Growing Chain End Research Articles

Photoredox Catalysis in Photocontrolled Cationic Polymerizations of Vinyl Ethers.

Advances in photocontrolled polymerizations have expanded the scope of polymer architectures and structures that can be synthesized for various applications. The majority of these polymerizations have been developed for radical processes, which limits the diversity of monomers that can be used in macromolecular design. More recent developments of photocontrolled cationic polymerizations have taken a step toward addressing this limitation and have expanded the palette of monomers that can be used in stimuli-regulated polymerizations, enabling the synthesis of previously inaccessible polymeric structures. This Account will detail our group's studies on cationic polymerization processes where chain growth is regulated by light and highlight how these methods can be combined with other stimuli-controlled polymerizations to precisely dictate macromolecular structure.Photoinitiated cationic polymerizations are well-studied and important processes that have control over initiation. However, we wanted to develop systems where we had spatiotemporal control over both polymer initiation and chain growth. This additional command over the reaction provides the ability to manipulate the growing polymer with an external stimulus during a polymerization, which can be used to control structure. To achieve this goal, we set out to develop a method to photoreversibly generate a cation at a growing chain end that could participate in a controlled polymerization process. We took inspiration from previous work on cationic degenerate chain transfer polymerizations of vinyl ethers that used thiocarbonylthio chain transfer agents. These polymerizations were initiated by a strong acid and gave well-defined poly(vinyl ether)s. We posited that we could remove the acid initiator in these systems and reversibly oxidize the thiocarbonylthio chain ends in these reactions with a photocatalyst to give a photocontrolled cationic polymerization of vinyl ethers. This Account will focus on our journey to discover cationic photocontrolled polymerizations. We will summarize our initial developments and detail our mechanistic understanding of these reactions using both organic and inorganic based photocatalysts, and we will outline more recent efforts to expand cationic degenerate chain transfer polymerizations to other thioacetal initiators. Finally, we will detail how these photocontrolled cationic polymerizations can be used to switch monomer selectivity in situ using light to control polymer structure. At the end of the Account, we will discuss our vision for future potential applications of these photocontrolled cationic polymerizations in the synthesis of novel block copolymers and next generation cross-linked networks.

Direct synthesis of highly reactive polyisobutylenes via cationic polymerization of isobutylene co‐initiated with TiCl4 in nonpolar hydrocarbon media

ABSTRACTLiving cationic polymerization of isobutylene (IB) with 1‐chlorine‐2,4,4‐trimethyl pentane (TMPCl)/TiCl4/isopropanol (iPrOH) or isoamylol (iAmOH) has been achieved in the presence of 2,6‐di‐tert‐butylpyridine (DtBP) at −80°C. Polyisobutylenes with nearly theoretical Mn based on TMPCl molecules and more than 90% of tert‐chlorine‐end groups could be obtained at high [TMPCl]. The β‐proton elimination from CH3 in growing chain ends increased with increasing polymerization temperature and decreasing solvent polarity. A chain‐transfer‐dominated cationic polymerization process with H2O/TiCl4/iAmOH could be achieved in n‐hexane at −30°C. The monomer conversion and content of exo‐olefin end groups increased while molecular weight decreased with increasing [iAmOH]. To the best of our knowledge, this is the first example to achieve the direct synthesis of highly reactive polyisobutylene with low Mn of 1200∼1600, carrying more than 80% of exo‐olefin terminals by a single‐step process via cationic polymerization co‐initiated by TiCl4 in nonpolar hydrocarbon. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42232.

Facile Preparation of Regioregular Poly(3-hexylthiophene) and Its Block Copolymers with π-Allylnickel Complex as External Initiator

Simply prepared π-allylnickel complexes were used as external initiators for promoting the polymerization of 2-bromo-3-hexyl-5-chloromagnesiothiophene in a living/controlled chain growth manner to afford regioregular poly(3-hexylthiophene) with an allyl terminus. The nickel species on the other chain end can initiate the block copolymerization of hexadecyloxylallene and 2-bromo-3-hexyl-5-chloromagnesiothiophene to give a well-defined triblock copolymer containing poly(3-hexylthiophene) and poly(hexadecyloxylallene) segments in one pot via mechanically distinct, sequential living polymerization. Furthermore, such π-allylnickel(II) complexes can also catalyze the polymerization of a range of vinyl monomers, including styrene, 1-methoxy-4-vinylbenzene, and 1-chloro-4-vinylbenzene as well as tert-butyl acrylate, in living/controlled fashion. The active nickel unit at the growing chain end of these vinyl polymers can also initiate the block copolymerization of 2-bromo-3-hexyl-5-chloromagnesiothiophene to give ...

Synthesis of highly reactive polyisobutylenes with exo-olefin terminals via controlled cationic polymerization with H2O/FeCl3/iPrOH initiating system in nonpolar hydrocarbon media

Cationic polymerizations of isobutylene (IB) with H2O/FeCl3/isopropanol (iPrOH) initiating system were conducted in nonpolar hydrocarbon media, such as n-hexane or mixed C4 fractions at −40 to 20 °C. This cationic polymerization is a chain-transfer dominated process via highly selective β-proton elimination from CH3 in the growing chain ends, leading to formation of highly reactive polyisobutylenes (HRPIBs) with large contents (> 90 mol %) of exo-olefin end groups (structure A). The content of structure A remained nearly constant at about 97 mol % during polymerization and isomerization via carbenium ion rearrangement could be suppressed in nonpolar media. First-order kinetics with respect to monomer concentration was measured for selective cationic polymerization of IB in the mixed C4 fraction feed at −30 °C and the apparent rate constant for propagation was 0.028 min−1. High polymerization temperature (Tp) or [FeCl3] accelerate β-proton elimination or isomerizations and simultaneously decrease selectivity of β-proton abstraction from CH3. Molecular weight decreased and molecular weight distribution (MWD) became narrow with increasing Tp or [FeCl3]. To the best of our knowledge, this is the first example to achieve high quality HRPIBs with near 100% of exo-olefin terminals and relatively narrow MWD (Mw/Mn = 1.8) by a single-step process in nonpolar hydrocarbon media. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4200–4212

Synthesis of high molecular weight polyisobutylene via cationic polymerization at elevated temperatures

A novel simple but effective initiating system of H2O/AlCl3/veratrole (VE) has been developed to synthesize high molecular weight polyisobutylene (PIB) at elevated temperatures via cationic polymerization of isobutylene (IB) in solvent mixture of hexane/methylene dichloride (n-Hex/CH2Cl2 = 2/1, V/V). VE played very important roles in decreasing cationicity of the growing chain ends, suppressing side reactions of chain transfer and termination during polymerization, leading to production of high molecular weight PIBs. PIBs with high yields, having very high weight-average molecular weight (M w) of 1117000 and 370000 g/mol could be synthesized with H2O/AlCl3/VE initiating system at VE concentration of 5.4 mmol/L at −80 and −60 °C respectively. Molecular weight of PIB increased remarkably with increasing VE concentration. The reaction order with respect to VE concentration was determined to be −3.52 via FTIR spectroscopy in combination with a diamond tipped attenuated total reflectance (ATR) immersion probe. The negative reaction order of VE was consistent with its retarding effect on IB polymerization by interacting with the propagating species. Molecular weight of PIB decreased with increasing polymerization temperature. The activation energy for polymerization degree (E DP) could be determined to be around −23 kJ/mol when VE concentration was 5.4 mmol/L or 6.4 mmol/L.

Polyisobutylene with High exo-Olefin Content via β-H Elimination in the Cationic Polymerization of Isobutylene with H2O/FeCl3/Dialkyl Ether Initiating System

The cationic polymerizations of isobutylene (IB) with H2O/FeCl3/dialkyl ether initiating system were conducted in dichloromethane (CH2Cl2) at temperatures from −20 to +20 °C, in which the dialkyl ether includes diethyl ether (Et2O), dibutyl ether (Bu2O) or diisopropyl ether (iPr2O). The highly reactive polyisobutylenes (HRPIBs) with high content of exo-olefin end groups (−CH2−C(CH3)═CH2) 82−91 mol % and acceptable monomodal molecular weight distribution (Mw/Mn = 1.7−2.3) could be successfully synthesized at low concentration of FeCl3 at 0.005 mol·L−1 at 0 or even 10 °C. These results are comparable to those of commercial HRPIBs produced industrially at far below 0 °C. The directly rapid β-proton elimination from −CH3 of the growing chain ends and chain transfer reaction to monomer were dependent on concentration of iPr2O·FeCl3 complex (1:1), concentration of free iPr2O ([free iPr2O] = [iPr2O] − [FeCl3]) if (iPr2O·FeCl3> 1) and polymerization time. The much higher concentration of PIB chains formed in the ...

A cost-effective process for highly reactive polyisobutylenes via cationic polymerization coinitiated by AlCl 3

The initiating system consisting of AlCl 3 with dialkyl ether such as di- n-butyl ether or diisopropyl ether has been successfully developed for providing a cost-effective process of synthesis of highly reactive polyisobutylenes (HRPIBs) with large proportion of exo-olefin end groups up to 93 mol% at temperatures ranging from −20 to +20 °C. The above dialkyl ethers played very important roles in promoting the directly rapid β-proton elimination from –C H 3 of the growing chain ends to create exo-olefin end groups and decreasing or even suppressing the carbenium ion rearrangements to form the double bond isomers. Very importantly, the highly reactive PIBs with 80–92 mol% of exo-olefin end groups, having low M ns of 1300–2300 g mol −1 and monomodal molecular weight distribution ( M w/ M n = 1.7–2.0) could be achieved at 0–20 °C. These results are comparable to those of commercial HRPIBs produced industrially by the best BF 3-based initiating system at far below 0 °C.

Stability of growing polymer chain ends for nitroxide-mediated photo-living radical polymerization

The stability of the growing polymer chain ends for the nitroxide-mediated photo-living radical polymerization of methyl methacrylate (MMA) was explored through block copolymerization with isopropyl methacrylate (iPMA). The block copolymerization of iPMA was performed with the PMMA prepolymer prepared by the photopolymerization of MMA using the racemic-(2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) (r-AMDV) as the initiator, 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator, and (4-tert-butylphenyl)-diphenylsulfonium triflate (tBuS) as the photo-acid generator. When the polymerization of MMA was carried out for 6.5 h, the resulting block copolymer showed a bimodal GPC due to the deactivation of part of the growing chain ends of the prepolymer. On the other hand, when the MMA polymerization was shortened to 5 h, the unimodal block copolymer was obtained without deactivation of the prepolymer.

Preparation of dendron-like polystyrenes from atom transfer radical polymerization (ATRP) and direct chain-end functionalization

Dendron-like branched polystyrenes are prepared using atom transfer radical polymerization (ATRP) and chain-end functionalization. The growing chain ends of polystyrene from ATRP are end-capped with 1,4-benzoquinone to incorporate branching points and ATRP initiating groups to the chain ends. Sequential ATRP of styrene initiated from the chain ends builds up the polystyrene arms. The number-averaged molecular weights of the first, second, and third generations of dendron-like polystyrene polymers calculated from their respective 1H NMR spectra are 5960, 6850, and 9150 g/mol, respectively. The NMR-data are consistent with the molecular weights determined by GPC. An approach to prepare dendron-like polymers through ATRP and chain-end functionalization is developed.

Controlling grafting density and side chain length in poly(n‐butyl acrylate) by ATRP copolymerization of macromonomers

AbstractAtom transfer radical polymerization (ATRP) was used for the preparation and subsequent copolymerization of two acryloyl‐terminated poly(n‐butyl acrylate) macromonomers with different degrees of polymerization (DPnBA = 25 and 42). Homopolymerization of the higher molecular weight macromonomer (MM1; PnBA42‐A, Mn = 5600, DPMM = 42, Mw/Mn = 1.18) resulted in preparation of a densely grafted polymer with a narrow molecular weight distribution (Mw/Mn = 1.14), but with the limited degree of polymerization DP = 12. The ultimate degree of homopolymerization for the lower molecular weight macromonomer (MM2; PnBA25‐A, Mn = 3400, DPMM = 25, Mw/Mn = 1.20) was higher, and DP increased from 12 to 22. The limited DP could be because of progressively increasing steric congestion for macromonomers in approaching the growing chain ends of densely grafted polymers. When MMs were copolymerized with nBA, the reactivity of MM was nearly the same as that of nBA monomer irrespective of the differences in the degree of polymerization of the MMs and the initial molar ratio of nBA to MM. Well‐defined graft polymers with different lengths of backbone and side chains, and different graft density were successfully prepared by “grafting through” ATRP. Tadpole‐shaped and dumbbell‐shaped graft polymers were also synthesized by ATRP. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5454–5467, 2006

Kinetic features of the cobalt dihalide/methylaluminoxane catalytic system in 1,3-butadiene polymerization

The kinetic features of polymerization with an active site comprising cobalt dihalides (CoX2, where X = Cl, Br, I) activated by methylaluminoxane (MAO)|were investigated in 1,3-butadiene polymerization. The catalytic system exhibited the characteristic features of living polymerization. The initiation (ki)|and propagation (kp)|rate coefficients were estimated using the kinetic model for slow initiation previously reported by Shionoet al.1 The energy of activation for the propagation reaction was calculated to be 27–30 kJmol−1. The marked changes in reaction rate observed with different halides could be adequately described in terms of variations in the initiation process, with the same Arrhenius curve fitting propagation rate coefficients estimated from all three halides, suggesting that the halide does not participate in the growing chain end.

Propylene Polymerization Using ansa‐Zirconocenes with Water/Pentafluorophenol‐Modified Methylaluminoxane

AbstractSummary: We have confirmed that water‐ and pentafluorophenol‐modified isobutylmethylaluminoxane (W/PM‐MMAO) enhances the productivity and stereoregularity of the yielded polymer and increases its molecular weight when propylene is polymerized using C2‐symmetric ansa‐zirconocenes. 13C NMR analysis shows that when W/PM‐MMAO is used as a cocatalyst with C2‐symmetric ansa‐zirconocenes, although the pentad peaks of the stereoerrors derived from enantiofacial misinsertion have still existed; those derived from the epimerization of the growing chain end that occurred with β‐hydride elimination disappear. Therefore, we propose that W/PM‐MMAO interacts more strongly to the cationic metal center of metallocene than does MMAO owing to steric and electronic effect, and W/PM‐MMAO suppresses β‐hydride elimination, resulting in the production of polypropylene with higher molecular weight and isotacticity. To the contrary, there was no significant difference in the structure of the yielded polymer when W/PM‐MMAO was used with Cs‐symmetric ansa‐zirconocenes. In the case of Cs‐symmetric ansa‐zirconocene, the stereoerror is mostly the result of monomer insertion error, and small amount of stereoerror caused by “skipped” insertion, not from the epimerization of the polymer chain end that occurs after β‐hydride elimination; therefore, there might not be any difference of the microstructure of yielded polymers between MMAO and W/PM‐MMAO. It can be concluded that the both cases of C2‐ and Cs‐symmetric ansa‐zirconocenes are used as a catalyst, and water and phenol modification of MMAO does not affect the monomer misinsertion which is one of the mechanisms of the stereoerrors.13C NMR spectra of isotactic PP. (a) Sample 1. (b) Sample 2.magnified image13C NMR spectra of isotactic PP. (a) Sample 1. (b) Sample 2.

Photoiniferter-Based Thermoresponsive Graft Architecture with Albumin Covalently Fixed at Growing Graft Chain End

The aim of this study was to develop a novel surface graft architecture in which albumin is covalently fixed at the growing chain end of the hydrophilic polymers: poly(N, N-dimethylacylamide), PDMAM, and poly(N-isopropylacrylamide), PNIPAM. Photoiniferter-based surface-grafted polymers were prepared using either an albuminated iniferter or a nonalbuminated iniferter, both of which were derivatized on glass surfaces, and ultraviolet (UV)-light-irradiated in the presence of a DMAM or NIPAM monomer. Surface chemical composition analysis by X-ray photoelectron spectroscopy, contact angle measurement, immunostaining using fluorescence labeled antibody and the measurement of graft thickness, as determined from force-distance curves obtained in water at 25 degrees C and 37 degrees C by atomic force microscopy, evidenced that the thickness of graft layer increased with photoirradiation time and albumin molecules exist at growing chain ends. For PNIPAM-grafted surfaces, the interconversion between swollen and collapsed graft chains was observed below and above the lower critical solution temperature of PNIPAM. The potential application of a thermoresponsive graft with albumin covalently fixed at its growing chain end was discussed in terms of "active" nonfouling surface design based on the temperature-dependent switching of phase transition.

Force Measurement for Antigen−Antibody Interaction by Atomic Force Microscopy Using a Photograft-Polymer Spacer

To determine the intermolecular force on protein-protein interaction (PPI) by atomic force microscopy (AFM), a photograft-polymer spacer for protein molecules on both surfaces of the substrate and AFM probe tip was developed, and its effectiveness was assessed in a PPI model of a pair of human serum albumin (HSA) and its monoclonal antibody (anti-HSA). A carboxylated photoiniferter, N-(dithiocarboxy)sarcosine, was derivatized on both surfaces of the glass substrate and AFM probe tip, and subsequently water-soluble nonionic vinyl monomers, N,N-dimethylacrylamide (DMAAm), were graft-polymerized on them upon ultraviolet light irradiation. DMAAm-photograft-polymerized spacers with carboxyl groups at the growing chain end but with different chain lengths on both surfaces were prepared. The proteins were covalently bound to the carboxyl terminus of the photograft-polymer chain using a water-soluble condensation agent. The effects of the graft-spacer length on the profile of the force-distance curves and on the unbinding characteristics (unbinding force and unbinding distance) were examined in comparison with those in the case of the commercially available poly(ethylene glycol) (PEG) spacer. The frequency of the nonspecific adhesion force profile was markedly decreased with the use of the photograft spacers. Among the force curves detected, a high frequency of single-peak curves indicating the unbinding process of a single pair of proteins and a very low frequency of multiple-peak profiles were observed for the photograft spacers, regardless of the graft chain length, whereas a high frequency of no-force peaks was noted. These observations were in marked contrast with those for the PEG spacer. The force peak values determined ranged from 88 to 94 pN, irrespective of the type of spacer, while the standard deviation of force distribution observed for the photograft spacer was lower than that for the PEG spacer, indicating that the photograft spacers provide a higher accuracy of force determination.

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

AbstractGas/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

Quasi-living surface graft polymerization with phosphorylcholine group(s) at the terminal end

A surface graft polymer with one or two phosphorylcholine (PC) polarheads at the terminus of the growing chain end was prepared by sequential reactions on a glass substrate. The dithiocarbamate group covalently bound to glass surfaces was derivatized with one or two PC groups and then irradiated with ultraviolet light in the presence of N, N-dimethylacrylamide (DMAAm). X-ray photoelectron spectroscopy, wettability measurements and dye staining experiment for the PC group showed that the resultant graft copolymers were produced via iniferter-based quasi-living radical polymerization, in which the polyDMAAm graft chain contains one or two PC groups at the terminal end of the graft chain. These polymer surface grafts may help provide biocompatibility.

Competitive complexation in the cationic polymerization of isobutylene in a nonpolar medium

AbstractThe cationic polymerization of isobutylene coinitiated by Al(i‐Bu)Cl2(Al) was carried out in mixed butane–butene fractions at −50 °C. The expected polymerization processes induced by the trace of moisture with Al system in the presence of a small amount of external electron‐donor modifiers, such as methyl acrylate (MA) and dimethyl sulfoxide (DMSO), were obtained. The experimental results showed that these polymerizations produced polymers with relatively high number‐average molecular weights and narrow molecular weight distributions (1.5–2.2). That the gel permeation chromatography traces of the polymers depended on the types and concentrations of external donors suggested that there existed competitive complexation reactions of various electron donors (H2O, MA, and DMSO) with the Al Lewis acid. The roles of external electron donors were to take part in the initiation step by competitive complexation and to modify the reactivity of the growing chain ends in the propagation step by mediation and/or solvation, which impaired the high reactivity of the original growing chain ends. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2209–2214, 2002

Bulk Polymerization of ε-Caprolactone in an Internal Mixer and in a Twin Screw Extruder

Abstract Bulk polymerization of ε-caprolactone has been carried out both in a laboratory internal mixer (Brabender Plasticorder) and in a modular intermeshing co-rotating twin screw extruder. Various initiators, such as titanium n-butoxide, aluminum triisopropoxide, and sodium hydride were used to polymerize ε-caprolactone in an internal mixer. The continuous polymerization of ε-caprolactone was performed using aluminum triisopropoxide under a range of processing conditions, including barrel temperature profiles, throughput, and screw speed. This polymerization was investigated for various ratios of monomer to initiator. GPC analysis demonstrated that significant quantities of oligomers were produced together with high molecular weight polymer at different reaction temperatures. H NMR detected the hydroxylation of growing chain end group for bulk polymerization at high barrel temperature. The molecular weight was reduced along the screw axis once the maximum molecular weight was obtained after the first kneading disk block. Increasing screw speed caused further reductions in molecular weight.

TiCl4 Reaction Order in Living Isobutylene Polymerization at Low [TiCl4]:[Chain End] Ratios

Isobutylene (IB) polymerization kinetics at −80 °C were monitored in real time using mid-infrared ATR-FTIR spectroscopy, with diamond-composite insertion probe and light conduit technology. Monomer concentration as a function of time was obtained by monitoring the absorbance at 887 cm-1 associated with the CH2 wag of IB. Polymerizations were initiated using 5-tert-butyl-1,3-bis(2-chloro-2-propyl)benzene (t-Bu-m-DCC) or 2-chloro-2,4,4-trimethylpentane (TMPCl) in conjunction with TiCl4 co-initiator, in hexane/methyl chloride or methylcyclohexane/methyl chloride (60:40 v/v) cosolvents. Either 2,4-dimethylpyridine (DMP) or 2,6-di-tert-butylpyridine (DTBP) was used as an electron donor (ED). Reaction conditions were [ED] = 2.00 × 10-3 M, [IB]0 = 1.0 M, and [TMPCl] (or 2[t-Bu-m-DCC]) = 2.08 × 10-2 M. Co-initiator concentrations were designed to be less than or equal to the growing chain end concentration and ranged from [TiCl4] = 7.20 × 10-3 to 2.28 × 10-2 M. Steady-state polymerization kinetics were found to b...

Photoinduced living cationic polymerization of tetrahydrofuran. II: Synthesis of four-armed star-shaped poly(tetrahydrofuran)

A four-armed star-shaped poly(tetrahydrofuran) (THF) having a pentaerythritol unit at the center of the molecule was synthesized from photoinduced cationic copolymerization of THF and pentaeryithritol tetrakis(3,4-epoxybutanoate) (PETE) in the presence of diphenyliodonium hexafluorophosphate. It was found that the molecular weight of the resulting polymer increases with increasing percent conversion and the rate of cationic polymerization of THF is remarkably increased by the addition of PETE when the concentration of PETE is kept much lower than that of THF. A similar enhancement effect was also observed in the photoinduced cationic polymerization of THF in the presence of epichlorohydrin (ECH). The enhancement effect of ECH and PETE was ascribed to the increased concentration of the cationic propagating species due to a rapid protonation process. The living nature was explained in terms of the stabilization of the cationic growing chain end by ion pair formation. Accordingly, the arm length of the polymer was determined by the molar ratio of THF and PETE. However, the polymer of well-defined structure was only obtainable in the early stage of the reaction because chain transfer to the polymer process plays an important role when the concentration of the polymer becomes high. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2637–2644, 1999