymerization reactions involving the carbon-carbon double bond can be
initiated principally by three types of initiators:7 (i) alkali metals, (ii) aromatic complexes
of alkali metals, and (iii) organoalkali compounds, mainly organolithiums. Organoalkali
compounds will be the primary focus for initiators herein. Initiation proceeds by addition
of the metal alkyl to monomer (a direct nucleophilic attack) (Figure 2.4) followed by
propagation (Figure 2.5). This mechanism leads to a monofunctional chain-growth
reaction.7 Y
C4H9Li + H2C CHY C4H9 CH2 C Li
H Figure 2.4 Monomer initiation by butyllithium.
11 Y Y
C4H9 CH2 C Li +n H2C CHY C4H9 CH2CHY CH2 C Li n H H
Figure 2.5 Chain propagation in an alkyllithium-initiated polymerization.
Kinetics of both the initiation and propagation steps have been found to be first
order with respect to monomer concentration, irrespective of solvent, temperature, and
other variables. But the rates of initiation and propagation have been found to have
various fractional order dependencies with respect to initiator concentration, depending
on the solvent used and monomer concentration, indicating a much more complex
mechanism. It is said that the fractional order dependence of the rate on initiator concentration is due to the tendency of organolithium compounds to aggregate in
solution.7 Experimental evidence shows that the observed energies of activation are too low to include the enthalpy of complete dissociation of the aggregates.15 Therefore, it has
been suggested that the aggregates undergo incomplete or step-wise dissociation to form
the unassociated alkyllithium species which then reacts with monomer.16 The initiating
species is a small amount of monomeric alkyllithium in equilibrium with a much larger
concentration of unreactive, aggregated species. When the aggregated initiator species
directly reacts with monomer, fractional kinetic orders are not observed, for example,
initiation with alkyllithium compounds in aliphatic solvents. Cross-association between
the initiator and growing chains is a complication in obtaining reliable kinetic results
because the kinetic orders change with conversion (Figure 2.6).
12 (RLi) + M [(RLi) (RMLi)] n n-1 Figure 2.6 Cross-association of monomer and initiator.
The reactivity of alkyllithium compounds as initiators is linked to their degree of
aggregation in solution. The less aggregated alkyllithiums are more reactive as initiators.
Figure 2.7 shows the degree of aggregation and relative reactivity (for the initiation
reaction) for various alkyllithium initiators in hydrocarbon solvent.17 Addition of Lewis
bases and the use of polar solvents lowers the degree of aggregation.1 The organolithium
alkyl groups have the same relative reactivity in styrene and butadiene polymerization,
except that the t-butyl group is considerably slower in styrene polymerization. This is
probably due to steric hindrance between the bulky t-butyl group and phenyl ring.7
Styrene polymerization: menthyllithium (2) > sec-BuLi (4) > i-PrLi (4-6) >
i-BuLi (4) > n-BuLi (6) > t-BuLi (4)
Diene polymerization: menthyllithium (2) > sec-BuLi (4) > i-PrLi (4-6) >
t-BuLi (4) > i-BuLi (4) > n-BuLi (6)
Figure 2.7 Reactivity and degree of aggregation (n) of alkyllithium compounds for styrene and diene polymerizations in hydrocarbon solvents.
Alkyllithium initiators are extensively used because they are soluble in
hydrocarbon media and hence can be used to prepare polydienes with high 1,4-
microstructure.1 n-Butyllithium is used commercially as an initiator to prepare elastomeric polydienes. The polymerization is carried out at temperatures greater than
50°C to increase the rate of initiation relative to propagation and thus to obtain a narrow
13 molecular weight distribution (MWD < 1.1). sec-BuLi is a faster reacting initiator and
gives polymers with narrower molecular weight distributions than does n-BuLi.18
2.1.5 Propagation
The rate of propagation is always first order with respect to monomer concentration and exhibits fractional orders with respect to chain end concentration.1 For
styrene polymerization with lithium as counterion, the kinetic order dependence on chain
end concentration is one-half.19 This can be explained in terms of the observation that
poly(styryl)lithium is predominantly associated into dimers in hydrocarbon solution, and
the unassociated species is the reactive entity for addition of monomer. There is a
controversy in propagation kinetics of poly(dienyl)lithiums about the relationship
between the degree of association of the chain ends in hydrocarbon solution and the
fractional kinetic order dependence on chain end concentration. This is due to a
disagreement about the predominant degree of association of chain ends in hydrocarbon
solution. Investigations on the association states of poly(butadienyl)lithium in benzene by
small-angle neutron scattering and both dynamic and static light scattering have shown that higher order aggregates (n >100) exist in equilibrium with dimers.20 The reaction
order dependence of the propagation rate on active center concentration is independent of
the identity of the hydrocarbon solvent, aromatic or aliphatic. However, the relative
propagation rates are faster in aromatic than in aliphatic solvents. The rate of
polymerization (Rp) in non-terminating systems can be expressed as the rate of
propagation as shown in the following equation:
1/n 1/n Rp = ─d [M]/dt = kpKd [PLi] [M] 2-12
14 where kp is the rate constant of propagation, Kd is the equilibrium constant of dissociation
of the aggregates, [PLi] is the total concentration of propagating centers, 1/n is the
fractional kinetic order (n = 2 for styrene polymerization), and [M] is the monomer concentration. Anionic polymerizations typically have [PLi] as high as 10-4-10-2M.
Propagation kinetics can be complicated by the presence of different propagating species in the polymerization media as shown in the Winstein spectrum.
- + - + - + (RMt)n nRMt R ,Mt R //Mt R + Mt 1 2 3 4 5 k [M] k [M] k4[M] k1[M] 2 3 k5[M]
Figure 2.8 Winstein spectrum of ionic species1
Going from left to right in the spectrum, the spectrum includes, aggregated
species, unaggregated species, contact ion pairs, solvent-separated ion pairs, and free
ions. Each can react with a different rate constant of propagation, e.g. free ions exhibit
the maximum value of kp while aggregated species have the minimum value. In
hydrocarbon media, the system has predominantly aggregated species (1), while in polar
media, there are predominantly solvent-separated ion-pairs (4). Addition of Lewis bases
shifts the spectrum to the right, thus increasing the rate of propagation. The addition of alkali metal alkoxides has a similar effect.1
2.2 Reactions of Polymeric Organolithium Compounds with Epoxides.21
2.2.1 Ethylene Oxide
The functionalization of poly(styryl)lithium (PSLi) with ethylene oxide in 15 benzene at 25°C has been shown to proceed quantitatively with no oligomerization to
produce the ω-hydroxy-functionalized polymer in the absence of polar additives (Figure
2.9).22
O C6H6 CH3OH PSLi + 4 PS-CH2CH2OLi PS-CH2CH2OH 12 h Figure 2.9 Reaction of poly(styryl)lithium with ethylene oxide.
This functionalization reaction is said to be a model functionalization reaction, because it proceeds quantitatively and in the absence of ethylene oxide oligomerization at the chain end. This conclusion was based on the absence of peaks at δ 69.1 and δ 72.2 ppm in the 13C NMR spectrum which would be expected for the dimeric ethylene oxide
chain end (PCH2CH2OCH2CH2OH). These assignments were confirmed by the work of
Mathers23 by direct synthesis of the model dimeric functionalization product. The ability to obtain the monoaddition product was attributed to the high degree of association of
lithium alkoxides formed even in polar media. Other alkali metals such as sodium and
potassium are also highly aggregated in solution, but they can promote polymerization of ethylene oxide.24 Thus alkoxylithium chain ends are more strongly aggregated than other
alkali metal alkoxides. Normal polar solvents and Lewis bases such as tetrahydrofuran
are not effective in promoting dissociation of the aggregates to form unassociated species
that are more reactive. The ω-hydroxy functionalized polymers obtained were
characterized by thin layer chromatography using toluene as an eluent on silica gel.
Unfunctionalized polymer was not detected in the product by this technique.
Quirk et al.23,25 recently reinvestigated the functionalization of PSLi with
ethylene oxide utilizing varying amounts of ethylene oxide and reaction times. When 4
16 molar equivalents of ethylene oxide were used in benzene at room temperature for 12
hours, no oligomerization was detected by 1H and 13C NMR and MALDI-TOF MS
analysis. When the amount of ethylene oxide was increased to 10 equilvalents and the reaction proceeded for longer reaction times, oligomerization was observed. Using 10 equivalents of ethylene oxide and a reaction time of 4 weeks, about 35 wt % dimeric oligomerization product [PSCH2CH2OCH2CH2OH], and 5 wt % of the trimeric oligomerization product [PS(CH2CH2O)2CH2CH2OH] were obtained. The formation of the dimeric and trimeric product was said to be possible because of dissociation of the aggregate with ethylene oxide since ethylene oxide is a very basic and polar ether.26
Mays and coworkers27 have studied the functionalization of poly(styryl)lithium
and poly(butadienyl)lithium with ethylene oxide in benzene at room temperature (4
equivalents of ethylene oxide, 24 h reaction time). The characterization was performed
using MALDI-TOF MS. In the case of poly(butadienyl)lithium, both the ω-hydroxyethyl
and hydroxyethoxyethyl functionalized products were observed. The ω-hydroxyethyl
functionalized polymer was the main product. With poly(styryl)lithium, only one
distribution of peaks was observed which corresponded to the ω-hydroxyethyl
functionalized product. When the functionalization was allowed to proceed for periods of
up to one month, oligomerization of ethylene oxide was observed. Thus, it seems that the
ability of a polymeric alkoxide to oligomerize depends on the nature of the backbone
chain. These surprising results were confirmed by the work of Y.Guo and co-workers28 who also found that oligomerization could be prevented by termination of the functionalization reaction with methanol after a few minutes.
Clear evidence was obtained by Ma and co-workers25 for the absence of ethylene
17 oxide oligomerization products for the functionalization of poly(styryl)lithium with 13C- labeled ethylene oxide (98 atom% 13C) for 12 h in benzene at room temperature. The 13C
NMR spectrum showed no resonances corresponding to the dimeric ethylene oxide unit at the chain end; only a doublet corresponding to the monohydroxyethylated product was observed. The doublet results from splitting of the carbon bonded to the hydroxyl group by the adjacent labeled carbon. Thus it can be concluded that the functionalization of polymeric organolithium compounds with ethylene oxide (3-4 equivalents) in hydrocarbon solution at room temperature results only in monoaddition of ethylene oxide with no oligomerization.
2.2.2 Propylene Oxide
The functionalization of polymeric organolithium compounds with propylene oxide is complicated by the asymmetry of the epoxide ring with respect to the regiochemistry of nucleophilic ring-opening (see Figure 2.10), and also the acidic nature of the protons of the methyl substituent on the epoxide ring that can react with strongly basic anions of the living polymer chain end to produce non-functional polymers (see
Figure 2.11).
(a) CH3OH P CH2 CHOH O CH PLi + 3 CH3OH CH P CH CH2OH 3 (b)
CH3 Figure 2.10 Reaction of polymeric organolithium compounds with propylene oxide: regiochemistry of ring opening. 18
Steric and electron-donating inductive effects of the methyl group make attack at
the less hindered carbon more favorable [pathway (a) in Figure 2.10)]. Attack at the more
hindered carbon [pathway (b)] is also possible. Additionally, oligomerization of
propylene oxide may occur.
O ROH P-H + H C CHCH OH PLi + HCH2 2 2 Figure 2.11 Reaction of polymeric organolithium compounds with propylene oxide: acidic nature of protons of the methyl substituent on the epoxide ring.
Quirk and Lizaragga29 studied the reaction efficiency, regioselectivity and extent
of oligomerization for the reaction of poly(styryl)lithium with three equivalents of
propylene oxide in the absence and in the presence of tetrahydrofuran (THF,
[THF]/[PLi]= 25) in benzene at room temperature for a period of 40 h (Figure 2.12).
O C6H6 ROH PSLi + 3 PS-CH2CHOH + PS-H 25oC CH3 CH3 Figure 2.12 Reaction of poly(styryl)lithium with propylene oxide.
The above functionalization formed the corresponding hydroxypropylated
polymer in 93% yield, accompanied by 7% of the unfunctionalized polymer, in the
absence of THF. In the presence of THF, the reaction was complete in 5 h and the yield
of functionalized polymer was 94% with 6% unfunctionalized polymer formed. The unfunctionalized polymer was ascribed to the abstraction of an acidic hydrogen from the 19 methyl substituent of the epoxide ring (see Figure 2.12). The 13C NMR spectrum of the
pure hydroxypropylated polystyrene showed peaks in the regions of δ 21.5-26 ppm and δ
64-67.5 ppm. The region between δ 21.5 and 26 ppm corresponded to the methyl carbon resulting from attack of the polymeric organolithium at the least hindered carbon to form a secondary alcohol chain end functional group [pathway (a) in figure 2.10]. The area between δ 64-67.5 ppm was assigned to the carbons bonded to oxygen.
PSCH2 CH CH2 CHOH
C6H6 CH3
Figure 2.13 Chain-end of polystyrene functionalized with propylene oxide.
13C NMR analysis did not show any peaks in the region of δ = 10-20 ppm
expected for the methyl group formed by attack of poly(styryl)lithium at the more
substituted carbon to form a primary alcohol [pathway (b) in Figure 2.10]. By effecting a
functionalization reaction of PSLi with 13C-labeled propylene oxide, it was found from
1H and 13C NMR spectra that only 3% of the reaction occurs by nucleophilic attack at the
more hindered carbon of the epoxide ring. No evidence for oligomerization was found. In
conclusion, the functionalization of polymeric organolithium compounds with propylene
oxide is of synthetic value ( ≥ 93% yield) and forms predominantly (97%) a secondary alcohol chain end, the product expected from nucleophilic attack at the least hindered carbon of the epoxide ring. This propylene oxide functionalization reaction is much slower (40 h reaction time at room temperature) than the analogous reaction with ethylene oxide (< 1h). 20
2.2.3 1-Butene Oxide
Compared to propylene oxide, the methylene group adjacent to the epoxide ring in 1-butene oxide is less reactive with respect to hydrogen transfer to the polymeric organolithium due to the steric and electronic effects of the adjacent methyl group. Thus, a high degree of functionalization is expected. A 9-fold excess of 1-butene oxide was reacted with poly(styryl)lithium (Mn =1900, Mw/Mn = 1.05) in benzene at room
temperature over a period of two days.30 The resulting products were found to have
monomodal, narrow molecular weight distributions by SEC analysis. Less than 1%
unfunctionalized product was detected by TLC and column chromatography analysis,
while the functional product was isolated in > 99 % yield. The high functionality of the
resulting polymer was confirmed by end-group titration (98.5%), 1H NMR (99%) and 13C
NMR (97%) analyses.
PSCH2 CH CH2 CHOH
CH CH C6H6 2 3 Figure 2.14 Chain-end of polystyrene functionalized with 1-butene oxide.
The regiochemistry of the chain-end was also of importance (see Figure 2.15).
The major product from this functionalization reaction was expected to be obtained by nucleophilic attack at the least hindered carbon of the epoxide ring. By 13C NMR
analysis, the two possible products were not distinguishable due to their similar
calculated chemical shifts. In order to distinguish between the chain-ends, the distortion
enhancement by polarization transfer (DEPT) 13C NMR method was used. This technique allows for the identification of the methyl carbons, methylene carbons, and methine 21 carbons. The results were consistent with complete regiochemical specificity to form the
secondary alcohol product resulting from attack at the least hindered carbon [pathway (a)
in Figure 2.15].
CH3OH (a) P CH2 CHOH O CH2CH3 PLi + 10 CH CH 2 3 CH3OH P CH CH2OH (b)
CH2CH3 Figure 2.15 Reaction of polymeric organolithium compounds with 1-butene oxide: regiochemistry of addition.
Oligomerization of 1-butene oxide was also studied for the above reaction over the normal two day reaction time at room temperature in benzene. Analysis by both 13C
NMR and MALDI-TOF MS showed no evidence for oligomerization. MALDI-TOF MS
showed some non-functional polymer in the product mixture as expected from the
column chromatography results.
Analogous functionalization reactions of poly(butadienyl)lithium with 1-butene oxide showed quantitative functionalization by column chromatography (98% yield) and
1H NMR (97% yield) analyses. Thus, the results were similar to those obtained for
poly(styryl)lithium functionalization.
Thus, it can be concluded that the efficiency of functionalization of homologous
epoxides is in the order: ethylene oxide (100%)> 1-butene oxide (99%)> propylene oxide
(93%). The reactivity order for these oxiranes is ethylene oxide > propylene oxide ≈ 1-
butene oxide. The properties of these epoxides are also of importance with respect to
their utility for functionalization reactions. Ethylene oxide is an explosive gas (b.p 22 10.7°C), while propylene oxide and 1-butene oxide are liquids with boiling points of
34.2°C and 63.3°C, respectively.31
2.2.4 Ring-opening Chemistry of Multi-functional Epoxides
The efficiency of the reactions of polymeric organolithium compounds with
epoxides21 suggests that multi-functional epoxides should be useful as linking agents
for polymeric organolithium compounds. Such linking agents can be used to prepare
functional polymers and star polymers with functional groups in the core. The
advantage of this kind of linking reaction is that it is a living linking reaction, that is, a
lithium alkoxide group is formed from each epoxide unit in the linking agent as shown in
Figure 2.18.
O O
Figure 2.16 Structure of 1,3-butadiene diepoxide.
OH O O CH OH + PSLi C6H6 3 PSLi + PS-CH2CH-CH-CH2-PS RT OH Figure 2.17 Coupling reaction of poly(styryl)lithium with 1,3-butadiene diepoxide.
Lithium alkoxides can be used to initiate the polymerization (oligomerization) of ethylene oxide.21,28
23 OLi O O + PSLi C6H6 PSLi + PS-CH2CH-CH-CH2-PS RT OLi
OLi O O[CH2CH2O]n/2H
n CH3OH PS-CH2CH-CH-CH2-PS PS-CH2CH-CH-CH2-PS
OLi O[CH2CH2O]n/2H Figure 2.18 Preparation of (PS)2-star-(PEO)2.
In this manner, it should be possible to obtain a heteroarm star polymer of styrene and ethylene oxide in a one-pot process.
2.3 Aggregation of Alkoxylithium chain ends:
Lochmann and coworkers32 have employed a thermoelectric method for the determination of the degree of association of organometallic compounds. A plot of ∆R versus concentration of alkoxide was obtained, where ∆R is the resistance offered by a
Wheatstone’s bridge which has a solution of the alkoxide connected to one of the arms.
From the data, a molecular weight, and from it the degree of association, α (ratio of molecular weight determined experimentally to theoretical molecular weight of a monomeric unit), was determined. The applicability of this method was proved by comparing the α-values obtained with those obtained by an ebulliometeric method for the t-butyllithium-benzene system. The values were found to be in good agreement.
Tertiary butoxides of lithium, sodium, and potassium, as well as lithium isopropoxide were some of the systems studied. With all systems, ∆R was directly
24 proportional to concentration. Thus, within the range of concentration and temperature
studied, α was practically constant. Lithium and sodium t-butoxides and lithium
isopropoxide are most associated in hydrocarbon solutions. Potassium t-butoxide is not
soluble except in polar solvents such as tetrahydrofuran and pyridine. The degree of
association of lithium t-butoxide in cyclohexane was found to be ≈6, and constant in the
range of concentration and temperature studied, which means that the hexamers are very
stable. This does not rule out negligible equilibrium concentrations of less associated
forms. Association of sodium t-butoxide was slightly more dependant on external
conditions. There was a stronger effect of solvent structure, and a just perceptible
dependence of temperature on α was observed.
The degree of association of all alkoxides measured in polar solvents such as
THF and pyridine decreases at 30 ºC to about α = 4. In the case of lithium, sodium and potassium t-butoxides, the solvation has been demonstrated by infrared spectroscopy by
the difference in the spectra in hydrocarbons and THF in the range of 900-1100 cm-1 or
by isolation of the pure crystalline co-ordination compounds, which were found to have
approximately one mole of THF per mole of the alkoxide. A co-ordination compound of
sodium t-butoxide with t-butyl isobutyrate has been isolated (α = 3.8).32 For the system, lithium t-butoxide in t-butyl isobutyrate, the co-ordination compound could not be isolated (α = 5.8). The infrared spectrum of this solution in the range of 400-600 cm-1 was
identical to that of the hydrocarbon solution, indicating that the lithium alkoxide is not as
strongly solvated as the sodium alkoxide. Therefore, it was concluded that so long as the lithium, sodium and potassium t-butoxides and lithium isopropoxide are well solvated, they form mainly tetrameric aggregates. The structure of such aggregates can be
25 visualized as a cube with atoms of oxygen and sodium alternating the corners with the sodium being solvated by the solvent molecule through its heteroatom. The solvent is weakly bound compared to other organoalkali compounds such as triphenylmethyl- lithium and sodium tetrahydrofuranates. The interaction of an alkoxide with the solvent, and thus the degree of association of the alkoxide, depend not only on the composition and structure of the alkoxide, but also on the electron-donating properties and structure of the solvent.
The degree of association of alkoxides dissolved in methacrylates (for anionic polymerization of methacrylates) can be estimated indirectly from the results obtained for t-butyl isobutyrates and isopropyl isobutyrates which are good models for t-butyl methacrylate and methyl methacrylate respectively.32 The values of α vary (α = 4-8) but do not decrease below α ≈ 4 (the least value found for strongly co-ordinating solvents).
Thus at the start of the polymerization, the alkoxides dissolved in the methacrylates are present predominantly as oligomers and not monomers.
26 CHAPTER III
EXPERIMENTAL
3.1 Inert Atmosphere Techniques
3.1.1 High Vacuum Line Techniques
A high vacuum line was used for monomer and solvent purification, removal of
air and moisture from ampoules and reactors, and for the degassing, drying and storage of
samples. It was used to create the vacuum inside the reactors used for all of the anionic
polymerizations. Morton and Fetters33 have provided a comprehensive review of high
vacuum techniques for anionic polymerization of vinyl monomers.
The vacuum line was made of Pyrex® glass and consisted of two horizontal manifolds, which were separated by two vertical 10 mm Teflon Rotoflo® stopcock valves. The lower manifold (15 mm glass tubing) had one grease trap and five 24/40 ground glass joints for attaching solvent or monomer flasks. Each item on the lower manifold could be exposed to vacuum by opening one of the Teflon Rotoflo® stopcock valves. The upper manifold (25 mm glass tubing) was attached to a liquid nitrogen trap
(Chemglass, Air-Free), a silicone oil (Dow Corning 705 Diffusion Pump Oil) diffusion
pump (Kontes), and a two-stage direct-drive mechanical pump (Edwards Model 18). The
vacuum in the line was on the order of 10-5 to 10-6 mm Hg.33 A Tesla coil (Fisher
Scientific) was used to test the quality of vacuum in the line by bringing it into close
27 proximity of the glass. If there was a lack of discharge (no noise), then there was high
vacuum.
Solution polymerizations were carried out in hand-blown, glass reactors with
ampoules and break-seals. The reactors were connected directly to the vacuum line
through a grease trap by using a hand torch. The detailed procedures have been described
by Morton and Fetters.33
3.1.2 Dry-Box Techniques
A dry-box (Vacuum Atmospheres, Model HE-43-2) was employed for
manipulation of air and moisture-sensitive reagents. It contained a circulation pump and
two separate purification columns. The drying column was filled with 13x molecular
sieves and Q5 copper deoxygenated catalyst (Schweizer Hall). The purification columns
were periodically regenerated using an automated system which heated the columns to
300ºC for 4 h, followed by a slow purge with a 5% hydrogen in 95% nitrogen mixture
(Praxair) to reduce the catalyst and remove the water vapor that is formed. The
regeneration cycle also evacuated the columns and then allowed them to cool to room temperature. The antechamber was equipped with a vacuum pump (Welch Duo Seal,
Model 1402) that enabled the antechamber to be brought to 30 mm Hg. All glassware that was placed in the dry-box was thoroughly cleaned and dried overnight in the oven at
140ºC, and was put into the antechamber while it was still hot. The antechamber was
evacuated and filled with argon gas (Argon technical grade, Praxair) three times before
the inner door of the dry-box was opened and materials could enter the dry-box. The condition of the atmosphere could be tested by cutting a piece of sodium metal to observe
28 its luster, and exposing a drop of a solution containing a titanium indicator to the
atmosphere, and monitoring the resulting color of the drop.34, 35
3.2 Purification of Reagents
3.2.1 Monomers
3.2.1.1 Styrene
Styrene (99%, Aldrich) was stirred over freshly ground CaH2 for about 24 h.
During this period it was purified by using atleast three freeze/degas/thaw cycles. A dry
ice-isopropyl alcohol slurry was used for freezing, and warm water was used to thaw
styrene. In each cycle, styrene was exposed to high vacuum after it was frozen, until no
gas was detected with the Tesla coil. The styrene was then distilled into a clean, dry flask
containing a small amount of neat dibutylmagnesium (12 wt.% in hexanes, FMC Lithium
Division). Liquid nitrogen cooling was required to transfer the styrene from the flask
containing CaH2 to the storage flask containing Bu2Mg. Styrene was thawed and stirred
over Bu2Mg for several hours until the color of the solution turned light yellow. It was
then stored in the refrigerator until needed. Styrene was distilled from the storage flask
into flame-dried, calibrated ampoules immediately before use. The ampoules were not
allowed to stand at room temperature for more than eight hours at a time.
3.2.1.2 Ethylene Oxide
Ethylene oxide ( ≥ 99.5%, Aldrich) was available as a liquid under pressure in steel cylinders. Its boiling point is 10.7ºC.36 About 30 mL was distilled into a dry, evacuated flask, containing freshly crushed CaH2, cooled by a dry ice-isopropyl alcohol
29 slurry, and stirred for 3 hours to dry it. It was then distilled into another clean flask
containing 1 mL of n-butyllithium and a pinch of 1,10-phenanthroline ( ≥ 99 %, Aldrich),
as indicator. The purple color of the solution indicated that the ethylene oxide was dry. It
was then distilled into flame-dried glass ampoules, and some benzene was distilled over
it. It was stored in the refrigerator until used.
3.2.2 Solvent (Benzene)
Benzene (reagent grade, Fisher Scientific) was first stirred over approximately
1L of concentrated sulfuric acid for atleast one week in a 4-L, Erlenmeyer flask to
remove all unsaturated impurities. The solvent was then poured into a separating funnel
leaving as much of the acid behind as possible. It was then neutralized by washing multiple times with a saturated solution of sodium bicarbonate (Fisher Scientific), followed by multiple washings with deionized water. After this it was dried over anhydrous MgSO4 (Fisher Scientific) overnight, filtered, and then poured into a 2-L, round-bottom flask equipped with a ground-glass joint, containing some freshly ground
CaH2 (Aldrich), and the flask was attached to the vacuum line. It was subjected to three freeze/degas/thaw cycles, and allowed to stir over CaH2 for at least 24 hours before being distilled into another flask containing sodium dispersion (50 wt.% in paraffin, Aldrich).
The benzene was stirred, frozen/degassed/thawed three times on sodium. It was distilled onto successive sodium mirrors until there was no degradation of the mirror. Finally it was transferred to a storage flask equipped with a Rotoflo® stopcock on the vacuum line,
which contained an orange-colored styryllithium oligomer (from sec-butyllithium and
styrene). It was distilled directly into reactors as required.
30
3.2.3 Linking Agent (1,3-Butadiene diepoxide)
The linking agent, 1,3-butadiene diepoxide, was obtained as a liquid (97% pure)
from Aldrich. It was handled in the dry-box. Partially decomposed epoxy compounds
could be present as impurities. The liquid was transferred to a flask containing freshly
crushed CaH2 (Aldrich), and subjected to three freeze/degas/thaw cycles on the vacuum
line before being distilled (through short-path distillation) into another flask equipped
with a Rotoflo® stopcock. It was stored under vacuum. A sample was prepared for 1H
NMR analysis in the dry-box, utilizing degassed CDCl3, to check for purity.
3.2.4 Other Reagents
3.2.4.1 sec-Butyllithium
A solution of sec-butyllithium was obtained from FMC, Lithium Division. The
Gilman37 double titration method with allyl bromide was used to titrate the initiator for
anionic polymerization. The concentration of carbon-bound lithium was determined by
this method. The organolithium compound was handled in the dry-box because it is
moisture and air sensitive. sec-Butyllithium was transferred into a septum-capped bottle
containing 20mL of dry benzene or cyclohexane, which had been dried over 4Å
molecular sieves. It was then taken out of the dry-box and 20 mL of water was added to
react with the organolithium compound. The mixture was titrated against 1.0 N
hydrochloric acid (aq) (Fisher) using phenolphthalein as an indicator to determine the
total base concentration. To determine the free base concentration, 4 mL of sec-
butyllithium was transferred into a septum-capped bottle containing 20 mL of dry
31 benzene or cyclohexane and 4 mL (excess) of allyl bromide was added to it. Allyl
bromide, which was dried over phosphorous pentoxide and distilled, reacts with the
carbon bound to lithium. The mixture was then taken out of the dry-box, 20 mL of water
and some phenolphthalein indicator was added to it. It was titrated against 1.0 N hydrochloric acid (aq) (Fisher). The difference between the total base concentration and
the free base concentration were used to calculate the concentration of active initiator
which was usually around 1.5 M. Each titration was done three times and the values were
averaged.
3.2.4.2 Methanol
Methanol (reagent grade, Fisher Scientific) was degassed on the vacuum line and
distilled into ampoules containing break-seals. Liquid nitrogen was used to cool methanol for degassing and distillation. The ampoules were then flame-sealed from the line and
attached to reactors to effect controlled termination.
3.3 General Polymerization Methods
3.3.1 Synthesis of polystyrene-b-poly(ethylene oxide)
A symmetric diblock copolymer of styrene and ethylene oxide of target
molecular weight 20,000 g/mol was prepared using the set-up shown in Figure 3.1.
32
Figure 3.1 Diagram of the reactor used for block copolymerization of styrene and ethylene oxide.
® Purified styrene over Bu2Mg, in a round-bottomed flask with a Teflon Rotoflo stopcock, was placed on the vacuum line, and transferred through short-path distillation into a clean, flame-dried calibrated ampoule with a break-seal. The required amount of styrene was transferred, the ampoule was flame-sealed, and then stored in the freezer until needed. Ampoules containing the required amount of purified ethylene oxide were prepared as stated in section 3.2.1. Methanol ampoules were prepared as stated in section
3.2.4.2. A phosphazene base (P4-t-butyl, Aldrich, 1.0M in hexane) ampoule was prepared in the dry-box. A glass ampoule equipped with a break-seal, a Teflon Rotoflo® stopcock and a ground-glass joint (for attaching it to the vacuum line) was prepared using a hand- torch and annealed. Phosphazene base (0.62 mL, 6.2093 × 10-4 moles) was injected into
33 the ampoule in the inert atmosphere of the dry-box, and the stopcock was closed tightly.
It was then taken out of the dry-box, placed on the vacuum line and evacuated while
cooling with an isopropyl alcohol/dry-ice bath. Some purified benzene was distilled into
it before it was flame-sealed. It was stored in the freezer until further use.
After all of the ampoules were ready, the reactor was assembled. All of the
ampoules were kept wrapped in wet paper towels and aluminum foil to keep them cold.
The reactor was assembled using a hand-torch. It was attached to the vacuum line
through one of the grease traps, flame-dried several times and checked for pinholes with the Tesla coil. Once the reactor was evacuated, 0.38 mL of sec-butyllithium in hexane
(1.494 mole/L) was injected into it under positive nitrogen pressure and then the injection
port was sealed with a septum. The vacuum was pulled once again to remove the solvent
from the initiator and the injection port was flame sealed. Benzene was then distilled
from poly(styryl)lithium into the reactor. Enough benzene was used to obtain a polymer
solution containing 5% solids (approximately 250 mL). The solvent was frozen, and
vacuum was pulled in order to ensure that there was no air inside the reactor before being
flame-sealed and taken off of the vacuum line. The solvent was thawed and the styrene
ampoule [5.64 g, 6.2 mL (ρ = 0.906 g/mL)] was opened. The solution turned from
colorless to orange-yellow immediately. The reactor was allowed to stand overnight until
the first block of polystyrene was formed. The following day, a sample was taken for
SEC analysis and that arm was sealed off and removed.
The second step in the reaction was polymerization of ethylene oxide. As soon
as the ethylene oxide ampoule was smashed open, the orange-yellow color of
poly(styryl)lithium disappeared. The phosphazene base ampoule was then opened, and
34 the reactor was kept in a warm water bath at 45-50 ºC for two weeks for the reaction to be completed. The reaction was then terminated with methanol and the reactor was opened and worked up. The polymer was precipitated into hexane, vacuum filtered, and then dried in a vacuum oven (Precision, Model 29) to constant weight. SEC and 1H NMR analysis were performed.
3.3.2 Synthesis of in-chain, hydroxyl-functionalized polystyrene
The reactor shown in Figure 3.2 was used for this synthesis. The linking agent ampoule was prepared in the dry-box in a way similar to that described for the phosphazene base. A glass ampoule equipped with a Teflon Rotoflo® stopcock, a break seal, and a ground glass joint (to attach it to the vacuum line) was prepared with a hand torch and annealed at 650 ºC. It was checked for pinholes on the vacuum line before being taken into the dry-box. Purified 1,3-butadiene diepoxide (0.15 mL, 1.905 × 10-3 moles, ρ = 1.113 g/mL) was injected into it in the inert atmosphere of the dry-box. The stopcock was tightly closed. Following this, it was taken out of the dry-box and attached to the vacuum line. Vacuum was pulled to remove inert gas from the ampoule while cooling the contents with an isopropyl alcohol/dry-ice bath. The ampoule was flame- sealed and stored in the freezer until needed. A styrene ampoule containing 8.2 mL of purified styrene (7.43g, ρ = 0.906 g/mL) was prepared by the procedure described in section 3.2.1. Methanol ampoules were prepared by the procedure described in section
3.2.4.2.
35
Figure 3.2 Diagram of the reactor used for the coupling reaction of poly(styryl)lithium with 1,3-butadiene diepoxide.
The ampoules were attached to the reactor using a hand torch. Moist paper towel and aluminum foil were used to cover the ampoules to keep them cool. The reactor was then connected to the vacuum line through the grease trap using a hand torch and allowed to be evacuated. It was checked for pinholes with the Tesla coil and flame dried several times. sec-Butyllithium (3.315 mL, 4.95 × 10-3 moles, 1.494 M in hexane, [sec-
BuLi]/[linking agent] = 2.3/1) was then injected into the reactor under a positive nitrogen
pressure. Vacuum was pulled to remove hexane while cooling with a bath of isopropyl
alcohol/dry-ice. The injection port was flame-sealed. About 200 mL of dry benzene
(polymer solution containing 5% solids) was distilled from poly(styryl)lithium into the
36 reactor. The benzene was frozen with the isopropyl alcohol/dry-ice bath, and the reactor
evacuated to remove any air that was present. The reactor was then flame-sealed and
taken off the vacuum line. The solvent was thawed using a warm water bath. The styrene
ampoule was opened. An orange-yellow solution of poly(styryl)lithium of target molecular weight 1500g/mol was obtained. It was allowed to stand overnight. A sample of the polymer was taken for SEC and the sampling tube was flame-sealed.
In the second step of the reaction, the ampoule containing 1,3-butadiene
diepoxide was opened. The orange-yellow color of poly(styryl)lithium disappeared. The reactor was allowed to stand for another day before being quenched with methanol and opened for work-up. The polymer was precipitated in methanol, vacuum filtered, and dried in the vacuum oven to constant weight. SEC and MALDI-TOF MS analysis were performed on the polymer. The polymer obtained was a mixture of polystyrene of
1500g/mol target molecular weight and in-chain, hydroxyl-functionalized polystyrene of
3088g/mol target molecular weight. The latter was the product of interest. These two products were separated by column chromatography using a silica gel column and a
50/50 v/v mixture of ethyl acetate and hexane as the eluent. SEC, 1H NMR, 13C NMR,
and MALDI-TOF MS analysis were performed on the hydroxyl-functionalized polymer.
3.3.3 Synthesis of (PS)2-star-(PEO)2 heteroarm star polymer
The coupling reaction of poly(styryl)lithium with 1,3-butadiene diepoxide, and
the polymerization of ethylene oxide to obtain the star polymer were performed in the
same reactor. A diagram of the reactor is shown in Figure 3.3 below.
37
Figure 3.3 Diagram of the reactor used for synthesis of (PS)2-star-(PEO)2 heteroarm star polymer.
An ampoule containing 0.1mL (1.32 × 10-3 moles, 0.1136g, ρ = 1.113 g/mL) of
1,3-butadiene diepoxide was prepared in the dry-box by the procedure stated in section
3.3.2. A phosphazene base ampoule (P4-t-butyl, Aldrich, 1.0 M in hexane) containing
2.9mL (2.9×10-3moles) of base was also prepared in the dry-box by the method described in section 3.3.1. Both of these ampoules were stored in the freezer until needed. Purified styrene (5.8 mL, 5.29 g, ρ = 0.906 g/mL), stored over Bu2Mg, was placed on the vacuum
line, and transferred through short-path distillation into clean, flame-dried calibrated
ampoule with a break-seal. The ampoule was flame-sealed, and then stored in the freezer
until needed. An ampoule containing 6.0 mL of purified ethylene oxide (5.29 g, ρ =
38 0.882 g/mL) was prepared as stated in section 3.2.1. Methanol ampoules were prepared as stated in section 3.2.4.2.
The ampoules were attached to the reactor using a hand torch. Moist paper towel
and aluminum foil were used to cover the ampoules to keep them cool. The reactor was
then connected to the vacuum line through the grease trap using a hand torch and allowed to be evacuated. It was checked for pinholes with the Tesla coil and flame dried several
times. sec-Butyllithium (1.77 mL, 2.646 × 10-3 moles, 1.494 M in hexane, [sec-
BuLi]/[linking agent] = 2/1) was then injected into the reactor under a positive nitrogen
pressure. Vacuum was pulled to remove hexane while cooling with a bath of isopropyl
alcohol/dry-ice. The injection port was flame-sealed. About 250 mL of dry benzene
(polymer solution containing 5 % solids) was distilled from poly(styryl)lithium into the
reactor. The benzene was frozen with the isopropyl alcohol/dry-ice bath, and the reactor
evacuated to remove any air that was present. The reactor was then flame-sealed and
taken off the vacuum line. The solvent was thawed using a warm water bath. The styrene
ampoule was smashed open with a hammer using a magnet. An orange-yellow solution of poly(styryl)lithium of target molecular weight 2000 g/mol was obtained. It was allowed to stand overnight. A sample of the polymer was taken for SEC and the sampling tube was flame-sealed.
In the second step of the reaction, the ampoule containing 1,3-butadiene
diepoxide was opened. The orange-yellow color of poly(styryl)lithium disappeared
almost instantaneously. Another sample of polymer (target molecular weight
4088g/mole) was taken for SEC and the sampling tube was removed by flame sealing
with a hand torch. After this the ethylene oxide ampoule was smashed open by breaking
39 the break-seal with a hammer and magnet. Following this, the phosphazene base ampoule
was opened in a similar manner. The reactor was kept in a warm water bath at 45-50°C
for two weeks before being quenched with methanol and opened for work-up. The
polymer of target molecular weight 8088g/mole, was precipitated into hexane, vacuum
filtered, and dried in the vacuum oven to constant weight. SEC, 1H NMR, 13C NMR and
MALDI-TOF MS analysis were performed on the polymer.
3.4 Characterization
3.4.1 Size Exclusion Chromatography (SEC)
The molecular weights and molecular weight distributions were obtained for the
samples of polystyrene-b-poly(ethylene oxide), in-chain hydroxyl functionalized polystyrene and the four-armed, (PS)2-star-(PEO)2 polymer using size exclusion
chromatography (SEC) at a flow rate of 0.5 mL/minute in THF at 32 ºC. These
measurements were taken on a Waters 150-C Plus instrument equipped with a differential
refractometer, a viscosity detector (Viscotek Model 150R) and four Phenomenex
Phenogel columns (500, 103, 104, and 105 Å). Polystyrene standards from Polymer
Laboratories, Ltd were used for calibration of the columns. Analyses were done using
universal calibration methods38 built into the TriSEC® software. Samples were prepared
by making 8.15 g/mL, 6.14 g/mL, and 1.63 g/mL solutions in THF for the 1500 g/mol
polystyrene, 3088 g/mol in-chain hydroxyl functionalized polymer, and 20,000 g/mol
polystyrene-b-poly(ethylene oxide) copolymer respectively. The solutions were filtered
through 0.45 μm Teflon filters before analysis.
40 3.4.2 Nuclear Magnetic Resonance Spectroscopy (NMR)
1H (300 MHz) and 13C NMR (75 MHz) spectra were measured on a Varian
Mercury-300 spectrometer. The solvent used was CDCl3 (99.8 % D, Cambridge Isotope
Laboratories). NMR sample concentrations were 10-20 wt% in this solvent. The solvent itself was used as an internal standard.
3.4.3 Matrix Assisted Laser Desorption Ionization - Time of Flight (MALDI-TOF) Mass Spectrometry
MALDI-TOF mass spectra were recorded using a Bruker REFLEX-III time-of- flight (TOF) mass spectrometer (Bruker Daltonics, Billerica, MA). The instrument had a
337 nm pulsed nitrogen laser (LSI model VSL-337ND, 3 nm pulse width), a single-stage pulsed extraction ion source and a two stage grid-less reflector. Three solutions: dithranol
(20 mg/mL), functionalized polymer (10 mg/mL), and silver trifluoroacetate (10 mg/mL), were made in THF and mixed in the ratio matrix: cationizing salt: polymer (10:1:2). A
0.5μL sample of this solution was deposited on the sample holder and taken for analysis after evaporation of the solvent. The attenuation of the nitrogen laser was set at 69 %.
The mass scale was calibrated externally with polystyrene standards and the mass accuracy was better than ±0.05 %.
The matrix absorbs strongly at the laser wavelength. As the solvent evaporates, a solid solution of the polymer in the matrix is formed which is bombarded by the laser.
The absorption of energy by the matrix, followed by transfer of energy to the polymer sample, results in formation of molecular ions of the polymer which escape into the gas phase. These ions are then accelerated in an electric field and separated according to their
41 mass-to-charge ratios (m/z) before being detected. Thus this technique identifies each polymer species based on its mass-to-charge ratio (m/z), and measures its abundance in the sample. The molecular ions in this case are singly charged (z =1), which means that the m/z values refer to the masses of each species present. All m/z values used in the analysis are monoisotopic, that is, they correspond to the species containing the lowest mass isotope of each element.
3.4.4 Thin Layer Chromatography (TLC)
Thin layer chromatography (TLC) analyses were carried out on silica gel plates
(Selecto Scientific Flexible TLC plates) that were activated an oven for 24 hours at 140
ºC. Suitable solvents were used as eluents. Samples were spotted on the plates using thin-
-drawn, glass tubing. The spots were viewed using a UVGL-25 Mineralight Lamp.
3.4.5 Column Chromatography
Column chromatography was used to separate the mixture of polymers obtained from the coupling reaction of poly(styryl)lithium with 1,3-butadiene diepoxide.
Poly(styryl)lithium was used in excess for this reaction. Silica (EM Science), the stationary phase, was activated by heating in an oven at 140 ºC for several hours. It was then cooled in a desiccator and stored there until further use. The column was packed by making a slurry of the stationary phase in the solvent of choice. This slurry was then poured into the column, which was already filled with solvent. The column was carefully packed with silica while solvent was eluting through it to ensure that there were no air bubbles present. The mixture of polymers was then dissolved in the eluent and carefully
42 loaded on top of the column. Some sand was sprinkled on top of the sample so as not to disturb the elution process. The column was eluted and the fractions were checked by
TLC for purity of the product. The solvent was then evaporated off from the fractions containing the desired polymer by means of a rotary evaporator.
43 CHAPTER IV
RESULTS AND DISCUSSION
4.1 Analysis of polystyrene-b-poly(ethylene oxide)
The reaction of polymeric organolithium compounds with ethylene oxide is a
quantitative functionalization reaction in hydrocarbon solution at room temperature (see
Section 2.2.1). No oligomerization of ethylene oxide is observed under these conditions.
In like manner, the anionic polymerization of ethylene oxide under the same conditions
results in initiation but no propagation. This effect is attributed to the strong aggregation
of lithium alkoxides due to the high charge density of the lithium cation. Normal polar
solvents and Lewis bases such as tetrahydrofuran are not effective in promoting the
dissociation of these aggregates to form reactive species.
Esswein and co-workers39 have shown that in the presence of a strong Lewis
t base, such as the phosphazene base Bu P4 (Figure 4.1), the polymerization of ethylene
t oxide using lithium as counterion can be achieved. The phosphazene base Bu P4 forms a strong complex with Li+, thus breaking-up the lithium alkoxide aggregates and
facilitating the polymerization of ethylene oxide.
44 C(CH3)3
(CH3)2N N N(CH3)2
(CH3)2N P N P N P N(CH3)2
N N(CH3)2 (CH3)2N (H3C)2N P N(CH3)2 N(CH3)2 t Figure 4.1 Structure of the phosphazene base Bu P4
Anionic polymerization of ethylene oxide in the presence of Li+ counterions is of particular interest for the synthesis of PEO-containing block copolymers by sequential monomer addition. Since most of the applicable monomers are commonly polymerized
t using organolithium initiators, the use of Bu P4 has the advantage that the block
copolymer can be prepared without exchange of the cation. Especially for the anionic
polymerization of butadiene, it is indispensable to use organolithium initiators to obtain
polydienes with high-1,4 microstructure. The objective of this study is to show that the
t use of Bu P4 in hydrocarbon solution results in a PEO-block of high molecular weight
and quantitative conversion of ethylene oxide to poly(ethylene oxide).
The polymerization of styrene and ethylene oxide to form a symmetric block
copolymer of molecular weight 20,000g/mole was carried out using the following
reaction scheme. Li sec-Bu n sec-BuLi + PSLi C6H6 n 10,000 g/mol RT, 12h
O m CH3OH PSLi PS CH2CH2O CH2CH2OH P4 base m-1 45 oC, 2 wks 20,000 g/mol Scheme 4.1 Synthesis of polystyrene-b-poly(ethylene oxide).
45 The yield of block copolymer was calculated based on the above reaction as follows.
Weight of styrene reacted = 6.2 mL × 0.906 g/mL = 5.64 g
Weight of ethylene oxide reacted = 6.4 mL × 0.882 g/mL = 5.64 g
Weight of sec-butyllithium used = 5.64×10-4 mol × 63 g/mol = 0.04 g
Weight of methanol reacted = 5.64×10-4 mol × 32 g/mol = 0.02 g
Total weight of block copolymer expected = 5.64 + 5.64 + 0.04 + 0.02 = 11.34 g
Weight of block copolymer obtained = 11.3 g
Yield of block copolymer = 11.3 × 100 = 99.7 % 11.34
Therefore the yield of polystyrene-b-poly(ethylene oxide) is greater than 99 % which indicates a high conversion of ethylene oxide to poly(ethylene oxide).
4.1.1 Size Exclusion Chromatography (SEC)
Size Exclusion Chromatography is a very useful tool in determining the number average molecular weight (Mn) and polydispersity index (PDI) of a polymer. This technique separates molecules based on their hydrodynamic volumes, which is related to the molecular weights of the polymer chains.
SEC-RI (refractive index detection) analysis was carried out after each step of the above reaction. Polystyrene standards were used to calibrate the instrument and universal calibration techniques were employed to adapt the data to poly(ethylene oxide). The chromatograms obtained are shown in Figure 4.2. Both chromatograms show good agreement between calculated and observed number average molecular weight (Mn) and
46 narrow molecular weight distribution (Mw/Mn; PDI ≤1.1).
Mn(calculated)=10,000 Mn(calculated)=20,000 Mn(observed) =10,700 Mn(observed) =22,500 PDI =1.01 PDI =1.05
Figure 4.2 SEC-RI chromatograms of base polystyrene and polystyrene-b-poly(ethylene oxide).
4.1.2 1H NMR Spectroscopy
The 1H NMR spectrum of the block copolymer is shown in Figure 4.3. The
spectrum shows peaks between δ = 6.2-7.2 ppm that are assigned to the aromatic protons
of polystyrene. There are two peaks in the region of chemical shift δ = 1-2 ppm that are assigned to protons of the polystyrene backbone. The peak at δ = 3.7 ppm is assigned to protons from the ethylene oxide portion of the block copolymer. Deuterated chloroform was used as the NMR solvent and also as the internal standard. A peak due to chloroform is seen at δ = 7.27 ppm.
47
Figure 4.3 1H NMR spectrum of polystyrene-b-poly(ethylene oxide) of molecular weight 20,000g/mol.
Integration ratios of the peaks assigned to the ethylene oxide protons and the aromatic protons were used to calculate the block copolymer composition. By denoting the mole fraction of poly(ethylene oxide) (PEO) as ‘x’ and the mole fraction of polystyrene (PS) as
‘y’, the following equations can be considered.
Mole fraction of PEO = x ≡ 55.36/4 4-1
Mole fraction of PS = y ≡ 28.69/5 4-2
x/y = (55.36/4)/(28.69/5) = 2.41 4-3
x + y = 1 4-4
Solving equations 4-3 and 4-4,
48 x = 0.71 and y = 0.29
Mn (PEO)/Mn (PS) = x×(Molecular weight of ethylene oxide) 4-5 y×(Molecular weight of styrene) = 0.71 × 44g/mol 0.29 × 104g/mol = 1.02
Thus each block is of equal molecular weight and the block copolymer is symmetric.
4.2 Analysis of in-chain, hydroxyl-functionalized polystyrene
The coupling reaction of poly(styryl)lithium with 1,3-butadiene diepoxide was used to prepare in-chain, hydroxyl-functionalized polystyrene. The chemical reactions involved in this synthesis are shown in Scheme 4.2.
Li sec-Bu n sec-BuLi + PSLi C6H6 n 1200 g/mol RT, 12h
OH 1)C6H6, RT 2.3 PSLi + O O PSCH2CHCHCH2PS + 0.3 PS H 2)CH3OH OH 2500 g/mol 1200 g/mol
Scheme 4.2 Synthesis of in-chain, hydroxyl-functionalized polystyrene.
Poly(styryl)lithium [Mn(calculated) =1200g/mol] was prepared in benzene at room temperature using sec-butyllithium as the initiator. The living polymer chains were then coupled using 1,3-butadiene diepoxide. A small excess of poly(styryl)lithium was used in the coupling step to avoid formation of any monofunctional polystyrene. The crude product contained a mixture of base polystyrene and in-chain, hydroxyl-functional
49 -ized polystyrene. The functional polymer was separated from the base polymer by
column chromatography using silica gel as the stationary phase and tetrahydrofuran as
the eluent. It was analyzed using SEC, 1H NMR spectroscopy, 13C NMR spectroscopy
and MALDI-TOF mass spectrometry.
4.2.1 Size Exclusion Chromatography (SEC)
The SEC-RI chromatograms of the base polystyrene and the crude product from
the coupling reaction are shown in Figure 4.4.
Mn(calculated) = 1200g/mol Mn(observed) = 1400g/mol PDI = 1.36 Mn(calculated) = 2400g/mol Mn(observed) = 2730g/mol PDI = 1.00
Figure 4.4 SEC-RI chromatograms of base polystyrene and the crude product from the coupling reaction of poly(styryl)lithium with 1,3-butadiene diepoxide.
The chromatogram for base polystyrene shows a slightly higher molecular weight than what was expected and a broader molecular weight distribution than is generally obtained for higher molecular weight polymers (Mw/Mn< 1.1). The chromatogram for the
50 product from the coupling reaction shows some low molecular weight material, since
excess poly(styryl)lithium was used in this step.
Mn(calculated) = 2400 g/mol Mn(observed) = 2800 g/mol PDI = 1.06
Figure 4.5 SEC-RI chromatogram of in-chain, hydroxyl-functionalized polystyrene of 3 Mn = 2.8×10 g/mol.
The SEC-RI chromatogram of the in-chain, hydroxyl-functionalized polystyrene
is shown in Figure 4.5. The observed molecular weight is in good agreement with the
molecular weight of base polystyrene and the molecular weight distribution is narrow.
4.2.2 Thin Layer Chromatography (TLC)
TLC analysis was carried out for the crude product from the coupling reaction of
poly(styryl)lithium with 1,3-butadiene diepoxide. The results are shown in Figure 4.6.
The substrate used was silica gel and the eluent was toluene. The analysis exhibited two spots. The front running spot was identified as unfunctionalized polystyrene since it
eluted with an identical Rf as the base polymer. Hydroxylated polystyrene eluted with an
Rf= 0.1. 51 A BC
TLC ANALYSIS Substrate: Silica Gel Eluent:Toluene A= Crude hydroxylated product
B= Base polystyrene
C= Pure hydroxylated polystyrene
Figure 4.6 TLC analysis of the product from the coupling reaction of poly(styryl)lithium with 1,3-butadiene diepoxide.
4.2.3 Column Chromatography
Flash column chromatography was used to separate unfunctionalized polystyrene from hydroxylated polystyrene. The system used was the same as for the TLC analysis.
After elution of the first component, namely unfunctionalized polystyrene, the eluent was
gradually changed to pure THF. Hydroxylated polystyrene eluted together with the solvent front in THF, and was subsequently removed from the column. The yield of functionalized polystyrene as calculated from the weights of functionalized and unfunctionalized material eluted from the column chromatographic separation was found to be 72.66 %.
52 4.2.4 1H NMR Spectroscopy
The 1H NMR spectrum of in-chain, hydroxyl-functionalized polystyrene is
shown in Figure 4.7(a).
Figure 4.7 1H NMR spectrum of in-chain, hydroxyl-functionalized polystyrene of 3 Mn=2.8×10 g/mol: (a) overall spectrum.
The spectrum is referenced at δ=7.27 ppm with respect to deuterated chloroform which is used as the NMR solvent. Protons of the polystyrene backbone are seen between
δ=1-2ppm and the aromatic protons are seen in the δ=6.4-7.4 ppm range. The spectrum shows three small peaks in the δ=2.8-3.2 ppm range which can be assigned to the methine and methylene protons of 1,3-butadiene diepoxide as shown in Figure 4.7(b). The peak at
δ=0.6 ppm is assigned to the methyl protons of the sec-butyl groups at the chain ends. In addition to these peaks, a sharp peak is seen at δ=7.4 ppm due residual benzene in the
53 sample, and another peak at δ=1.5 ppm is due to some moisture present in the
chloroform.
3 2 1 CH2OH * H3CCH2 CH* CH2 CH CH2 CH CH CH CH2 HC CH2 CH3 2 CH CH3 n/2 OH n/2 3
1≡ 2.9 ppm 2≡ 3.0 ppm 3≡ 3.1 ppm23
Figure 4.7 1H NMR spectrum of in-chain, hydroxyl-functionalized polystyrene of 3 Mn=2.8×10 g/mol: (b) chemical shifts and peak assignments.
The number average molecular weight (Mn) of functionalized polystyrene was
determined from calculations using the areas A under the peaks due to the methyl protons
of the sec-butyl groups at the chain-ends (δ=0.6 ppm) and the peaks due to the aromatic protons of polystyrene (δ=6.4-7.4 ppm), which correlate with the number of protons contributing to them. By denoting the degree of polymerization of polystyrene as ‘n’, the following equation can be considered.
AAr = Asec-Bu 4-6 5n 12
91.60 = 8.41 5n 12
n = 26.14
Mn = Degree of polymerization × Molecular weight of one styrene unit
= 26.14 × 104g/mol
= 2718.56g/mol
54 The molecular weight thus obtained is in good agreement with the SEC data
(Figure 4.5).
4.2.5 13C NMR Spectroscopy
The 13C NMR spectrum of in-chain, hydroxyl-functionalized polystyrene is shown in Figure 4.8. The spectrum is referenced at δ=77ppm with respect to deuterated chloroform which was used as the NMR solvent.
Figure 4.8 13C NMR spectrum of in-chain hydroxyl functionalized polystyrene of 3 Mn=2.8×10 g/mol
Some of the peaks observed in the spectrum can be assigned as shown in
Table 4.1.
55 10 CH OH 1 2 3 5 6 7 89 2 CH CH CH CH CH H3CCH2 CH CH2 CH CH2 CH CH2 CH CH CH CH2 2 2 3 n/2 -1 OH CH3 CH3 n/2 -1 4
Table 4.1 Observed 13C NMR chemical shifts for the in-chain, hydroxyl-functionalized 3 polystyrene of Mn= 2.8×10 g/mol. Carbon # Chemical Shifts (ppm) 1 11.0 2 28-31 3 30.9 4 19.7 5 44.3 6 43.3 7 45.2 8 40.2 9 40.8 10 67.3
There is no evidence for the formation of non-functional head-head dimer of polystyrene for which a peak would be observed at δ=49.5 ppm.40
4.2.6 MALDI-TOF Mass Spectrometry
The MALDI-TOF mass spectrum of the crude product from the coupling reaction is shown in Figure 4.9(a), (b) and (c). Figure 4.9(a) shows that there are two distributions in the product; the more abundant distribution is centered at m/z 2900, and the less abundant distribution is centered at m/z 1500.
56
Figure 4.9 MALDI-TOF mass spectrum of crude product from the coupling of excess poly(styryl)lithium with 1,3-butadiene diepoxide: (a) overall mass spectrum.
H Ag+ H3C 13 H3C A
Figure 4.9 MALDI-TOF mass spectrum of crude product from the coupling of excess poly(styryl)lithium with 1,3-butadiene diepoxide: (b) monoisotopic resolution in the range of m/z 1510-1630.
57 Ag+
OH CH3 CH H C 3 3 21-n n OH H3C B
Figure 4.9 MALDI-TOF mass spectrum of crude product from the coupling of excess poly(styryl)lithium with 1,3-butadiene diepoxide (continued): (c) monoisotopic resolution in the range of m/z 2490-2630.
Monoisotopic resolution for the smaller distribution is shown in Figure 4.9(b) and that for the bigger distribution is shown in Figure 4.9(c). The distance between successive peaks of each distribution is m/z 104 which corresponds to the mass of one styrene unit. The peak at m/z 1517.76 has the exact mass for the polystyrene 13-mer with
+ butyl and hydrogen end groups, C4H9(C8H8)13H.Ag ; the calculated monoisotopic mass is
+ {57.07 (C4H9) + 13×104.06 ([C8H8]17) + 1.01 (H) + 106.90 (Ag )} = 1517.76 Da.
A ≡ 1517.76 Da
H Ag+ H3C 13 H3C
58 The peak at m/z 2494.33 has the exact mass for the polystyrene 21-mer with butyl end-
+ groups and two in-chain hydroxyl groups C4H9(C8H8)n(C4H8O2)(C8H8)21-nC4H9.Ag ; the
calculated monoisotopic mass is {57.07 (C4H9) + 21×104.06 ([C8H8]17) + 88.05 (C4H8O2)
+ + 57.07 (C4H9) + 106.90 (Ag )} = 2494.35 Da.
B ≡ 2494.33 Da
Ag+
OH CH3 CH H C 3 3 21-n n OH H3C
The most abundant product is the in-chain, hydroxyl-functionalized polystyrene.
Base polystyrene, which has one-half the molecular weight of the coupled product and no
functional groups, is present as a side product since excess poly(styryl)lithium was used
in the coupling step. The functional polymer was obtained in its pure form by purification
of this product mixture by column chromatography. A MALDI-TOF mass spectrum of
the functional polymer in its pure form is shown in Figure 4.10(a) and (b). It shows only
a single product distribution of the appropriate molecular weight.
59
Figure 4.10 MALDI-TOF mass spectrum of in-chain, hydroxyl-functionalized 3 polystyrene of Mn= 2.8×10 g/mol: (a) overall mass spectrum.
Ag+
OH CH3 CH H C 3 3 21-n n OH H3C C
Figure 4.10 MALDI-TOF mass spectrum of in-chain hydroxyl-functionalized polystyrene 3 of Mn= 2.8×10 g/mol: (b) monoisotopic resolution in the range of m/z 2490-2630. 60 The peak at m/z 2494.42 has the exact mass for the polystyrene 21-mer with butyl end-
+ groups and two in-chain hydroxyl groups C4H9(C8H8)n(C4H8O2)(C8H8)21-nC4H9.Ag ; the
calculated monoisotopic mass is {57.07 (C4H9) + 21×104.06 ([C8H8]17) + 88.05 (C4H8O2)
+ + 57.07 (C4H9) + 106.90 (Ag )} = 2494.35 Da.
C ≡ 2494.42 Da
Ag+
OH CH3 CH H C 3 3 21-n n OH H3C
4.3 Analysis of (PS)2-star-(PEO)2 heteroarm star polymer
A four-armed, star polymer of polystyrene and poly(ethylene oxide) was
synthesized anionically in three steps.
The chemical reactions involved in this synthesis are shown in Scheme 4.3
below. Li sec-Bu n sec-BuLi + PSLi C6H6 n 2000 g/mol RT, 12h
OLi PS C H CH OH OH 2 PSLi + O O 6 6 3 PSCH2CHCHCH2PS RT PS OH OLi 1 4000 g/mol O m PS PS OLi CH3OH O[CH2CH2O]m/2H
PS OLi P4 base PS O[CH2CH2O]m/2H 45 oC, 2 wks 2 8000 g/mol Scheme 4.3 Synthesis of (PS)2-star-(PEO)2 heteroarm star polymer.
61
Poly(styryl)lithium of calculated number average molecular weight 2000 g/mol was prepared in benzene at room temperature and coupled with 1,3-butadiene diepoxide
(Section 3.3.3). The coupled product had in-chain alkoxylithium groups which were allowed to initiate the polymerization of ethylene oxide in the presence of the
t phosphazene base Bu P4 at 45°C for two weeks. The product (2) was terminated with methanol and precipitated out of solution using excess methanol. The products (1) and (2) were dried to constant weight and analyzed as described below.
4.3.1 Size Exclusion Chromatography (SEC)
The SEC-RI (refractive index detection) chromatograms of the in-chain, hydroxyl-functionalized polystyrene (product 1) and the final product of the reaction
(product 2) are shown in Figures 4.11 and 4.12 respectively.
The chromatogram for 1 shows some low molecular weight material, although there is good agreement between the calculated and observed number average molecular weight and the molecular weight distribution is narrow. The SEC chromatogram for 2 shows a large difference between calculated and observed molecular weights. The observed number average molecular weight of the product is much lower than the calculated molecular weight. The molecular weight distribution is broad. The chromatogram shows a broad peak with some material in the higher molecular weight region.
62 Mn(calculated) = 4000g/mol Mn(observed) = 3700g/mol PDI = 1.02
Figure 4.11 SEC-RI chromatogram of in-chain, hydroxyl-functionalized polystyrene of 3 Mn=3.7×10 g/mol.
By using the high molecular weight side of the chromatogram in Figure 4.11 as a mirror to draw the low molecular weight side, 6.84 % of the product was found to be uncoupled polystyrene.
63 Mn(calculated) = 8000g/mol Mn(observed) = 4790g/mol PDI = 1.28
Figure 4.12 SEC-RI chromatogram of product from (PS)2-star-(PEO)2 synthesis.
4.3.2 1H NMR Spectroscopy
The 1H NMR spectra for products 1 and 2 are shown in Figures 4.13 and 4.14 respectively.
The 1H NMR spectrum for the in-chain, hydroxyl-functionalized polymer shows peaks for all of the protons of polystyrene as well as for the methyl protons of the sec- butyl groups at the chain ends. NMR peaks corresponding to the methine and methylene protons of 1,3-butadiene diepoxide (discussed in Section 4.2.4) are not seen in the spectrum, probably because the molecular weight of the polymer is too high (3700g/mol;
Section 4.3.1) and hence these protons are very low in abundance.
The spectrum is referenced with respect to the peak at δ = 7.27 ppm which is assigned to chloroform, since deuterated chloroform was used as the NMR solvent. 64
Figure 4.13 1H NMR spectrum of in-chain, hydroxyl-functionalized polystyrene of 3 Mn=3.7x10 g/mol.
The 1H NMR spectrum for product 2 shows an extremely small peak in the region of 3.6-3.8 ppm which can be assigned to the methylene protons of the poly(ethylene oxide) chains. This peak is too small to be integrated with respect to the other peaks in the spectrum. Thus the extent of ethylene oxide polymerization is very small and not significant enough to be detected by proton NMR spectroscopy. The higher molecular weight material seen in the SEC-RI chromatogram (Figure 4.12) probably arises due to coupling of more than two poly(styryl)lithium chains with the linking agent.
This will result in a product which has three or four times the molecular weight of base polystyrene.
65
1 Figure 4.14 H NMR spectrum of product from the synthesis of (PS)2-star-(PEO)2.
4.3.3 13C NMR Spectroscopy
13 The C NMR spectrum for the product from (PS)2-star-(PEO)2 synthesis is shown in Figure 4.15.
13 Figure 4.15 C NMR spectrum of the product from (PS)2-star-(PEO)2 synthesis. 66
Some of the peaks observed in the spectrum can be assigned as shown in the
table below.
11 H[OCH2CH2]m/2O CH2O[CH2CH2O]m/2H 1 2 3 5 6 7 89 CH CH CH CH CH H3CCH2 CH CH2 CH CH2 CH CH2 CH C CH CH2 2 2 3 10 H n/2 -1 CH3 CH3 n/2 -1 4
13 Table 4.2 Observed C NMR chemical shifts for the product from (PS)2-star-(PEO)2 synthesis. Carbon # Chemical Shifts (ppm) 1 11.0 2 28-31 3 30.9 4 19.7 5 44.3 6 43.3 7 45.2 8 40.2 9 40.8 10,11 67.3
4.3.4 MALDI-TOF Mass Spectrometry
The MALDI-TOF mass spectra of product 1 are shown in Figures 4.16 (a), (b),
(c), (d) and (e). The complete distribution of products is shown in Figure 4.16(a). It shows two distributions; one centered at m/z 2310, and another centered at m/z 4060. The smaller distribution centered at m/z 2310 is magnified in Figure 4.16(b).
67
Figure 4.16 MALDI-TOF mass spectrum of in-chain, hydroxyl-functionalized 3 polystyrene of Mn=3.7×10 g/mol: (a) overall mass spectrum.
Figure 4.16 MALDI-TOF mass spectrum of in-chain hydroxyl-functionalized polystyrene 3 of Mn=3.7×10 g/mol: (b) magnified mass spectrum in the range of m/z 0-3000. 68 In Figure 4.16(b), the overall distribution contains four different distributions, labeled by four symbols, namely crosses, open circles, triangles and filled circles. The filled circles are a part of the bigger distribution centered at m/z 4060. The chemical structures for the other three products are assigned in Figure 4.16(c).
Figure 4.16 MALDI-TOF mass spectrum of in-chain hydroxyl-functionalized polystyrene 3 of Mn=3.7×10 g/mol (continued): (c) monoisotopic resolution in the range of m/z 0-3000.
Monoisotopic resolution was obtained for each of these peaks. The lowest mass isotope of each peak was taken for the purpose of calculations. The distance between successive peaks for each distribution is m/z 104, which corresponds to the expected mass of one styrene unit. The peak at m/z 2019.6575 has the exact mass for the polystyrene 17-mer with butyl and monohydroxyl, monoepoxide end groups,
69 + C4H9(C8H8)17C4H7O2.Ag ; the calculated monoisotopic mass is {57.07 (C4H9) +
+ 17×104.06 ([C8H8]17) + 87.04 (C4H7O2) + 106.90 (Ag )} = 2020.03 Da.
A ≡ 2019.6575 Da
O Ag+
H3C CH 17 H3C OH
The peak at m/z 2037.6869 has the exact mass for the polystyrene 18-mer with butyl and
+ hydrogen end groups, C4H9(C8H8)18H.Ag ; the calculated monoisotopic mass is {57.07
+ (C4H9) + 18×104.06 ([C8H8]18) + 1.01 (H) + 106.90 (Ag )} = 2038.06 Da.
B ≡ 2037.6869 Da
H Ag+ H3C 18 H3C
The peak at m/z 2051.5624 has the exact mass for the polystyrene 17-mer with butyl and
+ dihydroxyl, methoxy end groups, C4H9(C8H8)17C5H11O3.Ag ; the calculated
monoisotopic mass is {57.07 (C4H9) + 17×104.06 ([C8H8]17) + 119.07 (C5H11O3) +
106.90 (Ag+)} = 2052.06 Da.
C ≡ 2051.5624 Da
OH Ag+
OCH3 H3C 17 OH H3C
70 It appears that the most abundant product of the smaller distribution, centered at
m/z 2310 is polystyrene coupled to 1,3-butadiene diepoxide in which one epoxide ring is
still intact (peak A). Some of the poly(styryl)lithium gets terminated without any
coupling with the linking agent (peak B). Finally, the second epoxide ring of the product
corresponding to peak A can be cleaved open by methanol (terminating agent), in the
presence of phosphazene base (peak C).
A magnified spectrum of the distribution centered at m/z 4060 is shown in Figure
4.16(d). Once again there are three distributions in the spectrum as seen from Figure
4.16(e). The peak assignments can be made as discussed above for the smaller distribution.
Figure 4.16 MALDI-TOF mass spectrum of in-chain hydroxyl-functionalized polystyrene 3 of Mn=3.7×10 g/mol (continued): (d) magnified mass spectrum in the range of m/z 3000-6000.
71 The peak at 3935.95 has the exact mass for the polystyrene 35-mer with butyl end groups, an in-chain double bond and an in-chain hydroxyl group,
+ C4H9(C8H8)nC4H6O(C8H8)35-nC4H9.Ag ; the calculated monoisotopic mass is {57.07
+ (C4H9) + 35×104.06 ([C8H8]35) + 70.04 (C4H6O) + 57.07 (C4H9) + 106.90 (Ag )} =
3936.60 Da. A ≡ 3935.95 Da Ag+
OH CH3
CH3 H3C 35-n n H3C
The peak at m/z 3953.92 has the exact mass for the polystyrene 35-mer with butyl end
+ groups, and two in-chain hydroxyl groups, C4H9(C8H8)nC4H8O2(C8H8)35-nC4H9.Ag ; the calculated monoisotopic mass is {57.07 (C4H9) + 35×104.06 ([C8H8]35) + 88.05 (C4H8O2)
+ + 57.07 (C4H9) + 106.90 (Ag )} = 3954.27 Da.
B ≡ 3953.92 Da Ag+
OH CH3
CH3 H3C 35-n n OH H3C
72
Figure 4.16 MALDI-TOF mass spectrum of in-chain, hydroxyl-functionalized 3 polystyrene of Mn=3.7×10 g/mol (continued): (e) average masses in the range of m/z 3800-4100.
Monoisotopic resolution was not observed in this spectrum since the molecular
weight of the polymer is higher (twice that of the smaller distribution). The distance
between successive peaks in each distribution is m/z 104 which corresponds to the mass of one styrene unit. The most abundant product, and also the product of interest is the in- chain, hydroxyl-functionalized polystyrene represented by peak B. Loss of a molecule of water from this product gives a product represented by peak A, which has an m/z value
18 units lower than that corresponding to peak B. It was not possible to assign a chemical structure to the product represented by peak C.
Thus the product from the coupling reaction is a complex mixture of in-chain
hydroxyl-functionalized polystyrene, non-functional polystyrene and chain-end
73 functionalized polystyrene bearing a combination of hydroxyl, epoxide, and/or methoxy
groups at the chain end. This data alongwith the SEC results (Figure 4.11) indicate that the coupling reaction has a high efficiency (greater than 90 %).
The MALDI-TOF mass spectrum of the product from the (PS)2-star-(PEO)2
synthesis is shown in Figure 4.17 (a), (b) and (c). Figure 4.17(a) shows that the main
distribution is centered at m/z 4060. A smaller distribution is centered around m/z 2100
which corresponds to the unfunctionalized polystyrene formed in the coupling step.
Two small distributions are also seen in Figure 4.17(b), centered at m/z 6000 and m/z
8000, which can be interpreted as coupling of three or four functionalized polystyrene chains (each of mass around 2000 g/mol). Figure 4.17(c) shows a magnified spectrum of the main distribution.
Figure 4.17 MALDI-TOF mass spectrum of the product from (PS)2-star-(PEO)2 synthesis: (a) overall mass spectrum. 74
Figure 4.17 MALDI-TOF mass spectrum of the product from (PS)2-star-(PEO)2 synthesis (continued): (b) magnified mass spectrum in the range of m/z 5500-10,000.
Figure 4.17 MALDI-TOF mass spectrum of the product from (PS)2-star-(PEO)2 synthesis (continued): (c) average masses in the range of m/z 3800-4300. 75 In Figure 4.17(c), there are peaks corresponding to five distributions. The distance
between successive peaks of each distribution corresponds to the mass of one styrene
unit. The peak at m/z 3954.24 has the exact mass for the polystyrene 35-mer with butyl
+ end groups and two in-chain hydroxyl groups, C4H9(C8H8)nC4H8O2(C8H8)35-nC4H9.Ag ; the calculated monoisotopic mass is {57.07 (C4H9) + 35×104.06 ([C8H8]35) + 88.05
+ (C4H8O2) + 57.07 (C4H9) + 106.90 (Ag )} = 3954.56 Da.
A ≡ 3954.24 Da
Ag+
OH CH3
CH3 H3C 35-n n OH H3C
The peak at m/z 3998.14 has the exact mass for the polystyrene 35-mer with butyl end groups and two in-chain hydroxyl groups, containing one additional unit of ethylene
+ oxide than the product represented by peak A, C4H9(C8H8)nC6H12O3(C8H8)35-nC4H9.Ag ;
the calculated monoisotopic mass is {57.07 (C4H9) + 35×104.06 ([C8H8]35) + 132.08
+ (C6H12O3) + 57.07 (C4H9) + 106.90 (Ag )} = 3998.59 Da.
B ≡ 3998.14 Da Ag+
OH CH3
CH3 H3C 35-n n OCH2CH2OH H3C
The peak at m/z 4041.74 has the exact mass for the polystyrene 35-mer with butyl
76 end groups and two in-chain hydroxyl groups, containing two additional units of ethylene
+ oxide than the product represented by peak A, C4H9(C8H8)nC8H16O4(C8H8)35-nC4H9.Ag ;
the calculated monoisotopic mass is {57.07 (C4H9) + 35×104.06 ([C8H8]35) + 176.11
+ (C8H16O4) + 57.07 (C4H9) + 106.90 (Ag )} = 4042.08 Da.
C ≡ 4041.74 Da Ag+
OH CH3
CH3 H3C 35-n n OCH2CH2OH H3C 2
The peak at m/z 4084.47 has the exact mass for the polystyrene 35-mer with butyl end groups and two in-chain hydroxyl groups, containing three additional units of ethylene oxide than the product represented by peak A,
+ C4H9(C8H8)nC10H20O5(C8H8)35-nC4H9.Ag ; the calculated monoisotopic mass is {57.07
+ (C4H9) + 35×104.06 ([C8H8]35) + 220.14 (C10H20O5) + 57.07 (C4H9) + 106.90 (Ag )} =
4085.77 Da. D ≡ 4084.47 Da Ag+
OH CH3
CH3 H3C 35-n n OCH2CH2OH H3C 3
The peak at m/z 4128.30 has the exact mass for the polystyrene 35-mer with butyl end groups and two in-chain hydroxyl groups, containing four additional units of ethylene oxide than the product represented by peak A,
+ C4H9(C8H8)nC12H24O6(C8H8)35-nC4H9.Ag ; the calculated monoisotopic mass is {57.07
+ (C4H9) + 35×104.06 ([C8H8]35) + 264.17 (C12H24O6) + 57.07 (C4H9) + 106.90 (Ag )} = 77
4128.45 Da.
E ≡ 4128.30 Da Ag+
OH CH3
CH3 H3C 35-n n OCH2CH2OH H3C 4
The actual products may be a complex mixture of the above products and their isomers. The most abundant distribution is of the in-chain, hydroxyl-functionalized polymer, without any addition of ethylene oxide (peak A). This is followed by the products from the addition of one ethylene oxide unit (peak B), two ethylene oxide units
(peak C), three ethylene oxide units (peak D), and four ethylene oxide units (peak E) to the in-chain hydroxyl functionalized polystyrene, in decreasing order of abundance. After this, the intensity of peaks diminishes and the interference from more abundant peaks does not enable us to discern higher order peaks. Resolution of the spectrum is poor because of high molecular weight of the polymer and monoisotopic masses cannot be obtained.
The data from this reaction show that a symmetrical, four-armed, star polymer of polystyrene and poly(ethylene oxide) could not be synthesized by this procedure. The coupling reaction is more than 90 % efficient (from SEC results, Figure
4.11) but the linking agent must be added in exact stoichiometric amount to avoid formation of monofunctional polystyrene, bearing an epoxide group at the chain end.
Polymerization of ethylene oxide from the alkoxy lithium groups in the center of the coupled chain does not take place effectively probably due to strong aggregation
78 effects.32 Effect of epoxide in the product further complicates synthesis of the
amphiphilic star polymer and results in formation of three and four-armed star
polystyrene.
The high efficiency of the coupling reaction suggests that 1,3-butadiene
diepoxide can be used as a linking agent to synthesize in-chain, hydroxyl-functionalized polystyrene. The coupled product, after it is purified, can be used to reinitiate the polymerization of ethylene oxide by first metallating with sodium naphthalenide in tetrahydrofuran at -78°C (Scheme 4.4), as an alternative route to synthesis of the amphiphilic star polymer.41
OH O- Na+ + Na C10H10 PSCH2CHCHCH2PS PSCH2CHCHCH2PS THF, -78oC OH O- Na+ O- Na+ O O[CH2CH2O]m/2H 1) m PSCH CHCHCH PS 2 2 PSCH2CHCHCH2PS 2) CH3OH O- Na+ O[CH2CH2O]m/2H Scheme 4.4 Alternative route to synthesis of (PS)2-star-(PEO)2.
The use of sodium as a counterion instead of lithium is expected to reduce the aggregation effects since sodium is a larger cation than lithium and hence has a lower charge density than lithium.
79 CHAPTER V
SUMMARY
Living anionic polymerization was used to prepare a diblock copolymer of
styrene and ethylene oxide. Styrene was polymerized anionically in benzene at room
temperature using sec-butyllithium as an initiator. Poly(styryl)lithium was then crossed
over to ethylene oxide, which was polymerized at 45°C for two weeks in the presence of
a phosphazene base to reduce aggregation effects. The polymer thus obtained was
examined for molecular weight, polydispersity and block styrene content using SEC and
1H NMR analysis.
In-chain, hydroxyl-functionalized polystyrene was prepared by reacting living
poly(styryl)lithium with a difunctional epoxide linking agent, 1,3-butadiene diepoxide.
The polymer obtained was characterized by SEC, 1H NMR and 13C NMR spectroscopy,
MALDI-TOF mass spectrometry as well as with thin layer and column chromatography.
The reaction of poly(styryl)lithium with 1,3-butadiene diepoxide is a living linking reaction, and hence the possibility of synthesizing a symmetric, four-armed star polymer of styrene and ethylene oxide was examined. Living poly(styryl)lithium was crossed over to 1,3-butadiene diepoxide. The in-chain alkoxy lithium groups in the polymer were allowed to initiate the polymerization of ethylene oxide. The reaction was carried out at 45°C for two weeks in the presence of phosphazene base, as for the block
80 copolymer. The product obtained was characterized by SEC, 1H NMR and 13C NMR spectroscopy and MALDI-TOF mass spectrometry.
81 REFERENCES
1) Hsieh, H. L.; Quirk, R. P. Anionic Polymerization: Principles and Practical Applications; Marcel Dekker: New York, 1966.
2) Fetters, L.J. in Encyclopedia of Polymer Science and Engineering; 2nd ed., Kroschwitz, J. I., Ed., Wiley-Interscience: New York, 1987, Vol. 10, p. 19.
3) Flory, P. Principles of Polymer Chemistry; Cornell University Press: Ithaca, 1953.
4) Wakefield, B. The chemistry of Organolithium Compounds; Pergamon Press: New York, 1974.
5) Quirk, R. P.; Chen, W. C. J. Polym. Sci., Part A: Polym. Chem. 1984, 22, 2993.
6) Morton, M.; Fetters, L. J. Rubber Chem. Technol. 1975, 48, 359.
7) Morton, M. Anionic Polymerization: Principles and Practice; Academic Press: New York, 1983.
8) Swada, H. Thermodynamics of Polymerization; Marcel Dekker: New York, 1976.
9) Ivin, K.; Saegusa, T. General Thermodynamic and Mechanistic Aspects of Ring-Opening Polymerization; Elsevier: New York, 1984; Vol. 1.
10) Bergbreiter, D.; Blanton, J. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 4205.
11) Aldissi, M.; Schue, F.; Geckeler, K.; Abadie, M. Makromol. Chem. 1980, 181, 1425.
12) Heublein, G.; Stadermann, D. Prog. Polym. Sci. 1989, 14, 19.
13) Roovers, J. E. L.; Bywater, S. Macromolecules. 1968, 1, 328.
82 14) Gatzke, A. L. J. Polym. Sci., Part A: Polym. Chem. 1969, 7, 2281.
15) Brown, T. J. Organomet. Chem. 1966, 5, 191.
16) Graham, G.; Richtsmeier, S.; Dixon, D. A. J. Am. Chem. Soc. 1980, 102, 5759.
17) Hsieh, H. L.; Glaze, W. H. Rubber Chem. Technol. 1970, 43, 22.
18) Cunningham, R.; Treiber, M. J. Appl. Polym. Sci. 1968, 12, 23.
19) Worsfold, D. J.; Bywater, S. Can. J. Chem. 1960, 38, 1891.
20) Fetters, L. J.; Balsara, N. P.; Huang, J. S.; Jeon, H. S.; Almdal, K.; Lin, M. Y. Macromolecules 1995, 28, 4996.
21) Quirk, R. P.; Gomochak, D. L. Rubber Chem. and Technol. 2003, 76, 812.
22) Quirk, R. P.; Ma, J.-J. J. Polym. Sci., Part A: Polym. Chem. 1988, 26, 2031.
23) Quirk, R. P.; Mathers, R. T.; Wesdemiotis, C.; Arnould, M. A. Macromolecules, 2002, 35, 2912.
24) Boileau, S. Comprehensive Polymer Science; Eastmond, G.C.; Ledwith, A.; Russo, S.; Sigwalt, P.; Ed., Pergamon Press: Oxford, England, 1989, p 467.
25) Quirk, R. P.; Mathers, R. T.; Ma, J.-J.; Wesdemiotis, C.; Arnould, M. A. Macromol. Symp. 2002, 183, 17.
26) Chang, C. J.; Kiesel, R. F.; Hogen-Esch, T. E.; J. Am. Chem. Soc. 1973, 95, 8446.
27) Ji, H.; Sato, N.; Nonidez, W. K.; Mays, J. W. Polymer 2002, 43, 7119.
28) Quirk, R. P.; Guo, Y.; Arnould, M. A.; Wesdemiotis, C.; Polymer 2004, 45, 3423.
29) Quirk, R. P.; Lizaragga, G. M. Macromolecules 1998, 31, 3424.
83 30) Quirk, R. P.; Ge, Q.; Arnould, M. A.; Wesdemiotis, C.; Macromol. Chem. Phys. 2001, 201, 1761.
31) Budavari, S. The Merck index: An Encyclopedia of Chemicals, Drugs, and Biologicals; 12th ed.; Chapman & Hall: Whitehouse Station, 1996.
32) Halaska, V.; Lochmann, L.; Lim, D. Coll. Czech. Chem. Commun. 1968, 33, 3245.
33) Morton, M.; Fetters, L. J. Rubber Chem. Technol. 1975, 48, 359.
34) Sekutowski, D.; Stucky, G. J. Inorg. Chem. 1975, 14.
35) Sekutowski, D.; Stucky, G. J. J. Chem. Ed. 1976, 53, 110.
36) Budavari, S. The Merck index: An Encyclopedia of Chemicals, Drugs, and Biologicals; 12th ed.; Chapman & Hall: Whitehouse Station, 1996, Vol. 13, p. 3836.
37) Gilman, H.; Cartledge, F. K. J. Organomet. Chem. 1964, 181, 59.
38) Grubisic, Z.; Rempp, P.; Benoit, H. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 1707.
39) Esswein, B.; Mőller, M.; Angew.Chem. 1996, 108, 703.
40) Quirk, R. P.; Hasegawa, H.; Gomochak, D. L.; Wesdemiotis, C.; Wollyung, K. Macromolecules 2004, 37, 7146.
41) Piirma, I. Polymeric Surfactants; Marcell Dekker, Inc.: New York, 1992, p 27.
84