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C C h h a a p p t t e e r r 1 1
In I nt tr ro od du uc ct ti io on n
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1. Introduction
1.1 Free Radical Polymerization: General considerations
Free radical polymerization is the most widely used method of polymerization of vinylic monomers1. Free radical polymerizations are chain reactions in which every polymer chain grows by addition of a monomer to the terminal free radical reactive site called “active center”. The addition of the monomer to this site induces the transfer of the active center to the newly created chain end.
I2 2 I
I + M P1
kd ki
Initiation
Propagation P
n + M kp
Pn+1
Pn +
ktc
ktd
Pn+m
PnH + Pm
=
Pm Termination
Scheme 1.1 Mechanism of a free radical polymerization
The three basic mechanistic steps in a free radical polymerization are initiation, propagation and termination1,2 (Scheme 1.1). Initiation consists of the slow decomposition of initiator, I2 to give two radical species with a rate constant of kd. They quickly add to a monomer, M to form chain initiating radicals. During propagation, the later successively add to monomer units with a rate constant kp and the polymer chains grow until termination occurs. Termination is the reaction of two radicals either by combination (ktc) or by disproportionation (ktd). Combination occurs when two radicals couple together to form a longer dead chain. Disproportionation is the result of the abstraction of a ȕ -hydrogen from one radical chain by another to form a saturated and an unsaturated chain.
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species is transferred from a component of the reaction mixture, e.g. monomer, initiator or solvent to the growing radical. A new radical is generated which can reinitiate polymerisation. The direct consequence of chain transfer is the production of low molecular weight polymer.
The advantages of the free radical polymerization are (i) compatible with many monomers including functional monomers, (ii) versatile with regard to reaction conditions, and (iii) widely applied in industry for the above reasons.
The clear limitations are: (i) due to diffusion-controlled termination reactions between growing radicals, there is little control over molar mass distribution (MMD), (ii) since, the typical life time of a propagating chain is very short (typically less than a second or, at most, a few seconds), it is not possible to synthesize block copolymers or other chain topologies, and (iii) there is no control over the polymer tacticity.
Now, so as to retain the advantages of conventional free radical polymerization (FRP) and minimize its disadvantages, controlled radical polymerization (CRP) techniques were developed. The main similarity between CRP and FRP is the participation of free radicals in the chain growth. The main difference between CRP and FRP is that, in CRP, the steady concentration of free radicals is established by balancing rates of activation and deactivation, but in FRP this is realized by balancing the rates of initiation and termination.
1.2 Controlled Radical Polymerization (CRP)
Over the past few decades, controlled/living polymerization methodology has steadily expanded to include all of the chain-growth polymerization methods: cationic, anionic, transition metal-catalyzed, and, most recently, free radical polymerizations. The limited commercial application of living ionic polymerization systems stems from the need for high purity solvents and reagents, low reaction temperatures and solvents that do not induce chain transfer. Another important disadvantage of living ionic polymerization is that it can only exploit a limited variety of monomers. This makes CRP extremely important from commercial point of view, which combines the benefits of a living system with the versatility of a radical process.
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In the true sense, the controlled radical polymerization is a chain-growth polymerization that proceeds in the absence of irreversible chain-termination and chain- transfer steps. Thus, once a chain is initiated, it will grow until the monomer supply is depleted and will remain active either until more monomer is added, in which case chain growth will resume, or until a terminating agent is deliberately introduced. Provided that initiation is complete and exchange between species of various reactivities is fast, the final average molecular weight of the polymer can be adjusted by varying the initial conditions (DP = ǻ[Monomer]/[Initiator]o, where DP=degree of polymerization) while maintaining a narrow molecular weight distribution (1.0 < Mw/Mn < 1.5, where Mw= weight-averaged molecular weight and Mn = number averaged molecular weight). In 1956 Szwarc et al. reported the first observation of a polymerisation that had living characteristics3. He demonstrated that the rate of termination for the polymerisation of styrene using sodium naphthalenide as an anionic initiator system was zero. Furthermore, chain transfer activity was suppressed by the use of highly purified, dry aprotic reagents and so 100 % conversion of styrene to polystyrene was achieved. It was concluded that the lack of chain termination and chain transfer events was due to the charge repulsion between chain ends.
1.2.1 Classification of Controlled Radical Polymerization Methods
There are four different types of controlled radical polymerization methodologies known so far. These are (1) nitroxide mediated stable free radical polymerization (SFRP or NMP) (4-6), (2) reversible addition fragmentation chain transfer reaction (RAFT)7.8, (3) atom transfer radical polymerization (ATRP)9,10 and (4) degenerative chain transfer (DT)11,12. An outline for each of these is provided in the following pages. It is important to note that while NMP and ATRP are subject to the persistent radical effect (PRE),13-15 chain transfer processes, such as RAFT, do not conform to the PRE model because of the transfer-dominated nature of the reaction.
1.2.2 Persistent Radical Effect
The PRE is widely accepted to describe the kinetics of ATRP and NMP. The model
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was originally developed by Fischer13,14 in order to explain the high selectivities observed in some radical reactions. It should be noted clearly that RAFT polymerization does not follow PRE kinetics, and thus this section pertains only to ATRP and NMP. In keeping with presenting minimal equations, the discussion here will be qualitative in nature.
Starting with a dormant initiating species (I) which can be represented as containing two moieties: (1) an alkyl group that will form the propagating radical (R), and (2) the deactivating (or persistent) species (Y) (Scheme 1.2).
Y
R R Y
R
P + 2Y kt
kp M activation
deactivation (I)
Scheme 1.2 Reaction scheme used to describe the PRE
Thus, the compound I can be written as R-Y. For ATRP and NMP, the deactivating species (Y) is the oxidized metal catalyst and the nitroxide, respectively. The radical R may undergo propagation with monomer (M), deactivate with Y to reform R-Y, or react with another R entity to form a ‘dead’ species P. The persistent species, Y, only reacts with a radical R. An equimolar amount of radicals and persistent species are produced initially, during the activation step. However, radical-radical termination is unavoidable in radical polymerizations, even if the radical concentration is very low (e.g., <10-8 M).
Since two radicals (2R) are required for an irreversible termination reaction to occur, an equivalent number (two) of deactivator (Y·) molecules will also be produced subsequently. Hence, initially the concentration of R increases, and as a result more termination occurs, leading to a build-up of deactivating species Y·, as shown in Scheme 1.2. This in turn affects the position of the dormant/active species equilibrium; the equilibrium favors the dormant species more as Y is irreversibly produced. As a result fewer active radicals are produced, causing a slower rate of polymerization and less termination, and hence more control over chain growth (e.g. lower polydispersities).
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1.2.3 Nitroxide Mediated Stable Free Radical Polymerization (SFRP or NMP)
Nitroxide-Mediated Controlled Radical Polymerization was first discovered by Solomon et al., who patented their discovery in 198516. It is based on the reversible deactivation or ‘trapping’ of growing polymer radicals by a stable free nitroxide radical.
The reversible deactivation equilibrium in nitroxide mediated CRP is depicted in Scheme 1.3. As the trapping reaction of the stable nitroxide radical (2) with the growing polymer radical is near diffusion controlled, the equilibrium is strongly shifted towards the dormant (1) side. This ensures a low concentration of reactive radicals resulting in suppression of termination events relative to propagation events17,18.
(1) (2)
Scheme 1.3 Reversible deactivation equilibrium in nitroxide mediated CRP
To date, nitroxide mediated CRP appears to be best applicable in the controlled polymerization of styrene and derivatives thereof19-21. By modification of the alkyl groups on the nitroxide moiety it has been possible to polymerize acrylates22 as well as dienes23. The nitroxide-mediated polymerisation of acrylic acid has also been recently reported24.
1.2.4 Reversible Addition-Fragmentation Chain Transfer (RAFT)
This relatively new technique was reported by Rizzardo et al. in 1998 7,8. Simple organic compounds possessing the thiocarbonylthio moiety were effective in controlling the polymerization by reversible addition-fragmentation chain transfer process. There are four classes of thiocarbonylthio RAFT agents, depending on the nature of the activating (Z) group: (i) dithioesters (Z = aryl or alkyl), (ii) trithiocarbonates (Z = substituted sulfur), (iii) dithiocarbonates (xanthates), (iv) dithiocarbamates (Z = substituted
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combination of the activating and leaving groups are given in Figure 1.1. It is also shown, which combination suits for which class of monomers.
Initiator + Monomer + Z C S S
R R Pn S C Z
S
Z: Ph >> SCH3 ~ CH3 ~ N >> N
O
> OPh > OEt ~ N Ph
CH3 N
Et
Et
>
MMA VAc St, MA, AM, AN
Figure 1.1 Examples of different RAFT agents in relation to the monomers that can be polymerized in a well-controlled way. (St = styrene, MMA = methyl methacrylate, MA = methyl acrylate, AM = acrylamide, AN = acrylonitrile, VAc = vinyl acetate).
In a RAFT mechanism, initiation occurs via the decomposition of the free radical initiator leading to formation of propagating chains. This is followed by addition of the propagating radical to the RAFT chain transfer agent. Further, the fragmentation of the intermediate radical occurs, giving rise to a polymeric RAFT agent and a new radical.
kadd kadd
Pm S C
Z
S Pn kadd
kadd
Scheme 1.4 The central RAFT equilibrium
This radical reinitiates the polymerization to form new propagating radicals. The RAFT process relies on this rapid central addition fragmentation equilibrium between propagating and intermediate radicals, and chain activity and dormancy as shown in
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scheme 1.4. Many novel complex structures can be prepared using RAFT polymerization including blocks, gradients, stars and combs 26,27.
1.2.5 Degenerative Chain Transfer (DT)
Another route for achieving living free radical polymerization involves degenerative chain transfer11,12. A simplified mechanism for this process is shown in the Scheme 1.5.
AIBN 2 R
R +A X R X + A
R + nM
A + nM Mn
Mn+A X Mn X + A
M M
Scheme 1.5 Simplified mechanism of Degenerative Chain Transfer
where R is an initiator radical generated from the thermal decomposition of a x conventional free radical initiator such as 2,2’ azobisisobutyronitrile (AIBN), A-X is the transfer agent (Ax = an organic radical moiety e.g., PhCxH2, Ph(CH3) , (CH
Cx
3)2C COOEt etc. and X = iodine atom. The radicals present at certain low x concentration, or formed continuously during slow initiation, add to monomer and react with the transfer agent (A-X) reforming the growing radicals.
The main requirement for achieving living character is that an initiator radical (R ) or growing radical (x ) reacts rapidly and selectively with a transfer agent A- X to exchange X and form dormant species R-X or R-M
Mxn
R
n-X. and a new radical capable of chain growth. The later after addition of monomer unit will react with the transfer agent R-X or R-M
Ax
n-X. If this exchange is fast relative to propagation of radicals, polymers with narrow molecular weight distribution can be obtained.
