Investigations Into Chain‐Length‐Dependent Termination in Bulk Radical Polymerization of 1H, 1H, 2H, 2H‐Tridecafluorooctyl Methacrylate

The SP‐PLP‐EPR technique is used to carry out a detailed investigation of the radical termination kinetics of 1H, 1H, 2H, 2H‐tridecafluorooctyl methacrylate (TDFOMA) in bulk at relatively low conversion. Composite‐model behavior for chain‐length‐dependent termination rate coefficients, k_t^i,i, is observed. It is found that for TDFOMA, i_c ≈ 60 independent of temperature, and α_s ≈ 0.65 and α_l ≈ 0.2 at 80 °C and above. However, at lower temperatures the situation is strikingly different, with the significantly higher average values of α_s = 0.89 ± 0.15 and α_l = 0.32 ± 0.10 being obtained at 50 °C and below. This makes TDFOMA the first monomer to be found that exhibits clearly different exponent values, α_s and α_l, at lower and higher temperature, and that has both a high α_s, like an acrylate, and a high i_c, like a methacrylate.     

Chain‐Length‐Dependent Termination in Radical Polymerization of Acrylates

The technique of SPPLP EPR, which is single‐pulse pulsed‐laser polymerization (SPPLP) in conjunction with electron paramagnetic resonance (EPR) spectroscopy, is used to carry out a detailed investigation of secondary (chain‐end) radical termination of acrylates. Measurements are performed on methyl acrylate, n‐butyl acrylate, and dodecyl acrylate in bulk and in toluene solution at ?40 °C. The reason for the low temperature is to avoid formation of mid‐chain radicals (MCRs), a complicating factor that has imparted ambiguity to the results of previous studies of this nature. Consistent with these previous studies, composite‐model behavior for chain‐length‐dependent termination (CLDT) rate coefficients, , is found in this work. However, lower and more reasonable values of α_s, the exponent for variation of at short chain lengths, are found in the present study. Most likely this is because of the absence of MCRs, thereby validating the methodology of this work. Family‐type termination behavior is observed, with the following average parameter values adequately describing all results, regardless of acrylate or the presence of toluene: α_s = 0.79, α_l = 0.21 (long chains) and i_c ≈ 30 (crossover chain length). All indications are that these values carry over to termination of acrylate chain‐end radicals at higher, more practical temperatures. Further, these values largely make sense in terms of what is understood about the physical meaning of the parameters. Variation of the rate coefficient for termination between monomeric radicals, , is found to be well described by the simple Smoluchowski and Stokes–Einstein equations. This allows easy prediction of for different alkyl acrylates, solvent, and temperature. Through all this the unrivalled power of SPPLP EPR for measuring and understanding (chain‐length‐dependent) termination rate coefficients shines through.

     

Termination Kinetics of Dibutyl Itaconate Free‐Radical Polymerization Studied via the SP–PLP–ESR Technique

Summary: The termination kinetics of dibutyl itaconate (DBI) bulk polymerization was studied via SP–PLP–ESR single pulse–pulsed laser polymerization with time‐resolved detection of free‐radical concentration by electron‐spin resonance, at temperatures between 0 and 60 °C. As is characteristic of PLP experiments, termination rate coefficients, k_t(i,i), are measured for radicals of (almost) identical chain length (CL) i. CL‐averaged 〈k_t〉, for chain lengths up to 200 monomer units, and also kequation/tex2gif-stack-1.gif referring to termination of very small‐size radicals are directly deduced from measured DBI radical concentration vs time traces. At 45 °C, 〈k_t〉 is (3.4 ± 0.6) · 10⁵ L · mol^?1 · s^?1 and kequation/tex2gif-stack-2.gif is (7.2 ± 1.0) · 10⁵ L · mol^?1 · s^?1. Both rate coefficients are independent of monomer conversion up to the highest experimental conversion of 18%. The associated activation energies are E_A(〈k_t〉) = 23.0 ± 3.2 kJ · mol^?1 and E_A(kequation/tex2gif-stack-3.gif) = 27.6 ± 2.8 kJ · mol^?1, respectively. “Model‐dependent” and “model‐free” analyses of radical concentration vs time profiles indicate a pronounced CL dependence of k_t(i,i) for DBI radicals of moderate size, 5 < i < 50. The lowering of k_t(i,i) with CL corresponds to an exponent α close to 0.5 in a power‐law expression k_t(i,i) = kequation/tex2gif-stack-4.gif · i^?a. At higher chain lengths, the variation of k_t(i,i) with CL becomes weaker and may be represented by an α value of 0.16 or even below. These results are consistent with models according to which α varies to a larger extent at low CL and to a smaller extent at high CL with the crossover region between the two ranges being located somewhere around i = 100.

Conversion‐dependence of 〈k_t〉 and kequation/tex2gif-stack-5.gif from laser‐induced photopolymerization of DBI.     

Direct Synthesis of Controlled Poly(styrene‐co‐acrylic acid)s of Various Compositions by Nitroxide‐Mediated Random Copolymerization

Styrene and acrylic acid were copolymerized under controlled conditions, in 1,4‐dioxane solution at 120 °C and 2 bar, using an alkoxyamine initiator based on the N‐tert‐butyl‐N‐(1‐diethylphosphono‐2,2‐dimethylpropyl) nitroxide, SG1. A broad composition range from 90/10 to 10/90 was investigated. With slightly different initiator concentrations and a similar initial proportion of free SG1 (4.5 mol‐% with respect to the initiator) the polymerizations exhibited very similar rates, irrespective of the proportion of acrylic acid in the comonomer mixture (80% conversion within 8 h). In all cases, the copolymers presented number average molar masses, , that increased linearly with overall monomer conversion, and polydispersity indexes that ranged between 1.2 and 1.4. Moreover, followed the calculated values, based on the initial concentrations of monomers and initiator. The variation in the initiator concentration allowed to target various molar masses, but some limitation appeared at low initiator concentration owing to chain transfer to 1,4‐dioxane. From the kinetic data, the reactivity ratios were determined: r_A = 0.27 ± 0.07 for acrylic acid and r_S = 0.72 ± 0.04 for styrene. Depending on the initial comonomer composition, chains exhibited no or small composition drift, and hence a slightly pronounced gradient structure.

Reactivity ratios for acrylic acid and styrene.     

Hydrogen Abstraction Followed by Nitroxide‐Mediated Polymerization: Synthesis of 2,7‐Dibromofluorene‐Labeled Polystyrene

Chromophore end‐labeled polystyrene is synthesized using nitroxide‐mediated polymerization (NMP) by decomposing 2‐2′‐azoisobutyronitrile (AIBN) or benzoyl peroxide (BPO) in the presence of fluorene or fluorene derivatives. End‐labeling is dependent on the thermally produced radical species selectively abstracting a hydrogen atom from the 9‐position of the fluorene species prior to initiation of styrene. From gel permeation chromatography (GPC) data and UV–Vis analysis, it is found that AIBN initiation, compared to BPO, leads to a more controlled polymerization system, producing polymers with predictable molecular weights, narrower polydispersity index (PDI) values (<1.3), and higher amounts of fluorene end‐labeling. In terms of the reaction parameters, no consistent trend is observed as a function of the timing of styrene's addition or the temperature at which the hydrogen abstraction phase is performed. Analysis of the chromophore content by UV–Vis spectroscopy demonstrated that the presence of bromine atoms on the 2‐ and 7‐position of the fluorene species leads to higher percent labeling of the chromophore species, presumably due to a more facile abstraction of the hydrogen at the 9‐position.     

Formation of Soluble Hyperbranched Polymer Nanoparticles by Initiator‐Fragment Incorporation Radical Polymerization of Ethylene Glycol Dimethacrylate

Summary: Ethylene glycol dimethacrylate ( 1 ) was polymerized at 70 and 80 °C in benzene using high concentrations of the initiator, dimethyl 2,2′‐azobisisobutyrate ( 2 ). When the concentrations of 1 and 2 were 0.10 and 0.50 mol · L^?1, respectively, the polymerization proceeded homogeneously without any gelation to yield a soluble polymer. The polymerization system involved ESR‐detectable polymer radicals in high concentration (3.1 × 10^?6–7.5 × 10^?6 mol · L^?1 at 70 °C). The polymer formed at 80 °C after 4 h consisted of units of 1 with (4 mol‐%) and without (38 mol‐%) a double bond and a methoxycarbonylpropyl group (58 mol‐%) as the fragment of 2 . Thus, the initiator‐fragment incorporation radical polymerization proceeds in the present polymerization to yield a hyperbranched polymer. The polymer showed an upper critical solution temperature (31 °C on cooling and 32 °C on heating) in an acetone/water mixture (4:1 v/v). The intrinsic viscosity of the polymer was very low (0.047 dL · g^?1) at 30 °C in benzene despite its considerably high molecular weight ( = 5.7 × l0⁵ by MALLS). TEM confirmed that the individual polymer molecules were nanoparticles with diameters of 5–15 nm. The polymer was able to solubilize for the dye probe, Rhodamine 6G, by encapsulation in its hyperbranched structure.

Hyperbranched structure of the resulting polymer.     

Influence of Mid‐Chain Radicals on Acrylate Free Radical Polymerization: Effect of Ester Alkyl Group

Summary: The benzene solution polymerizations of tert‐butyl acrylate (tBA) and 2‐ethylhexyl acrylate (EHA) have been investigated with respect to the effects of mid‐chain radicals (MCRs) on the rate of polymerization, the total radical concentration, the apparent rate coefficients for propagation, termination, and β‐fragmentation, the contents of unsaturated end groups and branching, and the molecular weight distribution. The EHA polymerization involves a considerably higher concentration of MCR and is more significantly affected by MCR than the tBA polymerization. The MCR content as estimated by electron paramagnetic resonance (EPR) spectroscopy during photosensitized EHA polymerization was as high as 70% of the total radical concentration at 25 °C. However, no ¹H and ¹³C NMR resonances due to unsaturated ends and branching, respectively, were detected for the poly(EHA) obtained at 25 °C. These findings indicate that a high MCR content does not directly correspond to significant MCR effects on the polymer structure. The rate constants for mutual reaction ( ) and β‐fragmentation (k_f) of MCR were estimated for the two assumed cases where MCR was consumed solely by (i) mutual reaction, and (ii) β‐fragmentation, based on the after‐effect of the photosensitized EHA polymerization monitored by EPR spectroscopy at 25 °C; = 3.5 × 10³ L · mol · s^?1 and k_f = 4.2 × 10^?2 s^?1. These rate coefficients were compared with those for reactions of structurally similar radicals.

Reaction pathways of MCRs.     

Synthesis of Hydroxyl‐Terminated Poly(vinylcarbazole‐ran‐styrene) Copolymers by Nitroxide‐Mediated Polymerization

A series of poly(vinylcarbazole‐ran‐styrene) copolymers with terminal hydroxyl groups were synthesized using nitroxide mediated polymerization (NMP) with the hydroxyl‐functional initiator VA‐086 and TEMPO as the mediator at 130 °C. Polymerizations were studied as a function of vinylcarbazole feed content, target molecular weight, and VA‐086/TEMPO ratio. The characterization of the copolymers was done by GPC and NMR. For feed concentrations of 40 mol‐% vinylcarbazole, copolymers with vinylcarbazole concentration up to 33 mol‐% could be obtained with narrow molecular weight distributions (PDI = 1.35) and exhibit pseudo‐“living” character up to conversions of about 20% if the target molecular weight was >100 kg · mol^?1. ¹H NMR indicated that the hydroxyl group was retained sufficiently with a functionality typically of about 0.7 hydroxyl groups per chain. Copolymers synthesized with higher vinylcarbazole feed content exhibited slower kinetics and were less controlled, resulting in much broader molecular weight distributions. The absence of control could be attributed to the absence of thermal initiation by vinylcarbazole which is advantageous toward controlling the radical concentration during the polymerization.

     

Use of Original ω‐Perfluorinated Dithioesters for the Synthesis of Well‐Controlled Polymers by Reversible Addition‐Fragmentation Chain Transfer (RAFT)

A series of five fluorinated dithioesters PhC(S)SRCH₂C_nF_2n+1 (where R represents an activating spacer and n = 6 or 8) was obtained in fair to high yields (57–88%). These transfer agents were successfully used in reversible addition‐fragmentation transfer (RAFT) of styrene (S), methyl methacrylate (MMA), ethyl acrylate (EA) and 1,3‐butadiene. Well‐chosen fluorinated dithioesters were able to lead to a good control of the radical polymerization of these monomers (i.e., molar masses of the produced polymers increased linearly with the monomer conversion and the polydispersity indexes ranging between 1.1 and 1.6 remained low). The relationship between the structures of the dithioesters and the living behavior of the radical polymerization of these above monomers is discussed and it is shown that the nature of the R group influences the living behavior from different contributions to radical stabilization. Furthermore, the RAFT process also yielded PMMA‐b‐PS and PEA‐b‐PS block copolymers bearing a fluorinated moiety.     

Novel Access to Propagation Rate Coefficients of Radical Polymerization by the SP–PLP–EPR Method

A novel method for k_p measurement is presented, in which time‐resolved electron paramagnetic resonance detection of radical concentration after applying a single laser pulse (SP?PLP?EPR) is carried out in conjunction with measuring monomer conversion induced by the laser pulse. The method is particularly useful for the study of fully ionized monomers, as will be outlined by experiments on trimethylaminoethyl methacrylate chloride (TMAEMA, 2‐(methacryloyloxy)‐N,N,N‐trimethylethan‐1‐aminium chloride, 20 wt% in aqueous solution) at 60 °C. In addition to k_p, the method provides termination rate coefficient data. The reliability of the so‐obtained rate coefficients is demonstrated by chemically induced experiments and by measuring TMAEMA conversion after single‐pulse initiation via near‐infrared (NIR) spectroscopy.

     

A Tandem Controlled Radical Polymerization Technique for the Synthesis of Poly(4‐vinylpyridine) Block Copolymers: Successive ATRP,SET‐NRC,and NMP

Poly(methyl methacrylate)‐block‐poly(4‐vinylpyridine), polystyrene‐block‐poly(4‐vinyl pyridine), and poly(ethylene glycol)‐block‐poly(4‐vinylpyridine) block copolymers are synthesized by successive atom transfer radical polymerization (ATRP), single‐electron‐transfer nitroxide‐radical‐coupling (SET‐NRC) and nitroxide‐mediated polymerization (NMP). This paper demonstrates that this new approach offers an efficient method for the preparation of 4‐vinylpyridine‐containing copolymers.

     

Reactivities of ω‐Unsaturated Methacrylate Oligomers toward tert‐Butoxy Radicals: Investigation of the Effect of Degree of Polymerization and Ester Alkyl Group

The reactivities of ω‐unsaturated methacrylate oligomers (RMA‐n; n = 2–5) toward tert‐butoxy radicals (t‐BuO^·) as a model of the addition step in addition‐fragmentation chain transfer (AFCT) have been investigated by the nitroxide trapping technique in combination with HPLC and electrospray ionization mass spectrometry. The ratio of the rate coefficients for the addition of ω‐unsaturated methyl methacrylate oligomers (MMA‐n) to t‐BuO^· to the β‐fragmentation of t‐BuO^· (k_add/k_β), where k_β can be treated as a constant, has been shown to decrease with increasing n due to increased steric hindrance of the α‐substituents. However, the value of k_add/k_β reaches a constant value at n = 3–4, and the greatest extent of suppression of the addition rate compared with MMA is less than a factor of four. Comparison of the values of k_add/k_β for ω‐unsaturated cyclohexyl methacrylate oligomers (CHMA‐n; n = 2 and 3) with MMA‐n revealed that the extent of suppression increased with increasing n without regard to the ester alkyl group. Hydrogen abstraction by t‐BuO^· from RMA‐n appears to occur mainly at the ester alkyl groups, and the extent is much greater from CHMA‐n than from MMA‐n.

     

Synthesis of AB2 3‐ and AB4 5‐Miktoarm Star Copolymers Initiated from Dendritic Tri‐ and Penta‐Functional Initiators by Combination of Ring‐Opening Polymerization of ε‐Caprolactone and Nitroxide‐Mediated Radical Polymerization of Styrene

Summary: Well‐defined AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers were prepared by combination of ring‐opening polymerization (ROP) and nitroxide‐mediated radical polymerization (NMRP) using dendritic tri‐ and penta‐functional initiators. Initially, two kinds of dendritic initiators having one benzylic OH and two or four TEMPO‐based alkoxyamine moieties were prepared. Using them, ROP of ε‐caprolactone was carried out at room temperature to give poly(ε‐caprolactone)s carrying two or four alkoxyamine moieties. NMRP of styrene from the poly(ε‐caprolactone)s was carried out at 120 °C to give AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers, which were analyzed by ¹H NMR and SEC. The increased linearly with conversion and the were in the range 1.10–1.37, showing that well‐defined AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers were formed.

Well‐defined AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers were prepared by combination of ring‐opening polymerization (ROP) and nitroxide‐mediated radical polymerization (NMRP) using dendritic tri‐ and penta‐functional initiators.     

Structure and Properties of Poly(butyl acrylate‐block‐sulfone‐block‐butyl acrylate) Triblock Copolymers Prepared by ATRP

Summary: A series of telechelic OH polysulfones (PSU) were converted to atom transfer radical polymerization (ATRP) macroinitiators by reaction with 2‐bromoisobutyryl bromide. Three macroinitiators with different chain lengths were extended with poly(butyl acrylate) (PBA) to form ABA triblock copolymers. The structure and dynamics of the ABA triblock copolymers with PSU central segments and various molecular weight PBA side chains were investigated by small‐angle X‐ray scattering and rheology. The block copolymers form micelles with a PSU core and PBA corona. The length of each block has an important effect on the structure and resulting dynamics of the copolymers. Dynamic mechanical measurements indicate three relaxation modes: (i) PBA segmental relaxation at high frequency; (ii) PBA relaxation of the corona block at intermediate frequency; (iii) an additional relaxation process related to structural rearrangement of the micelles at low frequency. The shear modulus plateau corresponding to a soft rubbery state extends over a very broad time or temperature range because of this slow additional relaxation.

Schematic illustration of the structural elements and the bulk supramolecular structure for a symmetric triblock copolymer with a stiff central segment strongly incompatible with the other constituent.     

Novel Fluorinated Hybrid Polymer by Radical Polyaddition of Bis(α‐trifluoromethyl‐β,β‐difluorovinyl) Terephthalate onto Dialkoxydialkylsilanes

To develop the radical polyaddition of bisperfluoroisopropenyl esters, the reactions of bis(α‐trifluoromethyl‐β,β‐difluorovinyl) terephthalate [CF₂?C(CF₃)OCOC₆H₄COOC(CF₃)?CF₂] (BFP) with dialkoxydialkylsilane were examined to prepare fluorinated hybrid polymers bearing dialkylsilyl groups in the main chain. Prior to polyaddition, the radical addition reaction of 2‐benzoyloxypentafluoropropene [CF₂?C(CF₃)OCOC₆H₅] (BPFP) has been investigated to afford the results that diethoxydimethylsilane (DEOMS) or dimethoxydimethylsilane with BPFP initiated by oxo radical are the best combination for the preparation of polymers. The mechanism of the addition reaction was proposed. Radical polyaddition of BFP with DEOMS initiated by benzoyl peroxide or di‐tert‐butyl peroxide has yielded polymers of up to molecular weight 1 × 10⁶ with rather broad molecular weight distribution. A mechanism for the polyaddition reaction is proposed based on the radical addition reaction between BPFP and DEOMS. The step‐growth polymerization is initiated by hydrogen abstraction of DEOMS to add a perfluoroisopropenyl group, followed by a 1,7‐shift of the radical in the intermediate. The relationship between addition reaction mechanism and polyaddition mechanism was also discussed.

     

Nanostructured hybrid hydrogels prepared by a combination of atom transfer radical polymerization and free radical polymerization

A new method to prepare nanostructured hybrid hydrogels by incorporating well-defined poly(oligo (ethylene oxide) monomethyl ether methacrylate) (POEO₃₀₀MA) nanogels of sizes 110–120 nm into a larger three-dimensional (3D) matrix was developed for drug delivery scaffolds for tissue engineering applications. Rhodamine B isothiocyanate-labeled dextran (RITC-Dx) or fluorescein isothiocyanate-labeled dextran (FITC-Dx)-loaded POEO₃₀₀MA nanogels with pendant hydroxyl groups were prepared by activators generated electron transfer atom transfer radical polymerization (AGET ATRP) in cyclohexane inverse miniemulsion. Hydroxyl-containing nanogels were functionalized with methacrylated groups to generate photoreactive nanospheres.¹H NMR spectroscopy confirmed that polymerizable nanogels were successfully incorporated covalently into 3D hyaluronic acid-glycidyl methacrylate (HAGM) hydrogels after free radical photopolymerization (FRP). The introduction of disulfide moieties into the polymerizable groups resulted in a controlled release of nanogels from cross-linked HAGM hydrogels under a reducing environment. The effect of gel hybridization on the macroscopic properties (swelling and mechanics) was studied. It is shown that swelling and nanogel content are independent of scaffold mechanics. In-vitro assays showed the nanostructured hybrid hydrogels were cytocompatible and the GRGDS (Gly–Arg–Gly–Asp–Ser) contained in the nanogel structure promoted cell–substrate interactions within 4 days of incubation. These nanostructured hydrogels have potential as an artificial extracellular matrix (ECM) impermeable to low molecular weight biomolecules and with controlled pharmaceutical release capability. Moreover, the nanogels can control drug or biomolecule delivery, while hyaluronic acid based-hydrogels can act as a macroscopic scaffold for tissue regeneration and regulator for nanogel release.     

Synthesis of Blockcopolymers P3HT‐b‐PS Using a Combination of Grignard‐Metathesis and Nitroxide‐Mediated Radical Polymerization

The synthesis of π‐conjugated NMRP‐macroinitiators using GRIGNARD‐metathesis polymerization in combination with azide/alkyne‐“click” chemistry has been investigated. Alkoxyamine‐functionalized poly(3‐hexylthiophene)s (P3HTs) have been used for block copolymer preparations in presence of styrene. Molecular weight and molecular weight distribution of the polymers have been determined in SEC‐measurements, while end‐group determination was performed with MALDI‐ToF‐MS. The molecular weight of the P3HT macroinitiators was influenced by the amount of Ni‐catalyst during the GRIM reaction. Those macroinitiators have been used to prepare block copolymers in subsequent nitroxide‐mediated radical polymerization (NMRP). Thin‐layer‐morphologies of the block copolymers were investigated using tapping‐mode AFM. Short and disordered rods were observed, as well as continuous and parallel fibrils.

     

The Kinetics of Surface‐Initiated RAFT Polymerization of Butyl acrylate Mediated by Trithiocarbonates

Stationary and time‐resolved electron spin resonance spectroscopy measurements are employed to investigate the kinetics of the surface‐initiated reversible addition fragmentation chain transfer (RAFT) polymerization of n‐butyl acrylate from silica nanoparticles using both R‐ and Z‐group‐attached trithiocarbonates as RAFT agents. The obtained kinetic parameters reveal that the addition rate coefficient in the main equilibrium of RAFT graft polymerizations is significantly smaller than the one for comparable RAFT polymerizations in solution phase, as translational diffusion of surface‐attached molecules is limited. In comparison to the R‐group approach, the equilibrium constants of the Z‐group approach are about one to two orders of magnitude smaller due to a stronger shielding of the RAFT moieties.

     

Stable Free‐Radical Polymerization of Styrene in Combination with 2‐Vinylnaphthalene Initiation

The elevated temperatures required for the stable free‐radical polymerization (SFRP) process lead to thermal initiation of styrene via the classic Mayo mechanism. Studies have shown the utility of styrene thermal initiation in controlled autopolymerization processes. In addition, the thermal polymerization of 2‐vinylnaphthalene (2VN), a styrenic derivative with an additional fused ring, was examined. An Arrhenius analysis of the 2VN thermal polymerization process using in‐situ FT‐IR spectroscopy demonstrated a greater propensity for 2VN thermal polymerization compared to styrene. A modified Mayo mechanism was proposed based on additional resonance stabilization in 2VN derived radicals compared to a single ring of resonance stabilization for styrene. The utility of 2VN as an initiator in the SFRP of styrene was investigated. 2VN initiation resulted in polymers that are end‐functionalized with a single 2VN unit or a polycyclic unit that results from Diels‐Alder reaction in the initiation mechanism. Variation of the styrene/2VN molar ratio in the presence of 2,2,6,6‐tetramethylpiperdinyl‐1‐oxyl enabled molecular weight control and narrow molecular weight distributions (ca. 1.1–1.2). The integrity of the polymer end groups was confirmed using ¹H NMR and UV‐vis spectroscopy. In addition, the UV‐vis absorbance of the 2VN initiating unit was used for construction of a calibration curve for molecular weight prediction based on the intensity of the absorbance at 312 nm associated with 2VN initiation. The observed rate constant for the polymerization initiated with 2VN (1.73 × 10^?5 s^?1) was comparable to that of benzoyl peroxide (k_obs = 1.99 × 10^?5 s^?1) initiation.

SFRP of styrene thermally initiated with 2VN.     

Synthesis and Characterization of Well‐Defined Mid‐Chain Functional Macrophotoinitiators of Polystyrene by Combination of ATRP and “Click” Chemistry

A combination of ATRP and “click” chemistry is employed for efficient preparation of a novel well‐defined mid‐chain functional macrophotoinitiator of polystyrene. Bromo‐terminated polystyrene (PSt‐Br) is prepared by ATRP of styrene using a methyl‐2‐bromopropanoate initiator with CuBr/PMDETA. Subsequently, PSt‐Br is converted to PSt‐N₃ by a simple nucleophilic substitution reaction. A dialkyne‐functionalized photoinitiator (alkyne‐PI‐alkyne) is synthesized using a dihydroxy‐functional photoinitiator and propargyl bromide. Then the “click” reaction between PSt‐N₃ and alkyne‐PI‐alkyne is performed by Cu(I) catalysis. Spectroscopic studies reveal that low‐polydispersity polystyrene with the desired photoinitiator functionality in the middle of the chain (PSt‐PI‐PSt) is obtained.