a semi-batch process for nitroxide mediated radical polymerization

A Semi-Batch Process for Nitroxide Mediated

Radical Polymerization

Yanxiang Wang, Robin A. Hutchinson,* Michael F. Cunningham*

Department of Chemical Engineering, Queen’s University, Kingston, Ontario K7L 3N6, CanadaE-mail: [email protected]

Received: September 28, 2004; Revised: January 26, 2005; Accepted: January 28, 2005; DOI: 10.1002/mame.200400273

Keywords: butyl acrylate; coatings; living; nitroxide mediated polymerization; semi-batch; stable free radical polymerization;styrene

Introduction

In order to improve the performance of polymer materials

and their applications, synthesis of polymers with narrow

molecular weight distribution and controlled composition

has long been a goal of polymer researchers. As recent as a

decade ago, low polydispersity polymers and well-defined

copolymers could only be produced by ionic polymeriza-

tion (anionic and cationic). However, the discovery of living

(controlled) free radical (LRP) polymerization has changed

this situation. There are three important LRP types; stable

free radical polymerization (SFRP) or nitroxide mediated

radical polymerization,[1] atom transfer radical polymer-

ization (ATRP),[2] and reversible addition fragmentation

transfer process (RAFT).[3] Living radical polymerization

is based on an alternating activation/deactivation process of

Summary: A semi-batch process using nitroxide mediatedpolymerization, was explored for the design of low molecularweight solvent-borne coatings, typical of those used in theautomotive industry. While living radical polymerization(LRP) offers many advantages in the control of polymer chainmicrostructure that may confer important physical and chem-ical property benefits to coatings, adapting LRP to a semi-batch process poses significant challenges in the design andoperation of the process. Using styrene monomer, varioustwo-component initiating systems (free radical initiator,4-hydroxy-TEMPO) were studied to understand the effects ofdifferent initiators on the course of polymerization. In addi-tion, an alkoxyamine was synthesized and used as theinitiating source. The initiators Luperox 7M75 and Luperox

231 give higher polymerization rates and reasonable controlover polymerization, while benzoyl peroxide (BPO), Vazo67, and the alkoxyamine are less effective. The number ofpolymer chains in the final product is always less than thetheoretical value, reflecting poor initiation efficiency, prob-ably resulting from undesirable termination reactions thatbecome important due to the nature of the semi-batchprocess. Adding camphorsulfonic acid (CSA) or charginginitiator concurrently with monomer during semi-batch feed,can increase the polymerization rate while maintaining theliving character of the polymerization. The copolymerizationof styrene and butyl acrylate is also shown to exhibit livingcharacter.

Schematic representation of the exchange reaction to produce N-TEMPO cappedpolymer chains.

Macromol. Mater. Eng. 2005, 290, 230–241 DOI: 10.1002/mame.200400273 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

230 Full Paper

the propagating chains. Details of LRP kinetics and mech-

anisms can be found elsewhere.[4,5] Recent years have

witnessed numerous publications on the synthesis of low

polydispersity polymers and polymers with various struc-

tures (block, brush, star, etc.,). As this technique, by its

nature, is a radical process it has much stronger tolerance to

functional groups, water, and protic media than ionic living

polymerizations.

Due to the versatility and synthetic ease of living radical

polymerization, it has attracted enormous attention for its

potential applications in industry. In the past twenty years,

the paint and coating industry has faced regulatory pressure

to reduce the amount of volatile organic compound (VOC)

in the products, and even stricter regulations on VOC are

anticipated in the future.[6] This has acted as an impetus to

develop new environmentally compliant products such as

water-borne coatings and powder coatings, to resolve the

problems concerning VOC emissions. However, automo-

tive coatings, which are still primarily solvent-based mate-

rials produced by solution polymerization, cannot be easily

replaced by other techniques because of the advantageous

product properties of solvent-borne materials. Past appro-

aches to decrease the VOC content of automotive coatings

include, reducing the molecular weight of polymers (to

2 000 Da) and increasing the solid content from about

20 wt.-% in older formulations to 70 wt.-% in current

formulations.[7]

In this paper, the 4-hydroxy-2,2,6,6,-tetramethylpiper-

idinyloxy) (4-hydroxy-TEMPO) mediated SFRP of styrene

and styrene-co-butyl acrylate, was explored using a semi-

batch process. This approach is used industrially because it

permits easier control of temperature and superior control

of copolymer composition. In a batch conventional radical

copolymerization, if the consumption rates of two mono-

mers are considerably different, the copolymer composition

will change with reaction time. Using a semi-batch process,

copolymers with a more homogeneous copolymer compo-

sition distribution can be produced. In a living radical

polymerization, there is another potentially important ad-

vantage in chain microstructure that may be realized. LRP

allows variation of the copolymer composition within the

same chain, thereby allowing polymer chains with tapered

gradients in composition to be produced. This could be

advantageous in product design as it would allow desired

monomers (e.g., functional monomers used for subsequent

cross-linking) to be placed at specific locations along the

chain. This study is one of the first reports of semi-batch

living radical polymerization.

Experimental Part

Materials

The alkoxyamine PE-T [1-(2,2,6,6-tetramethylpiperidiny-loxy)-1-phenylethane] and the nitroxide N-TEMPO [4-(1-

naphthoyloxy)-2,2,6,6-tetramethylpiperidine-1-oxyl] weresynthesized in our laboratory according to literature proce-dures.[8] 4-Hydroxy-TEMPO (Aldrich Chemical), BPO (97%,Aldrich Chemical), Vazo 67 (2,20-azo(2-methylbutyronitrile)(DuPont), Luperox 231 [1,1-bis(tert-butylperoxy)-3,3,5-tri-methylcyclohexane 92%] (Lupersol), Luperox 7M75 (tert-butyl peroxyacetate 75 wt.-% solution in aliphatic hydro-carbons) (Lupersol), styrene (Aldrich Chemical), butylacrylate (Aldrich Chemical), and xylenes (Aldrich Chemical)were used without further purification.

Polymerization Procedure

The polymerizations were conducted in a 1 L automaticlaboratory reactor (METTLER TOLEDO LabMaxTM) con-trolled by CAMILE TG software (Camille Products). For theconventional free radical polymerization experiments, 210 gxylenes were first added to the reactor. After the temperaturestabilized at 138 8C, a mix of 496 and 13.8 g of styrene andLuperox 7M75, respectively was fed to the reactor in semi-batch mode at a constant rate over 6 h, and then the solution washeld at this temperature for another 30 min. The thermallyinitiated polymerizations used the same recipe except thatno initiator was added. Samples for analysis were collect-ed from the bottom valve of the reactor at specified times tobottles containing ca. 1–1.5% inhibitor (4-methoxyphenol)solution.

For semi-batch SFRPs using the alkoxyamine (unimer) PE-T as initiator, 27.8, 28, 179 g of PE-T, styrene, and xylenes,respectively were charged to the reactor and the temperaturewas increased to 138 8C. 397 g styrene was then fed in semi-batch mode over 6 h. The other semi-batch experiments usingvarious two-component initiating systems were based on thefollowing basic recipe, which is comprised of three stages(Figure 1). In Stage I, 210 g xylenes was charged into the reac-tor at room temperature and the temperature increased to138 8C. 21.6 g of 4-hydroxy-TEMPO, 15.8 g of Luperox 7M75,and 34 g of styrene (mole ratio of 4-hydroxy-TEMPO/Luperox7M75¼ 1.39) were charged in sequence and held for 1 h. InStage II, 467 g styrene was semi-batch fed over 6 h. In Stage III,the reaction mixture was held at this temperature to increase themonomer conversion. The same 4-hydroxy-TEMPO/initiatormolar ratio of 1.39 was consistently used with all initiators. Inthe solution batch polymerizations, 141 g of xylenes, 334 g ofstyrene, 14.4 g of 4-hydroxy-TEMPO and Luperox 7M75,were charged concurrently to the reactor and then the tem-perature was increased to 138 8C. In experiments varying theLuperox 7M75/4-hydroxy-TEMPO ratio, the amounts of Lupe-rox 7M75 were 10.6, 9.3, and 7.9 g, respectively, which gavethe corresponding mole ratios of nitroxide/initiator of 1.39,1.59, and 1.87. A summary of experiments is shown in Table 1.

Livingness Analysis

In the exchange reaction between alkoxyamine and N-TEMPO,0.11 g dry polymer, 0.66 g N-TEMPO, and 1.98 mL chloro-benzene were charged in a 5 mL flask. After the solution wasdegassed by three freeze-pump-thaw cycles, the flask wasimmersed in oil of about 123 8C. The exchange reaction

A Semi-Batch Process for Nitroxide Mediated Radical Polymerization 231

Macromol. Mater. Eng. 2005, 290, 230–241 www.mme-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

lasted 154 min, with stirring, under a nitrogen blanket. Theexchanged products were dried before further analysis.[9]

Characterization

The concentration of residual monomers was determined byusing a Varian CP-3800 gas chromatograph installed with aModel 8410 autosampler and a flame ionization detector set at250 8C. A 30M Chrompack Capillary Column (CP-Sil 8 CB)was used for the separation and the injector temperature washeld at 200 8C. The samples were diluted in acetone and exter-nal calibration was made before the measurements.

For gel permeation chromatography (GPC) measurements,the dry samples were dissolved in THF, which was also used aseluant at a flow rate of 1 mL �min�1. Before injection, thesolutions were filtered through a Chromspec syringe filter(25 mm nylon, 0.2 mm non-Sterile). The molecular weightmeasurements were performed using a Waters 2690 Separa-tions Module equipped with Waters Styragel HR columns(HR0.5, HR1, HR3, HR4, HR5) in THF at 35 8C and a Waters410 differential refractometer. The molecular weight valueswere reported relative to polystyrene standards using Millen-ium software. For the measurement of polymer livingness, theWaters 410 differential refractometer was operated in series

Figure 1. Schematic representation of the procedure of semi-batch 4-hydroxy-2,2,6,6,-tetramethylpiperidinyloxy (4-hydroxy-TEMPO) mediated radical polymerization usingbimolecular initiators.

Table 1. Summary of experiments. T¼ 138 8C for all runs. Complete details in the Experimental Part.

Experiment ID Monomer(s) Initiator OH-TEMPO:I(molar ratio)

Operating mode

1 (Conventional) Styrene Luperox 7M75 0 Semi-batch2 Styrene Luperox 231 1.39 Semi-batch3 Styrene Luperox 7M75 1.39 Semi-batch4 Styrene benzoyl peroxide (BPO) 1.39 Semi-batch5 Styrene Vazo 67 1.39 Semi-batch6 Styrene [1-(2,2,6,6-tetramethylpiperidinyloxy)-

1-phenylethane] (PE-T)– Semi-batch

7 (Thermal) Styrene None – Semi-batchModified 1a) Styrene Luperox 7M75 1.39 Semi-batchModified 2b) Styrene Luperox 7M75 1.39 Semi-batchModified 3 Styrene Luperox 7M75 1.39 Semi-batch8 Styrene Luperox 7M75 1.87 Batch9 Styrene Luperox 7M75 1.59 Batch10 Styrene Luperox 7M75 1.39 Batch11 Styrene/butyl acrylate

(50/50 mass ratio)Luperox 7M75 1.39 Semi-batch

a) 9.8 g camphorsulfonic acid (CSA) added in the beginning of Stage II.b) Additional 4.1 g Luperox 7M75 added concurrently with styrene during the semi-batch feed.

232 Y. Wang, R. A. Hutchinson, M. F. Cunningham


with a Waters 474 scanning fluorescence detector, which usedan excitation wavelength of 280 nm and an emission wave-length of 355 nm. The responses of RI and fluorescencedetectors are related to sample mass and number of alkoxy-amine chains, respectively, and a calibration was made beforethe calculation of livingness.[9]

Results and Discussion

Although alkoxyamine initiators offer greater control of the

number of chains in an LRP, we conducted most of our

experiments using a two-component initiating system, as

this is the approach most likely to be used by the industry. In

Stage I (Figure 1), the initiator thermally decomposes to

give primary radicals, which can undergo a variety of side

reactions in addition to the intended propagation reaction

with monomer.[10] These side reactions include transfer to

solvent and monomer by hydrogen abstraction, termination

with other radicals, and decomposition to smaller radicals

and fragments. It may also be possible that reactive initia-

tors such as peroxides will react directly with nitroxides.

While nitroxides readily react with carbon-centered radi-

cals, they do not easily react with oxygen-centered radicals

such as those derived from peroxides. Therefore, Stage I

may be a complex radical process and, depending on the

initiator used, various species can be formed. A noteworthy

and potentially problematic feature of Stage I is the

unusually high initiator concentrations required. In LRP,

all the chains are created at the outset of polymerization,

while in the analogous semi-batch FRP process, chains are

created continuously throughout the process. Furthermore,

the low desired molecular weight in the final product gives a

high number of chains. Consequently, the initial initiator

and nitroxide concentrations in semi-batch SFRP are an

order of magnitude higher than those in batch SFRP. The

implications of this issue will become apparent later in

Results and Discussion.

In Stage II, the propagation of the chains takes place

during the reversible activation/deactivation process as

shown in Figure 2. Successful control over the polymeriza-

tion depends largely on an appropriate equilibrium between

the activation and deactivation process. Even though K is

very small, both kact and kdeact should be large enough to

provide a reasonable polymerization rate while still

providing good control. Bimolecular termination between

propagating radicals also occurs, albeit at a much lower rate

than in conventional free radical polymerization. At the

beginning of the reaction, the nitroxide and propagating

radical concentrations increases due to the reversible homo-

lytic cleavage of the newly initiated chains. After a short

time (less than a few minutes), the system enters a quasi-

stationary state, where the nitroxide and propagating radical

concentrations change slowly with reaction time. The time

required to achieve this state depends on a number of

factors, including the initiation rate, temperature, and the

activation/deactivation equilibrium constant. Simulation

and experimental results have shown other reactions exist in

this system,[11–14] including styrene thermal initiation, and

disproportionation of dormant chains to yield hydroxyla-

mine and terminally unsaturated (dead) polymer. Two

routes are shown in Figure 2 for hydroxylamine formation,

but the bimolecular is likely to be the dominant route. The

propagating radical can also abstract hydrogen from hydro-

xylamine to regenerate nitroxide. These side reactions are

the main factors contributing to the loss of livingness

and the increase of polymer polydispersity at high

conversion.[11]

In this paper, we explored five different initiating systems

in semi-batch nitroxide mediated styrene polymerization.

The bimolecular initiators were 4-hydroxy-TEMPO com-

bined with peroxides (Luperox 7M75, Luperox 231, BPO),

an azonitrile (Vazo 67), and a unimolecular alkoxyamine

(PE-T). The alkoxyamine is the simplest system because

many of the complex reactions related to initiator decom-

position in Stage I are avoided.

Figure 3 shows experimental monomer concentration

profiles. For comparison, besides the nitroxide mediated

runs, three additional runs are shown. These are: (1) semi-

batch conventional free radical polymerization (no nitr-

oxide); (2) thermally initiated polymerization; and (3) the

monomer concentration profile without polymerization.

For the recipes using the initiators BPO and Vazo 67, or the

alkoxyamine PE-T, the polymerization rate was very slow

and no conversion was observed in the semi-batch feed

stage. For recipes using Luperox 7M75 and Luperox 231,

some conversion was observed during the semi-batch feed

stage, with higher conversion observed for the recipe using

Luperox 231. In all cases, the polymerization rate is lower

than that of conventional radical polymerization. The diffe-

rent polymerization rate is also reflected from the temper-

atures of the reaction mass and reactor jacket. In semi-batch

conventional radical polymerization, the temperature of the

reaction mass was maintained at 138 8C but the jacket

temperature was about 2 8C lower during the semi-batch

feed due to the exothermic nature of polymerization. This

temperature difference became gradually smaller in the

30 min holding time, because the polymerization was

close to completion and hence monomer concentrations

were low. In the nitroxide mediated polymerization using

Luperox 7M75 as initiator, after the addition of 4-hydroxy-

TEMPO, styrene and initiator at 138 8C, the color of the

reaction mass changed gradually from reddish to light

brown during the first hour (4-hydroxy-TEMPO gives a

reddish solution). During the addition of styrene in the

following semi-batch feed, the jacket temperature was

always slightly higher than the reactor temperature, which

indicated that the polymerization is slow, and less heat was

evolved.

Figure 4 shows the number average molecular weight

(Mn) and polydispersity profiles. (The runs using Vazo 67



and BPO are not shown as their rates were negligibly small.)

Compared to conventional free radical polymerization,

polydispersities were significantly lower, and molecular

weight increased approximately linearly with conversion.

These two features indicate the polymerization proceeded

in a living manner under reasonably good control. In an

ideal LRP, Mn is equal to the ratio of consumed monomer

divided by the number of chains, and the molecular weight

should increase linearly with conversion. The deviation

from linearity, especially at high conversion, indicates that

the polymerization was affected by side reactions. The

nature of the curvature, which is concave down, shows that

the number of chains increases as the polymerization pro-

gresses, while in the ideal case it should remain constant.

This is attributable to the presence of thermal initiation.

Another important consideration is how the experimentally

observed number of chains compares to the theoretically

expected number. Table 2 shows the number of polymer

chains in the final products, calculated using experimental

conversion and molecular weight. Theoretical values were

calculated, based on the moles of nitroxide added. These

values were generally low, typically in the range of 0.3–0.5.

The characteristics of semi-batch nitroxide mediated

polymerization using the five different initiators can be

interpreted as follows. First, the recipes using Luperox

7M75 and Luperox 231 as initiators give a reasonable

polymerization rate, while the number of polymer chains is

much lower than the number of nitroxide molecules added

in the start-up stage. The low number of polymer chains can

be caused by low initiator efficiency or the formation of

inactive alkoxyamine in the start-up stage. For initiator

Luperox 7M75, it is believed that decomposition occurs by

one-bond homolysis to an acyloxy and an alkoxy radical[15]

as shown in Figure 5. These radicals cannot be directly

capped (deactivated) by 4-hydroxy-TEMPO as they are

oxygen-centered. Possible reactions for these two radical

Figure 2. Reaction mechanisms for semi-batch 4-hydroxy-TEMPO mediated radicalpolymerization.



species are: (1) propagation with styrene (desired); (2)

hydrogen abstraction from xylene (transfer to solvent); and

(3) decomposition to methyl radicals. Luperox 231 decom-

poses, to give two tert-butoxy radicals, which follow the

processes described for Luperox 7M75. If a methyl radical

is capped by 4-hydroxy-TEMPO, it will probably be too

stable to contribute to the polymerization since the equili-

brium constant K is too small.[16]

The recipes using the initiators Vazo 67, or BPO, or the

alkoxyamine PE-T give little conversion in the semi-batch

feed stage and during the holding period in comparison to

the other runs, suggesting high residual nitroxide levels.

The initiator efficiencies of these runs (Table 2) are compa-

rable to the Luperox 231 and Luperox 7M75 runs. Similar

number of chains are thus formed for all initiators, yet with

Vazo 67, BPO, and PE-T the rates remain distinctly lower.

Therefore, the lower rates with these three initiators com-

pared to the Luperox 231 and Luperox 7M75 cannot be

explained solely by lower initiator efficiencies. It is true that

within the series of runs using Vazo 67, BPO, and PE-T,

higher initiator efficiencies (PE-T>Vazo 67>BPO) cor-

respond to lower rates, which is consistent with higher

initial nitroxide levels. The high nitroxide concentrations in

these recipes could be a result of bimolecular termination,

because of the very high initial concentration of initiator or

alkoxyamine (10� higher than is commonly used in batch

polymerizations) and low concentration of monomer. Ter-

mination leads to high amount of nitroxide being built-up in

the reactor at the early stage of polymerization, with the

consequent suppression of the polymerization rate [Equa-

tion (1) and (2)].

K ¼ kact

kdact

¼ ½P�n�½T��

½Pn-T�ð1Þ

Rp ¼ kp½P��½M� ¼ Kkp½M�½Pn-T�½T�� ð2Þ

In the recipes with Vazo 67, BPO, and PE-T, it is probable

that the high concentration of nitroxide built-up in the early

stages as a result of bimolecular termination, causes the rate

to be low for the duration of the polymerization. We ob-

served that during these runs, the solution was initially

colorless and a light red color appeared shortly after the

temperature was increased to 138 8C, which indicates the

accumulation of free nitroxide. Only limited polymeriza-

tion occurred after the semi-batch feed stage. Strong

evidence for the role of termination, early in the polymeri-

zation, is the low number of chains observed when the

alkoxyamine PE-T was used. In this case, we are beginning

with a known number of chains, and previous experience

with alkoxyamines in batch systems suggests that there

Figure 3. Monomer concentration profiles for semi-batch nitr-oxide mediated styrene polymerization using different initiationsystems. Conventional radical polymerization (*), Luperox 231/4-hydroxy-TEMPO (~), Luperox 7M75/4-hydroxy-TEMPO(^), benzoyl peroxide/4-hydroxy-TEMPO ( ), Vazo 67/4-hydroxy-TEMPO ( ), unimer PE-T (&), thermal initiation( ), without polymerization (�).

Figure 4. Number average molecular weight (solid symbols)and polydispersity index (open symbols) profiles for semi-batchnitroxide mediated styrene polymerization using different initia-tion systems. Conventional radical polymerization (*),Luperox1 231/4-hydroxy-TEMPO (~), Luperox1 7M75/4-hydroxy-TEMPO (^), unimer PE-T (&).

Table 2. Experimental polymer chain number in the final product and theoretical value for the recipes using five different initiationsystems.

Formula Luperox 7M75/4-hydroxy-2,2,6,6,-tetramethylpiperidi-

nyloxy) (4-hydroxy-TEMPO)

Luperox 231/4-hydroxy-TEMPO

Vazo 67/4-hydroxy-TEMPO

BPO/4-hydroxy-TEMPO

PE-T

Experimental/theoretical(mol/mol)

0.04/0.125 0.04/0.107 0.05/0.107 0.03/0.125 0.06/0.106



should be no more than 10% deviation from the expected

chain number. That so many chains could be ‘‘lost’’ with

PE-T, can only be reasonably explained by high initial

termination rates, caused by the high PE-T concentration. A

recent report by Georges et al.[17] offers an explanation for

the BPO results. At low temperatures, promoted initiator

dissociation of the BPO, causes the loss of a significant

portion fraction of the nitroxide. At the unusually high

initiator and nitroxide concentrations used in this study (a

necessary condition of operating in semi-batch mode), this

problem is even more severe. For the other initiator systems,

we believe that high rates of primary radical termination

lead to an excess of free nitroxide in the system, and

consequently a prolonged induction period.

In nitroxide mediated styrene polymerizations, thermal

initiation plays an important role. The polymerization rate

is often seen to be independent on the concentration of

alkoxyamine and instead depends on the thermal initiation

rate.[11,12] In Figure 3, it can be seen that the polymerization

rate of the run recipe using Luperox 7M75/4-hydroxy-

TEMPO is comparable to the corresponding thermally

initiated run, consistent with literature findings for batch

systems.[11,12] It is also common for nitroxide mediated

polymerizations to be slower than conventional thermal

polymerization, if there is excess nitroxide present which

acts to suppress the active radical concentration (and

therefore the rate) by shifting the equilibrium toward the

dormant state. This can happen with two-component initi-

ating systems if initiation efficiency is low, resulting in an

initial excess of nitroxide. However, the recipe using Lupe-

rox 231/4-hydroxy-TEMPO gives a much higher polymer-

ization rate. That the rate is higher for the run using Luperox

231 indicates the active radical concentration, which is

higher for this run than even for the thermally initiated run

(without nitroxide). This observation implies that there is a

deficiency of nitroxide after the initiation stage. It is un-

clear, why there should be such a deficiency, given that the

nitroxide/initiator ratio used (1.39), which is typical to give

optimal results. For example, if the initiator has an effi-

ciency of �0.7, we have added just enough nitroxide to cap

(deactivate) the newly formed chains. One possible expla-

nation is that the efficiency for the Luperox 231 is much

greater that �0.7, thereby leading to a nitroxide deficiency.

However, examination of the number of chains (Table 2)

indicates that in fact the efficiency is lower with the Luperox

231 compared to Luperox 7M75, disqualifying that expla-

nation. Therefore an unexplained question is why the

Luperox 231 and Luperox 7M75 runs do progress reason-

ably well. Their initiation efficiencies are within the range

seen for the Vazo 67, BPO, and PE-T runs, and there-

fore they should have comparable residual nitroxide levels.

The higher rates observed with both Luperox initiators may

be caused by consumption of nitroxide by side reactions

involving the initiators. The chemistry of peroxide initiators

is complex, and they are known to be very reactive. Direct

reaction between the Luperox initiators and the hydroxyl

group of 4-hydroxy-TEMPO, would explain the faster than

expected rates with these two initiators. Despite the loss of

nitroxide, however, the polydispersity remains low (com-

parable to Luperox 7M75), and a well-controlled poly-

merization is maintained.

Based on the above results, we used Luperox 7M75 as the

initiator of choice for further experimental work, as it most

closely followed the expected rate (equal to the thermal

Figure 5. Reaction mechanisms in the start-up stage with Luperox 7M75/4-hydroxy-TEMPO initiation.



initiation rate). Luperox 231 would have also been a suit-

able choice, although the unexplained high initial rate and

the lower initiation efficiency led us to choose the Luperox

7M75. As previously discussed (Figure 4), a characteristic

of these semi-batch living radical polymerizations is that

the polydispersity is low at the beginning of reaction and

increases with conversion. This is different from the batch

LRP experiments where the polydispersity is higher at the

beginning of polymerization and decreases with conver-

sion. This can be interpreted by the polymerization degree

(run length) obtained by a propagating radical each time it is

activated as shown in Equation (3),

Run length per activation cycle ¼ Rp

Rdeact

¼ kp½M�½P��kdeact½T��½P��

¼ kp½M�kdeact½T�� ð3Þ

which is the ratio of its propagation and deactivation rate.

This equation represents the mean polymerization degree

increase of a chain for each activation/deactivation cycle,

and is related to the monomer and nitroxide concentrations.

In a semi-batch solution polymerization, the monomer

concentration is much lower than that in a batch bulk poly-

merization, and therefore the increase in polymerization

degree, for each activation cycle is smaller than that for the

batch case. Consequently, the polydispersity in semi-batch

solution polymerization is low at the beginning. The

observed increase in polydispersity at higher conversions

is caused by the increasing number of chains due to thermal

initiation and the decrease in nitroxide concentration due to

dilution.

Effect of Using Modified Recipes

A high polymerization rate is mandatory for the application

of LRP in industrial applications. In order to overcome the

low polymerization rate of the basic recipe using 4-

hydroxy-TEMPO/Luperox 7M75, process modifications

were made. In ‘‘modified recipe I,’’ about 9.8 g camphor-

sulfonic acid (CSA),[18] was added in the beginning of Stage

II. CSA is known to enhance the rate of TEMPO mediated

polymerizations, primarily by reducing the free nitroxide

concentration. The color of the reaction mass changed

immediately from light brown to deep black after the addi-

tion of CSA. In ‘‘modified recipe II,’’ an additional 4.1 g

Luperox 7M75 was added concurrently with styrene during

the semi-batch feed. The additional initiator feed was

intended to reduce the nitroxide concentration by generat-

ing additional radicals at a controlled rate. Because the

initiator half-life is short at the reaction temperature, the

Luperox 7M75 decomposition rate is approximately equal

to its addition rate. Finally, in order to improve the effi-

ciency of 4-hydroxy-TEMPO (or increase the number of

polymer chains in the final product), in ‘‘modified recipe

III,’’ we designed a new charging procedure for the initia-

tors in Stage I designed to maintain a low 4-hydroxy-

TEMPO concentration. In this recipe, during Stage I, all

xylenes were first charged to the 1 L reactor and, after

increasing the temperature to 138 8C, a mixture of 4-

hydroxy-TEMPO, Luperox 7M75, and 107 g styrene was

fed by semi-batch over 80 min and then the solution was

held for another 10 min. 397 g styrene was charged by semi-

batch feed over 5 h in Stage II. The differences with the

original procedure are; (1) the 4-hydroxy-TEMPO,

Luperox 7M75, and styrene mixture was charged slowly

(over 80 min) in the new process, and quickly in the original

process; and (2) 107 g styrene was used in the new process

versus 34 g in the original process. These modifications are

intended to promote the formation of TEMPO-capped

styrene oligomers (increased styrene charge) during Stage I

while reducing the occurrence of side reactions that

consume initiator (slower initiator addition rate).

Figure 6 shows the free monomer concentration profiles

of the basic and modified recipes. For comparison, the

conventional free radical polymerization (bottom) and the

semi-batch process without polymerization (top) are also

shown. Addition of CSA or feeding Luperox 7M75 concur-

rently with monomer improves the polymerization rate

considerably, with the initiator addition having a more

pronounced effect. The unreacted monomer concentration

profile of modified recipe II (Luperox 7M75 addition)

appears similar to the conventional free radical polymer-

ization but with a higher free monomer concentration. In the

run using modified recipe I (CSA addition), the free

monomer concentration curve displays a differently shaped

profile, rising to 300 mg � g�1 and then staying at that level

for approximately 300 min before beginning to decline.

These differences between adding CSA and Luperox 7M75

reflect their differing roles. Luperox 7M75 initiates new

Figure 6. Monomer concentration profiles for the modifiedrecipes. Conventional radical polymerization (*), basic recipe( ),modified I (^), modified II (~), modified III (&), withoutpolymerization (�).



chains, and will therefore cause a direct and immediate

decline in the nitroxide concentration. CSA is known to

have only a pronounced effect when there is considerable

excess of nitroxide, and to have a much smaller effect when

free nitroxide levels are low.[19] In these runs, the free

nitroxide concentrations are likely, not initially high enough

for the CSA addition to exhibit a significant effect. Modified

recipe III slightly increases the rate, however recall its pri-

mary purpose that was to enhance the initiation efficiency

rather than increase rate.

Figure 7 shows the molecular weight and polydispersity

profiles for the basic and modified profiles. It can be seen

that for all the LRP runs the molecular weight still increases

with conversion and the polydispersity is also lower than

that of conventional radical polymerization, which indica-

tes that the living characteristic of the reaction is maintained

in the modified recipes. It is interesting to observe that the

basic recipe and all modified recipes have similar poly-

dispersity at the same conversion. However, there are major

differences in the Mn profiles between the various recipes,

affecting both the linearity of the curves and the slope.

Modified recipe I (CSA addition) shows the steepest slope

with some downward curvature at higher conversions.

Modified recipe II (Luperox 7M75) has a slope slightly

steeper than the basic recipe, also with some downward

curvature at higher conversions. At low conversions

however, Mn is noticeably higher than the basic recipe,

which corresponds to fewer chains. Modified recipe III,

which was intended to increase the initiation efficiency,

displayed an Mn profile very similar to the basic recipe.

Recipe III was thus not successful in improving efficiency;

if it had been the observedMn values would have been lower

than the basic recipe at a given conversion. The downward

curvature seen at higher conversions in modified recipes I

and II, is a result of thermally generated chains and

additional chains arising from added initiator. The steeper

slopes, which signify fewer chains present in the mixture,

have a different underlying cause. Bimolecular termination

reduces the number of chains if termination is predomi-

nantly by combination, as it is with styrene. With modified

recipe I, the rapid increase in Mn at low conversions

followed by a linear growth suggests a burst of termination

occurs when the CSA is added, and after that the number of

chains remains approximately constant. With modified

recipe II, the slow Luperox 7M75 addition results in a more

gradual loss of chains by termination.

Effect of Recipe Modifications on Polymer Livingness

In living radical polymerization, the livingness of polymer

chains (fraction of polymer chains that are capped by nitro-

xide) is an important issue. Recently, our group developed

an approach to study the livingness of polymer chains.[9] In

this method, 4-hydroxy-TEMPO capping the polymer

chains end is replaced by the fluorescent N-TEMPO

through an exchange reaction, as shown in Figure 8. The

percentage of livingness can then be obtained by using GPC

coupled with calibrated RI and fluorescence detectors.

Figure 9 shows the mole percent livingness for recipe modi-

fication II (basic recipe using Luperox 7M75 as initiator).

Both conditions show that, as expected, the livingness

decreases as the conversion increases. Interestingly, the

slopes of the two curves are very different, with modified

recipe II declining more slowly. However the livingness of

the basic recipe is higher at low conversions. Extrapolating

to high conversions, the livingness is expected to be com-

parable for the two recipes. It is surprising that at the higher

reaction rate (modified recipe 2), the slope of livingness

versus conversion is so shallow. This finding may at first

consideration appear anomalous; higher rates imply higher

active radical concentrations and therefore higher termina-

tion rates. However, at these temperatures, disproportiona-

tion of the growing chains to yield a dead unsaturated chain

and a hydroxylamine is an important reaction at long

reaction times. We have previously shown that the majority

of dead chains are formed by disproportionation and not

termination.[20] Because disproportionation is a first order

reaction, longer reaction times (i.e., lower polymerization

rates) promote its effect. Therefore, the disadvantage of

increasing the rate (greater termination rates) is partially or

wholly offset by reduced disproportionation rates. We have

not optimized the initiator addition process; it may in fact be

possible to achieve higher livingness with the modified

recipe, as has been found in mini-emulsion SFRP.[21]

Batch Solution Polymerization With Varying4-Hydroxy-2,2,6,6,-tetramethylpiperidinyloxyLuperox 7M75 Ratio

One of the issues noted in previously described experiments

is the low initiation efficiency. A possible solution is to

Figure 7. Number average molecular weight (solid symbols)and polydispersity index (open symbols) for modified recipes.Symbols as in Figure 6.



increase the amount of nitroxide present. Batch solution

polymerizations were run using three different 4-hydroxy-

TEMPO/Luperox 7M75 ratios to better understand the

effect of this variable on the course of polymerization. We

elected to use batch, and not semi-batch, conditions because

it simplifies interpretation of the results. The monomer

concentration profiles, molecular weight and its polydis-

persity are shown in Figure 10 and 11, respectively. In the

recipe using a nitroxide/initiator ratio of 1.39, the poly-

merization proceeded vigorously. The rate is high at the

beginning and slows down gradually. Mn increases rapidly

with conversion initially, and then subsequently displays

linear growth. This behavior is indicative of poor control at

an early stage, followed by a well-controlled polymeriza-

tion. The polydispersity remains constant at �1.4. At a

nitroxide/initiator ratio of 1.59, the rate is somewhat slower,

and the final conversion lower than at the 1.39 ratio. How-

ever, Mn is again seen to increase rapidly with conversion

initially, and then exhibit linear growth. Polydispersity is

much improved at 1.2. In the recipe using a nitroxide/

initiator ratio of 1.87, a long induction period was observed,

and the final conversion lower than in runs with a lower

nitroxide/initiator ratio. TheMn profile more closely resem-

bled the ideal curve, and polydispersity was 1.1. Along with

differences in the kinetics and polydispersities, the number

of chains initiated at the three nitroxide/initiators also

differed, with more chains being initiated at higher nitro-

xide/initiator ratios. Initiation efficiencies are 65, 55, and

46% for nitroxide/initiator ratios of 1.87, 1.59, and 1.39,

respectively, based on 4-hydroxy-TEMPO. Better control

of the polymerization was also achieved at higher nitroxide/

initiator ratios, as reflected by lower polydispersities. These

runs illustrate that better control of the molecular weight

distribution can be achieved by increasing the amount of

nitroxide used. This benefit unfortunately comes at the

expense of a significantly reduced rate. The relative impor-

tance of each of these attributes would have to be assessed

prior to deciding on an appropriate nitroxide/initiator ratio.

Copolymerization of Styrene and Butyl Acrylate

Copolymerization experiments were also performed to

investigate system behavior in the presence of acrylates, a

common monomer in coatings formulations. Acrylates do

Figure 8. Schematic representation of the exchange reaction to produceN-TEMPO cappedpolymer chains.

Figure 9. Comparison of livingness (mol-%) for the basic recipe(^), and modified recipe II (&).

Figure 10. Monomer concentration profiles for batch polymer-izations. 4-Hydroxy-TEMPO/TBPA¼ 1.87 (*), 4-hydroxy-TEMPO/TBPA¼ 1.59 (&), 4-hydroxy-TEMPO/TBPA¼1.39 (^).



not polymerize easily with TEMPO-derived nitroxides,

primarily because the near absence of thermal initiation in

these system results in the gradual accumulation of nitro-

xide and subsequent rate suppression. Copolymerizations

with styrene are possible since the styrene still contributes

to thermally generated radicals but the rates tend to be low.

Using a recipe similar to the basic recipe for styrene

homopolymerization, a mixture of 50/50 (mass basis)

styrene/butyl acrylate was used in the copolymerization

experiments.

The monomer concentration profiles in Figure 12, reveal

the overall concentration of BA and ST in copolymerization

is similar to the monomer concentration profile in styrene

homopolymerization. During copolymerization, styrene is

consumed slightly faster than butyl acrylate because of their

different copolymerization reactivity ratios. Figure 13

shows that the mass of styrene in the copolymer is about

1.2 times that of butyl acrylate. Figure 14 shows that Mn

increases approximately linearly with conversion to about

40% conversion, after which downward curvature in the

plot is seen. This indicates an increasing number of chains

caused by thermally generated radicals, similar to what was

observed in styrene homopolymerization. The molecular

weight distribution is slightly broader than that of the

corresponding styrene homopolymerization, and increases

gradually during polymerization to a final value of 1.5.

Although no attempt was made to optimize this formula-

tion, these results demonstrate the feasibility of conducting

nitroxide mediated styrene/acrylate copolymerizations in a

semi-batch process.

Figure 11. Number average molecular weight (solid symbols)and polydispersity index (open symbols) for batch polymeriza-tions. Symbols as in Figure 10.

Figure 12. Monomer concentration profiles for homopolymer-ization of styrene (*) and styrene/butyl acrylate copolymeriza-tion: overall monomer (~), styrene (&) and butyl acrylate (^).

Figure 13. Weight ratio of styrene and butyl acrylate in solution(^) and in polymer (&).

Figure 14. Number average molecular weight (solid symbols)and polydispersity index (open symbols) for homopolymerization(^) and copolymerization (&).



Conclusion

It has been demonstrated that semi-batch nitroxide medi-

ated polymerization is feasible. The unique operating

conditions in semi-batch, present challenges that do not

exist with batch processes, including high initial initiator/

alkoxyamine concentrations (�10� greater than batch

processes) and low monomer concentrations throughout the

process. The high initiator/alkoxyamine concentrations,

result in increased rates of bimolecular termination in the

very early stages of the polymerization, and are reflected in

low initiation efficiencies. Low monomer concentrations

lead to reduced reaction rates. The five different initiating

systems (four two-component systems and one alkoxya-

mine) displayed distinctly different kinetic behavior.

Among the five recipes using different initiation appro-

aches, Luperox 7M75/4-hydroxy-TEMPO and Luperox

231/4-hydroxy-TEMPO give the highest rates with reason-

able control over the polymerization, although they are

slower than conventional free radical polymerization. The

slow polymerization rate can be partially overcome by

adding CSA or charging initiator concurrently with mono-

mer during the semi-batch feed stage, with initiator addition

being the preferred route. Using a higher ratio of nitroxide/

initiator can increase the initiating efficiency but at the

expense of a lower rate. The copolymerization of styrene

and butyl acrylate (50/50) was also demonstrated, and was

shown to yield a well-controlled polymerization with

the molecular weight increasing linearly with conversion.

The type of process reported in this work is suitable for the

preparation of low molecular weight coatings such as those

used in the automotive industry.

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