Temporally Controlled Ultrasonication‐Mediated Atom Transfer Radical Polymerization in Miniemulsion

Due to the increasing requirement for more environmentally and industrially relevant approaches in macromolecules synthesis, ultrasonication‐mediated atom transfer radical polymerization (sono‐ATRP) in miniemulsion media is applied for the first time to obtain precisely defined poly(n‐butyl acrylate) (PBA) and poly(methyl methacrylate) (PMMA) homopolymers, and poly(n‐butyl acrylate)‐block‐poly(tert‐butyl acrylate) (PBA‐b‐PtBA) and poly(n‐butyl acrylate)‐block‐poly(butyl acrylate) (PBA‐b‐PBA) copolymers. It is demonstrated in the reaction setup with strongly hydrophilic catalyst copper(II) bromide/tris(2‐pyridylmethyl)amine (Cu^IIBr₂/TPMA) responsible for two principal mechanisms – interfacial and ion‐pair catalysis reflecting single‐catalyst approach. This solution turns out to be an excellent tool in controlled preparation of well‐defined polymers with narrow molecular weight distribution (up to Ð = 1.28) and preserves chain‐end functionality (DCF = 0.02% to 0.32%). Temporal control over the polymer chain growth is successfully conducted by turning the ultrasonication on/off. Taking into consideration long OFF stage (92.5 h) during ultrasonication‐induced polymerization in miniemulsion, synthesis is efficiently reinitiated without any influence on controlled characteristics maintaining the precise structure of received PBA homopolymers, confirmed by narrow molecular weight distribution (Ð = 1.26) and high retention of chain‐end functionality (DCF = 0.01%). This procedure constitutes an excellent simple and eco‐friendly approach in preparation of functional polymeric materials.     

Application of Nitroxide‐Terminated Polymers Prepared by Sonochemical Degradation in the Synthesis of Block Copolymers

Summary: The ultrasonic irradiation of a polymer solution results in the breakage of macromolecular C? C bonds. In the presence of radical scavengers the formed macroradicals are prevented from termination reactions as combination or disproportionation. Using nitroxides as trapping agents the polymer is transformed into a macroinitiator, which can be used in controlled free‐radical polymerization to synthesize block copolymers. In this work several polymers were exposed to sonochemical degradation and terminated with various nitroxides, e.g. OH‐TEMPO and TIPNO. In a second reaction step the prepared polymer‐nitroxide‐adducts were applied as macroinitiators in controlled free‐radical polymerizations with styrene. The obtained products were mixtures of block copolymer and the corresponding homopolymers. The visco‐elastic properties were investigated by rheological analysis. A special separation technique with selective solvents was applied to determine the content of block copolymer.

Synthesis of block copolymers with sonochemically prepared macroinitiators.     

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.     

Low Conversion 4‐Acetoxystyrene Free‐Radical Polymerization Kinetics Determined by Pulsed‐Laser and Thermal Polymerization

Summary: The free‐radical polymerization kinetics of 4‐acetoxystyrene (4‐AcOS) is studied over a wide temperature range. Pulsed‐laser polymerization, in combination with dual detector size‐exclusion chromatography, is used to measure k_p, the propagation rate coefficient, between 20 and 110 °C. Values are roughly 50% higher than those of styrene, while the activation energy of 28.7 kJ · mol⁻¹ is lower than that of styrene by 3–4 kJ · mol⁻¹. With known k_p, conversion and molecular weight data from 4‐AcOS thermal polymerizations conducted at 100, 140, and 170 °C are used to estimate termination and thermal initiation kinetics. The behavior is similar to that previously observed for styrene, with an activation energy of 90.4 kJ · mol⁻¹ estimated for the third‐order thermal initiation mechanism.

Joint confidence (95%) ellipsoids for the frequency factor A and the activation energy E_a from non‐linear fitting of k_p data for 4‐AcOS (black) and styrene (grey).     

Nitroxide‐Mediated Radical Polymerization with Bisaminooxy Compounds as Initiators – Controlled Biradical Polymerization

Summary: The bisaminooxy compounds Bis‐TEMPO and Bis‐TIPNO derived from 2,2,6,6‐tetramethyl‐piperidine‐1‐oxyl (TEMPO) and 2,2,5‐trimethyl‐4‐phenyl‐3‐azahexane‐3‐oxyl (TIPNO) were applied as “biradical initiators” for the nitroxide‐mediated radical polymerization (NMRP) of styrene and n‐butyl acrylate. It was shown by comparison with analogous alkoxyamines as unimolecular initiators and mixing experiments of mono‐ and biradical species, that in the case of the biradical initiators chain growth occurs at both sides under NMRP conditions. This enables a two‐step synthesis of A‐B‐A‐triblock copolymers. Kinetics and molecular mass development were investigated for the controlled biradical polymerization of styrene at different initiator concentrations, temperatures, and with addition of acetic anhydride as accelerator. For the controlled biradical polymerization of n‐butyl acrylate with Bis‐TIPNO, the effect of added free nitroxide relative to the initiator concentration was studied. The poly(styrene‐block‐n‐butyl acrylate‐block‐styrene) copolymers with higher block length prepared by this method show two glass transition temperatures, which indicates microphase separation of the polymer blocks.

Structure of poly(styrene‐block‐n‐butyl acrylate‐block‐styrene), synthesized by nitroxide‐mediated radical polymerization with Bis‐TIPNO as initiator.     

Surface‐Initiated Controlled Radical Polymerization from Silica Nanoparticles with High Initiator Density

The surface of silica nanoparticles is modified using the “grafting from” technique. A multi‐step reaction is conducted to modify their surface properties. (3‐glycidoxypropyl) trimethoxysilane (GPS) is used as the coupling agent for the fixation of atom transfer radical polymerization (ATRP) initiator. The grafting efficiency of GPS mixed with aqueous suspension of silica nanoparticles is studied, followed by the coupling efficiency towards ATRP initiator. The bromide concentration of ATRP initiator is kept constant for comparative kinetic studies of styrene and MMA polymerizations. The consequences at high conversions and the particle size distribution are studied. The behaviour of the glass transition temperature of either polymer‐modified particles and the nature of dispersion of polymer‐coated silica particles are analyzed.     

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.

     

Nitroxide‐Mediated Radical Polymerization of N‐tert‐Butylacrylamide

The nitroxide‐mediated polymerization of N‐tert‐butylacrylamide (TBAM) in DMF at 120 °C using SG1/DEPN and AIBN has been investigated. Linear growth in number‐average molecular weight ( ) versus conversion and narrow molecular weight distributions (MWDs) with high livingness were obtained up to ≈8 000 g · mol^?1. For higher molecular weights, the MWDs gradually became broader with low molecular weight tailing, and deviated downwards from theoretical values. Quantitative analyses of MWDs, along with specifically designed conventional radical polymerizations at 120 °C, were consistent with chain transfer to monomer limiting the attainable . This finding can be equally applied to existing literature polymerizations of TBAM.

     

Xanthate‐Mediated Controlled Radical Polymerization of N‐Vinylcarbazole

Summary: Well‐defined poly(N‐vinylcarbazole) [poly(NVC)] was synthesized by macromolecular design via interchange of the xanthates (MADIX)/reversible addition‐fragmentation chain transfer (RAFT) polymerization. The homopolymers with controlled molecular weights ( = 3 000–48 000) and low polydispersities indices ( = 1.15–1.20) were obtained by the polymerization of NVC with AIBN in the presence of O‐ethyl‐S‐(1‐phenylethyl) dithiocarbonate as a xanthate‐type chain transfer agent (CTA). Good control of the polymerization was confirmed by the linear first‐order kinetic plot, the molecular weight controlled by the monomer/CTA molar ratio, linear increase in the molecular weight with the conversion, and the ability to extend the chains by the second addition of the monomer.

Radical polymerization of NVC in the presence of CTA and plot of number‐average molecular weight (circles) and polydispersity (squares) as a function of conversion.     

Visible Light Controlled Polymerization of Azide‐Derived Monomers: A Facile,Metal‐Free PET‐ATRP Route to Construct Azide Polymers

Well‐defined azide polymers are successfully synthesized by visible‐light‐induced metal‐free electron transfer–atom transfer radical polymerization (PET‐ATRP) at room temperature. This technique uses Eosin Y/Et₃N as the reductive quenching photocatalyst system, which can effectively prevent the destruction of the azide group in polymerization. Four kinds of azide‐derived monomers participate well in this reaction and obtain satisfactory results. The kinetic behavior, “ON/OFF” experiment, and chain‐extension experiment confirm the living feature of this visible light controlled polymerization. Moreover, random copolymers obtained by this protocol can be used as surface modifier which further demonstrates the utility and reliability of this method.     

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.

     

Synthesis of Branched Polymers by Means of Living Anionic Polymerization, 9. Radical Coupling Reaction of 1,1‐Diphenylethylene‐Functionalized Polymers with Potassium Naphthalenide and Its Application to Syntheses of In‐Chain‐Functionalized Polymers and Star‐Branched Polymers

Radical coupling reactions of both 1,1‐diphenylethylene (DPE)‐chain‐end‐ and DPE‐in‐chain‐functionalized polymers with potassium naphthalenide have been studied under the conditions mainly in THF at –78°C. Chain‐end‐functionalized polymers having M̄_n values of less than 10 kg/mol were very efficiently coupled in more than 90% yield to afford the polymeric dianion that were dimeric coupled products with two 1,1‐diphenylalkyl anions in the middle of the chains. However, the dimer yield decreased with increasing the molecular weight. The dimer was obtained in 59% yield with use of the chain‐end‐functionalized polymer having M̄_n of 33.9 kg/mol. Well‐defined in‐chain‐functionalized polymers with two benzyl bromide and DPE moieties each have been successfully synthesized by the reaction of the polymeric dianion thus obtained with 1‐(4‐bromobutyl)‐4‐(tert‐butyldimethylsilyloxymethyl)benzene and 1‐[4‐(4‐bromobutyl)phenyl]‐1‐phenylethylene, respectively. The radical coupling reaction of in‐chain‐functionalized polymers with DPE (M̄_n ca. 20 kg/mol) with potassium naphthalenide also proceeded efficiently to afford the coupled products that were A₂A′₂ and A₂B₂ four‐arm star‐branched polymers with well‐defined structures (M̄_n ca. 40 kg/mol).     

Terpyridine‐Terminated Homo and Diblock Copolymer LEGO Units by Nitroxide‐Mediated Radical Polymerization

Summary: Novel block copolymers were synthesized in a controlled fashion by nitroxide‐mediated radical polymerization starting from a terpyridine‐modified alkoxyamine. An important feature for controlling the efficiency of the polymerization is the presence of excess nitroxide, responsible for the initial rate of deactivation, which eventually leads to a decrease of the polydispersity indices of the desired block copolymer. The materials obtained were characterized by means of ¹H NMR, UV‐vis spectroscopy, and GPC. The complexation of the terpyridine ligands resulted in the formation of A‐B‐[Ru]‐C, A‐B‐[Ru]‐B‐A, and A‐B‐[Fe]‐B‐A metallo‐supramolecular block copolymers.

Telechelic polymers bearing a terpyridine end‐group at the α‐position and a nitroxide at the ω‐position were prepared in a living fashion by nitroxide‐mediated polymerization.     

Critically Evaluated Rate Coefficients for Free‐Radical Polymerization, 4

Propagation rate coefficients, k_p, which have been previously reported by several groups for free‐radical bulk polymerizations of cyclohexyl methacrylate (CHMA), glycidyl methacrylate (GMA), benzyl methacrylate (BzMA), and isobornyl methacrylate (iBoMA) are critically evaluated. All data were determined by the combination of pulsed‐laser polymerization (PLP) and subsequent polymer analysis by size‐exclusion chromatography (SEC). This so‐called PLP‐SEC technique has been recommended as the method of choice for the determination of k_p by the IUPAC Working Party on Modeling of Polymerisation Kinetics and Processes. The present data fulfill consistency criteria and the agreement among the data from different laboratories is remarkable. The values for CHMA, GMA, and BzMA are therefore recommended as constituting benchmark data sets for each monomer. The data for iBoMA are also considered reliable, but since SEC calibration was established only by a single group, the data are not considered as a benchmark data set. All k_p data for each monomer are best fitted by the following Arrhenius relations: CHMA: , GMA: , BzMA: , iBoMA: . Rather remarkably, for the methacrylates under investigation, the k_p values are all very similar. Thus, all data can be fitted well by a single Arrhenius relation resulting in a pre‐exponential factor of 4.24 × 10⁶ L · mol^?1 · s^?1 and an activation energy of 21.9 kJ · mol^?1. All activation parameters refer to bulk polymerizations at ambient pressure and temperatures below 100 °C. Joint confidence intervals are also provided, enabling values and uncertainties for k_p to be estimated at any temperature.

95% joint confidence intervals for Arrhenius parameters A and E_A for cyclohexyl (CHMA), glycidyl (GMA), benzyl (BzMA), and isobornyl (iBoMA) methacrylate; for details see text.     

Free‐Radical Polymerization of N‐Vinylimidazole and Quaternized Vinylimidazole in Aqueous Solution

Aqueous‐phase free‐radical batch polymerizations of N‐vinylimidazole (NVI) and quaternized N‐vinylimidazole (QVI) are conducted with varying initial monomer and initiator concentrations at 70 and 85 °C. The polymerization rate of NVI is very slow at the natural pH of 9 due to degradative radical addition to monomer. The rates are increased by lowering the pH, wherein the degradative addition to NVI monomer is partially (at pH 4) and completely (at pH 1) hindered, with the polymerization rate matching that of QVI at pH 1. The initial rates of polymerization for both NVI and QVI are independent of temperature. A kinetic model developed in Predici that includes the pH‐dependent side reactions can reasonably represent both QVI and NVI polymerization.

     

A Suitable Photoinitiator for Pulsed Laser‐Induced Free‐Radical Polymerization

Detailed kinetic studies into free‐radical polymerization via pulsed laser experiments ideally require photoinitiators which almost instantaneously dissociate into primary free‐radical fragments that rapidly add to monomer molecules and thus induce macromolecular growth. 2‐Methyl‐4′‐(methylthio)‐2‐morpholinopropiophenone (MMMP) is shown to be such a suitable photoinitiator. Measurement of monomer conversion induced by a single laser pulse, within the so‐called single‐pulse pulsed laser polymerization (SP–PLP) experiment, provides direct information about the chain‐length dependence of the termination rate coefficient if MMMP is used as the photoinitiator.

Relative monomer concentration vs time trace of a methyl acrylate homopolymerization at 40 °C and 2 000 bar where MMMP was used as the initiator. The primary radical concentration from MMMP photo‐decomposition increases from curve (a) to (b) to (c) by the ratios 1:2.2:5.7.     

Direct Monitoring of the Cross‐Over from Diffusion‐Controlled to Decomposition‐Controlled Initiation in Free Radical Polymerization

High‐resolution free radical polymerization kinetics was obtained using automatic continuous online monitoring of polymerization reactions (ACOMP). A sharp cross‐over from diffusion‐controlled initiation at low [monomer] to initiator decomposition control at higher [monomer] was found, and agrees with the quasi‐steady state approximation (QSSA). The cross‐over was also measurable within individual experiments. The kinetic implications for polymer weight average molecular weight and intrinsic viscosity (η)_w were analyzed, and the QSSA‐predicted trends for the ratios of final to initial , and (η)_w, confirmed. Analytical expressions for conversion are contrasted, and it was found that first‐order fits, while not fully justified theoretically, nonetheless are robust, which simplifies calculations needed for controlling molecular weight distributions in “semi‐batch” reactions, where reagents are fed to the reactor through programmable flow profiles. At the low monomer concentrations used, there was no evidence that propagation or termination rate coefficients changed during the reactions.

     

How to Fight Kinetics in Complex Radical Polymerization Processes: Theoretical Case Study of Poly(divinyl ether‐alt‐maleic anhydride)

Products of free‐radical polymerization (FRP) are usually not regulated on the molecular scale, consisting of blocks obtained through the fastest kinetic scheme pathways. The side or kinetically restricted products can be a source of impurities in a complex FRP case, or possess new properties if isolated solely. FRP synthesis of poly(divinyl ether‐alt‐maleic anhydride), known as “DIVEMA”, serves as a polymerization example with such kinetic and thermodynamic complexities. Uncertainty in factors regulating polymer structure is a challenge in advancement “DIVEMA” derivatives toward medical practice. In‐depth investigation via quantum‐chemical and molecular mechanics methods unveils mechanistic aspects of polymer stereoisomerism and confirms possible isolation of thermodynamically or kinetically controlled products on a large data set. Strategies toward regulation of 5‐exo/6‐endo cycloisomerism are theorized and then studied via microkinetic modeling. Thermodynamically controlled products can be isolated utilizing lower monomer concentrations, in range of 10^?3 to 10^?1 m , and/or application of a complexing agent that is better to realize via solvents, capable of formation π‐ and σ‐radical complexes. Change of electrophilic monomer is proposed as an approach for designing more molecularscale adjustable copolymerization processes. Methodology, obtained results, and conclusions for “DIVEMA” can be valuable to control other FRP processes on the molecular scale, unlocking polymers with improved or new functionalities.     

Visible Light‐Induced Atom Transfer Radical Polymerization

Visible light‐induced reverse and simultaneous reverse and normal initiation (SR&NI) atom transfer radical polymerizations of vinyl monomers are examined using various dyes and type I photoinitiators. The effect of photoinitiator types on the control of molecular weight and distribution is described. In both dye and type I photoinitiator sensitized SR&NI ATRP systems, the molecular weights increase linearly with conversion. However, the experimental molecular weights are considerably higher than the theoretical values and the polymers show broad‐molecular‐weight distributions ranging from 1.28 to 1.60 in the dye‐sensitized SR&NI ATRP. However, the polymers obtained by SR&NI ATRP using type I photoinitiator system had molecular weight values close to the theoretical ones and very narrow‐molecular‐weight distributions ranging from 1.11–1.18.     

Critically Evaluated Rate Coefficients for Free‐Radical Polymerization, 5,

Summary: Propagation rate coefficients, k_p, for free‐radical polymerization of butyl acrylate (BA) previously reported by several groups are critically evaluated. All data were determined by the combination of pulsed‐laser polymerization (PLP) and subsequent polymer analysis by size exclusion (SEC) chromatography. The PLP‐SEC technique has been recommended as the method of choice for the determination of k_p by the IUPAC Working Party on Modeling of Polymerization Kinetics and Processes. Application of the technique to acrylates has proven to be very difficult and, along with other experimental evidence, has led to the conclusion that acrylate chain‐growth kinetics are complicated by intramolecular transfer (backbiting) events to form a mid‐chain radical structure of lower reactivity. These mechanisms have a significant effect on acrylate polymerization rate even at low temperatures, and have limited the PLP‐SEC determination of k_p of chain‐end radicals to low temperatures (<20 °C) using high pulse repetition rates. Nonetheless, the values for BA from six different laboratories, determined at ambient pressure in the temperature range of ?65 to 20 °C mostly for bulk monomer with few data in solution, fulfill consistency criteria and show excellent agreement, and are therefore combined together into a benchmark data set. The data are fitted well by an Arrhenius relation resulting in a pre‐exponential factor of 2.21 × 10⁷ L · mol^?1 · s^?1 and an activation energy of 17.9 kJ · mol^?1. It must be emphasized that these PLP‐determined k_p values are for monomer addition to a chain‐end radical and that, even at low temperatures, it is necessary to consider the presence of two radical structures that have very different reactivity. Studies for other alkyl acrylates do not provide sufficient results to construct benchmark data sets, but indicate that the family behavior previously documented for alkyl methacrylates also holds true within the alkyl acrylate family of monomers.

Arrhenius plot of propagation rate coefficients, k_p, for BA as measured by PLP‐SEC.