Nitroxide-Mediated Polymerization in Miniemulsion - American

Chapter 31

Nitroxide-Mediated Polymerization in Miniemulsion: A Direct Way from Bulk to Aqueous Dispersed Systems

Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch031

B. Charleux Laboratoire de Chimie Macromoléculaire, Université Pierre et Marie Curie, T44, E1 4, Place Jussieu, 75252 Paris Cedex 05, France (email: [email protected])

Owing to fundamental differences with bulk polymerization, the special features of nitroxide-mediated CRP in miniemulsion and the key for success are discussed in this article. The miniemulsion process, where polymerization is initiated by a SG1-based alkoxyamine, is examined in terms of initiation / nucleation, kinetics, monomer transport, and their consequences on control of molar mass and distribution.

438

© 2003 American Chemical Society

Matyjaszewski; Advances in Controlled/Living Radical Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


439 Since the very first developments, controlled free-radical polymerization (CRP) methods proved to be extremely powerful for the synthesis of a variety of new Macromolecules with increased complexity (1,2). When these chemically and architecturally well-defined polymers are prepared at the scale of the laboratory, homogeneous polymerizations like bulk or solution are particularly well suited. Additionnaly, these simple conditions are also the best choice for kinetic and mechanistic investigation. However, it is of industrial concern to develop CRP in aqueous dispersed systems (3,4). Emulsion polymerization is indeed a widespread process for the production of polymers via free-radical chemistry. Combination of the properties imparted by control of the polymer chains at the molecular level along with the advantages of this process will constitute a great achievement for the future developments. The most significant progresses of CRP in aqueous dispersed systems were however not directly done in emulsion polymerization but in miniemulsion (5,6). Indeed, the miniemulsion process can be regarded as a simple model for emulsion polymerization (7-9). In miniemulsion, the initial monomer in water emulsion is strongly sheared in order to divide the organic phase into small droplets that remain stable throughout the reaction. In addition to classical surfactants that ensure stability against coalescence, the use of a hydrophobe (such as hexadecane and/or polymer) was shown to enhance droplet stability via inhibition of Oswald ripening. The complex nucleation step that exists in emulsion polymerization is replaced by droplet nucleation (the presence of micelles should be avoided). For this reason, droplets behave as individual bulk reactors with ideally no exchange between them. The process allows the use of oil-soluble initiators and is tolerant to thermal auto-initiation, which is not possible in emulsion because undesirable polymerization would take place in the large non-stabilized monomer droplets. This polymerization process is nowadays developing very fast for manyAppl.ications:in addition to its use in CRP, it was shown to be very useful for the synthesis of organic/organic and organic/inorganic hybrid particles; moreover, other chemistries than free-radical can beAppl.ied,such as polyadditions (8) and anionic polymerization (10). The most direct pathway from bulk to miniemulsion CRP is to use the same reagents and particularly the same initiator and control agent. In the simplest case, as far as nitroxide-mediated polymerization in miniemulsion is concerned, the initiator is a monomer-soluble alkoxyamine, either a low molar mass one or a polymeric one, and the control agent is an oil-soluble nitroxide (TEMPO or SGI for instance, see Figure 1) with low water-solubility (11,12). A classical oilsoluble radical initiator can also be selected in conjunction with added free nitroxide; such bicomponent initiating system forms oil-soluble alkoxyamines and hence, behaves very similarly to the previous system. Another way to perform nitroxide-mediated CRP in miniemulsion is to use a water-soluble initiator in conjunction with an oil-soluble nitroxide. In this case, the initiating system can be either a bicomponent one with classical initiator and free nitroxide, or a monocomponent one with specially designed water-soluble alkoxyamine. Owing to fundamental differences with bulk polymerization, the special features of nitroxide-mediated CRP in miniemulsion and the key for success are discussed in this article. The miniemulsion process is examined here in terms of


440 initiation / nucleation, kinetics, monomer transport and their consequences on control of molar mass and distribution. We will focus on SGl-mediated polymerization and examine more particularly the use of alkoxyamine initiators that allow the best control over molar mass and architecture (13).


A Brief Description of Nitroxide-Mediatcd CRP and its

Appl.icat

Nitroxides are stable radicals that are able to trap carbon centered radicals at a nearly diffusion controlled rate (1,2). At low temperatures, the formed alkoxyamine is stable and therefore the nitroxide behaves as an inhibitor. However, at elevated temperature, the C-O bond may undergo homolytic cleavage, leading back to the propagating radical and to the nitroxide. This equilibrium between propagating radical and inactive alkoxyamine is the key step in nitroxide-mediated polymerization. Moreover, owing to the stability of their alkoxyamine end-group, the dormant Macromolecules can be isolated and further used as macroinitiators for the polymerization of the same or a different monomer. TheAppl.icationof nitroxides to control Macromolecular architecture has been recently reviewed by Hawker et al. (13). Initially, TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxy) (Figure 1) was the most widely used and studied nitroxide for CRP of styrene and derivatives (1416% enabling the synthesis of well-defined block copolymers and star-shaped structures. However, the Appl.ication of this nitroxide to other monomers appeared to be less straightforward (13). A new class of acyclic nitroxides was more recently proposed (17-20). One of them is the N-/€?tf-butyl-N-(ldiethylphosphono-2,2-dimethy!propyl) nitroxide (also called SGI) (Figure 1) (18-20).

TEMPO

SGI

Figure 1. Structure of TEMPO (2,2,6,6-tetramethylpiperidmyl-l-oxy) and SGI fN'tert'butyl-N'(l'diethylphosphono-2,2'dimeth^ nitroxide). Faster kinetics than with TEMPO were observed for styrene polymerization and this nitroxide was also shown to be particularly well suited for the controlled polymerization of acrylic esters such as η-butyl acrylate. This feature opened the


441


way to the synthesis of complex copolymer architectures using nitroxidemediated polymerization (13 J1-23), as it was also the case with the other CRP techniques, namely atom transfer radical polymerization (ATRP) (24,25) and reversible addition-fragmentation transfer (RAFT) (26). TEMPO and derivatives have been used as mediators in aqueous dispersed systems such as suspension (27-50), seeded emulsion (31), ab initio batch emulsion (32,33) and miniemulsion polymerizations (34-42). Styrene was the most studied monomer, but more recently, the CRP of η-butyl acrylate was also made possible in a miniemulsion system (37,38). Nevertheless, TEMPO presents many drawbacks, which are not in favor of its use in aqueous dispersed systems and the more recent progresses have been done with SGI as a mediator (12,4348). For this reason, the examples presented in this article will be based on S G l mediated polymerization of η-butyl acrylate (BA) and styrene (S) in miniemulsion.

Initiation and Nucleation Oil-Soluble Alkoxyamine Initiator When this type of initiator is used, transposition of bulk polymerization to miniemulsion process is quite straightforward. The oil-soluble alkoxyamines that have been Appl.ied as initiators in miniemulsion are TEMPO-capped polystyrene (PS-TEMPO) (36-38), SGI-capped poly(n-butyl acrylate) (PBA-SG1)(47) and SGl-based low molar mass alkoxyamines such as MONAMS (12,44,45,47) and D1AMS (48) (Figure 2). ÇH

MONAMS:

3

H, C — O - C - C H - S G l H 3

A ÇH D1AMS:

CH

3

3

SGl—CH-g-0-(CH ) —O-^-CH-SGl 2

6

Figure 2. Structrure ofMONAMS and D1AMS SGl-based alkoxyamines. An advantage of the polymeric macroinitiators is that they act as reactive hydrophobic agents against Oswald ripening, avoiding the use of other molecules that can be considered as volatile organic compounds. However, to



442 reduce the number of steps and avoid a preliminary bulk polymerization, low molar mass alkoxyamines are of great interest too. When an oil-soluble alkoxyamine is used, exhibiting fast initiation because of fast dissociation rate, droplet nucleation is necessarily a fast process too. However, this ideal situation can be complicated by the dispersed state of the system with possible partitioning of the alkoxyamine or/and of the initiating carbon-centered radical. This actually does not Appl.y for polymeric macroinitiators because neither the alkoxyamine nor the initiating radical are water-soluble; hence they are trapped in the droplets/particles and cannot undergo exit. Low molar mass alkoxyamines behave however quite differently. For instance, M O N A M S has been used to initiate η-butyl acrylate polymerization in miniemulsion at 112 and 125 °C, in the presence of a small concentration of free SGI (44-47). Whereas partition coefficient of the alkoxyamine in water is low (49% it is not the same for the 1(methoxycarbonyl)eth-l-yl (MCE) primary radical formed upon homolytic cleavage. Indeed, M C E is known to be quite hydrophilic: for instance, water solubility of methyl acrylate at saturation at 50 °C is 0.6 mol.L" and partition coefficient, i.e. ratio of concentration in water over concentration in the organic phase, is 0.05 (50). Therefore, after dissociation of the alkoxyamine, the initiating radical has three possibilities: it can either exit, initiate, or recombine with free SGI (to simplify, termination with another carbon-centered radical is not considered; such hypothesis actually holds if sufficient concentration of free nitroxide is initially introduced in the reaction medium). If one compares first, initiation (rate = kp.[BA] .[MCE]) and recombination (rate = k .[SGl]o.[MCE]), the ratio of initiation rate over recombination rate, (k .[BA]o)/(k 3.5x10" mol.L" (saturation concentration is 0.05 mol.L" ), this ratio is then lower than 1, i.e. much lower than in the monomer phase. For styrene, because water-solubility is slightly lower than that of B A , the 1

3

1

7

1

1

5

aq

5

aq

aq

1

aq

1


445 propagation step is even less favored. When deactivation of the water-soluble oligoradicals is faster than propagation (here again self-termination of carbon centered radicals is considered as negligible), i.e. when p, the probability of propagation with respect to recombination (Eq. 3), is lower than 1, then initial chain growth in the aqueous phase is strongly slowed down. VtMJaq P

k .[M] p

a q +

a u

k -[X]

a q

p

k .[X] c

k -[M] a q

*

c

From now on, X represents the nitroxide deactivator, RX is the water-soluble alkoxyamine initiator (initial concentration [RX] ,X M is the monomer, RMjX is an oligomeric alkoxyamine with i monomer units, and the concentrations are expressed per volume unit of water-phase. According to the activationdeactivation equilibrium, lg is the rate constant of alkoxyamine dissociation and kc is the rate constant of coupling between a propagating radical and X (both are supposed to be chain length independent). With the assumptions that ρ < 1 and that the RMj* radicals are totally watersoluble until they reach a critical degree of polymerization z+1, it is possible to calculate the rate of entry of these radicals in the monomer phase: Re ) = kp.[M] .[RM '] . This means that the radicals with i = ζ + 1 irreversibly enter the oil-phase, while those with i < ζ propagate in the aqueous phase only. The step of entry itself is not rate determining. The critical value ζ is a function of the nature of the initiator and of the monomer. Such a theoretical approach was proposed by Maxwell et al. (56) for the rate of entry in classical emulsion polymerization. To simplify, the RM,X alkoxyamines will be considered as also water-soluble for i < z, although a partition coefficient should be Appl.ied for every chain length. The concentration of each species RMjX and RMi* in the water-phase can be calculated as a function of time as given in Eqs 4-6 (they follow the Poisson distribution). As all the considered events take place at low conversion, the monomer concentration remains constant in the water-phase, and equal to the saturation concentration (thus ρ is a constant too).


aq(

(2

aq

z

aq

[RX]

[RMiX]

a q

aq

= [RX]

( P

k

= ' ?j'

t ) l

a q 0

·exp(-p · k · t)

-[RX1

(4)

d

aq0

-exp(-pk

d

t)


(5)

446 For a given monomer with critical value z, the rate of entry can then be calculated as given in Eq.7.

Re = p . k . {2)

d

( p , k

^

t ) Z

.[RX]

a q 0

.exp(-p.k

t)

d

(7)

The integrated form of this equation represents the amount of alkoxyamine that has entered the monomer phase at a given time. An example is given in Figure 5, for various values of ζ (between 0 and 4, corresponding to usual hydrophobic monomers (4)), using kd = 3 χ 10* s" , that is a common value for SGl-based alkoxyamines, and ρ = 0.3. Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch031

3

1

time (h)

Figure 5. Proportion of alkoxyamine that has entered the monomer phase at a given time, for various values ofz. /:,/ = 3 χ 10' s'* and p- 0.3. s

It appears very clearly that entry of the alkoxyamines, which corresponds to the true initiation step (i.e. the real start of chain growth), is significantly retarded when ζ increases. Such situation would be very unfavorable to good control over molar mass and distribution, due to an effect of apparent slow initiation. For a given monomer and a given water-soluble initiator, i.e. a given value of z, very few experimental parameters can be modified. To favor water-phase propagation and fast entry, the concentration of free SGI should be reduced. Additionnally, if possible, an alkoxyamine initiator exhibiting fast dissociation rate should be selected.


447


Polymerization Kinetics Because of radical segregation, conventional emulsion and miniemulsion polymerizations usually exhibit slow termination reactions resulting in much faster polymerization than in the homogeneous systems under similar conditions. However, the controlled radical polymerizations operating via a reversible termination reaction (nitroxide-mediated polymerization and ATRP) do not follow the same kinetics as a conventional radical polymerization. They are governed instead by the activation-deactivation equilibrium. The transient and persistent radical concentrations are regulated by the persistent radical effect (PRE) (57,58) rather than by a steady state resulting from an initiation/termination balance. As a consequence, reversible termination is favored due to the large concentration of deactivator, which continuously increases with conversion. A theoretical kineticAnal.ysisbased on the persistent radical effect (no other source of radicals than the activation reaction) was Appl.ied to miniemulsion systems (59). To simplify the model, an oil-soluble alkoxyamine initiator was considered, the propagating radicals were supposed to be compartmentalized (true for macroradicals), whereas free nitroxide could exchange between the particles due to molecular diffusion through the water-phase. The instantaneous rate of polymerization is proportional to [Ρ'], the concentration of propagating radicals, and inversely proportional to the concentration of nitroxide deactivator at a given time. The latter depends on both the initial concentration, and the concentration that is released owing to the PRE (equal to the concentration of irreversibly terminated chains). It can then be calculated for a miniemulsion system, as a function of monomer conversion, x, according to Eq. 8. 2

[XIMEI = - A + V(A + [ X ] ) - B . l n ( l - x )

(8)

0

.. . with A =

k

t

N .v .k A

p

„ 4.K.[RX] .k and Β = — k 0

c

t

p

The concentrations are expressed per volume unit of the overall organic phase. [X]o represents the initial concentration of nitroxide in the system; k , kc, and kp are the rate constants of termination, recombination with nitroxide and propagation respectively; Κ is the activation-deactivation equilibrium constant, [RX] is the initial concentration of alkoxyamine, v is the particle volume and Ν is the Avogadro's number. t

0

p

Λ

In a bulk polymerization the same relationship leads to:


448 x

[XW , 2

(9)

2

From simple mathematics, [X] ULK) = [X] (MH) + 2.A.([X] i;) - [Χ]ολ so that [X](BULK> ^ [X](ME)- Therefore, the difference between the concentrations of free nitroxide released at a given conversion in bulk and miniemulsion is to a large extent determined by the value of parameter A , which is dependent on the size of the particles for a given monomer and a nitroxide. When the particles are very small, A is large and [ X ] is larger in bulk than in miniemulsion, indicating the greater degree of irreversible termination. As a consequence, the polymerization should be faster in miniemulsion than in bulk, the proportion of dead chains should be smaller, but the molar mass distribution should be broader. The polydispersity index can be reduced by addition of free nitroxide, at the expense however of the polymerization rate. In contrast, when particle volume is large, A becomes small and the concentration of released nitroxide is nearly the same in bulk and in miniemulsion. Consequently, the compartmentalization effect does not operate any longer, and the kinetics should be the same in both systems. In other words, the overall concentration of propagating radicals (relative to the organic phase volume) is not much larger in a miniemulsion system than in the corresponding bulk polymerization. Therefore, the average number of radicals per particle is simply inversely proportional to the number of particles in the system, N , and usually far below I, while the average number of deactivator molecules per particle is much larger than 1 (typically from a few tens to a few hundreds). In other words, a pseudo-bulk kinetics operates and polymerization rate is independent of N . (B


2

= V[ ]o -B.!n(!-x)

Nitroxide-Mediated Polymerization in Miniemulsion - American

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