Photodissociation Rate Constants of New Light Sensitive Alkoxyamines

The photochemical properties of two new photocleavable alkoxyamines bearing a benzophenone‐derived chromophore were studied by electron spin resonance (ESR). The C O bond cleavage has been demonstrated and the photolysis rate constants (k_d) determined over a large range of light intensity through the monitoring of the nitroxide concentration in aerated conditions. The obtained kinetic data highlight for the first time the linear dependence of k_d on the light intensity for alkoxyamines: this should be a driving factor for nitroxide mediated radical photopolymerization (NMP²).

     

Improving the control of styrene polymerization at 60 °C using a dialkylated α‐hydrogenated nitroxide

A new dialkylated α‐hydrogenated linear nitroxide and the corresponding 1‐phenylethyl alkoxyamine were synthesized in two and three steps, respectively. The alkoxyamine was involved in the polymerization of styrene at 60 °C, and the in situ concentration of nitroxide was monitored by electron spin resonance spectroscopy. The enhanced characteristics of these new alkylated alkoxyamine and nitroxide (k = 1.5 × 10^?4 s^?1 and k = 5.7 × 10⁴ L mol^?1 s^?1) yielded a monomer consumption one order of magnitude higher than styrene thermal polymerization. This resulted in well‐defined polystyrenes up to 70,000 g mol^?1 and the observation of a control occurring through the establishment of the radical persistent effect, that is, ln([M]₀/[M]) = t^2/3. Experimentally determined kinetic constants were involved in PREDICI modelings to investigate the influence of temperature and initial alkoxyamine concentration on the kinetics as well as on the livingness and the controlled character of the polymerization. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Nitroxide‐mediated living free‐radical polymerization of styrene: A systematic study of the variation of the alkoxyamine concentration

The effect of the variation of the alkoxyamine concentration on the conversion and polydispersity of the nitroxide‐mediated living free‐radical polymerization of styrene is discussed. Four different alkoxyamines ( 1 – 4 ) have been used for these studies. For an alkoxyamine with a small equilibrium rate constant (K), such as styryl–TEMPO 2 , the conversion is governed by the autopolymerization of styrene. For efficient alkoxyamines 1 , 3 , and 4 , the conversion at high alkoxyamine concentrations is higher than the conversion obtained by autopolymerization. At high alkoxyamine concentrations, the conversions vary to a small extent for all the alkoxyamines studied. As long as the conversion remains high, the polydispersity index is small. In addition, simulations of polymerizations with a program for modeling nonlinear dynamics are discussed. Polymerizations with efficient alkoxyamines at high alkoxyamine concentrations are well described by the kinetic scheme applied. K for alkoxyamines 1 and 4 has been estimated with the simulations. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3342–3351, 2004     

Re‐formation Reaction of Cyclic Nitroxide‐Based Alkoxyamines: Steric and Polar/Stabilization Effects

In nitroxide‐mediated radical polymerization, the polymerization times decrease with the increasing re‐formation rate constant of the C? ON bond (→ alkoxyamine) between the growing polymer chain and the nitroxide radical. The factors influencing the re‐formation rate constant are of considerable interest, but up to now, the polar/stabilization effects have not been addressed thoroughly. The combination of new data with previously reported data now showed that the re‐formation rate constant k_c increases with the increasing polar character of the substituents attached to the nitroxide moiety. The polar/stabilization effects are weaker for the re‐formation than for the homolysis of the C? ON bond, and may be mainly attributed to the relocation of the odd electron onto the O‐atom of the N? O moiety, i.e., the stabilization of the nitroxide moiety. Hence, it is possible to predict the values of k_c by combining both the polar/stabilization (σ_I) and steric effects (E ), i.e., log(k_c/M ^?1 s^?1) = 9.86 + 0.57 ? σ_I + 0.40 ? E_s.     

Synthesis of polymer grafted carbon nanotubes by nitroxide mediated radical polymerization in the presence of spin-labeled carbon nanotubes

Multiwalled carbon nanotubes (MWCNT) were grafted with polystyrene by in situ nitroxide mediated radical polymerization in the presence of TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl) functionalized MWCNT, which was synthesized by the reaction between 4-hydroxyl-TEMPO (HO-TEMPO) and carbonyl chloride groups on the MWCNT. Although the controllability of the polymerization was not high, highly soluble grafted MWCNTs were indeed obtained, indicating that the graft polymerization was efficient. The resulting polystyrene grafted MWCNTs were easily defunctionalized at room temperature using 3-chloroperoxybenzioc acid. TEM, SEM, and TGA were employed to determine the structure, morphology, and the grafting quantities of the resulting products.     

Controlled Synthesis and Degradation of Poly(N‐(isobutoxymethyl) acrylamide) Homopolymers and Block Copolymers

The homopolymerization of the water‐insoluble N‐(isobutoxymethyl)acrylamide (IBMA) is investigated for the first time by nitroxide‐mediated polymerization. The homopolymerization is characterized by a linear increase in number average molecular weight (M_n) versus conversion (X) to X > 0.80 while maintaining dispersities of M_w/M_n < 1.30. A strong Arrhenius relationship correlates the apparent rate constants and the homopolymerization temperatures between 105 and 120 °C. All poly(IBMA) homopolymers are then successfully chain‐extended with styrene (S) to form well‐defined block copolymers of poly(IBMA)‐b‐poly(S) suggesting a high degree of livingness of the poly(IBMA) macroinitiators. Thermogravimetric analysis and differential scanning calorimetry are both used to characterize the thermal properties of the homopolymers and block copolymers and identify possible unique degradation of the poly(IBMA) block through imide formation at elevated temperatures.

     

Genesis of the CSIRO polymer group and the discovery and significance of nitroxide‐mediated living radical polymerization

The background to the formation of the Commonwealth Scientific and Industrial Research Organization (CSIRO) polymer group is discussed. In particular, the challenges of working with high‐conversion polymerization, as found in commercial systems, and the need to explain variations in polymer properties led to important advances in the theory of radical polymerization and control over both the initiation and termination steps. Studies on the fate of the macromonomer, formed in termination by disproportionation, led to an early form of addition/fragmentation now known as reversible addition–fragmentation chain transfer, whereas detailed studies on initiation pathways using nitroxide trapping led to nitroxide‐mediated living radical polymerization. These studies contributed to the renaissance in free‐radical polymerization studies. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5748–5764, 2005     

Synthesis of a series of SG1 2‐[N‐tert‐butyl‐N‐(1‐diethoxyphosphoryl‐2,2‐dimethylpropyl)aminoxyl] based alkoxyamines,SG1‐CH(Me)CO2R,and measurement of the homolysis rate constants of the C?ON bond

During nitroxide‐mediated polymerization, the polymerization time decreases with an increasing rate constant of the cleavage of the NO? C bond of dormant alkoxyamines. Thus, knowledge of the factors influencing this cleavage is of considerable interest. We have prepared a series of SG1 2‐[N‐tert‐butyl‐N‐(1‐diethoxyphosphoryl‐2,2‐dimethylpropyl)aminoxyl] based alkoxyamines [SG1‐CH(Me)CO₂R] with various R groups (alkyl or aryl) and measured the homolysis rate constants (k_d). k_d decreases with the bulkiness and increases with the polarity of the R group. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3504–3515, 2004     

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     

Controlled Positioning of Activated Ester Moieties on Well‐Defined Linear Polymer Chains

The successful sequence‐controlled installation of an activated ester using a newly designed monomer pentafluorophenyl 4‐maleimidobenzoate is demonstrated. Pentafluorophenyl 4‐maleimidobenzoate is kinetically installed at different stages of a nitroxide‐mediated polymerization, namely, near the α‐chain end and in the middle of a PS chain. In addition, successful installation of apolar and polar functional groups is achieved via post‐polymerization functionalization, which demonstrated the versatility of the synthesis of a universal precursor for locally functionalized polymers.     

Nitroxide‐Mediated Polymerization of Styrene in a Continuous Tubular Reactor

Summary: Nitroxide‐mediated polymerization of styrene in a continuous tubular reactor has been demonstrated for the first time. The polymerization kinetics in the tubular reactor are similar to those in a batch reactor. The number average molecular weight increases linearly with conversion, and chain extension experiments were successful, indicating that the living nature of the polymerization is maintained in the tubular reactor.

Evolution of molecular weight as measured by GPC for chain‐extended latex in continuous tubular reactor.     

Chemically Triggered C–ON Bond Homolysis in Alkoxyamines. Part 2: DFT Investigation and Application of the pH Effect on NMP

In recent work, a 15‐fold increase in the C–ON bond homolysis rate constant k_d of 4‐pyridylethyl‐SG1‐based alkoxyamine was observed upon protonation of the pyridyl moiety in organic solvent. In this report, the pH dependence of k_d (pK_a = 4.7) is investigated in D₂O/CD₃OD (v/v 1:1). A 64‐fold increase in k_d is observed at acidic pH. Calculations show that the increase in k_d upon protonation is due to both an increase in the stabilization of the protonated 4‐pyridylethyl radical and an increase of the destabilization of the starting materials through an increase in the polarity of the alkyl fragment. This new alkoxyamine is applied to NMP of styrene and sodium styrene sulfonate.     

Well‐defined polyoxazoline‐based alkoxyamines as efficient macroinitiators in nitroxide‐mediated radical polymerization of styrene

The synthesis of two well‐defined 2,2,5‐trimethyl‐4‐phenyl‐3‐azahexane‐3‐nitroxide‐terminated poly(2‐methyl‐2‐oxazoline) with narrow dispersity (M_w/M_n = 1.1) has been achieved for the first time. The insertion of the alkoxyamine end groups at one or both ends of poly(2‐methyl‐2‐oxazoline) (PMEOX) chains has been successfully done using a method based on “terminating reagent method.” These macroinitiators have molecular weights ranging from 6.3 × 10³ to 9.4 × 10³ g mol^?1. In contrast, attempt to introduce the alkoxyamine group at one end of PMEOX chain through the “initiator method” has furnished a mixture of alkoxyamine‐graft polyoxazolines because of rearrangement of alkoxyamine occurring during the synthesis of PMEOX. The macroinitiators obtained by terminating reagent method have been used successfully for polymerization of styrene by nitroxide‐mediated radical polymerization (NMP), which exhibited all the expected features of a controlled system. The control of NMP has been proved by a good agreement between theoretical and experimental molecular weights and by narrow dispersity (M_w/M_n < 1.2). Different types of well‐defined multiblock copolymers have been prepared: diblock copolymers poly[(2‐methyl‐2‐oxazoline)‐b‐(styrene)] (PMEOX‐b‐PS) and, for the first time, triblock copolymers poly[(styrene)‐b‐(2‐methyl‐2‐oxazoline)‐b‐(styrene)] (PS‐b‐PMEOX‐b‐PS). © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.     

Nitroxide‐Mediated Polymerization of Bio‐Based Farnesene with a Functionalized Methacrylate

Farnesene (Far) is a bio‐based terpene monomer that is similar in structure to commercially used dienes like butadiene and isoprene. Nitroxide‐mediated polymerization (NMP) is adept for the polymerization of dienes, but not particularly effective at controlling the polymerization of methacrylates using commercial nitroxides. In this study, Far is statistically copolymerized with a functional methacrylate, glycidyl methacrylate (GMA), by NMP using N‐succinimidyl modified commercial BlocBuilder (NHS‐BB) initiator. Reactivity ratios are determined to be r _Far = 0.54 ± 0.04 and r _GMA = 0.24 ± 0.02. The ability of the poly(Far‐stat‐GMA) chains to reinitiate for chain extension with styrene showed a clear shift in molecular weight and monomodal distribution. Copolymerizations using a new alkoxyamine, Dispolreg 007 (D7), is explored as it is shown to homopolymerize methacrylates, but not yet reported for statistical copolymerizations. Bimodal molecular weight distributions are observed when an equimolar ratio of Far and GMA is copolymerized with D7 due to slow decomposition of the initiator, but chain ends are active as shown by successful chain extension with styrene. Both NHS‐BB and D7 initiators are used to synthesize poly[Far‐b‐(GMA‐stat‐Far)] and poly(Far‐b‐GMA) diblock copolymers. While the NHS‐BB initiated polymer chains have lower dispersity, D7 exhibits more linear polymerization kinetics and maintains more active chain ends.     

Photosensitized alkoxyamines as bicomponent radical photoinitiators

The photopolymerization ability of photosensitized alkoxyamines has been investigated. These compounds behave as interesting two‐component photoinitiators. Laser flash photolysis, electron spin resonance, and density functional theory allow to support the interactions encountered between the photosensitizer (benzophenone and isopropylthioxanthone) and the alkoxyamines (C? O bond breaking and hydrogen transfer) and the side reactions of the nitroxide radical with photosensitizer (electron transfer). © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2910–2915, 2010     

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     

Fast Controlled Radical Polymerization of Styrene Mediated by Oligomeric Nitroxides Formed In Situ

In the presence of an oligomeric hindered secondary amine added with peracetic acid as the oxidant, radical polymerization of styrene is fast and controlled at 110 °C. Under these experimental conditions, an oligomeric nitroxide is formed in situ. This polymerization is 2.5 faster than polymerization mediated by the alkoxyamine derivated from TIPNO (2,2,5‐trimethyl‐4‐phenyl‐3‐azahexane‐3‐nitroxide), which generates a low molar mass nitroxide. Similarly, substitution of a low molar mass secondary amine, 2,2,6,6‐tetramethylpiperidone (4‐oxo‐TMP), for the oligomeric secondary amine maintains the control on the polymerization, which is however 4.6 times slower, all the other conditions being the same. The in situ formation of the oligomeric nitroxide has been confirmed by electron spin resonance (ESR).

     

Alkoxyamines of Stable Aromatic Nitroxides: N?O vs. C?O Bond Homolysis

A series of stable 2,2‐disubstituted 3‐(phenylimino)indol‐1‐oxyls, the alkoxyamines 3 , were prepared, characterized, and tested as possible candidates in controlled radical polymerization (CRP). The sturctures of 3d and 10 were additionally solved by X‐ray diffraction. The lability of the N? O(C) and (N)O? C bonds of compounds 3 were compared, and the possibility of N? O vs. O? C bond cleavage was evaluated by thermal degradation, ESR spin trapping, MS experiments, and DFT calculations. Alkoxyamines with a primary‐ or secondary‐alkyl group bound to the O‐atom of the nitroxide function (hexyl and i‐Pr) mainly underwent (undesired) N? O bond homolysis. When the O‐alkyl radical was a tertiary or a benzyl group (crotonyl or styryl), O? C bond cleavage occurred as the main process, thus suggesting a possible use of these compounds in CRP processes.     

Amphiphilic Poly(4‐acryloylmorpholine)/Poly[2‐(N‐carbazolyl)ethyl acrylate] Random and Block Copolymers Synthesized by NMP

A series of random copolymers and block copolymers containing water‐soluble 4AM and fluorescent VAK are synthesized by NMP. The homopolymerizations of 4AM and VAK and 4AM/VAK random copolymerization are performed in 50 wt% DMF using 10 mol% SG1, resulting in a linear increase in versus conversion, and final polymers with narrow molecular weight distributions ( < 1.4). Reactivity ratios r_VAK = 0.64 ± 0.52 and r_4AM = 0.86 ± 0.66 are obtained for the 4AM/VAK random copolymerization. In addition, a poly(4AM) macroinitiator is used to initiate a surfactant‐free suspension polymerization of VAK. After 2.5 h, the resulting amphiphilic block copolymer has = 12.6 kg · mol^?1, = 1.48, molar composition F_VAK = 0.38 with latex particle sizes between 270 and 475 nm.