Cationic Bottlebrush Polymers from Quaternary Ammonium Macromonomers by Grafting‐Through Ring‐Opening Metathesis Polymerization

Cationic bottlebrush homopolymers are polymerized using a grafting‐through approach by ring‐opening metathesis polymerization (ROMP) to afford well‐defined polymers. Quaternary ammonium macromonomers (MMs) are prepared by quaternizing tertiary amine MMs synthesized by reversible addition‐fragmentation chain transfer (RAFT) polymerization. The quaternary ammonium MMs undergo ROMP to target molecular weights (M_n = 30 000–100 000 g mol^?1) and a low dispersity (? = 1.10–1.30). Halide‐ligand exchange between the third generation Grubbs catalyst (G3) and halide counter ions (bromide and iodide ions) of MMs changes the catalyst activity throughout ROMP, causing it to deviate from pseudo‐first order kinetic behavior; however, the polymerization still follows controlled behavior without significant catalyst termination. Increasing steric bulk of the MMs decreases the polymerization rate as well. Amphiphilic block copolymers are synthesized by sequential polymerization of quaternary ammonium MMs and polystyrene (PS) MMs. Using a PS macroinitiator affords block copolymers with lower ? values as compared to the less active cationic macroinitiator.     

The “Grafting‐to” of Well‐Defined Polystyrene on Graphene Oxide via Nitroxide‐Mediated Polymerization

The grafting of well‐defined polystyrene to graphene oxide (GO) using nitroxide‐mediated polymerization (NMP) is demonstrated by a two‐step reaction. In the first step, GO is functionalized with glycidyl methacrylate (GMA) to yield GO‐GMA. Polystyrene (PS), previously synthesized via SG1‐based NMP, is then grafted to GO‐GMA by a simple reaction between the SG1 end group and the GMA double bond to yield GO‐GMA‐g‐PS. ¹H, heteronuclear single‐quantum correlation (HSQC), nuclear magnetic resonance (NMR), X‐ray photoelectron spectroscopy (XPS), Raman, Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), and thermogravimetric analysis (TGA) are consistent with attachment of the GMA group to the GO surface and with polystyrene being grafted to the GO surface to form the GO‐GMA‐g‐PS nanocomposite (NC). GPC analysis shows a number‐average molecular weight of 3330 g mol^?1 for the PS with molecular weight dispersity (Ð) of 1.13. Up to 28 mass% of PS has been introduced into the GO NC. The present “grafting‐to” methodology holds promise for the facile and clean synthesis of graphene oxide polymer NCs.

     

Grafting‐from Afforded Cyclic Graft Copolymers from Cyclic Anionic Macroinitiator via Lithiation of Tolyl Groups

Cyclic graft copolymers consisting of poly(p‐methylstyrene) backbone and polystyrene branches with relatively low molecular weight distribution are synthesized with grafting‐from approach by living anionic polymerization of styrene from lithiated cyclic poly(p‐methylstyrene)s as multifunctional anionic macroinitiators for vinyl polymerizations. Precursors of the macroinitiators are prepared by ring‐closing metathesis reaction of α,ω‐divinyl‐terminated telechelic polymers. Subsequently, selective lithiation of tolyl groups in the cyclic polymers is conducted by s‐BuLi in the presence of tetramethylethylenediamine in cyclohexane in order to prepare the macroinitiators. Addition of styrene monomer into the cyclic macroinitiators provides shift in SEC to higher molecular weight due to graft polymerization from the macroinitiators to form cyclic graft copolymers. The unique polymer architecture of the obtained cyclic graft copolymers is confirmed by unimolecular observation by using atomic force microscopy.     

Nickel‐Mediated Surface Grafting From Polymerization of α‐Amino Acid‐N‐Carboxyanhydrides

Summary: The feasibility of a living grafting from polymerization of α‐amino acid‐N‐carboxyanhydrides (NCA) from a surface using nickel initiators was shown. The polymerization has been carried out on commercially available polystyrene resins as spherical substrates in two different ways. Firstly L ‐glutamic acid was bound to the surface as γ‐ester via a UV‐labile linker and transferred into the NCA by treatment with triphosgene. The grafting from polymerization was then carried out as a “block copolymerization” by reaction of the surface bound NCA with an excess of the Ni amido‐amidate complex initiator and subsequent addition of free NCA to grow the polymer chain. By this procedure polymer was formed at the surface and can be isolated after photolysis of the linker. The characterization of the polymer by size exclusion chromatography indicates a living polymerization at the surface. The second approach employs N‐alloc‐amides at the surface to prepare an initiating Ni amido‐amidate complex directly at the surface. It can be shown that the latter approach is much more straightforward and gives smaller quantities of non‐tethered polypeptide.

Surface bound polypeptides were obtained by ring opening polymerization of α‐amino acid‐N‐carboxyanhydrides initiated by nickel amido‐amidate complexes installed at surfaces of commercially available polystyrene resins.     

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.     

New Functional Block Copolymers via RAFT Polymerization and Polymer‐Analogous Reaction

The synthesis of block copolymers consisting of nonfunctional and reactive blocks is reported. The precursor polymers are ABA triblock copolymers, consisting of S/VBC or MMA/GMA and prepared via RAFT polymerization. The reactive blocks are converted into blocks with new functionalities that are hard to achieve by direct polymerization. The new polymers are either amphiphilic, with acidic or basic blocks on the outside and a nonfunctional core, or have functionalities that are useful for further reactions like thiol‐ene or alkyne‐azide click reactions. Reaction success and degree of functionalization are determined via FTIR, elemental analysis, and MALDI‐TOF MS. The stability of the RAFT functionalities during the modification reactions is analyzed.     

One‐Pot Synthesis of Polyester–Polyolefin Copolymers by Combining Ring‐Opening Polymerization and Carbene Polymerization

The ring‐opening polymerization (ROP) of a cyclic ester using alkyl acetate carbene (ROCOCH:) is generated from diazoacetate as organocatalyst under microwave irradiation, which enables the one‐pot preparation of copolymers of polyester and polyolefin. The chemical structure of the polymerized product is characterized by NMR, Fourier transformed infrared (FTIR), and UV–vis spectroscopy. The incorporation of the azo group into the obtained copolymer is determined by elemental analysis, which indicates that 1.38–6.21% nitrogen is contained in the obtained copolymers. The influences of catalyst and microwave irradiation parameters on the polymerization are investigated. Both the microwave power and irradiation time have great influences on the copolymerization. Moreover, the molar mass of the obtained polymers is calculated with polystyrene standards, which gradually increases from 600 to 36 100 g mol^?1 as the reaction temperature increases from 60 to 120 °C. Poly­mer with of 36 100 g mol^?1 and PDI of 1.86 is produced under optimized conditions. The combination of ROP and carbene polymerization offers a new and convenient pathway to synthesize copolymers of polyesters and polyolefins.

     

Cyclic Poly(l‐lactide) via Ring‐Expansion Polymerization by Means of Dibutyltin 4‐Tert‐Butylcatecholate

Five new catalysts are prepared from dibutyltin oxide and catechol (HCa), 2,3‐dihydroxynaphthalene (NaCa), 4‐tert‐butyl catechol (BuCa), 4‐cyano catechol (CyCa), and 4‐benzoyl catechol (BzCa), but only BuCa gives useful results. When benzyl alcohol is used as an initiator, linear chains having benzyl ester end groups are formed in a slow polymerization process. In contrast to cyclic or noncyclic dibutyltin bisalkoxides, neat BuCa yields cyclic poly(l ‐lactide)s via a fast ring‐expansion polymerization. Under certain conditions, a high‐melting crystalline phase (T _m = 191 °C) is obtained. At 160 °C and short reaction times even‐numbered cycles are slightly prevailing, but, surprisingly, at 120 °C, odd‐numbered cycles are predominantly formed. These results definitely prove that a ring‐expansion mechanism is operating.     

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).     

Surface‐Initiated RAFT Polymerization of Clay Nanoparticles with Polystyrene: New Insights Using MALDI‐TOF MS and 1H NMR

Reversible addition‐fragmentation chain transfer (RAFT)‐mediated grafting of polystyrene from clay is investigated using advanced analytical methods to elucidate the polymerization mechanism occurring at the clay surface and in solution. The chain‐end structures of the grafted polymer grown from a surface‐attached cationic functionalized RAFT agent, and the free polymer chains generated from a non‐functionalized RAFT agent are investigated using size exclusion chromatography, matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) mass spectrometry, MALDI‐TOF–collision‐induced dissociation mass spectrometry, and ¹H NMR spectroscopy. The results obtained show that free and surface‐confined polymer chains undergo significant conventional chain‐transfer reactions in addition to reversible addition–fragmentation chain transfer. This information provides additional new insight into the polymerization mechanism.

     

Anionic Synthesis of Epoxy End‐Capped Polymers

The reaction of living anions of polystyrene (PS) or poly(methyl methacrylate) (PMMA) with epibromohydrin for the synthesis of well‐defined epoxy end‐functionalized polymers is reported. Polyanions were reacted with an excess of epibromohydrin in tetrahydrofuran (THF) at ?78 °C. The functionalities of the resulting polymers were analyzed by matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS), NMR, and size exclusion chromatography (SEC). The epoxy end groups were reacted with 1,1‐diphenyl‐ hexyllithium, and MALDI‐TOF MS and NMR before and after this chemical modification were used to determine the presence of the epoxy end groups. The presence of the epoxy end group was confirmed by anionically polymerizing ethylene oxide from these epoxy end group. The formation of a block copolymer due to the epoxy end groups was proved by SEC analysis. The combined MALDI‐TOF MS, ¹H NMR, and SEC results indicate that epoxy end‐capped PS was obtained in quantitative yield. The method was extended to the synthesis of epoxy end‐capped PMMA. With this polymer the extent of end‐functionalization was high but not quantitative, with non‐dimeric byproducts detected by MALDI‐TOF MS.

     

MALDI‐TOF Analysis of Halogen Telechelic Poly(methyl methacrylate)s and Poly(methyl acrylate)s Prepared by Atom Transfer Radical Polymerization (ATRP) or Single Electron Transfer‐Living Radical Polymerization (SET‐LRP)

Poly(methyl methacrylate)s (PMMA)s and poly(methyl acrylate)s (PMA)s are prepared by atom transfer radical polymerization (ATRP) or single electron transfer‐living radical polymerization (SET‐LRP) using methyl dichloroacetate (MDCA) and ethyl dibromoacetate (EDBA) as bifunctional initiators. The chain‐end functionality is determined by MALDI‐TOF mass spectrometry. The target PMMA (M_n = 2000 g mol^?1) and PMA (M_n = 2000 g mol^?1) samples obtained by ATRP of MMA and MA with MDCA as initiator have 12 and 81 mol% bis‐chloro end groups, respectively; those prepared by SET‐LRP have 57 and 100 mol% bis‐chloro end groups, respectively. The PMMAs obtained by ATRP or SET‐LRP with EDBA have no bromine end groups.

     

Preparation of Monodisperse Crosslinked Organic‐Inorganic Hybrid Copolymer Particles by Dispersion Polymerization

This study presents a new method for producing monodisperse crosslinked organic‐inorganic hybrid polymer particles in micron‐size by using simple dispersion polymerization. Firstly, highly monodisperse hybrid copolymer particles were prepared by conventional dispersion polymerization of styrene and 3‐(trimethoxysilyl)propyl methacrylate (TMSPM) in a methanol/water medium. It was very interesting to find that slightly crosslinked hybrid polymer particles were formed by this simple dispersion polymerization due to a partial condensation reaction between the adjacent silanol groups of the TMSPM during the polymerization. Secondly, for the hydrolytic condensation of the remaining trimethoxysilyl groups, post treatment by sol‐gel process was carried out to form an inorganic siloxane network, which provides the particle with high crosslinking density. In the proper conditions, monodisperse crosslinked particles could be produced in high TMSPM concentrations, up to 20 wt.‐%. In addition, the size monodispersity of the hybrid polymer particles was maintained after the sol‐gel process. Crosslinking with inorganic networks formed by the sol‐gel process was confirmed by a thermal analysis and a ²⁹Si NMR spectroscopy.

Synthesis of crosslinked organic‐inorganic hybrid polymer particles by dispersion copolymerization and post sol‐gel process.     

Post‐Polymerization Thiol Substitutions Facilitated by Mechanochemistry

The use of mechanical force to facilitate post‐polymerization, solvent‐free thiol substitution reactions is described. These reactions are amenable to halogen‐containing materials prepared using ring opening metathesis polymerization and free radical polymerization reactions. Reactions of these polymers with various thiols can be carried out in a ball mill and are complete in a matter of minutes. Further, ¹H NMR and GPC analysis show no significant decomposition of the polymer chain.     

Functionalization of Shortened Single‐Walled Carbon Nanotubes with Poly(p‐dioxanone) by “Grafting‐From” Approach

Summary: It has been a real challenge to form carbon nanotube (CNT)/polymer composites where CNTs are well‐dispersed in the polymer matrix and the interactions between CNTs and polymers are effectively strong. In this paper, we applied surface‐initiated, ring‐opening polymerization (SI‐ROP) of p‐dioxanone (PDX) to shortened single‐walled carbon nanotubes (s‐SWCNTs) and successfully formed s‐SWCNT/PPDX composites (see Figure). Due to intimate interactions between s‐SWCNTs and PPDX, we observed dramatic changes in PPDX properties upon the formation of the composites: 10%‐weight‐loss‐temperature of PPDX increased by 20 °C (measured by thermogravimetric analysis) and the patterns of T_g and T_m were greatly altered. We did not observe any noticeable peaks from the composite up to 120 °C in differential scanning calorimetry (DSC), while DSC data of PPDX itself showed T_g and T_m at ?13.4 and 103 °C respectively.

Schematic representation of the procedure for formation of s‐SWCNT/PPDX composites.     

Formation of Inorganic/Organic Nanocomposites by Nitroxide‐Mediated Polymerization in Bulk Using a Bimolecular System

Summary: A series of organic‐inorganic nanoparticles were synthesized by nitroxide‐mediated polymerization (NMP) of butyl acrylate initiated by a self‐assembled monolayer of an azo initiator. The azo initiator was immobilized on silica particles in the presence of a stable nitroxide radical, SG1 (an acyclic β‐phosphonylated nitroxide, N‐tert‐butyl‐N‐(1‐diethylphosphono‐2,2‐dimethyl)propyl nitroxide). After preliminary qualitative characterization by X‐ray spectroscopy (XPS) and Fourier‐transform infrared (FTIR) measurements, the nanoparticles were studied by thermogravimetric analysis (TGA) to determine the polymer grafting density and to permit a comparison with corresponding values of the initiator monolayer. It was demonstrated that the grafting from polymerization exhibits a controlled character with a low polydispersity ( < 1.2) in a large range of molecular weights of the grafted chains (from 4 000 up to 145 000 g · mol^?1) under the conditions when the stable radical SG1, acting as chain growth moderator tethered to the inorganic core, was used.

     

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.     

Anionic Polymerization of Ethylene Oxide in the Presence of the Phosphazene Base ButP4 – Kinetic Investigations Using In‐Situ FT‐NIR Spectroscopy and MALDI‐ToF MS

Fourier‐transform near‐infrared (FT‐NIR) fiber‐optic spectroscopy was successfully used to monitor the anionic polymerization of ethylene oxide (EO). Kinetic data are provided for the polymerization of EO with the sec‐BuLi/Bu^tP₄ initiating system under different reaction conditions. In addition, the influence of different initiators and reaction conditions on the polymerization of EO is investigated. Online monitoring using NIR spectroscopy reveals an unexpected induction period present in EO homopolymerizations as well as in the synthesis of PEO containing block copolymers with [Li/Bu^tP₄]⁺ counterions. The resulting polymers are characterized by size exclusion chromatography (SEC). A low‐molecular‐weight polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) diblock copolymer was synthesized to gain more insight into the observed induction period by matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry (MALDI‐ToF MS) on samples taken during EO polymerization. The induction period is believed to be a result of different factors involved in the formation of active centers, for example, the break up of lithium alkoxide aggregates by the phosphazene base Bu^tP₄, and chain length effects. It depends on reaction temperature, concentration of the phosphazene base Bu^tP₄, as well as the structure of the initiator.

SEC traces for EO homopolymerizations using different initiating systems at 50 °C.     

Activation–Deactivation Equilibrium Associated With Iron‐Mediated Atom‐Transfer Radical Polymerization up to High Pressure

The equilibrium constant, K_model, of iron‐mediated atom‐transfer radical polymerization (ATRP) is investigated for FeBr₂/α‐bromoester model systems in the absence of monomer. Quantitative analysis is carried out in solution of either N‐methylpyrrolidin‐2‐one (NMP) or acetonitrile (MeCN) via high‐pressure online VIS/NIR spectroscopy monitoring the formation of the Fe^III species. The reaction volume is determined to be ΔV_R = 13 ± 3 cm³ mol^?1, which indicates that K_model decreases with pressure. This observation points to preferred formation of catalytically inactive or less active iron–solvent complexes toward higher pressure. Thus suitable solvent selection is important for the development of efficient iron‐based ATRP catalysts.     

Controlled Grafting‐From of Polystyrene on Polybutadiene: Mechanism and Spectroscopic Evidence of the Functionalization of Polybutadiene with 4‐Oxo‐TEMPO

Polybutadienes functionalized with nitroxide (multifunctional macroalkoxyamines) were synthesized by heating PB in the presence of a nitroxide radical (N) and a radical initiator (I). Another functional PB was prepared by using only nitroxide. These functionalized polymers were characterized by FT‐IR, GPC, and NMR and as a result it was possible to elucidate the resulting structure of the macroalkoxyamines, to propose likely functionalization mechanisms, and to estimate functionalization characteristics. One of these polymers was heated in the presence of styrene to obtain PB grafted with polystyrene, which was characterized in detail in order to investigate its structure and level of grafting control.