TEMPO-terminated polystyrene

Later on, these same research groups started using pre-formed TEMPO-terminated polystyrene as a one component initiator system instead of a bicomponent nitroxide/initiator system. Pan et al. [224] prepared TEMPO-termin-ated polystyrene in bulk and isolated this to use it as the initiator in their miniemulsions. This led to slower polymerization rates, molecular weights lower than predicted and relatively broad molecular weight distributions. Keoshkerian et al. [225], on the other hand, reported very high conversions in six hours (99.6%) and narrow polydispersities (1.15) by preparing TEMPO-terminated polystyrene in bulk up to a conversion of about 5% and applying... 

The application of these procedures to 1,3-dienes has presented problems. The rates of polymerization were observed to decrease and then stop due to a buildup of excess free nitroxide (Keoshkerian et al., 1998). An effective procedure for the controlled polymerization of isoprene at 145°C involved the addition of a reducing sugar such as glucose in the presence of sodium bicarbonate to react with the excess nitroxide (Keoshkerian et al., 1998). After 4 h, polyisoprene with M = 21,000 and My,/M = 1.33 was obtained in 25% yield. The reaction of TEMPO-terminated polystyrene with either butadiene or isoprene resulted in the formation of the corresponding diblock copolymers that were characterized by NMR and SEC (Georges et al., 1998). No evidence for either polystyrene or polydiene homopolymers was reported. 

Block Copolymers. Well defined block copolymers of 4-hydroxystyrene and styrene were prepared by firstly, the synthesis of low molecular weight blocks of styrene capped with TEMPO. The molecular weights of these styrene blocks were controlled by the ratio of unimolecular initiator (4) relative to styrene monomer (2). For example, 1-phenyl-l-(2, 2, 6, 6 -tetramethyl-r-piperidinyloxy)ethane (4) (1.67g, 0.0064 mol) was added to styrene (2) (20.0g, 0.192 mol) and heated to 125-130°C, under N2, for 48 hours. The reaction was then cooled to room temperature and the polymer dissolved in tetrahydrofuran (100 mL), and isolated by precipitation into methanol (1000 mL). The TEMPO terminated polystyrene (7) was then filtered, washed with methanol and dried in a vacuum oven overnight at 50°C. Isolated yield 90% of theory. M = 2764, M = 3062, PD = 1.10 (Theoretical A.M.U = 3120). 

Dynamic formation of graft polymers was synthesized by means of the radical crossover reaction of alkoxyamines by using the complementarity between nitroxide radical and styryl radical (Fig. 8.13) [40]. Copolymer 48 having alkoxyamine units on its side chain was synthesized via atom transfer radical polymerization (ATRP) of TEMPO-based alkoxyamine monomer 47 and MMA at 50°C (Scheme 8.9). The TEMPO-based alkoxyamine-terminated polystyrene 49 was prepared through the conventional nitroxide-mediated free radical polymerization (NMP) procedure [5,41], The mixture of copolymers 48 and 49 was heated in anisole... 

Scheme 8.10 Dynamic formation of graft polymer 50 prepared form copolymer 48 and TEMPO-based alkoxyamine-terminated polystyrene 49 [40].

NMP is based on the concept of a dynamic equilibration between dormant alkoxyamines and propagating radicals as shown in eqn [55].The choice of the persistent radical is cmcial for controlled polymerization. While styrene can be easily moderated by 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO), other monomers required the development of nitroxides that contain hydrogen atoms at the a-C. There are two different initiation methods for NMP. Conventional radical initiators (i.e., AIBN, BPO) in conjunction with a persistent radical were initially used to prepare polymers by NMP, but these systems were limited in the choice of monomer. Functionality could be incorporated via a functionalized initiator or a functionalized persistent radical. For example, Baumert and Mulhaupt prepared carboxylic acid-terminated polystyrene, poly(styrene-co-acrylonitrile), and polystyrene-b-poly (styrene-co-acrylonitrile) by the use of the functionalized initiator 4,4 -azobis(4-cyanopentanecarboxylic acid). The polymerization was controlled by the addition of 2,2,6,6-tetramethyl-l-piperidyloxyl radical, and polymers with... 

Polymerization of styrene with these initiators gave hydroxy and protected amino-terminated polymers with narrow polydispersities (i.e., MJM 1.1-1.6). The r-Boc amino group in the latter case can be deprotected by trifluoroacetic acid to eventually yield monofunctional amino-terminated polystyrene. An alternative procedure is based on the use of a functional initiator possessing the desired groups (Scheme 26) together with TEMPO. 

C ) with a 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL) terminated polystyrene (PS) (M = 12000gmor M /M = 1.16) at 95°C in toluene [19]. In a similar reaction, but with 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) as the stable counter radical, a di-adduct was formed in high yield even when a four-fold excess of Qo per PS-TEMPO was used [20]. The two chains are attached in the 1,4 positions on the same six-membered ring, only one double bond is opened and no TEMPO is present on the fullerene (Scheme 5.2). 

Figure 6.23 Reversible termination of polystyrene chain radical by TEMPO radical. (After Georges et al., 1993.)...

The use of stable free-radical polymerization techniques in CO2 represents an emerging new area of research. Odell and Hammer have demonstrated the use of reversibly terminating free radicals generated by systems such as benzoyl peroxide or AIBN and 2,2,6,6,-tetramethyl-l-piperidinyloxy free radical (TEMPO) to polymerize styrene at a temperature of 125°C and pressures of 245-280 bar in CO2 [92]. At low monomer concentrations (10% by volume), the polymerization resulted in low conversions of PS with an of about 3000 g/mol and a narrow molecular weight distribution (PDI <1.3). NMR analysis of the resulting polymer confirmed that the precipitated polystyrene chains are predominantly end-capped with TEMPO. Additionally, the polymer could be isolated and later extended by the addition of more monomer under an inert argon blanket. It was also determined that the precipitated PS could be extended while still in the CO2 continuous phase simply by increasing monomer concentration in the reactor. 

Polybutadiene-f)-polystyrene [54-56], poly(dimethyIsiloxane)-h-polystyrene [57], PEO-b-PSt [58], and PEO-h-poly(4-vinyl pyridine) [59] copolymers were synthesized by terminating the corresponding hving anionic polymerization with a suitable TEMPO derivative and subsequent NMRP. 

In the mid-1980s, the first technique that relies on the reversible termination of radicals with a stable free radical was developed in the group of E. Rizzardo at CSIRO in Australia. Rizzardo and co-workers found that nitroxide-stable free radicals were able to add to carbon-centered radicals to form alkoxy amines (9). In certain cases these alkoxy amines are thermally unstable, so that they enter into an equilibrium between (transient) carbon-centered radical and (persistent) nitroxide radical on one side, and alkoxy amine on the other side. TEMPO was initially the most frequently used nitroxide in conjunction with the polymerization of styrene and its derivatives. The TEMPO-polystyrene adduct requires temperatures of 120° C or above in order to establish an equilibrium at which polymerization takes place. Around the mid-1990s Georges and co-workers focused on the TEMPO-mediated pol5unerization of styrene (10), and developed various strategies to overcome intrinsic weaknesses of the system. They used camphor sulfonic acid to enhance the rate of polymerization (11). This rate enhancement was later elucidated to be due to the destruction of excess nitroxide that builds up during the polymerization. 

The key to success in synthesizing polymers with narrow polydispersity and well-de ned chain end structure by carrying out free-radical polymerization in the presence of nitroxide SFRs such as TEMPO, is the essentially simultaneous initiation and reversible termination of the polymer radical with the SFR (Georges et al., 1994). However, the dissociation such as depicted in Fig. 11.4 for the polystyrene (PSt)-TEMPO adduct is known to occur in a limited number of systems (at high temperatures). A versatile use of the simple TEMPO-based SFRP is therefore not possible. For example, attempts to perform SFRP of monomers such as acrylonitrile (AN), methyl and ethyl acrylates (MA and EA), and 9-vinylcarbazole (VCz) with benzoyl peroxide (BPO) and TEMPO have not been successful. Interestingly, however, styrene has been successfully copolymerized (see Section 11.2.4) with these monomers using BPO initiator and TEiMPO under a living fashion (Fukuda etal., 1996). 

Fukuda describes an experiment in which Polystyrene terminated with TEMPO (PSt-T) of Mn= 1700 is heated in the presence of styrene, and in the gel permeation chromatography (GPC) of the product, the peaks of unreacted and reacted PSt-T ([I]o) could be observed separately. Then, -d[I]/dt = fed[I]/ giv-... 

Another example for block copolymer synthesis is the preparation of poly (methylene-fc-styrene), which was achieved by using a hydroxyl-terminated living polystyrene obtained by TEMPO-mediated living radical polymerization [75] (Scheme 55). The chain end hydroxyl group was transformed into an allyl ether moiety, which was subjected to hydroboration with BH3 to afford polystyrene macroinitiator for the polymerization of 11. After chain elongation with 11 oxi-dation of the C-B chain ends furnished poly(methylene- -styrene) bearing TEMPO and hydroxyl group at chain ends. 

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