High-Molecular-Weight Poly( tert -butyl acrylate) by Nitroxide-Mediated Polymerization: Effect of...

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High-Molecular-Weight Poly(tert-butylacrylate) by Nitroxide-MediatedPolymerization: Effect of Chain Transfer toSolvent

Benoıt Lessard, Christopher Tervo, Milan Maric*

Tert-Butyl acrylate (TBA) was polymerized by nitroxide-mediated polymerization (NMP) usingBlocBuilder initiator and 4.5 mol-% additional SG1 (N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide) relative to BlocBuilder (2-methyl-2-[N-tert-butyl-N-(diethoxylpho-sphoryl-2,2-(dimethylpropyl)aminooxy]propionic acid) at 115 8C in bulk and in varioussolvents. In all cases, number average molecular weight (Mn) increased linearly up to 35%conversion.kpKvalues (kp¼propagationrateconstant,K¼ equilibriumconstant) forTBAagreedwellwith literaturedata.ForhighertargetMn >50kg �mol�1, solution polymeriza-tions used to reduce viscosity were pro-blematic as chain transfer reactionsbecame noticeable, particularly whenblock copolymers with styrene weredesired. A better strategy to obtain highMn block copolymers involved a semi-batch feeding in bulk of styrene mono-mer to a poly(TBA) macroinitiator whichresulted in highMn gradient blocks withlow polydispersity (Mn¼ 54.7 kg �mol�1,Mw=Mn ¼1.3).

Introduction

Well-defined and complex polymeric microstructures have

been made more easily accessible due to advances in

controlled radical polymerization (CRP).[1–3] These con-

B. Lessard, M. MaricDepartment of Chemical Engineering, McGill University, McGillInstitute of Advanced Materials (MIAM), Centre for Self-Assembled Chemical Structures (CSACS), 3610 University Street,Montreal, Quebec, Canada H3A 2B2Fax: (514) 398-6678; E-mail: [email protected]. TervoDepartment of Chemical Engineering and Materials Science,Minneapolis, Minnesota 55455-0431

Macromol. React. Eng. 2009, 3, 245–256

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

trolledmoleculararchitectureshavetraditionallyonlybeen

attainable via ionic or other ‘‘living’’ polymerizations,

which is a technique faced with challenges such as air free

transfers and extensive reagent purifications.[4] The

increase in popularity of CRP methods is partly due to

the potential ease associated with industrial scale-up of

radical polymerizations while still maintaining a high

degree of microstructural control. Well-defined, functional

polymers made by CRP have been utilized in applications

such as plastic solar cells,[5–7] bio-sensors,[8,9] nanotemplat-

ing,[10–12] and hydrogen storage systems.[13,14] Some of

theseapplications, suchas separationsmembranes, require

functional groups like acrylic acid (AA) to introduce

hydrophilic segments into the block copolymer.

DOI: 10.1002/mren.200900014 245

B. Lessard, C. Tervo, M. Maric

246

The incorporation of AA into block copolymers by

nitroxide-mediated polymerization (NMP) (one of the types

of CRP) has been previously accomplished.[15,16] The direct

polymerization of AAwas previously determined to be less

effective in terms of controlling the molecular weight

distribution compared towhen itwas used in the protected

formof tert-butyl acrylate (TBA).[17,18] The tert-butyl groups

can then be de-protected by a mild acid treatment to yield

thecarboxylic acid functionality.[19]Ourgrouphasexplored

routes to obtain poly(AA)-b-poly(styrene) block copoly-

mers. The route which resulted in the narrowest molecular

weight distribution involved homopolymerizing TBA and

then using the resulting poly(TBA) as a macroinitiator to

reinitiate a fresh batch of styrene (S) to yield a poly(TBA)-b-

poly(styrene) block copolymer. The poly(AA)-b-poly(styr-

ene)wasobtainedafter cleavageof the tert-butylgroups.[17]

In all reported syntheses of TBA in bulk by CRP, rarely

were polymerizations done with higher target molecular

weights exceeding 50 kg �mol�1 nor were they taken to

conversions above 40% due to viscosity issues and

irreversible termination reactions.[20,21] Higher molecular

weight polymers are particularly appealing for better

mechanical properties in the finalmaterial application and

for potentially easier processing.[22] The purpose of this

paper is to determine a viable solution to reach higher

molecular weights without excessively deleterious side

effects such as long chain branching and broad molecular

weight distributions. This study will focus on applying

the unimolecular initiator, 2-methyl-2-[N-tert-butyl-N-

(diethoxylphosphoryl-2,2-(dimethylpropyl)aminooxy]pro-

pionicacid,knownasBlocBuilder1 (Arkema),withadditional

N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)]

nitroxide (SG1) to attain high molecular weight poly(TBA)-

b-poly(styrene) block copolymers with relatively narrow

molecular weight distributions. The use of solvents will be

studied as a means of avoiding viscosity problems

associated with bulk polymerization and hopefully allow

attainment of higher conversions and ultimately higher

molecularweights. In contrast tosolutionpolymerization, a

semi-batch approach to make tapered or gradient block

copolymers was also examined where the second batch of

monomer is effectively used to reduce the viscosity.

Examination of thesemethodswill hopefully lead to better

methods of producing high molecular weight poly(TBA)-b-

poly(styrene) block copolymers using relatively simple

protocols.

Experimental Part

Materials

The monomers, styrene (99%) and TBA, 98%, in addition to

basic alumina (Brockmann, Type 1, 150mesh) and calciumhydride

(90–95% reagent grade) were purchased from Aldrich. P-xylene



(99%), dimethylformamide (DMF, 99.8%), anisole (99%), methanol

(99.8%) and tetrahydrofuran (99.9%), were obtained from

Fisher. Purification of TBA and styrene was done by passage

through a column of 5% calcium hydride/basic alumina. The

purified monomers were stored in a sealed flask in a refrigerator

under a head of nitrogen until required. 2-[N-tert-butyl-2,2-

(dimethylpropyl)aminooxy] proponic acid (99%, BlocBuilder)

was purchased from Arkema and was used as received. N-tert-

butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide, also

knownasSG1, (>85%)waskindlydonatedbyNoahMacyofArkema

and was also used as received.

Synthesis of Poly(tert-butyl acrylate) (Poly(TBA))Homopolymers

The syntheses were all performed in a 100mL three neck round

bottom glass flask similar to previous NMP syntheses.[17,18] The

reactor was equipped with a condenser that was connected to a

chilling unit (Neslab 740 – using a 50wt.-% ethylene glycol/water

mixtureas coolant), a thermalwellwitha thermocouple connected

to a controller, and rubber septa to allow sampling by syringe. The

flaskwas placed inside a heatingmantle, whichwas placed on top

of a magnetic stirrer. Themixing was performed using a magnetic

Teflon stir bar. All experimental formulations can be found in

Table 1. Once the reactorwas set up, BlocBuilder and SG1were both

addedwiththestirrerandthereactorwasthensealedwitha rubber

septum. The previously purified TBA monomer was added to the

reactor by injection with a syringe and the amount of TBA relative

to BlocBuilder was added so that the target number average

molecularweights (Mn)at completeconversionwasapproximately

25or65 kg �mol�1 inall cases.At thispoint, theappropriateamount

ofp-xylene,DMForanisolewasadded toobtaina50wt.-%solution.

An ultra pure nitrogen purgewas applied aftermonomer injection

by bubbling for 30min at room temperature prior to increasing the

temperature. The reactor was then heated to 115 8C at a rate of

about 10 8C �min�1 while maintaining a nitrogen purge. The time

at which the reactor reached 110 8C was taken as the start of the

reaction and samples were taken periodically. The sample

polymers were precipitated with a 1:5 mixture of distilled

water/methanol followed by drying overnight in a vacuum oven

at 70 8C to remove any solvent or unreacted monomer.

Synthesis of Poly(tert-butyl acrylate)/Poly(styrene)(Poly(TBA)-b-PS) Block Copolymer using Poly(tert-butyl acrylate) (Poly(TBA)) Macroinitiators

All chain extensions of poly(TBA) macroinitiators with styrene

were performed using the identical set-up as the homopolymer-

izationsof themacroinitiators described in theprevious section.All

experimental formulations can be found in Table 2. Once the

reactor was set up, the poly(TBA)macroinitiators were addedwith

the stirrer into the reactor. For example, experiment TD25-PS-1was

performed by adding themacroinitiator TD25 (2.0 g, 0.19mmol) to

the reactor and sealed with a rubber septum. The previously

purified styrene was injected into the reactor. For TD25-PS-1, the

mass of styrene added (16.7 g, 160mmol) was such that if a 40%

conversion of styrene was obtained, the final block copolymer

molar composition, FTBA, would be�0.20,whichwould correspond

to a cylindrical morphology consisting of poly(TBA) cylinders in a

DOI: 10.1002/mren.200900014

High-Molecular-Weight Poly(tert-butyl acrylate) by Nitroxide-Mediated Polymerization . . .

Table 1. Tert-butyl acrylate (TBA) homopolymerization formulations done at 115 8C in a) Various solvents and b) Gradient copolymerizationof TBA with styrene.

Expt. IDa) [BlocBuilder]0 [SG1]0 rb) [TBA]0 Solvent [Solvent]0 Solvent Mn; target

mol � L�1 mol � L�1 mol � L�1 mol � L�1 wt.-% kg �mol�1

TB25 0.036 0.002 0.054 6.89 bulk – – 24.6

TB65 0.013 0.001 0.044 6.89 bulk – – 69.1

TA25 0.019 0.002 0.046 3.65 anisole 4.32 50 24.9

TA65 0.007 0.001 0.044 3.65 anisole 4.33 49 64.6

TD25 0.018 0.002 0.047 3.56 DMF 6.24 50 24.9

TD65 0.007 0.001 0.044 3.56 DMF 6.24 50 64.7

TX25 0.017 0.002 0.050 3.38 p-xylene 4.13 50 25.0

TX65 0.007 0.001 0.044 3.44 p-xylene 4.06 51 65.8

Expt. ID [BlocBuilder]0 [SG1]0 rb) [TBA]0 [Styrene]0 Solvent Mn; target; PTBAd) Mn; target; total

d)

mol � L�1 mol � L�1 mol � L�1 mol � L�1 wt.-% kg �mol�1 kg �mol�1

TB25-t-Sc) 0.006 2.2� 10�4 0.037 1.21 7.21 Bulk 27.2 157.2

a)Experimental Identification is as follows: T¼ tert-butyl acrylate and the second letter refers to the solvent used in the polymerization

(B¼bulk, X¼p-xylene, D¼dimethylformamide (DMF) and A¼ anisole) b)Molar concentration ratio of free nitroxide to BlocBuilder given

as r¼ [SG1]0/[Bloc Builder]0.c)Tapering or gradient copolymerization experiment where TBA was polymerized first and then styrene was

added later. d)Mn; target; PTBA is the target number average molecular weight of the first block consisting of TBA. Mn; target; total is the target

number average molecular weight of the entire block.

styrenic matrix.[23] Such a morphology would be expected to be

useful for nanoporous membranes. The addition of solvent was

done to obtain a 50wt.-% solution for the syntheses performed in

solution.For thisexample,DMF(18.7 g, 260mmol)wasaddedas the

solvent. Similar to the homopolymerizations, a nitrogenpurgewas

applied during monomer injection, 30min post-injection at room

temperature andduring theentire polymerization. The reactorwas

heated to 115 8C at a rate of about 10 8C �min�1 and the time at

which the reactor reached 110 8C was taken as the start of the

reaction. Samples were taken periodically and precipitated in

methanol. The samples were then dried overnight in a vacuum

oven at 70 8C.

Synthesis of Poly(tert-butyl acrylate-tap-styrene)(Poly(TBA)-tap-Poly(S)) Tapered Block Copolymer

The tapering or gradient experimental formulation can also be

found in Table 1. A 250mL three neck round bottom reactor was

used for the synthesis with the same condensing, heating and

controlling methods as the previously described syntheses except

no solvent was used. The experimental procedure was identical to

thehomopolymerizationsofpoly(TBA)done inbulkwitha targetof

25 kg �mol�1. BlocBuilder (0.208g, 0.55mmol), additional SG1

(0.006g, 0.05mmol) and TBA (14.7 g, 49.8mmol) were all added

to the reactor at the start of the polymerization. Once the reaction

had reached a point where the conversion reached approximately

40% (based on estimates from a prior bulk kinetic study), styrene



(71.1 g, 684mmol) was added in one shot by syringe without

stopping the reaction. Samples were drawn periodically and the

reactionwas carried out until the viscosity of the reactionmixture

increased to the point that taking samples became difficult.

Precipitation, drying and processing of the samples and product

were performed in a similar fashion to the homopolymerizations

and chain extensions.

Chain Transfer Constant Determination

Chain transfer constants to solvent (Ctr) were not readily available

forTBAat thepolymerizationtemperatureusedherefor thevarious

solvents examined so these needed to be determined. These

experiments were identical to the TBA homopolymerizations

described previously except that no initiator or additional free

nitroxide was added. Similar protocols were used for acrylic acid

homopolymerizations in 1,4 dioxane.[16] Only the monomer (TBA)

andthesolvent (anisole,DMForp-xylene)wereaddedto the reactor

along with a magnetic stirrer. Note that polymerization was

effected by trace impurities in the reagents which could serve as

initiators. The solutionswere approximately 50wt.-%monomer in

solvent (3.4–3.7mol � L�1 TBA in 4.1–6.2mol � L�1 of the solvent).

The reactor was sealed and the contents were purged with ultra

pure nitrogen for 30min. The heating was then commenced at a

rate of 10 8C �min�1 up to the desired set point of 115 8C. Samples

were removed periodically for determination of conversion and

molecular weight to be used for Ctr calculations. Gravimetry was

www.mre-journal.de 247


Table 2. Formulations of styrene chain extensions from Poly(tert-butyl acrylate) macroinitiators at 115 8C in various solvents.

Experiment IDa) Macroinitiator IDa) [Macroinitiator]0 [Styrene]0 Solvent [Solvent]0 Solution Mn; targetb)

mol � L�1 mol � L�1 mol � L�1 wt.-% kg �mol�1

TB25-PS-1 TB25 5.34 8.74 bulk – – 170.1

TB25-PS-2 TB25 3.80 8.74 bulk – – 239.1

TB25-PS-3 TB25 2.95 8.74 bulk – – 307.8

TB65-PS-1 TB65 4.29 8.74 bulk – – 212.1

TB65-PS-2 TB65 3.37 8.74 bulk – – 270.0

TB65-PS-3 TB65 2.62 8.74 bulk – – 346.8

TA25-PS-1 TA25 2.07 4.30 anisole 4.67 50 215.8

TA25-PS-2 TA25 1.51 4.38 anisole 4.59 50 301.7

TA65-PS-1 TA65 1.06 4.31 anisole 4.66 50 421.2

TA65-PS-2 TA65 0.78 4.40 anisole 4.57 50 587.9

TD25-PS-1 TD25 2.76 4.20 DMF 6.70 50 158.4

TD25-PS-2 TD25 2.05 4.27 DMF 6.61 50 216.3

TD65-PS-1 TD65 1.93 4.21 DMF 6.70 50 227.0

TD65-PS-2 TD65 1.40 4.26 DMF 6.62 50 315.6

TX25-PS-1 TX25 1.95 3.82 p-xylene 4.57 48 203.7

TX25-PS-2 TX25 1.48 4.10 p-xylene 4.31 50 287.0

TX65-PS-1 TX65 1.47 3.99 p-xylene 4.41 50 281.8

TX65-PS-2 TX65 1.06 4.08 p-xylene 4.33 50 399.4

a)Experimental identification is as follows. The first letter refers to the first monomer polymerized where T¼ tert-butyl acrylate (TBA) and

the second letter refers to the solvent used in the polymerization (B¼bulk, X¼p-xylene, D¼dimethylformamide (DMF) and A¼ anisole).

The number after the first two letters signifies the number averagemolecular weightMn; target at complete conversion. The PS refers to the

poly(styrene) added onto the PTBA macroinitiator. b)Mn; target is the target molecular weight of the second block (styrene) if its

polymerization went to complete conversion.

248

usedtoestimateconversionwhilegelpermeationchromatography

(GPC) was used to estimate the molecular weight of each sample

taken.

Characterization

Gravimetry was used to determine the monomer conversion. The

molecularweight distributionwasmeasuredusinggel permeation

chromatography (GPC, Waters Breeze system) with tetrahydro-

furan (THF) as the mobile phase. Both homopolymers and block

copolymers were soluble in organic solvents and therefore did not

need further treatment prior to GPC analysis. The flow rate of the

mobilephasewas0.3mL �min�1andtheGPCwasequippedwith, in

addition to a guard column, three Waters Styragel1 HR columns

(HR1 with molecular weight measurement range of 102–

5�103 g �mol�1, HR2 with molecular weight measurement range

of 5� 102–2�104 g �mol�1 and HR4 with molecular weight

measurement range 5� 103–6�105 g �mol�1). The columns were

heated to 40 8C during the analysis. The molecular weights were

determined by calibration with linear narrow molecular weight

distribution poly(styrene) standards. The following Mark–Hou-

wink parameters K and a were used to estimate the molecular



weights of the poly(tert-butyl acrylate) homopolymers in THF. For

poly(TBA), KP(TBA)¼3.33� 10�5 dL � g�1 and aP(TBA)¼0.80[24] while

for poly(styrene), KPS¼ 11.4�10�5 dL � g�1, aPS¼0.716.[25] The GPC

was equipped with both ultra-violet (Waters UV 2487) and

differential refractive index (Waters RI 2410) detectors. The UV

detector was set to a wavelength of 255nm to detect the aromatic

rings in the poly(styrene) containing copolymers.

Results and Discussion

Tert-Butyl Acrylate Homopolymerization: Effect ofSolvent on Kinetics

A series of homopolymerizations of TBA in bulk at 115 8Cwere carried out with two different target Mn’s of 25 and

65 kg �mol�1, and a ratio of SG1 to BlocBuilder of 4.5mol-%.

The addition of 4.5wt.-% of SG1 (N�) relative to BlocBuilder

has previously been shown to significantly improve control

in similar systems.[18,26] Inaddition topolymerizing inbulk,

a series of solution polymerizations in anisole, DMF and p-

DOI: 10.1002/mren.200900014


Figure 1. Semi-logarithmic plot of scaled conversion (ln((1� x)�1)(x¼ conversion) versus time for homopolymerizations of tert-butyl acrylate in bulk and in different solvents at 115 8C. Thevarious solution polymerizations correspond to the experimentalidentifications in Table 1: TB25 (^), TB65 (^), TA25 (&), TA65 (&),TD25 (�), TD65 (*), TX25 (~), and TX65 (D).

xylene were conducted (Table 1). Figure 1 shows the semi-

logarithmic plot of [ln((1� x)�1)] (where x¼ conversion)

versus time for all homopolymerizations of tert-butyl

acrylate at 115 8C. From the ln[(1� x)�1] versus polymer-

ization time plots, it is apparent that the slope of all

polymerizations with or without the use of solvent tend to

Figure 2. Number average molecular weight Mn versus conversion (x) for varioushomopolymerizations of TBA done at 115 8C with a target molecular weight ofa) 25 kg �mol�1, in bulk (experiment TB25) (^)); in 50 wt.-% anisole (experiment TA25

be similar at low conversion (x< 0.4;

50min polymerization time) but at

higher conversion the effect of chain

transfer reactions and other side reac-

tions becomes evident and conversion

tends to deviate from the expected

‘‘pseudo-living’’ linear increase. The

slopes of the ln[(1� x)�1] versus time

plots at low conversions give the appar-

ent rate constants kp[P�] where kp is the

propagation rate constant and [P�] is theconcentration of propagating radicals. In

addition to the propagation of the

radicals, the equilibrium between the

dormant and the active chains affects the

controlof thepolymerizationasshownin

Equation (1)

(&)); in 50 wt.-% dimethylformamide (experiment TD25 (�)) and in 50 wt.-% p-xylene(experiment TX25 (~)). Homopolymerizations of TBA with a target Mn ¼65 kg mol�1

are shown in b) for polymerizations done in bulk (experiment TB65 (^)); in 50 wt.-%anisole (experiment TA65 (&)), in 50 wt.-% DMF (experiment TD65 (*)), and in 50 wt.-%

Macrom

� 2009

K ¼ P�½ � N�½ �P � N½ � (1)

p-xylene (experiment TX65 (D)). The straight solid lines indicate the theoretical Mn

versus x based on the monomer to initiator ratio for the particular experiment.Complete characterization data for the experiments is listed in Table 4.

K is defined as the ratio of the product

between the concentration of propagat-

ol. React. Eng. 2009, 3, 245–256

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ing radicals [P�] and free nitroxide [N�] to the concentra-

tion of the dormant species [P�N].[16,27,28] At low

conversions, the large concentration of initial free SG1

suggests the free nitroxide concentration does not change

significantly and [N�]� [N�]0. In addition, the system is

assumed to behave in a pseudo-‘‘living’’ manner and

therefore [P�N] is equivalent to the initial concentration of

initiator during the early stages of the polymerization

([P�N]¼ [BlocBuilder]0). Electron spin resonance (ESR)

measurements would easily confirm whether the

pseudo-‘‘living’’ assumption is justified. In this case, the

validity of this pseudo-‘‘living’’ assumption in a given

conversion range is reasonable if Mn increases linearly

with conversion, as shown in Figure 2 at low conversion.

Therefore, the pseudo-‘‘living’’ assumption is suitable here

in the low conversion regime and allows the determina-

tion of the product of the propagation rate constant with

the equilibrium constant, kp K, as shown in Equation (2)

kPK ¼ kp P�½ � SG1�½ �0BlocBuilder½ �0

� kP P�½ �r (2)

where the parameter r is the ratio [SG1�]0/[BlocBuilder]0.The kp K values are listed in Table 3. The kp K’s obtained

experimentally ranged from (3.4� 0.3)� 10�6 � s�1 to

(5.9� 0.4)� 10�6 � s�1 in bulk and (4.5� 0.1)� 10�6 � s�1

to (7.6� 0.4)� 10�6 � s�1 for those done in solution

(Table 3). These kp K values agree relatively well with

those done in bulk at 115 8C reported elsewhere (kpK¼ 3.0� 10�6 s�1).[18] Becer and al.[29] reported kpK for TBA

homopolymerizations in a parallel synthesizer with



Table 3. Tert-Butyl acrylate (TBA) polymerization kinetic data from homopolymerizations done at 115 8C in various solvents.

Experiment IDa) rb) Solvent kp[P�]c) kp Kc)

s�1 s�1

TB25 0.054 bulk (1.1� 0.1)� 10�4 (5.9� 0.4)� 10�6

TB65 0.044 bulk (7.7� 0.1)� 10�5 (3.4� 0.3)� 10�6

TA25 0.046 anisole (9.8� 0.1)� 10�5 (4.5� 0.1)� 10�6

TA65 0.044 anisole (1.1� 0.1)� 10�4 (5.0� 0.1)� 10�6

TD25 0.047 DMF (1.2� 0.1)� 10�4 (7.6� 0.4)� 10�6

TD65 0.044 DMF (1.0� 0.1)� 10�4 (4.5� 0.01)� 10�6

TX25 0.050 p-xylene (1.2� 0.1)� 10�4 (5.9� 0.4)� 10�6

TX65 0.044 p-xylene (1.1� 0.1)� 10�4 (4.9� 0.3)� 10�6

a)Experimental ID identifies the polymerization as follows. The first letter T refers to TBA and the second letter refers to the solvent used in

the polymerization (B¼bulk, X¼ p-xylene, D¼dimethylformamide (DMF) and A¼ anisole). The numbers after the first two letters refer

to the number average molecular weight at complete conversion [¼] kg �mol�1. b)Molar concentration ratio of SG1 free nitroxide relative

to BlocBuilder given by r¼ [SG1]0/[BlocBuilder]0.c)kp[P�] (kp¼propagation rate constant, [P�] is the concentration of propagating

macroradicals) is the apparent rate constant taken from the slopes of the ln[(1�x)�1] (x¼ conversion) versus time plots taken in the

linear region. The error in kp[P�] is given by the standard error in the slope from the linear fit of ln[(1�x)�1] versus time. kp K

(K¼ equilibrium constant) was calculated from kp[P�] and r using Equation (2).

250

various free nitroxide concentrations at 110 8C. Their

reported kp K values were kp K¼ 4.3� 10�7–

1.0� 10�6 � s�1. For the TBA homopolymerizations con-

ducted, therewas no significant difference in apparent rate

constant between bulk and solution polymerizations. In

addition, no significant difference in apparent rate

constant was observed for the polymerizations done in

different solvents (Table 3).

Figure 2 shows theMn versus x for all homopolymeriza-

tions, where the homopolymerizations with a targetMn of

25 kg �mol�1 are presented in Figure 2a andb for thosewith

a targetMn ¼ 65 kg �mol�1. In Figure 2a, it is apparent that

homopolymerizations done in bulk exhibited a linear

increase inMn with conversion. The homopolymerizations

which were done in bulk became highly viscous when

reaching higher x� 0.4. To compensate for the increased

viscosity, solventswere used to push the polymerization to

higher conversions. The homopolymerizations done in

solvent however, exhibit a noticeable loss of this linear

behavior, particularly when DMF was used. For example,

the polymerization of TBA in DMF exhibited a pronounced

levelling at even very low conversion (x< 0.2) (Figure 2a).

For all homopolymerizations with a target Mn of

65 kg �mol�1, virtually all of the Mn versus conversion

plots indicatedaplateau, regardless ofwhether solventwas

used (Figure 2b). At these higher target Mn, the effect of

solvent on the Mn versus x was less apparent, due to the

increase in other more significant terminating or transfer

effects suchas intramolecular chain transfer topolymer (eg.

backbiting), propagation to terminal double bond, chain

transfer to monomer and termination by combination,



which are all common for acrylic polymerizations.[30]

Regardless of the solvent used (anisole, p-xylene or DMF)

a tailing off ofMn versus x took place at similar conversions

(x> 0.4). These findings prove that even though viscosity

wasno longer an issue, substantial irreversible termination

takes place, causing Mn versus x to deviate from ideal

behavior at higher conversions.

Depending on the solvent used in the homopolymeriza-

tion of TBA, the molecular weight distribution varied

significantly. Theuseofaverypolar solvent, likeDMF, in the

homopolymerizationgave rise tobroadermolecularweight

distributions (Mn ¼ 10.7 and 16.0 kg �mol�1,Mw=Mn ¼ 2.11

and 2.28) compared to the homopolymerizations done in a

lesspolar solvent likeanisoleatnearly thesameconversion,

which resulted in homopolymers with a relatively nar-

rower molecular weight distribution (Mn ¼ 15.3 and

30.7 kg �mol�1, Mw=Mn ¼ 1.56 and 1.62) (Table 4). Our

findings prove that in attempts to reach higher conversion

and higher molecular weights, the use of solvent is not

necessarily beneficial. Homopolymerizations of TBA were

unable to reach x> 0.4 without exhibiting irreversible

termination or loss of ‘‘livingness’’.

Chain Transfer to Solvent

Tobetterunderstand the limitationsofpolymerizingTBA in

solutionbyNMP, knowledgeof the chain transfer to solvent

constant (Ctr¼ ktr/kp, with ktr the rate constant for chain

transfer) for the various solvents was essential. However,

Ctr values for TBA were not necessarily available. Thus,

determination of Ctr values for TBA homopolymerizations

DOI: 10.1002/mren.200900014


Table 4. Characterization of Poly(tert-butyl acrylate) (PTBA) homopolymers from polymerizations done at 115 8C in various solvents.

Experiment IDa)Mn; target Mn

b) Mw=Mnb) Conversion(x)

kg �mol�1 kg �mol�1

TB25 24.6 21.0 2.27 0.51

TB65 69.1 23.9 1.51 0.50

TA25 24.9 26.7 1.56 0.78

TA65 64.6 51.8 1.62 0.83

TD25 24.9 19.0 2.11 0.85

TD65 64.7 27.9 2.28 0.94

TX25 25.0 25.0 1.64 0.81

TX65 65.8 35.1 1.97 0.82

TB25-t-Sc) 157.2 54.7 1.30 0.34

a)Experimental identification is as follows. The first letter refers to the first monomer polymerized where T¼ tert-butyl acrylate (TBA) and

the second letter refers to the solvent used in the polymerization (B¼bulk, X¼p-xylene, D¼dimethylformamide (DMF) and A¼ anisole).

The number after the first two letters signifies the number average molecular weightMn; target at complete conversion. b)Number average

molecular weight Mn and polydispersity index (Mw=Mn) determined by gel permeation chromatography (GPC) relative to linear

poly(styrene) standards in tetrahydrofuran (THF) at 40 8C. The following Mark-Houwink parameters K and a were used for the

determination of the molecular weight of PTBA from the PS standards in THF: KPTBA¼3.33� 10�5 dL � g�1, aPTBA¼ 0.80[24] and

KPS¼ 11.4�10�5 dL � g�1, aPS¼ 0.716.[25] c)The experiment TB25-t-S refers to the polymerization of TBA followed by injection of styrene

after TBA conversion�40% to produce a tapered or gradient copolymer. The number averagemolecular weight at complete conversion of

all monomers is given by Mn; target.

wasperformedwithno initiatorormediator ineachsolvent

(Table 5). With no initiator or mediator present, Ctr for a

given solvent can be estimated given the conversion,

polymer chain concentration [Chains] and initial solvent

concentration [Solvent]0 are known (Equation (3)).

Tab

Exp

TA

TD

TXC

a)Expe

used i

ChTS¼taking

Handb

Macrom

� 2009

Ctr ¼ln 1� Chains½ �

Solvent½ �0

h i

ln 1� x½ �0(3)

To apply Equation 3, the concentration of the chains and

the solvent are needed. Couvreur et al. defined the

le 5. Chain transfer to solvent experiments for TBA homopolyme

eriment IDa) [TBA]0 Solvent

mol � L�1

ChTS 3.65 anisole

ChTS 3.56 DMF

hTS 3.39 p-xylene

rimental identification is given as follows. The first letter represe

n the polymerization (B¼bulk, X¼p-xylene, D¼dimethylform

chain transfer to solvent experiment (no initiator was used). b

the slopes of the � ln 1� ½chains�0½solvent�0

� �versus -ln(1�x) plots show

ook, 4th ed., 2002 (Brandup, J.; Immergut, E. H.; Grulke, E. A., e

ol. React. Eng. 2009, 3, 245–256

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

concentration of polymer chains to be equivalent to

monomer conversion multiplied by the monomer concen-

tration divided by the number average degree of poly-

merization DPn ([Chains]¼ x[monomer]0/DPn).[16] The DPn

values were approximated from GPC analysis for each

polymer–solvent system. Thus, the slopes from the plots of

ln(1�[Chains]/[Solvent]0)versus ln(1(�x) give theCtr values

forTBAat115 8Cinp-xylene, anisoleandDMF(Figure3).The

largest Ctr reported in this study corresponded to poly-

merizations in DMF while the smallest Ctr value corre-

sponded to polymerizations in anisole (Table 5). Generally,

the chain transfer constant is directly proportional to the

rization at 115 8C.

[Solvent]0 Ctrb) dc)

mol � L�1 MPa1/2

4.32 4.7� 10�4 20.1

6.24 8.6� 10�4 24.7

4.12 6.3� 10�4 18.2

nt T¼ tert-butyl acrylate and the second letter refers to the solvent

amide (DMF) and A¼ anisole) while the last four letter refers to)Chain transfer to solvent constants (Ctr) values were obtained by

n in Figure 3. c)Solubility parameters d taken from the Polymer

ds.).


M


Figure 3. Plots of � ln 1 � ½polymer chains�½solvent�

� �versus �ln(1� x)

(x¼ conversion) for homopolymerizations of TBA in anisole, p-xylene and dimethylformamide (DMF) at 115 8C where the slopesgives Ctr, the chain transfer constant to solvent. The varioussolution polymerizations are represented symbolically by theexperiments: TAChTS (&), TDChTS (�), and TXChTS (~) andcorrespond to the experiment identifications listed in Table 5.

252

stability of the solvent radical formed after transfer. If the

solvent radical is unstable, the effect of chain transfer to

solvent will be negligible. By examining the solvent

molecular structure, it suggests that DMF produces the

most stable radical trap due to the number of possible

resonance forms. Anisole’s alkoxy group which resides on

the benzene ring has a strong electron-donating resonance

Figure 4. Number average molecular weight Mn versus conversion (x) for varioushomopolymerizations of TBA done at 115 8C: a) in 50 wt.-% anisole with a targetMn ¼ 25 kg �mol�1 (experiment TA25 (&)) and 65 kg �mol�1 (experiment TA65 (&));b) in 50 wt.-% DMF with a target Mn ¼ 25 kg �mol�1 (experiment TD25 (�)), and65 kg �mol�1 (experiment TD65 (*)); c) in 50 wt.-% p-xylene with a targetMn ¼ 25 kg �mol�1 (experiment TX25 (~)), and 65 kg �mol�1 (experiment TX65 (D)).The solid lines indicate the theoretical Mn versus x that includes solvent transfer effectsaccording to Equation 4 and using the chain transfer constants determined fromFigure 3. The dashed and dotted lines indicate the theoretical Mn versus x withoutany chain transfer effects for target Mn ¼65 and 25 kg �mol�1, respectively. All charac-terization of experiments is listed in Table 3 and 5.

effect that outweighs its weaker elec-

tron-withdrawing inductive effect, mak-

ing it much more reactive and less

favorable for chain transfer to solvent.

The effect of Ctr on the molecular weight

distribution can be observed by the

severe plateau of Mn as x increased for

polymerizationsdone inDMF (Figure4b).

A similar trend for othermonomers such

as styrene was observed with Ctr values

for free radical homopolymerization

at 60 8C. The Ctr values for styrene

were much higher when polymerized

in DMF (Ctr, p-xylene¼ 0.8� 10�4 and

Ctr DMF¼ 4.0� 10�4).[31]

We tried to incorporate the effect of

chain transfer to solvent on Mn for

controlled radical polymerization from

a relationship originally derived for

RAFT polymerization by Loiseau

et al.[32] and then adapted for NMP



(Equation 4)

n¼x TBA½ �0

BlocBuilder½ �0þ Solvent½ �0 1� 1� xð ÞCtr� �MWTBA

þMWBlocBuilder

(4)

In Equation (4), [BlocBuilder]0, [Solvent]0, [TBA]0 are the

initial concentration of initiator, solvent, and monomer,

respectively, used in the polymerization, x is the

conversion and Ctr is the chain transfer to solvent

constant. The theoretical Mn values were plotted against

x using the experimentally determined Ctr’s for each

solvent (Figure 4) and the levelling and even decrease in

Mn with increase in the conversion was reasonably

predicted (Table 5). Note that the final polymers could

contain branches, due to chain transfer reactions, which

could potentially give deviations in the reported Mn

values. Branching was present in the final polymer

samples (�4% long chain branching was the highest level

as measured by 13C NMR for TBA polymerizations in

DMF) so the Mn’s reported by GPC relative to linear PS

standards may not provide absolute molecular weights

but may also not be excessively erroneous.

Equation (4) was based on the assumption that chain

transfer to solvent was the dominating irreversible

terminating step which proved to be the case when using

relatively low target Mn (�10 kg �mol�1) for acrylic acid

polymerizations in 1,4 dioxane.[16] In this work however,

the target Mn’s were much higher and we can see that in

Figure 4 for the case where anisole (Figure 4a)) and p-

xylene (Figure 4c)) were used as solvents, the experimental

DOI: 10.1002/mren.200900014


points for the 65 kg �mol�1 target seem to tail off at lower

conversions than predicted by the equation where chain

transfer to solvent was only considered. Other chain

breaking effects in Figure 4a and c are obviously present at

higher target Mn’s besides chain transfer to solvent. For

example, acrylates are considerably prone to ‘‘backbiting’’

reactions, which irreversibly terminate the radicals and

lead to long chain branching.[33] Lovell and co-workers

showed that in concentrated solutions of n-butyl acrylate

(where initial monomer concentration [M]0> 10% (w/w)),

the mole percent of branches increased with conversion

due to the presence of intermolecular chain transfer to

polymer.[34] In addition to inter- and intra-molecular chain

transfer reactions, Hutchinson and co-workers showed

that at temperatures above 30 8C the effect of chain

transfer to monomer must also be considered as a

potential source for irreversible termination.[35] All of

these other factors contributed to the Mn being lower for

the TBA polymerization in DMF as the fraction of long

chain branching associated with chain transfer to polymer

was measurable, being approximately 4% as determined

by 13C NMR (13C NMR has previously been used to

quantitatively determine the fraction of long-chain

branching[28,36]). Only in the case of polymerizations done

in DMF is the fit to Equation (4) relatively good regardless

of the target Mn, thereby suggesting chain transfer to

solvent was a considerable source for the irreversible

termination reactions and may have masked the effect of

other chain transfer events which were also present

(Figure 4b).

Chain Extension of Poly(tert-butyl acrylate) (PTBA)Macroinitiators with Styrene: Effect of Solvent onKinetics and ‘‘Livingness’’

Poly(TBA) macroinitiators, synthesized previously using

SG1 free nitroxide and BlocBuilder initiator, were used to

polymerize a fresh batch of styrene at 115 8C. This

temperature was selected for the chain extension with

styrene to avoid significant thermal initiation which has

been reported to become evident at temperatures which

exceed 120 8C.[37–39] This chain extension would demon-

strate whether these macroinitiators were significantly

‘‘living’’ enough to polymerize a second batch of

monomer and how the nature of the solvent used to

make the macroinitiator affected the chain extension. All

of the molecular weight characterizations for the chain

extension experiments are summarized in Table 6.

In all cases, an increase inMn was observed, regardless of

whether the chain extension was done in bulk or in

solution. The increase in Mn for chain extensions done in

solution was not large. The Mw=Mn generally increased

after chain extension. The shift of the peaks as shown in the

GPC chromatograms to lower elution times signified that



the various macroinitiators possessed some ability to

reinitiate a fresh batch of monomer. However, when

examining the GPC traces for the chain extensions that

were performed in bulk, the difference in Mn between the

macroinitiator and the chain extended species was clearer

and the final GPC chromatograms clearly exhibited a more

bimodal nature. The bimodality is a result of the dead

chains present in the macroinitiators used and was

apparent from the significant low molecular weight tails

in the macroinitiator chromatograms. Thus, for chain

extensions done in bulk or in solution, virtually all of the

macroinitiators possessed a high fraction of dead chains.

These results show that when proper conditions are not

met, it is difficult obtainamacroinitiatorwithahighdegree

of livingness.

‘‘Semi-Batch’’ Like Tapered or Gradient Poly(tert-butyl acrylate)-tap-Poly(styrene) Block Copolymer(P(TBA)-tap-PS))

To avoid the viscosity issues present when polymerizing

TBA to high conversion in bulk and to avoid chain

transfer to solvent, a ‘‘semi-batch’’ approach was

attempted to produce tapered or gradient copolymers

such as those described by the Torkelson group.[40,41] Such

a gradient copolymer can reasonably approximate the

microstructure expected when the interface between the

two segments is not too diffuse. Reactivity ratios for TBA/

styrene random copolymerization previously determined

by NMP (rTBA¼ 0.09–0.12 and rS¼ 0.40–0.49)[18] suggest

that the resulting copolymer will have a transition zone

that is not overly diffuse.

The experiment was designed to initially mimic the

TBA homopolymerizations but at a certain point, an

injection of styrene monomer was added (here it was done

when the TBA conversion �0.4). The growth of the pure

PTBA chains followed a linear increase in Mn with

conversion as seen in Figure 5a. Note that the higher Mn

at low conversions is probably due to the use of gravimetric

analysis as the smaller chains can be slightly soluble in

the methanol and get discarded with the unreacted

monomer, thereby biasing the results in favor of higher

molecular weight chains at low conversion. Just before the

injection of styrene, PTBA had a Mw=Mn of 1.27 and a

Mn ¼ 12.7 kg �mol�1. After the injection of styrene just after

100minpolymerization, the chains continued togrowwith

a linear increase of Mn with conversion and followed the

second predicted Mn versus conversion plot shown in

Figure 5b. Immediately after the injection of styrene was

done (Figure 5c) theMw=Mn increased sharply, as a result of

the introduction of fresh monomer. Consequently, the

Mw=Mn was reduced as the chains grew. Thefinal sample at

polymerization time t¼ 140min after the injection point



Table 6. Molecular weight characterization of Poly(tert-butyl acrylate)/Poly(styrene) (P(TBA)-b-PS) copolymers synthesized at 115 8C invarious solvents.

Experiment IDa) Macroinitiator Copolymer

IDa)Mn

c) Mw=Mnc) Mn; target

b) Mnc) Mw=Mn

c) xstyrened)

kg �mol�1 kg �mol�1 kg �mol�1

TB25-PS-1 TB25 21.0 2.27 170.1 257.1 1.67 0.38

TB25-PS-2 TB25 21.0 2.27 239.1 191.8 2.42 0.28

TB25-PS-3 TB25 21.0 2.27 307.8 280.9 1.73 0.26

TB65-PS-1 TB65 23.9 1.51 212.1 155.4 1.39 0.34

TB65-PS-2 TB65 23.9 1.51 270.0 168.6 1.51 0.31

TB65-PS-3 TB65 23.9 1.51 346.8 194.7 1.68 0.29

TA25-PS-1 TA25 26.7 1.56 215.8 117.5 1.39 0.47

TA25-PS-2 TA25 26.7 1.56 301.7 96.9 1.62 0.38

TA65-PS-1 TA65 51.8 1.62 421.2 168.8 1.73 0.45

TA65-PS-2 TA65 51.8 1.62 587.9 101.6 1.65 0.19

TD25-PS-1 TD25 19.0 2.11 158.4 93.5 1.75 0.38

TD25-PS-2 TD25 19.0 2.11 216.3 85.4 1.80 0.36

TD65-PS-1 TD65 27.9 2.28 227.0 94.1 2.11 0.30

TD65-PS-2 TD65 27.9 2.28 315.6 119.0 2.03 0.28

TX25-PS-1 TX25 25.0 1.64 203.7 75.9 1.71 0.46

TX25-PS-2 TX25 25.0 1.64 287.0 115.1 1.71 0.43

TX65-PS-1 TX65 35.1 1.97 281.8 109.9 1.93 0.34

TX65-PS-2 TX65 35.1 1.97 399.4 102.9 2.15 0.29

a)Experimental identification is given as follows. The first letter refers to the first block (T¼ tert-butyl acrylate) and the second letter refers

to the solvent used in the polymerization (B¼bulk, X¼p-xylene, D¼dimethylformamide (DMF) and A¼ anisole). The number after the

first two letter refers to the target number average molecular weight of the first block [¼] kg �mol�1. b)Target number average molecular

weightMn; target of the second block (styrene). c)Number averagemolecular weight (Mn) and polydispersity index (Mw=Mn) determined by

GPC relative to linear poly(styrene) standards in tetrahydrofuran (THF) at 40 8C. The following Mark-Houwink parameters K and a were

used to obtain the molecular weights of the poly(tert-butyl acrylate) homopolymers: poly(TBA) (KP(TBA)¼ 3.33�10�5 dL � g�1,

aP(TBA)¼0.80)[24] and poly(styrene) (KPS¼ 11.4�10�5 dL � g�1, aPS¼ 0.716)[25] in THF. d)Conversion of the styrene added in the second

batch of monomer is given by xstyrene.Denotes bimodal (>5% dead chains) molecular-weight distributions.

254

(243min from the start of the polymerization) maintained

a narrow molecular weight distribution with Mw=Mn

of 1.30 and a relatively high molecular weight of

Mn ¼ 54.7 kg �mol�1. The increase in Mn with time can

be seen in Figure 5d by the GPC chromatograms of the

tapering experiment where the point c) is the

injection point (t¼ 0min from injection or t¼ 103min

from start) (points a), b), and c) represented in Figure 5

are the same samples in all plots shown in Figure 5a–d)).

The addition of styrene was deemed successful since the

final Mn was higher than the initial target Mn of the first

block of TBA (Mn;final ¼ 54.7 kg �mol�1, Mn;target; PTBA block ¼27.2 kg �mol�1). It should be noted that with rTBA< rS, the

resulting copolymers are likely to possess a relatively broad



transition zone between the TBA and styrene segments

where the second block never becomes pure styrene but is

very rich in styrene. The composition of the transition zone

will be nearly the azeotropic composition since the

monomer feed composition after styrene injection would

be fTBA� 0.15, which is nearly the copolymer composition

predicted from the literature reactivity ratios. These

potentially large transitional zones could still result in

gradient or tapered PTBA-b-P(TBA-ran-S)-b-PS block copo-

lymers with structures that may reasonably approach that

of PTBA-PS diblocks while using a relatively simple semi-

batch feeding technique. Future experiments will examine

how closely themorphologies of such gradient copolymers

will resemble the neat diblock copolymer.

DOI: 10.1002/mren.200900014


Figure 5. Tert-butyl acrylate/styrene (TBA/S) tapered copolymerization experiment doneat 115 8C in bulk with r¼ [SG1]0/[BlocBuilder]0¼0.1. In a), a feed of pure TBA waspolymerized and the number average molecular weight Mn versus conversion x isshown before injection of styrene in a) where the theoretical target Mn for TBA is shownas the solid line. In b), styrene has been injected at t¼ 103 min and Mnversus x of thestyrene/TBA mixture post-injection is shown where the solid line is the new target Mn

and the conversion x is defined as the conversion of the second monomer. In c), theMw=Mn of samples taken during various conversions of the TBA is shown. In d), theMw=Mn of samples taken during various conversions after the injection of styrene isshown. Figure 5e) shows the GPC chromatograms of samples taken at various timeslabeled a)-e) in Figure 5a) and b). The samples labeled a)–e) correspond to samples takenat polymerization times of a) 40 min, b) 60 min, c) 103 min (initial point of styreneinjection), d) 143 min, and e) 300 min.

Conclusion

In attempts to reachhighermolecularweights for poly(tert-

butyl acrylate) (PTBA) and ultimately poly(TBA)-b-poly

(styrene) copolymers using nitroxide-mediated polymer-

ization, itwas found that the polymerization of TBA in bulk

resulted in increasingly viscous solutions as conversion

increased, resulting in difficult removal of the product and

limitation of themolecularweight that could be practically

attained. At this limit, substantial termination was

observed from GPC experiments by the considerable

concentration of dead PTBAmacroinitiators resulting after

attempted re-initiation with styrene. Therefore, the use of

solvents was then employed but was not necessarily

beneficial. The effect of chain transfer to solvent was

particularly noticeable for the homopolymerization of TBA

inDMFsolutionas indicatedby theplateau in theMn versus

conversion plot at conversions greater than about 40%.

Other chain transfer reactions contributed to the levelling

of Mn such as long-chain branching as witnessed by the

fraction of branched chains measured with 13C NMR (�4%

for TBA in DMF). Subsequent reinitiation of the TBA

macroinitiators with styrene resulted in a significant

fraction of ‘‘dead’’ macroinitiators. Therefore, when done

in bulk or in solution, a similar plateau inMn was observed.

These results suggest that attempts to reach largeMn while

maintaining low Mw=Mn via NMP must be attained by

stopping the polymerization at low conversions �40%. An



alternative approach to obtain desirable

PTBA-PS diblocks could be approximated

by polymerization in a semi-batch mode

to produce tapered or gradient copoly-

mers which may resemble the morphol-

ogy expected of the neat diblock. The

results presented here show that a

relatively high Mn � 55 kg �mol�1 gradi-

ent or tapered copolymer with a low

Mw=Mn ¼ 1.30 could be attained. Such an

approachmight be particularly attractive

for scale-up as intermediate steps

required for crossing over to the second

blockwould be eliminated. Further inves-

tigations into the semi-batch operation

will be targeted to determine whether

the tapered products will have similar

microstructures and functionality to the

segmented block copolymers.

Acknowledgements: We thank the CanadianFoundation for Innovation (CFI) New Opportu-nities Fund and NSERC Discovery Grant forfinancial support.We also thank Scott Schmidtand Noah Macy of Arkema, Inc. for their aid inobtaining the BlocBuilder initiator and SG1

nitroxide and for useful discussions regarding acrylic acidpolymerization with nitroxides.

Received: February 27, 2009; Revised: May 8, 2009; Accepted: May11, 2009; DOI: 10.1002/mren.200900014

Keywords: block copolymers; chain transfer to solventcontrolled radical polymerization (CRP); nitroxide-mediatedpolymerization (NMP); tert-butyl acrylate (TBA)

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DOI: 10.1002/mren.200900014

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