Controlled Radical Polymerization - American Chemical Society

Chapter 15

Downloaded by NANYANG TECHNOLOGICAL UNIV on October 18, 2015 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch015

Nitroxide-Mediated Controlled Radical Polymerization: Toward Control of Molar Mass Stefan A . F. Bon, Frank A . C . Bergman, J. J. G . Steven van Es, Bert Klumperman, and Anton L. German Eindhoven Polymer Laboratories, Eindhoven University of Technology, Laboratory of Polymer Chemistry, P.O. Box 513, 5600 M B Eindhoven, Netherlands

The mechanism of the TEMPO-mediated controlled radical polymerization o f styrene in bulk is discussed. It is shown that the isotropic correlation time (τ ) o f a nitroxide can be used as a measure o f the diffusive rate coefficient o f trapping (ket ). A general empirical relationship for the density o f polystyrene as a function of molar mass and temperature is established to correct concentration data obtained from C R P experiments for volume contraction. It is demonstrated that the overall rates o f polymerization of styrene in bulk do not show a dependence upon alkoxyamine concentration. Broadening o f the molar mass distribution in a C R P experiment is ascribed to a low rate o f alkoxyamine C-O bond homolysis and permanent chain-stopping reactions, e.g. bimolecular termination. c

D

The quest for a polymerization technique that can be applied to a wide variety o f vinyl monomers and affords the possibility to design and synthesize a polymer material with control o f the molar mass distribution as well as of the configurational and compositional monomer sequences may be a mirage. Fact is that numerous efforts are being undertaken to develop novel polymerization systems, for which one or preferably more o f the above criteria are met. One o f the systems that presumably has the ability to fulfill all these expectations and has therefore received great attention lately, is the nitroxidemediated controlled radical polymerization (CRP)(7). The concept of this novel technique is essentially an elementary living radical polymerization system, consisting of a constant number o f dormant species that can be reversibly activated, at a rate fast compared to the overall rate o f polymerization. In the activated form it undergoes

236

© 1998 American Chemical Society

In Controlled Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

237

propagation and as a result o f the reversibility o f the process this occurs via "stepwise" insertion o f monomer. Scheme 1 represents the reversible homolytic dissociation o f an alkoxyamine (i.e. 2-tert-butoxy-l-phenyl-l-(l-oxy-2,2,6,6tetramethylpiperidinyl)ethane; compound (1)) to yield the corresponding carboncentered radical and nitroxide (i.e. l-oxy-2,2,6,6-tetramethylpiperidine; T E M P O ) .


Scheme 1. Reversible Homolytic Dissociation of an Alkoxyamine

TEMPO

compound (1)

This fundamental step is implemented into the picture o f a conventional free-radical polymerization. Consequently, the activation o f the dormant species, i.e. the rate o f the C - 0 alkoxyamine bond homolysis, needs to be a fast process, preferably instantaneous on the time scale o f the overall rate o f polymerization. Moreover, the deactivation process, that is the trapping o f the polymeric carbon-centered radical by a nitroxide, is required to be competitive with the generally diffusion-controlled permanent bimolecular termination (2, 3). In this paper the impact on the kinetics o f the free-radical polymerization o f styrene in bulk due to the introduction o f two novel compounds, i.e. the alkoxyamineend-functionalized polymeric chain and the nitroxide, is thoroughly evaluated. The rotational correlation time ( T ) o f a nitroxide calculated from the E S R spectrum o f this compound is proposed as a measure for the diffusive rate coefficient o f the neardiffusion controlled process o f trapping ( k ) (2). A n Arrhenius expression for the rate coefficient o f the C - 0 bond homolysis (k d) o f compound (1) in toluene is given (4). The Arrhenius parameters were determined by so-called nitroxide exchange experiments, based upon the alkoxyamine half-life measurements as performed by Solomon et al.(5). Furthermore, an empirical equation to describe the density o f polystyrene as a function o f molar mass and temperature is postulated to correct concentration data obtained from controlled radical polymerization experiments for volume contraction. Finally, the mechanism o f the TEMPO-mediated controlled radical polymerization o f styrene in bulk is described, with respect to the overall rate of polymerization and to the control o f the molar mass distributions. c

D

et

e

Rate of Trapping It is generally accepted that the rate o f trapping o f a carbon-centered radical with a nitroxide is near-diffusion controlled (2). This implies that both the intrinsic chemical reaction as well as the preceding diffusion process o f the two reactants need to be taken into account.


238

The rate coefficient for trapping (ke ) therefore has to be described as: t

J_ K

_

_ L ι D et

_

ι

1 +

ι chem et

K

et

0)

K

D

Diffusion of Τ · and R: The diffusive rate coefficient for trapping ( k ) is a function of the diffusion coefficient o f the two species involved, i.e. R and Τ · . Since the nitroxide usually is a compound o f small size, its diffusion coefficient (D) can be described by the Stokes-Einstein equation, with ke/ J K " as the Boltzmann constant, Τ/ Κ as the absolute temperature, η / Pa-s as the micro viscosity, r/ m as the hydrodynamic radius(d, 7) and a constant β = 4 for stick boundary conditions or β = 6 for slip boundary conditions (8): et

e


1

D

=

2

- ^ L βπητ

( )

Data obtained in various solvents for the diffusion coefficient o f l-oxy-2,2,6,6tetramethylpiperidine ( T E M P O ) reveal that the activation energy for diffusion merely coincides with the activation energy for viscosity changes o f the solvent, indicating that no specific interactions between solvent and nitroxide could be observed (6). However, a complication arises in finding a correct value for the microscopic viscosity that is perceived by the nitroxide in a polymer solution. It is plausible that the viscosity o f a medium experienced by a diffusing molecule depends on its size and shape. Instead o f the macroscopic viscosity, a value for viscosity should be used that fits the geometric scale o f the solute. Rotational Correlation Time (x ). A s a measure o f the microscopic viscosity of the nitroxide during a controlled radical polymerization, the rotational correlation time (x ) can be used. Its value can be calculated from E S R spectra, e.g. the spectra obtained when monitoring the nitroxide concentration vs. time during a C R P experiment. During the first stages o f polymerization the rotational motion o f the nitroxide w i l l be rapid, which allows a simple analysis for x , with AH( =+i)/ G as the peak-to-peak distance o f the low-field absorption line, I( =+i)/ a.u. as the peak-to-peak height o f the low-field absorption line, I( =.i)/ a.u. as the peak-to-peak height o f the high-field absorption line (9): c

c

c

m

m

m

=

1

10

6.6xlO" ΔΗ,

The isotropic rotational diffusion can be described by the equation, which yields a value for the microscopic viscosity:

(3)

Stokes-Einstein-Debye

3

4πτ η

If the viscosity o f the system becomes high, the rotational motion o f the nitroxide is restricted. This induces anisotropy implying that severe deformations o f the E S R spectrum w i l l occur, and, therefore, a more complex analysis to describe the microscopic viscosity and, consequently, the diffusion coefficient, is required (10).


239

e

The other species involved i n the trapping reaction is the carbon-centered radical R . Due to the presence o f monomer, these radicals w i l l be able to propagate to yield species o f variable chain length. The chain length distribution o f these carboncentered radicals depends on the kinetic events that occur throughout the polymerization. Together with the solute specificity o f the microscopic viscosity, this results in an intricate task to describe the diffusive rate coefficient for trapping. A n approximation that can be made is to consider only a chain length dependence for the diffusive rate coefficient o f trapping ( k e ) o f oligomeric species (77). In addition it is assumed that the dimensions for the microscopic viscosity are identical for these small species (In general, however, the diffusion coefficient for R w i l l have a minor influence on the mutual diffusion coefficient, D = D R . + Dj.). Thus, it can be assumed that the diffusive rate coefficient o f trapping ( k e ) is solely determined by D T If k e is expressed as a Smoluchowski equation and the isotropic correlation time T is used as a measure o f the microviscosity, with σ / m as the non-reversible distance o f interaction, N A/ mol" as Avogadro's number, ρ as the spin factor, its combination with Equations 2 and 4 yields (with σ = r, β = 4 and ρ = A.)\ D


t

e

D

t

D

t

c

Γ

1

l

Γ

k°

=

4πσ N Γ

A

p(D

+D )

R

T

=

(5)

From this equation it can be observed that x can be used as a measure of the diffusive rate coefficient ( k e ) o f trapping. The monitoring o f the nitroxide concentration by quantitative E S R measurements during a C R P experiment could therefore be used to investigate a decrease in k e . c

D

t

D

t

Intrinsic Chemical Reaction between R* and Τ · . The overall rate o f trapping (ke ) also depends on the intrinsic chemical reaction between R* and Τ·(72). Therefore, to study the corresponding intrinsic chemical rate o f trapping ( k e ) both species, i.e. R and Τ · , have to be considered. One o f the relevant aspects that has to be taken into account is the two canonical forms o f the nitroxide bond (see Figure 1). Polar solvents w i l l stabilize the dipolar form, thereby increasing the spin density on nitrogen. This effect decreases the reactivity o f the nitroxide for the trapping reaction, as has been confirmed by Beckwith et al.(13) for the trapping o f benzyl radicals with T E M P O . t

c h e m

t

e

\

y _Q.N

\ • yj-OI

A

Β

Figure 1. Canonical structures o f a nitroxide. Rate of Alkoxyamine C - O Bond Homolysis The rate coefficient o f homolytic dissociation (ked) o f the alkoxyamine C - 0 bond is one o f the fundamental aspects that has to be considered i n a controlled radical polymerization (4). W e have developed a method to obtain data for ked, inspired by


240

the half-life measurements o f Rizzardo et al.(5). These so-called nitroxide-exchange experiments are based upon a system that obeys pseudo-first order kinetics. In a typical experiment an alkoxyamine ( L i ) is dissociated into radical species R and the corresponding nitroxide T i * . A n excess o f a different nitroxide ( Τ 2 · ) present in the reaction mixture assures that the only fate for Κ· is to be trapped with this nitroxide Τ 2 · to yield the alkoxyamine L 2 . In this study the dissociation o f compound (1) in toluene with 4-benzoyloxy-l-oxy-2,2,6,6-tetramethylpiperidine as Τ 2 · was investigated by following its consumption and the corresponding production of L by reversed phase H P L C analysis, leading to ke (4). The following Arrhenius equation for the rate coefficient o f homolytic dissociation ( k ) o f compound (1) in toluene was obtained (333 Κ < Τ < 373 Κ ) : 9


2

d

ed

k

e d

=

1 4

9.1 xl0 exp 2

'-138., χ 10

3 Λ

RT

(6)

Noteworthy is that a hydrogen atom abstraction by 4-benzoyloxy-l-oxy-2,2,6,6tetramethylpiperidine from toluene to yield a benzyl radical and the corresponding hydroxyl amine, has been observed as a non-intervening side-reaction. Relative Inertness of Nitroxides. It is known that nitroxides are able to abstract hydrogen atoms readily under photochemical conditions. The hydrogen atom is abstracted by the photoexcited nitroxyl group in the η ^ π * excited state (14). Thermal hydrogen atom abstraction, as has been observed, is possible at elevated temperatures for conventional nitroxides, such as T E M P O and derivatives thereof (75, 16). (Nitroxides with electron-withdrawing substituents are much more reactive towards hydrogen atom abstraction and addition reactions to vinyl groups, e.g. l-oxy-1,1bis(trifluoromethyl)amine (7 7)). This confirms that nitroxides are not completely inert and capable o f hydrogen atom abstraction. The hydrogen atom abstraction from the styrene Diels-Alder dimer, observed by M o a d et al. (18) for T E M P O and T M I O would influence the initiation process markedly. However, it is likely to assume that the hydrogen atom abstraction o f the Diels-Alder dimer is a side reaction o f minor importance in a C R P experiment, since the overall concentration o f Τ · is low in comparison with the systems for which the hydrogen atom abstraction was observed. However, the capability o f nitroxides to abstract hydrogen atoms may lead to trapping by disproportionation, that is hydrogen atom abstraction from R» to yield an unsaturated end-functionalized dead polymeric chain and the corresponding hydroxyl amine. This indeed is observed clearly for systems having an α-methyl group as substituent next to the alkoxyamine bond (5,19). Since absolute inertness o f the nitroxide species is ruled out, any possible addition o f a nitroxide to a monomer to yield a carbon centered radical species also has to be considered. Whether this occurs at all, and i f so as a simple addition or via a concerted mechanism induced by polarization o f the vinyl group by the nitroxide is still unsolved (16-18, 20).


241

Impact of Reversible Homolysis of Alkoxyamine C - O Bond on Fundamental Free-Radical Polymerization Kinetics The occurrence o f permanent chain stopping reactions, i.e. bimolecular termination and transfer, cannot be excluded in a free-radical polymerization. If no specific interaction between Τ · and R occurs, the alkoxyamine C - 0 bond homolysis and trapping can be incorporated as additional reactions in the events occurring during a free-radical polymerization. Imagine a system composed o f an alkoxyamine dissolved in an inert solvent. The three fundamental reactions that can occur in this system are the homolytic dissociation o f the alkoxyamine, trapping o f R* by Τ · and permanent bimolecular termination o f two R*. It is easy to understand that the occurrence o f permanent bimolecular termination leads to a continuous production o f Τ · , resulting in [Τ·] being considerably higher than [R«]. It can be inferred that, due to a continuous decrease in the probability o f bimolecular termination, a quasi-equilibrium w i l l be reached when time goes to infinity (4):


e

|LJ

k

t-**

q et

The continuous decrease in [R»] results in a decreasing overall rate o f polymerization, eventually leading to long reaction times and incomplete conversion. To suppress the production o f [ Τ · ] , as a direct result o f bimolecular termination, and to simultaneously increase the overall rate o f polymerization, an additional radical flux, i.e. the production o f radicals by a source other than the homolytic dissociation of the alkoxyamine, can be added. In Figure 2, [ Τ · ] and [ R ] vs. time are plotted for an imaginary system discussed above, having an additional radical flux o f σ = 1.0 χ 10" m o l - L ' V with k = 1.0 χ 10" s" , k = 5.0 χ 10 L - m o l ' V , k = 2.5 χ 10 L - m o l ' * " and [ L ] = 0.01 mol-L* , [ R « ] = [ Τ · ] = 0 mol-L" . e

1

8

5

1

1

8

e d

1

et

1

9

t

1

1

0

0

0

It can be observed from this picture that the current system reaches a steadystate after a certain period o f time with a value o f the overall radical concentration corresponding to: R

=

\2k

(8) t

The process eventually leading to steady-state conditions is also referred to as the "persistent radical effect" (27, 22). Furthermore, the steady-state implies: [R I S . IT ]

γ

•

=

[LL

S

fcr

S =

(Q\

EQ

These simulations indicate that the overall radical concentration in a C R P experiment with a properly adjusted additional radical flux is independent o f the radical production by homolytic dissociation o f the alkoxyamines present.


242

r 1.6


1.2

0.8

^

h 0.4

a

o.o

40

30

—ι— 50

60

time/s F i g u r e 2. Imaginative system o f an alkoxyamine dissolved in an inert solvent, in which the three fundamental reactions that can occur are the homolytic dissociation o f the alkoxyamine, trapping o f R* by Τ · and permanent bimolecular termination o f two R«. Simulated [ Τ · ] and [R»] vs. time (Δ and • , resp.) with ke = 1.0 χ 10" s" , 5

1

d

k

1

= 5.0 χ 10 L - m o l ' V , k = 2.5 χ 10 L - m o l " * ' , σ = 1.0 χ 10" mol-L" -s" and 8

et

9

1

1

8

1

1

t

1

1

[ L ] = 0.01 mol-L" , [ R » ] = [ Τ · ] = 0 mol-L" . 0

0

0

It can be calculated that for chain length-independent values for ket and k , the ratio o f t

radicals

derived

from the

additional radical flux

over radicals

derived

from

alkoxyamine homolysis (r ) equals: CT

σ _ Γ σ

=

(10)

k [L] e d

A s Γ is small, the major part o f the radicals propagating are derived from an alkoxyamine. This results i n a controlled chain-growth o f the constant number o f alkoxyamines leading, under ideal conditions, to a linear relation between average molar mass vs. conversion ( X ) and narrow molar mass distributions. σ

w

Volume Contraction To investigate the kinetics of a TEMPO-mediated controlled radical polymerization of styrene in bulk, e.g. by monitoring the overall gravimetrically determined conversion, X , vs. time or the concentration o f T E M P O , [ Τ · ] , it is important to mention that the data obtained need to be corrected for volume contraction, as a direct result o f polymerization. w


243

It is assumed that a negligible volume contraction occurs upon mixing o f polymer and monomer (23). Ideal behavior o f mixing leads to the following expression for the average density o f the system: P M PP

(11) wPM The molar mass distribution ( M M D ) o f a controlled radical polymerization is a function o f conversion, as can be observed in ,e.g., Figures 9a and 9b. Consequently, the molar mass dependence o f the density o f the polymer has to be taken into account. A s a measure o f the molar mass, the theoretical weight average molar mass can be taken: ( l -


P

I

1

(

=

-

Ρ

X

X

w ) p P

w )

X

+

X

w

+

PM

(

1 2

)

P ((Mr)) P

However, a more accurate description representing the polymer solution is obtained, when using the normalized weight M M D determined from S E C analysis, yielding:

Ρ

PP(

PM

M

)

To correct the data o f the polymerization reactions o f styrene for volume contraction, an empirical relationship for the density o f polystyrene as a function o f molar mass and temperature has been established (24): p(M,T)

=

A

-

4

Bxl(T T

(14)

with: A

Β

=

=

a

a

B

+

A

+

2

ο ξ

+

οξ

ο ξ-'

+

ο ξ~

Α

Β

ξ

=

+

Α

2

+

Β

d ^

3

d^"

3

(15)

ln(log(M))

The values for the constants aA, de are given i n Table I. Figure 3 represents the density o f polystyrene, p, vs. log ( M ) at 393.15 K . A s can be observed, a large dependence o f ρ on molar mass is observed for low molar mass material up to about 5.0 χ 10 g-mol" . 4

1

Table I. Constants of p ( M , T ) for Polystyrene Index: A Β

a\

bj

0.96402817 6.289622

0.42456027 -1.6441887

Cj -0.23245121 0.16572399

dj 0.045159034 1.5493251


244


1.05-,

0.80 2.0

3.00

6.0

5.0

4.0

log ( M / g - m o F ) 3

1

1

Figure 3. Density (ρ χ 10" / g-L" ) vs. log molar mass ( M / g-mol" ) o f polystyrene at 393.15 K . These results indicate that a constant value for the density o f polystyrene cannot be assumed implying that molar mass dependent volume contraction has to be taken into account when analyzing concentration data. Controlled Radical Polymerization of Styrene in Bulk Initial Overall Rate of Polymerization. In Figure 4 the concentration o f monomer, [M], vs. time for both a thermally initiated spontaneous polymerization o f styrene and C R P s at 393 Κ with [ L ] = 9.63 χ 10" m o l - L , 9.02 χ 10" mol-L" and 4.51 χ 10" mol-L" are given. The initial overall rates o f polymerization are presented in Table II. The data were corrected for volume contraction on the basis o f their theoretical molar mass assuming an ideal " l i v i n g " polymerization system, using Equations 12, 14 and 15. 4

-1

3

1

0

1

Table II. Initial Overall Rates of CRPs of Styrene in Bulk at 393 Κ

[L]o/ m o l - L-i

R , o

0.0 9.63 9.02 4.51 4.51 4.51

1.1 1.0 1.0 1.2 1.2 1.1

χ χ χ χ χ

10"r4 ΙΟ1-3 10"1-2 ΙΟ1-2 10"r2 -

-

P

t=

x

4

1

10 /mol-L" -s"


2

245

8.00-. 7.757.50-

_

7.25-

7.00-


ο g 6.75-

t

6.506.256.00~60~

300

240

180

120

time/ min F i g u r e 4. [M] vs. time for both a thermally initiated spontaneous polymerization o f styrene (•) and C R P s in bulk at 393 Κ with [ L ] = 9.63 ''10" ~' mol-L — - (x), — 9.02 — 4 A

_11

0

3

10" m o l - L

-1

2

1

( A ) and 4.51 χ 10" m o l L " ( O ) .

From these results it can be seen that the initial rates o f polymerization merely coincide for all cases within experimental error and, therefore, do not show a significant dependence upon alkoxyamine concentration. This behavior is properly described by the imaginative system represented previously (see also Equation 8). The steady-state overall radical concentration depends on the spontaneous thermal selfinitiation o f styrene (25) and the average rate coefficient o f bimolecular termination. Since the overall rates o f polymerization o f the experiments carried out merely coincide and the additional radical flux is presumed not to be influenced markedly by the presence o f the alkoxyamines and nitroxides (see section on alkoxyamine C - 0 bond homolysis) the average rate coefficients for bimolecular termination have to be identical within experimental error. However, the chain length distribution o f R* is different in each case and strongly depends on the amount o f alkoxyamines present. Therefore, it can be concluded that for the initial stages o f polymerization the chain length dependence o f the average rate coefficient o f bimolecular termination is leveled out. E S R Measurements: [Τ·] vs. time and x . In Figure 5 the [Τ·] vs. time is plotted for a C R P o f styrene in bulk at 393 Κ with [ L ] = 9.0 χ 10" mol-L" . The E S R spectra measured consisted only o f a conventional signal for T E M P O , indicating that a large excess o f Τ · , in comparison with R% is present and no specific interactions occur throughout the controlled radical polymerization. The slight increase in [Τ·] vs. time can be ascribed to a decrease in the rate o f thermal self-initiation o f styrene (25) and to the volume contraction o f the system, for which the data acquired have not been corrected. c

3

1

0


246

3.0

Ί

2.5 Η Ο ο

2

4^ · ° 1

ο

(Ρ

ο


ο

0.5-

ο.ο όο

60

120

180

240

300

360

420

time/ min F i g u r e 5. T E M P O concentration ([Τ·]) vs. time for a C R P o f styrene in bulk at 393 Κ with [Lo] = 9.0x 10" mol-L" . 3

1

N o distinct differences between the intensities o f the high-field and low-field absorption lines o f the E S R spectra could be detected during polymerization up to high conversion. Therefore, a value o f the rotational correlation time needed to calculate a value for k (see Equations 3 and 5) cannot be obtained from these spectra at these elevated temperatures. This indicates that no severe changes in the diffusion coefficient o f T E M P O occur, i n other words the microscopic viscosity for T E M P O does not show a pronounced change when conversion increases. This can partly be ascribed to the relatively low molar mass material and to the strongly reduced thermal self-initiation o f styrene at high conversion levels. (As comparison the diffusion coefficient o f toluene in different solutions o f polystyrene (26) and the diffusion o f styrene-analog penetrants in different solutions o f polystyrene can be given (27). A sharp decrease o f the diffusion coefficient at moderate temperatures is observed for toluene and styrene at high weight fractions o f polystyrene.) Consequently, the ket can be considered as a constant under the present experimental conditions. D

e t

D

T r o m m s d o r f f Effect. In Figure 6 the concentration o f monomer, [ M ] , vs. time for both a thermal spontaneous polymerization o f styrene and a C R P with [L]o = 4.40 χ 10" mol-L" at 413 Κ are given. The data were corrected for volume contraction on the basis o f their theoretical molar mass assuming an ideal " l i v i n g " polymerization system, using Equations 12, 14 and 15. The average molar mass produced in a C R P is much lower than i n a spontaneous thermal polymerization, as a result o f the monomer distribution over the relatively large number o f alkoxyamines (see e.g. Figures 7a, 7b and 7c). This difference is also reflected in the chain length distribution ( C L D ) o f the propagating radicals (R«). Together with a lower microviscosity for a specific Rj* in the C R P experiment in 2

1


247

comparison with the thermal polymerization at the same X , the Trommsdorff effect is less pronounced in the C R P experiment. (Noteworthy is that when all radical species are considered to be identical, the process o f reversible homolytic dissociation could be described as randomly removing a radical from the system by trapping with Τ · , and reintroducing it at a new position o f a randomly chosen alkoxyamine. Whether or not this process contributes to the suppression o f the Trommsdorff effect in a highly microviscous system, by enhancing the probability for two radicals to terminate at a specific time scale, is uncertain and needs to be investigated.) Downloaded by NANYANG TECHNOLOGICAL UNIV on October 18, 2015 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch015

w

0

'

60

'

120

'

180

'

240

'

300~

time/ min Figure 6. [M] vs. time for both a thermal spontaneous polymerization o f styrene (•) and a C R P with [ L ] = 4.40 χ 10" mol-L" ( O ) in bulk at 413 K . 2

1

0

Fundamental C R P Kinetics. From the data on the overall rate o f polymerization and the [Τ·] determined for a C R P o f styrene at 393 K , with [ L ] = 9.0 χ 10" mol-L" , a value o f 1.33 χ 10" mol-L' for Ke can be calculated (k = 2045 L - m o l ' V , [ M ] = 7.86 mol-L" , R = 1.1 χ 10" mol-L" s" and [Τ·] = 1.75 χ 10" mol-L" ). (Since a specific interaction between T E M P O and the carbon-centered radical or monomer is not observed it is assumed that the benchmark values o f the Arrhenius parameters for the k o f the bulk polymerization o f styrene can be applied (28).) The ked o f compound (1) at 393 Κ calculated with Equation 11 is 3.3 χ 10" s" . This results in ket = 2.5 χ 10 L-morV . In comparison with literature values o f ket for analogous systems (2, 12, 13), the obtained value is an underestimate. This, however, was to be expected, because the rate coefficient o f alkoxyamine C - 0 bond homolysis (ked) is markedly larger for longer chain lengths, implying that a higher value o f the rate coefficient o f alkoxyamine homolysis during a C R P experiment must be used. A large enhancement in ked is expected and ascribed to the disappearance o f the stabilizing electrostatic effect o f the electrophilic ter/-butoxy group on the C - 0 bond. Moreover, the entropie effects for a higher chain length presumably result in a 3

1

0

11

1

1

q

1

4

p

1

0

1

5

1

p

p

4

7

1

1


248

small increase in the pre-exponential factor o f the Arrhenius equation describing k d (29). Both effects w i l l result in a higher calculated value for the rate coefficient o f homolytic dissociation (k d). e

e


"Stepwise" G r o w t h . A n important requirement to synthesize a uniform polymer material by C R P with proper control o f both the molar mass distribution and the chemical composition distribution, is that the average number o f monomelic units polymerized per event o f chain activation (v) needs to be smaller than 1 to assure "step wise" growth of the propagating radicals. k [M][R] p

V

"

k ,[R][T]

+

e

2k,[R]

2

16

< > x

A value o f 37 for ν can be calculated for the system given above (assuming: k = 1.0 10 L - m o l ' V ) , which would rule out control o f the M M D . Figures 7a, 7b and 7c represent the number and weight average molar masses, < M > and < M > , together with theoretical molar mass of an ideal living polymerization system ( < M > = , γ = 1) for both a thermally initiated spontaneous polymerization o f styrene and C R P s at 393 Κ with [ L ] = 9.63 χ 10" mol-L" and 4.51 χ 10* mol-L" . In Figures 7b and 7c the polydispersity o f the M M D (γ) is included. From these plots it can be concluded that a reasonable control o f the M M D is still possible. This confirms the earlier statement that a higher ked is to be expected for polymeric alkoxyamines. t

7

1

n

W

t h e o r

n

t h e o r

w

4

1

2

1

0

7.0. 6.05.0J 4.03.0-1 2.01.00.00.00

0.05

0.10

0.15 X

Figure 7a. < M > (•) and < M > ( O ) vs. X polymerization o f styrene in bulk at 393 K . n

W

0.20

0.25

0.30

/ -

w

for a thermally initiated spontaneous


249

3.0

3.0

χ

2.5

2.5

2.0


1.5-1

/

8

ο

1.0

/

2.0

χ •

m

1.5

0.5 0.0 0.00

0.10

0.05

0.20

0.15 Χ /-

1.0 0.30

0.25

Figure 7b. < Μ > (•), ( Ο ) and γ (χ), together with theoretical molar mass of an ideal living polymerization system (—) ( = , γ = 1) vs. X for a C R P of styrene in bulk at 393 Κ with [ L ] = 9.63 χ 10" mol-L" . η

W

theor

theor

n

w

w

4

1

0

0.00

0.05

0.10

0.15 X

0.20

0.25

0.30

/ -

w F i g u r e 7c. (•), (O) and γ (χ), together with theoretical molar mass of an ideal living polymerization system (—) ( = , γ = 1) vs. X for a C R P o f styrene in bulk at 393 Κ with [ L ] = 4.51 χ 10" mol-L" . n

W

theor

theor

n

w

2

w

1

0


250

However, the control of the M M D is inadequate and a low polydispersity (γ) is not obtained, in terms of what could be expected from a proper C R P experiment. This could be ascribed to both the slow rate of homolytic dissociation and to the occurrence o f permanent chain-stopping reactions. The ked o f compound (1) at 393 Κ calculated from Equation 11 is 3.3 χ 10" s" and, therefore, (1) has a corresponding half-life o f about 35 min. Since the overall rate of polymerization and, therefore, the monomer consumption rate is fixed, this slow activation leads to broadening o f the M M D as a result o f the non-instantaneous rate o f activation. The molar mass o f the polymer material produced during the initial stages of polymerization w i l l be higher than the theoretical value calculated for an ideal " l i v i n g " radical polymerization system. This effect is clearly seen in Figures 7b and 7c. From Figure 8 the slow dissociation o f the initial alkoxyamine, monitored as the outer right peak in the S E C plots, is clearly observed for a C R P o f styrene at 393 K , with [ L ] = 4.51 χ 10" mol-L' .


4

2

1

1

0

e l u t i o n v o l u m e / a.u. Figure 8. Dissociation o f compound (1) monitored as the outer right peak for a C R P o f styrene in bulk at 393 K , with [ L ] = 4.51 χ 10" mol-L" ( X = 1.4 χ 10" , 9.7 χ 10" , 4.0 χ 10" , 0.10 and 0.21). 2

0

3

1

3

w

2

Permanent chain-stopping events restrict the molar mass that can be achieved and broaden the M M D . The production o f dead polymer material can largely be ascribed to the additional radical flux. The cumulative fraction o f the dead polymer chains has to be kept low to yield a uniform polymer material that is able to undergo further polymerization (e.g. to synthesize a blockcopolymer). Since the overall rates o f polymerizations are similar up to moderate conversion, the total number o f radicals produced by thermal self-initiation is equal in all cases, implying a similar number o f dead polymer chains in each system. Consequently, the fraction o f dead polymer material is higher in C R P systems with a lower amount o f alkoxyamines. This w i l l be reflected in a broader M M D for these systems. This is confirmed by the polydispersity


251

(γ) o f the M M D from the C R P experiments with [ L ] = 9.63 χ 10" mol-L" and 4.51 χ 10" mol-L" plotted in Figures 7b and 7c. To reduce the effect o f the low rate o f alkoxyamine bond homolysis on the M M D , experiments were carried out at higher temperatures. In Figure 9a the x ( M ) vs. log ( M ) at different stages o f conversion for a C R P o f styrene with [L]o = 4.40 χ 10" mol-L" and [Τ·]ο = 1.0 10" mol-L" is presented. A small amount o f T E M P O is added to prevent the average number o f monomeric units polymerized per chain activated (v) to exceed unity at the initial stages o f polymerization. In Figure 9b the number and weight average molar masses, < M > and < M > , together with the corresponding polydispersity γ are given. The line represents the theoretical molar mass for an ideal living polymerization system ( < M > = , γ = 1). 4

1

0

2

1

2

1

χ

4

1


n

W

t h e o r

t h e o r

n

I 2.5

'

1

«

3.0

1

«

3.5

w

1

'

1

4.0

4.5

log (M/g-moW) F i g u r e 9a. x(M) vs. log (M) at different stages o f conversion for a C R P o f styrene in bulk at 413 K , with [ L ] = 4.40 χ 10" mol-L" and [ Τ · ] = 1.0 χ 10" mol-L" . 2

1

4

0

1

0

A t this temperature the measured values for and at the initial stages o f polymerization are comparable to the theoretical values for an ideal " l i v i n g " polymerization system. Therefore, the ked for compound (1) at 413 Κ calculated from Equation 6, being 2.6 χ 10" s" , which corresponds to a half life o f about 4.5 min, is sufficiently fast. The comparable average molar masses also indicate that the average number of monomeric units polymerized per event of chain activation (v) is small. If R /(ked[L]o) is taken as an estimate for ν (R , =o 4.3 χ 10" mol-L" -s" ) , a value o f 3.8 would be obtained. Since the ked value for compound (1) is an underestimate for its value for a polymeric alkoxyamine, it can be presumed that "stepwise" insertion o f monomer takes place. This is confirmed by the S E C results shown in Figure 9a. n

3

1

=

P

p

4

1

1

t


252


14

w Figure 9b. < M > ( • ) , < M > ( O ) and γ ( χ ) , together with theoretical molar mass o f an ideal living polymerization system (—) ( < M > = , γ = 1) vs. X for a C R P o f styrene in bulk at 413 Κ with [ L ] = 4.40 χ 10' mol-L' . n

W

t h e o r

t h e o r

n

w

2

w

1

0

The occurrence o f permanent chain stopping reactions is reflected in negative deviation for < M > and < M > vs. conversion ( X ) , from the dependence o f the average molar masses on conversion for the ideal " l i v i n g " polymerization system. Due to the thermal self-initiation o f styrene (25), short-short and short-long bimolecular termination (3, 11) are in direct competition with the trapping o f R* by Τ · (2, 12, 13). This results in the production o f dead polymer material and the formation of novel alkoxyamines of, relatively, low molar mass. Consequently, a broadening o f the M M D w i l l occur, acting as a cumulative effect throughout the polymerization. This effect is more pronounced in the number M M D . This, indeed, is observed i n Figures 9a and 9b. n

W

w

Conclusions Further progress in molar mass control of nitroxide-mediated controlled radical polymerization requires detailed insight in the impact on the kinetics and mechanism upon addition of the alkoxyamine and nitroxide. Several aspects to investigate these influences were presented in a study on the TEMPO-mediated radical polymerization o f styrene in bulk. It has been shown that the isotropic correlation time (x ) of a nitroxide can be used as a measure o f the diffusive rate coefficient of trapping (ket ) and that under the conditions o f the C R P experiments performed a constant value of ket can be assumed. The Arrhenius parameters obtained for model compound (1), accurately determined by the nitroxide-exchange experiments, give an underestimated value o f the c

D


253

ked of polymeric alkoxyamines. A general empirical relationship for the density of polystyrene as a function o f molar mass and temperature has been established to correct concentration data obtained from C R P experiments for volume contraction. Finally, it has been found that the overall rates of polymerization o f styrene in bulk do not show a dependence upon alkoxyamine concentration. Broadening of the molar mass distribution in a C R P experiment has been ascribed to a low rate o f alkoxyamine C - 0 bond homolysis and permanent chain-stopping reactions, e.g. bimolecular termination.


Experimental Syntheses. The syntheses o f the nitroxides and alkoxyamines have been described elsewhere (4). Simulations. The differential equations were solved using a Gear algorithm for solving a set o f stiff differential equations, modified from the N A G library subroutine D 0 2 A E F . A l l calculations were performed on a Silicon Graphics Challenge X L Supercomputer (30). Quantitative E S R (31). A l l measurements were carried out on a Bruker E R 2 0 0 D S R C spectrometer, operating with an X-band standard cavity and interfaced to a Bruker Aspect 3000 data system. Temperature was controlled by a Bruker ER4111 variable temperature unit. The T E M P O concentrations were determined by double integration of the E S R spectra and the data were calibrated with stock solutions o f T E M P O measured in ter/-butylbenzene under identical conditions. Procedure. Prior to use ter/-butylbenzene was distilled and stored over molsieves (4Â). Styrene was distilled and passed over a column o f inhibitor remover (Aldrich) before use. A ITO" M solution o f compound (1) in styrene was prepared (0.0333 g in 10 m L ) . Approximately 1 m L was put i n an N M R tube and deoxygenated by purging helium through the solution in the N M R tube for several minutes. Next, the tube was placed under an argon atmosphere and sealed. Timeresolved measurements were performed at 393 K . 2

Controlled Radical Polymerizations of Styrene in Bulk. Styrene was distilled and passed over a column of inhibitor remover (Aldrich) before use. Compound (1) was purified by recrystallization from methanol. Procedure. Styrene, compound (1) and T E M P O were charged into a threenecked 100 m L round-bottomed flask equipped with a Teflon-coated magnetic stirrer. The reaction mixture was deaerated by three freeze-pump-thaw cycles in order to place the reaction mixture under an argon atmosphere. The flask was placed i n a thermostated o i l bath at the required temperature. Conversion was determined gravimetrically. S E C Analysis. The molar mass distributions ( M M D ) o f the polymers produced were determined by S E C . 0.1 w/v % Solutions in tetrahydrofuran (THF, stabilized, Biosolve,


254

A R ) were prepared o f each sample which was isolated by freeze drying. The solutions were filtrated through 0.2 μπι syringe filters. The S E C analyses were carried out with two Shodex K F - 8 0 M (linear) columns or two P L gel 10 and 500, at 40 °C. The eluent was T H F at a flow rate o f 1 m L m i n " . A Waters 410 differential refractometer and a Waters 440 U V detector (254 nm) were used for the detection. Narrow-distribution polystyrene standards (Polymer Labs) with molar masses ( M ) ranging from 370 to 6.5 χ 10 g-mol" were used for calibration o f the columns. After a baseline correction, the S E C chromatograms were converted to the differential log molar mass distributions (x(M) vs log(M)), weight M M D (w(M) vs. M ) and number M M D (n(M) vs. M ) accord ing to the procedure described by Shortt (32). 3

1


6

1

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Controlled Radical Polymerization - American Chemical Society

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