Synthesis of star polymers via nitroxide mediated free‐radical polymerization: A “core‐first” approach using resorcinarene‐based alkoxyamine initiators

The synthesis of new octafunctional alkoxyamine initiators for nitroxide‐mediated radical polymerization (NMRP), by the derivatization of resorcinarene with nitroxide free radicals viz TEMPO and a freshly prepared phosphonylated nitroxide, is described. The efficiency of these initiators toward the controlled radical polymerization of styrene and tert‐butyl acrylate is investigated in detail. Linear analogues of these multifunctional initiators were also prepared to compare and evaluate their initiation efficiency. The favorable conditions for polymerization were optimized by varying the concentration of initiators and free nitroxides, reaction conditions, etc., to obtain well‐defined star polymers. Star polystyrene thus obtained were further used as macro‐initiator for the block copolymerization with tert‐butyl acrylate. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5559–5572, 2007     

“Living” radical graft polymerization of styrene to styrene butadiene rubber (SBR) with 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO)

Well‐defined graft copolymers with styrene butadiene rubber (SBR) backbones and polystyrene branches were synthesized by living free radical polymerization (LFRP) techniques. Thus 1‐ benzoyl‐2‐phenyl‐2‐(2′,2′,6′,6′‐tetramethyl‐piperidinyl‐1′‐oxy)ethane (BZ‐TEMPO) was synthesized and hydrolyzed to the corresponding 1‐hydroxyl derivative. This functional nitroxyl compound was coupled with brominated SBR (SBR‐Br). The resulting macroinitiator (SBR‐TEMPO) for “living” free radical polymerization was then heated in the presence of styrene for the formation of the controlled graft copolymer. ¹H‐NMR and IR spectroscopy were used to investigate the structure of the polymers. Copyright © 2004 John Wiley & Sons, Ltd.     

Jacketed polymers: Controlled synthesis of mesogen‐jacketed polymers and block copolymers

The controlled radical polymerization of mesogen‐jacketed liquid crystalline polymers has triggered great interests in synthesis of complex structures as well as well‐defined linear homopolymers with controlled molecular weight and narrow molecular weight distributions. This review highlights the synthetic strategies of controlled radical polymerization of linear homopolymers, star polymers, superbranched polymers, graft polymers, block copolymers, star block copolymers, and so on. The employed living methods include nitroxide‐mediated radical polymerization and atom transfer radical polymerization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 319–330, 2009     

Deactivation reactions in the modeled 2,2,6,6‐tetramethyl‐1‐piperidinyloxy‐mediated free‐radical polymerization of styrene: A comparative study with the 2,2,6,6‐tetramethyl‐1‐piperidinyloxy/acrylonitrile system

The competitiveness of the combination and disproportionation reactions between a 1‐phenylpropyl radical, standing for a growing polystyryl macroradical, and a 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) radical in the nitroxide‐mediated free‐radical polymerization of styrene was quantitatively evaluated by the study of the transition geometry and the potential energy profiles for the competing reactions with the use of quantum‐mechanical calculations at the density functional theory (DFT) UB3‐LYP/6‐311+G(3df, 2p)//(unrestricted) Austin Model 1 level of theory. The search for transition geometries resulted in six and two transition structures for the radical combination and disproportionation reactions, respectively. The former transition structures, mainly differing in the out‐of‐plane angle of the N? O bond in the transition structure TEMPO molecule, were correlated with the activation energy, which was determined to be in the range of 8.4–19.4 kcal mol^?1 from a single‐point calculation at the DFT UB3‐LYP/6‐311+G(3df, 2p)//unrestricted Austin Model 1 level. The calculated activation energy for the disproportionation reaction was less favorable by a value of more than 30 kcal mol^?1 in comparison with that for the combination reaction. The approximate barrier difference for the TEMPO addition and disproportionation reaction was slightly smaller for the styrene polymerization system than for the acrylonitrile polymerization system, thus indicating that a β‐proton abstraction through a TEMPO radical from the polymer backbone could diminish control over the radical polymerization of styrene with the nitroxide even more than in the latter system. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 232–241, 2007     

Controlled polymerization of N‐isopropylacrylamide with an activated methacrylic ester

A copolymer of N‐isopropylacrylamide with the N‐hydroxysuccinimide ester of methacrylic acid has found use in a variety of applications. Here we report our efforts to gain control over the molecular weight distribution of this copolymer with controlled radical polymerization methods, such as atom transfer radical polymerization, reversible addition–fragmentation transfer (RAFT), and nitroxide‐mediated polymerization. We have found that RAFT is capable of affording these copolymers with a polydispersity index of 1.1–1.2. Our results for all three polymerizations are reported. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6340–6345, 2004     

Some novel accelerating agents for nitroxide‐mediated living free‐radical polymerization

Malononitrile (MN), trifluoroacetic acid anhydride, acetylacetone, acetoacetic ester, and diethyl malonate have been identified as novel rate‐accelerating additives for nitroxide‐mediated living free‐radical polymerization. Among these additives, MN has the greatest accelerating effect. Adding MN at an MN/2,2,6,6‐tetramethylpiperidine‐oxyl (TEMPO) molar ratio of 4.0 results in a nearly 20 times higher rate of polymerization of styrene (St), and adding MN at an MN/TEMPO molar ratio of 2.5 results in a nearly 15 times higher rate of copolymerization of St and methyl methacrylate. The polymerization of St proceeds in a living fashion, as indicated by the increase in the molecular weight with time and conversion and the relatively low polydispersity. The polymerization rate of St is so quick that the conversion reaches 70% within 1 h at 125 °C when the molar ratio of MN to TEMPO is 4:1. Moreover, the reaction temperature can be reduced to 110 °C. A possible explanation for this effect is that the formation of hydrogen bonds between the MN and TEMPO moiety weakens the C? ON bond at the end of the polymer chain. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5246–5256, 2005     

Polymerization of styrene with tetramethylthiuram disulfide as an initiator in the presence of 2,2,6,6‐tetramethyl‐1‐piperidinyloxy

The bulk polymerization of styrene was investigated with tetramethylthiuram disulfide (TMTD) as an initiator in the presence of 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) at 123 °C. The polymerization proceeded in a controlled/living way; that is, the polymerization rate was first‐order with respect to the monomer concentration, and the molecular weight increased linearly with conversion. The molecular weights of the polymers obtained were close to the theoretical values, and the molecular weight distributions were relatively low (weight‐average molecular weight/number‐average molecular weight = 1.1–1.3). The rate of polymerization with TMTD as an initiator was faster than that with benzoyl peroxide, and the rate was independent of the initial concentration of TMTD in the presence of TEMPO. The obtained polystyrene was functionalized with ultraviolet‐light‐sensitive ? SC(S)N(CH₃)₂ groups, which was characterized with ¹H NMR spectroscopy. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 543–551, 2005     

Thermal self‐initiation of styrene in the presence of TEMPO radicals: Bulk and miniemulsion

In TEMPO (2,2,6,6,‐tetramethyl‐1‐piperidinyloxy) controlled styrene radical polymerizations, the thermal self‐initiation reaction of styrene monomer is one of the main sources for the deviations from ideal living polymerization. However, it is also important because it continuously generates radicals to compensate for the loss of radicals caused by the termination reactions and thereby maintains a reasonable reaction rate. In this report, different initial TEMPO concentrations were used in styrene miniemulsion polymerizations without any added initiator. The consumption rate of TEMPO or radical generation rate was calculated from the length of the induction period and the increasing total number of polymer chains. It was found that there is little difference between the miniemulsions and the corresponding bulk systems in terms of the length of the induction period, which increases linearly with initial TEMPO concentration. After the induction period, the consumption rate of TEMPO or radical generation rate was reduced to a lower level, and a faster initial polymerization rate was found in the bulk system compared to the corresponding miniemulsion system. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4921–4932, 2004     

Influence of media polarity on the rate of activation of “dormant” chains in nitroxide‐mediated radical polymerization

Rate constants of decomposition of “living” polymeric adducts polystyrene (PS)‐TEMPO, poly‐4‐vinylpyridine‐TEMPO, PS‐SG1, and polyacrylic acid‐SG1 in different solvents were determined by the method of a competitive inhibition by means of electron spin resonance. It was found that with the increase in polarity of solvent, the values of constants become higher for all systems studied. This effect opens a new way of activating chains in nitroxide‐mediated radical polymerization. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Polymerization of o‐quinodimethanes. IV. Radical polymerization of 1‐cyano‐o‐quinodimethane in the presence of TEMPO

The radical polymerization behavior of 1‐cyano‐o‐quinodimethane generated by thermal isomerization of 1‐cyanobenzocyclobutene in the presence of 2,2,6,6‐tetramethylpiperidine‐N‐oxide (TEMPO) and the block copolymerization of the obtained polymer with styrene are described. The radical polymerization of 1‐cyanobenzocyclobutene was carried out in a sealed tube at temperatures ranging from 100 to 150 °C for 24 h in the presence of di‐tert‐butyl peroxide (DTBP) as a radical initiator and two equivalents of TEMPO as a trapping agent of the propagation end radical to obtain hexane‐insoluble polymer above 130 °C. Polymerization at 150 °C with 5 mol % of DTBP in the presence of TEMPO resulted in the polymer having a number‐average molecular weight (M_n ) of 2900 in 63% yield. The structure of the obtained polymer was confirmed as the ring‐opened polymer having a TEMPO unit at the terminal end by ¹H NMR, ¹³C NMR, and IR analyses. Then, block copolymerization of the obtained polymer with styrene was carried out at 140 °C for 72 h to give the corresponding block copolymer in 82% yield, in which the unimodal GPC curve was shifted to a higher molecular weight region. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3434–3439, 2000     

Expanding the supramolecular polymer LEGO system: Nitroxide‐mediated living free‐radical polymerization as a tool for mono‐ and telechelic polystyrenes

Nitroxide‐mediated, controlled living radical polymerization was employed to introduce terpyridine ligands at one or two chain ends of polystyrene. For this purpose, a unimolecular initiator bearing both a terpyridine ligand as well as a mediating nitroxide was synthesized and used for the controlled polymerization of styrene. Moreover, a maleimide‐functionalized terpyridine was prepared in order to synthesize telechelic polymers, utilizing nitroxide substitution reactions. Kinetic studies of the polymerization of styrene were carried out. In all polymerizations, special attention was focused on the retention of end‐group functionality, in light of the effects of autoinitiation and autopolymerization. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4016–4027, 2004     

TEMPO‐mediated dispersion polymerization of styrene in the presence of camphorsulfonic acid

The TEMPO‐mediated polymerization of styrene in the presence of camphorsulfonic acid (CSA) is carried out using controlled radical dispersion polymerization. In the absence of TEMPO and CSA, 92% of conversion was achieved within 3 h of polymerization. When TEMPO is solely used, broadening of particle size with narrow PDI was observed because of the prolonged polymerization time. However, when 1:1 molar ratio of CSA/TEMPO was added, the fairly monodisperse PS microspheres having 5.83 μm average size and 3.42% C_V (coefficient of variation) were successfully achieved because of the narrow molecular weight of intermediate oligomers and shortening of the polymerization time. This result obviously indicates that the addition of CSA in TEMPO‐mediated dispersion polymerization not only shortens the polymerization time but also greatly improves the uniformity of the microspheres. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 62–68, 2006     

Synthesis and characterization of novel photoactive polymer poly(vinyl alcohol)‐graft‐poly(vinyl naphthalene)

A copolymer of poly(vinyl naphthalene) grafted onto poly(vinyl alcohol) has been synthesized with nitroxide‐mediated controlled radical polymerization. By separating the processes of the generation of grafting sites and polymerization, we can avoid the formation of the homopolymer. Because of its architecture, the polymer is soluble in water, despite the high content of hydrophobic groups. The naphthalene chromophores tend to aggregate, forming hydrophobic microdomains in an aqueous solution. Those aggregates exist in a very constrained environment that leads to extraordinarily large redshifts of both the absorption and emission of the polymer. The polymer acts as an efficient photosensitizer in photoinduced electron transfer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2675–2683, 2006     

Synthesis of poly(chloromethylstyrene‐b‐styrene) block copolymers by controlled free‐radical polymerization

The controlled free‐radical polymerization of styrene and chloromethylstyrene monomers in the presence of 2,2,6,6‐tetramethyl‐1‐piperidinyloxyl (TEMPO) has been studied with the aim of synthesizing block copolymers with well‐defined structures. First, TEMPO‐capped poly(chloromethylstyrene) was prepared. Among several initiating systems [self‐initiation, dicumyl peroxide, and 2,2′‐azobis(isobutyronitrile)], the last offered the best compromise for obtaining a good control of the polymerization and a fast polymerization rate. The rate of the TEMPO‐mediated polymerization of chloromethylstyrene was independent of the initial concentration of TEMPO but unexpectedly higher than the rate of the thermal self‐initiated polymerization of chloromethylstyrene. Transfer reactions to the chloromethyl groups were thought to play an important role in the polymerization kinetics and the polydispersity index of the resulting poly(chloromethylstyrene). Second, this first block was used as a macroinitiator in the polymerization of styrene to obtain the desired poly(chloromethylstyrene‐b‐styrene) block copolymer. The kinetic modeling of the block copolymerization was in good agreement with experimental data. The block copolymers obtained in this work exhibited a low polydispersity index (weight‐average molecular weight/number‐average molecular weight < 1.5) and could be chemically modified with nucleophilic substitution reactions on the benzylic site, opening the way to a great variety of architectures. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3845–3854, 2000     

Rate enhancement for nitroxide stable free radical polymerization of n‐butyl acrylate by dodecylbenzenesulfonic acid

Dodecylbenzenesulfonic acid, DDBSA, was chosen as a new rate‐accelerating additive for 2,2,6,6,‐teramethyl‐1‐piperidinyloxy (TEMPO)‐mediated stable free radical polymerization of n‐butyl acrylate (n‐BA) monomers with 2,2′‐azobisisobutyronitrile (AIBN). It was found that the number‐average molecular weight of polymers could reach about ten thousand with a narrow polydispersity index (PDI) of 1.4 in a few minutes, which was faster than other systems reported previously. But, at higher conversion, the molecular weight distribution of polymers became broad, and a bimodal distribution occurred. The macro‐initiators isolated from the former polymers with narrow PDI could be extended by polymerization with monomers by the addition of DDBSA. Furthermore, a proposed kinetic model demonstrated that the decay of the concentration of DDBSA would reduce the living polymer concentration and retard the growth of the polymers, which could be further propagated by the supplement of DDBSA. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 42–49, 2005     

End‐group fidelity in nitroxide‐mediated living free‐radical polymerizations

In this study, new nitroxides based on the 2,2,5‐trimethyl‐4‐phenyl‐3‐azahexane‐3‐oxy skeleton were used to examine chain‐end control during the preparation of polystyrene and poly(t‐butyl acrylate) under living free‐radical conditions. Alkoxyamine‐based initiators with a chromophore attached to either the initiating fragment or the mediating nitroxide fragment were prepared, and the extent of the incorporation of the chromophores at either the initiating end or the propagating chain end was determined. In contrast to 2,2,6,6‐tetramethyl piperidinoxy (TEMPO), the incorporation of the initiating and terminating fragment into the polymer chain was extremely high. For both poly(t‐butyl acrylate) and polystyrene with molecular weights less than or equal to 70,000, incorporations at the initiating end of greater than 97% were observed. At the terminating chain end, incorporations of greater than 95% were obtained for molecular weights less than or equal to 50,000. The level of incorporation tended to decrease slightly at higher molecular weights because of the loss of the alkoxyamine propagating unit, which had important consequences for block copolymer formation. These results clearly show that these new α‐H nitroxides could control the polymerization of vinyl monomers such as styrene and t‐butyl acrylate to an extremely high degree, comparable to anionic and atom transfer radical polymerization procedures. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4749–4763, 2000     

Physically controlled,free‐radical polymerization of vaporized fluoromonomer on solid surfaces

Gas/vapor‐deposition polymerization (GDP) of vinyl monomer is expected to exhibit a unique polymerization behavior different from its polymerization in the liquid phase. Free‐radical GDP of 2,2,3,3,3‐pentafluoropropyl methacrylate (FMA) was carried out with a conventional free‐radical initiator (azobisisobutyronitrile) on substrate surfaces. A linear relationship between the number‐average molecular weight and polymer yield was observed, and the consecutive copolymerization of methyl methacrylate (MMA) and FMA led to the formation of block copolymer P(MMA‐block‐FMA). These results suggested that the GDP process on substrate surfaces has a living nature. During the process, the active species at growing chain ends may be immobilized on the deposit surface and restricted from the chain‐transfer reactions, resulting in a continuation of the propagation reaction. The GDP on substrate surfaces is therefore a physically controlled polymerization process. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2621–2630, 2004     

Synthesis of graft copolymers with “V‐shaped” and “Y‐shaped” side chains via controlled radical and anionic polymerizations

A combination of nitroxide‐mediated radical polymerization and living anionic polymerization was used to synthesize a series of well‐defined graft (co)polymers with “V‐shaped” and “Y‐shaped” branches. The polymer main chain is a copolymer of styrene and p‐chloromethylstyrene (PS‐co‐PCMS) prepared via nitroxide‐mediated radical polymerization. The V‐shaped branches were prepared through coupling reaction of polystyrene macromonomer, carrying 1,1‐diphenylethylene terminus, with polystyryllithium or polyisoprenyllithium. The Y‐shaped branches were prepared throughfurther polymerization initiated by the V‐shaped anions. The obtained branches, carrying a living anion at the middle (V‐shaped) or at the end of the third segment (Y‐shaped), were coupled in situ with pendent benzyl chloride of PS‐co‐PCMS to form the target graft (co)polymers. The purified graft (co)polymers were analyzed by size exclusion chromatography equipped with a multiangle light scattering detector and a viscometer. The result shows that the viscosities and radii of gyration of the branched polymers are remarkably smaller than those of linear polystyrene. In addition, V‐shaped product adopts a more compact conformation in dilute solution than the Y‐shaped analogy. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4013–4025, 2007     

Low temperature TEMPO‐mediated styrene polymerization in miniemulsion

The 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO)‐mediated stable free radical polymerization of styrene in miniemulsion at 100 °C is demonstrated. Although this temperature is 20–35 °C lower than typical temperatures used for TEMPO‐mediated polymerizations, reasonable reaction rates were achieved by the addition of ascorbic acid or a free radical initiator. More importantly, the living character of the chains was preserved; the degree of polymer “livingness” was comparable to polymerizations conducted at 135 °C. Polydispersities were broader than that observed in well‐controlled systems, ranging from ～1.4–1.6, and consistent with expectations for systems having a low activation rate. The results are significant for two reasons. They will facilitate TEMPO‐mediated minemulsion polymerizations in nonpressurized (or minimally pressurized) reactors, and they reveal the potential to expand the traditional temperature range of TEMPO and possibly other nitroxides in bulk, solution, and miniemulsion. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 232–242, 2006     

Photoresponsive poly(methyl methacrylate)2–(polystyrene)2 miktoarm star copolymer containing an azobenzene moiety at the core

We prepared a novel miktoarm star copolymer with an azobenzene unit at the core via combination of atom transfer radical polymerization (ATRP) and nitroxide‐mediated free radical polymerization (NMP) routes. For this purpose, first, mikto‐functional initiator, 3 , with tertiary bromide (for ATRP) and 2,2,6,6‐tetramethylpiperidin‐1‐yloxy (TEMPO) (for NMP) functionalities and an azobenzene moiety at the core was synthesized. The initiator 3 thus obtained was used in the subsequent living radical polymerization routes such as ATRP of MMA and NMP of St, respectively, to give A₂B₂ type miktoarm star copolymer, (PMMA)₂‐(PSt)₂ with an azobenzene unit at the core with controlled molecular weight and low polydispersity (M_w/M_n < 1.15). The photoresponsive properties of 3 and (PMMA)₂‐(PSt)₂ miktoarm star copolymer were investigated. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1396–1403, 2006