Controlled/living free radical polymerizations

Free radical polymerization is flexible and less sensitive to the polymerization conditions and functional groups compared to ionic analogues. However, classical

free radical processes yield polymers without control of molecular weight and chain end. Competing coupling and disproportionation reactions and the inefficiency of the initiation step lead to loss of functionalities. Recent developments in controlled/living radical polymerization provided the possibility to synthesize well-defined polymers with controlled functionality also with radical routes [49]. The controlled/living free radical polymerization techniques depends on minimization of chain breaking reactions and growth of all chains at the same time. Therefore, fast initiation and very slow termination reactions are main requirements of such systems in which active propagating radicals are periodically generated. The first examples of these systems were seen consecutively in cationic ring opening [50], carbocationic [51] and anionic polymerizations [52].

Each controlled/living free radical polymerizations method depends on one of the following equilibria between propagating active radicals and dormant species:

(1) Active radicals are reversibly deactivated into dormant species in a process called deactivation/activation (Figure 2.3). In the first approach depending on persistent radical effect, newly formed radicals are immediately stabilized (with a rate constant of deactivation, k_da) by a group (i.e. nitroxide) or a transition metal complex (commonly based on Cu, Fe, Co, Mo, Os and Cr) leading to formation of dormant species. Subsequently, dormant species are activated (with a rate constant of activation k_a) thermally [37, 39, 53], photochemically [54, 55], electrochemically [56] or by redox [57] to form active species again. Most of these radical species undergo propagation (kp), and the others can terminate (kt).

Figure 2.3 : General mechanism of controlled/living free radical polymerization by activation/deactivation process.

The crucial point of the system is that persistent radicals (X) do not react with each other but only cross-couple with the growing species. However, termination by both radical coupling and disproportionation should be also included in the mechanism due to the magnitude of the related rate constant. While, the propagating radicals mostly couple with X whose concentration is 1000 times higher. The amount of

radicals decreases when amount of X increases with time leading to drop in the probabilty of termination.

NMRP and transition metal-catalyzed free radical polymerizations including OMRP, ATRP and CMRP are all rely on this mechanism.

(2) In an alternative mechanism [58], radicals are involved in a rapid degenerative exchange process between dormant and active polymer chain ends (Figure 2.4).

These processes do not rely on persistent radical effect. The initiation and termination reactions cause a steady state concentration. Sometimes, long-lived intermediates can participate in the exchange process leading to retardation of polymerization or side reactions such as termination of propagating radicals.

Figure 2.4 : Controlled/living free radical polymerization via degenerative exchange process.

ITP and RAFT are the most widely used techniques that follow degenerative exchange process.

Most commonly used controlled/living polymerizations, namely ATRP; NMRP; and RAFT, will be described briefly here.

2.1.1.1 ATRP

Among the controlled/living radical polymerization strategies, ATRP, coined by Matyjaszewski et al. [59] and Sawamoto et al. [60] in 1995, has turned out to be the most promising method allowing good control on the molecular weight and microstructure of the polymers in a targeted manner. As illustrated in Figure 2.5, a typical ATRP system must contain an initiator (RX), a transition metal halide (Mt^zXz) coordinated with ligand(s) (L), and obviously, monomer (M). The transition metal of the complex can exist in two different oxidation states. ATRP process begins reaction of the lower oxidation state metal complex (Mt^z/L) with the initiator leading to formation a radical (R^●) and the corresponding higher oxidation state metal complex with a coordinated halide anion X-Mt^z+1/L, in a process termed activation. In the consequent step namely deactivation, the latter complex transfers the halogen atom back to the radical to re-form the alkyl halide and the lower

oxidation state metal complex. However, prior to deactivation some of the active radicals can react with the monomer (M) yielding polymer chains in the propagation step. A little fraction of the radicals can be coupled with each other instead of X-Mt^z+1/L. The most important difference of the ATRP compared to conventional radical polymerization is structure of the resulted polymer containing halogen-terminated end. This halide functionality allows preparation of block copolymers when suitable condition for ATRP of another monomer is provided.

Figure 2.5 : General mechanism of ATRP

In this method, control is achieved by the equilibrium maintained between an active (Pn●

) and a dormant chain (Pn-X). The mechanism involves reversible homolytic cleavage of a carbon–halogen bond by a redox reaction between an dormant species and a metal/ligand complex, such as copper (I) complexes with 2,2-bipyridine.

ATRP catalyzed by copper complexes has been applied successfully to the controlled/living polymerization of several monomers including styrenes, acrylates, methacrylates, acrylonitrile, and other monomers so far [61]. Table 2.1 summarizes trnasiton metals of complex catalysts system employed in ATRP of diverse monomers.

Table 2.1 : Transition metals used in ATRP of various monomers.

ATRP Monomer Ref.

Cu-based Styrenes, acrylates, methacrylates, acrylonitrile [61]

Ru/Al-based Styrenes, acrylates, methacrylates [62, 63]

Fe-based Styrenes, methacrylates, vinyl acetate [64-66]

Ni-based Methacrylates [67-69]

2.1.1.2 NMRP

Another controlled/living radical polymerization method developed in recent years is NMRP, also called SFRP by some authors [39, 70]. This type polymerization, which relies on persistent radical effect, can be realized through reversible deactivation of

propagating chain end by relatively stable radical nitroxides such as TEMPO (Figure 2.6).

Figure 2.6 : General mechanism of NMRP.

According to general mechanism, the control is assured by the reversible termination of the propagating polymeric chain and reduced concentration of the growing radical chain end. The exceptionally low concentration of reactive radicals ends because of the absence of other reactions leading to initiation of new polymer chains results in decrease in irreversible termination reactions, such as coupling or disproportionation.

This allows formation of desired amount of initiating species and growing chains which is observed in a living fashion. Like the other controlled/living radical polymerization, this method produces well-defined polymers due to the these properties. The key to the success of this approach is employment of a wide variety of persistent radicals such as (arylazo)oxy, substituted triphenyls, verdazyl, triazolinyl, nitroxides, etc. in a reaction at near diffusion controlled rates with the carbon-centered free radical of the propagating chain end in a thermally reversible process [39].

2.1.1.3 RAFT

RAFT polymerization is one of the most recent entrants and one of the most efficient methods in controlled/living radical polymerization techniques [40, 71-74]. A major advantage of the RAFT process over other controlled radical polymerization techniques is its tolerance of protic and other functionalities that can be incorporated into the chain transfer agent [72, 75-77]. In this technique, after the initiation, the RAFT agents reversibly deactivate the polymer chains as the rate constant of chain transfer is faster than the rate constant of propagation (Figure 2.4). In the polymerization, the functional group present on the CTA is retained at the end of the polymerization. This enables the incorporation a wide range of functional end-groups such as -OH, -COOH, -NR₂, -CONR2 -SCO3Na etc. α-Functional groups can be introduced via the R group while the Z group of the CTA contribute to the incorporation of ω-functional groups (Figure 2.7).

Figure 2.7 : Synthesis of functional polymers by RAFT polymerization.

The key to succesful RAFT polymerization is a order of addition-fragmentation equilibria as presnted in Figure 2.8 [40, 73]. In initiation and termination steps, there is no difference between RAFT and conventional polmerizations. However, after initiation, addition of a propagating radical (Pn●

) to the thiocarbonylthio compound [RSC(Z)=S] completely change the nature of the polymerization. In the subsequent step, fragmentation of the intermediate radical yields a macromolecular thiocarbonylthio compound [PnS(Z)C=S] and a new initiating radical (R^●). A new growing radical (Pm●

) is generated by the reaction of the radical (R^●) with monomer.

The crucial point is establishment of rapid equilibrium between the active growing radicals (Pn●

and Pm●

) and the dormant polymeric thiocarbonylthio compounds assuring equal possibility for all chains to grow. This situation allows formation of polymers with narrow molecular weight distribution. After completion of the polymerization, polymers with thiocarbonylthio end group are obtained; therefore, this way block copolymers can be synthesized by sequential addition of another monomer without any post-modification process [78].

Figure 2.8 : General mechanism of RAFT polmyerization.

RAFT polymerization differs considerably from NMRP and ATRP by the means of the mechanism for achieving control. In ATRP and NMRP, propagating radicals are successively deactivated by atom transfer and radical-radical coupling and activated by homolytic cleavage of the labile bonds of dormant species. The position of the deactivation-activation equilibria and the “persistent radical effect” play key role in the rate of polymerization and, in turn, the control over the kinetics. Whereas, initial radicals are necessary to initiate and retain RAFT polymerization, since radicals are neither generated nor deactivated in this approach.