Good Molecular Weight Control Research Articles

Coordination of ε-Caprolactone to a Cationic Niobium(V) Alkoxide Complex: Fundamental Insight into Ring-Opening Polymerization via Coordination-Insertion.

We report three niobium-based initiators for the catalytic ring-opening polymerization (ROP) of ε-caprolactone, exhibiting good activity and molecular weight control. In particular, we have prepared on the gram-scale and fully characterized a monometallic cationic alkoxo-Nb(V) ε-caprolactone adduct representing, to the best of our knowledge, an unprecedented example of a metal complex with an intact lactone monomer and a functional ROP-initiating group simultaneously coordinated at the metal center. At 80 °C, all three systems initiate the immortal solution-state ROP of ε-caprolactone via a coordination-insertion mechanism, which has been confirmed through experimental studies, and is supported by computational data. Natural bond orbital calculations further indicate that polymerization may necessitate isomerization about the metal center between the alkoxide chain and the coordinated monomer. The observations made in this work are expected to inform mechanistic understanding both of amine tris(phenolate)-supported metal alkoxide ROP initiators, including various highly stereoselective systems for the polymerization of lactides and of coordination-insertion-type ROP protocols more broadly.

Fast Ring-Opening Metathesis Polymerization of Tricyclic Oxanorbornene Derivatives

A new type of tricyclic oxanorbornene monomer was developed for living ring-opening metathesis polymerization (ROMP). A methyl group located at the endoposition of the oxanorbornene bicycle resulted in polymerization kinetics orders of magnitude slower than the corresponding hydrogen-substituted monomers because of steric effects. Both monomers showed well-controlled molecular weights and excellent dispersities with Grubbs’ third-generation catalyst. The slower endomethyl substituted monomers could even be polymerized in a well-controlled manner using Grubbs’ second-generation catalyst. Investigations of random copolymers demonstrated that the steric hindrance only affects the rate of propagation rather than the rate of initiation. Block copolymers between the fast and slow monomers could be prepared with good molecular weight and dispersity control.

Vinyl-Addition Polymerizations of Borylated Allenes

Borylated motifs hold immense potential for functional soft materials, as they can facilitate a range of reaction manifolds (e.g., analyte binding or cross-coupling). Traditional boron-containing materials are predominantly composed of either aryl boronates or main-chain embedded vinylic boronates. Progress toward expanding the architectural (and, by extension, functional) scope of borylated polymers remains slightly limited. To address this, we report the vinyl-addition polymerization of an allenylboronic acid pinacol ester. The base metal catalyst, [(η3-C3H5)Ni(OC(O)CF3)]2, was found to readily polymerize this allene monomer with good regioselectivity and molecular weight control. Additionally, the allenylboronate monomer could be copolymerized with other allenes (e.g., 1,2-nonadiene or β-pinadiene) to access an array of borylated materials. The borylated polyallenes could also be chemically tailored under mild conditions (e.g., using Pd-catalyzed cross-coupling), which suggests that they could serve as general platforms for accessing diverse architectures.

Simple polymerization through oxygen at reduced volumes using oil and water

AbstractAn enduring question is: what is the simplest and easiest way to obtain tailored polymers? This communication explores a robust photoiniferter polymerization with only two active ingredients that requires no prior deoxygenation and can be performed on the milliliter scale or sub‐milliliter scale. Rather than leaving headspace in the polymerization vessel or scaling reactions up to fill the vessel, this approach fills the headspace of the reaction vessel with mineral oil or inert solvents. This approach can also be applied to polar monomers in aqueous media, using oil as the inert solvent, or to hydrophobic monomers with water as the inert solvent. This method removes enough ambient oxygen that the photoiniferter reaction proceeds with no deoxygenation step, and achieves high conversion and good molecular weight control in 10–20 h in both aqueous and organic solvents. Complex polymer architectures such as multiblock copolymers and gradient polymers were successfully synthesized by this approach.

Synthesis, Characterization, and Reactivity of N-Alkyl Phenoxazines in Organocatalyzed Atom Transfer Radical Polymerization.

Core-modified N-alkyl phenoxazines were synthesized and evaluated as photoredox catalysts (PCs) in organocatalyzed atom transfer radical polymerization (O-ATRP). Each PC was characterized and found to strongly absorb UVA and visible light, undergo reversible oxidation to radical cation species, and exhibit high quantum yields of fluorescence (Φf > 77%). PCs found to exhibit intramolecular charge transfer in their excited state were capable of the controlled synthesis of poly(methyl methacrylate) with good molecular weight control (I* ~ 100%) and moderately low dispersity (Đ < 1.30).

Cascade Ring-Opening/Ring-Closing Metathesis Polymerization of a Monomer Containing a Norbornene and a Cyclohexene Ring.

Norbornene is polymerized extremely fast when reacted with Grubbs' first (G1) or third generation catalyst (G3) because of its very high ring strain energy. Cyclohexene, on the other hand, cannot be polymerized using G1 or G3 due to its very low ring strain energy. Subsequently, the sequence-selective polymerization of these two monomers is extremely challenging. A sequence-selective cascade ring-opening/ring-closing metathesis polymerization of the monomer M containing both the norbornene and the cyclohexene ring using G1 or G3 is reported. The polymer structure was analyzed by 1H NMR, 1H-1H COSY, and 1H-1H ROESY spectroscopy and MALDI-ToF mass spectrometry. Polymers with moderate molecular weight dispersities and good molecular weight control were achieved by varying the ratio between monomer M and G1.

Zinc Complexes for PLA Formation and Chemical Recycling: Towards a Circular Economy.

A series of ZnII complexes, based on propylenediamine Schiff bases, have been prepared and fully characterized. X-ray crystallography and NMR spectroscopy identified significant differences in the solid and solution state for the ZnII species. All complexes have been applied to the ring-opening polymerization of l-lactide with emphasis on industrial conditions. High conversion and good molecular weight control were generally achievable for Zn(A-D)2 , and high-molecular-weight poly(lactic acid) (PLA) was prepared in 1 min at a 10 000:1:33 [lactide]/[Zn]/[BnOH] loading. The more active ZnII catalysts were also applied to PLA degradation to alkyl lactate under mild conditions. Zn(A-B)2 demonstrated high activity and selectivity in this process with PLA being consumed within 1 h at 50 °C. Zn(C-D)2 were shown to be less active, and these observations can be related to the catalysts' structure and the degradation mechanism. Initial results for the degradation of poly(ethylene terephthalate) and mixed feeds are also presented, highlighting the broader applicability of the systems presented.

Dinuclear Pentacoordinated Organoyttrium Biphenolates as Initiators for the Synthesis of High-Molecular Weight Isotactic Poly(2-vinylpyridine).

Dinuclear organoyttrium biphenolato complexes have been synthesized for the first time starting from Y(CH2SiMe3)3(THF)2 and readily available biphenols with bulky substituents at the 3,3'-positions. XRD analysis revealed that these complexes are formed as homochiral dimers even though racemic biphenols were employed. Such dinuclear penta-coordinated yttrium complexes were evaluated as initiators in the polymerization of 2-vinylpyridine (2-VP). High activity and excellent isotacticity (mm > 99%) were observed. DSC analysis of poly(2-VP) showed a Tm = 221 °C, which is the highest value reported to date. Attempts to improve the low initiation efficiency of the present system were made by co-use with several organo-main-group metal reagents. Specifically, good molecular weight control within the high Mn region was achieved by the addition of Et3Al (Al/Y = 10).

Catalytic living ring-opening metathesis polymerization with Grubbs' second- and third-generation catalysts.

In a conventional living ring-opening metathesis polymerization (ROMP), an equal number of ruthenium complexes to the number of polymer chains synthesized are required. This can lead to high loadings of ruthenium complexes when aiming for shorter polymers. Here, a reversible chain-transfer agent was used to produce living ROMP polymers from norbornene derivatives using catalytic amounts of Grubbs' ruthenium complexes. The polymers obtained by this method showed all of the characteristics of a living polymerization (that is, good molecular weight control, narrow molecular weight dispersities and the ability to form block copolymers). Monomers carrying functional moieties such as ferrocene, coumarin or a triisopropylsilyl-protected primary alcohol could also be catalytically polymerized in a living fashion. The method presented follows a degenerative chain-transfer process and is more economical and environmentally friendly compared with previous living ROMP procedures as it utilizes only catalytic amounts of costly and toxic ruthenium complexes.

ATRP of N‐Hydroxyethyl Acrylamide in the Presence of Lewis Acids: Control of Tacticity, Molecular Weight, and Architecture

Good control of tacticity, molecular weight, and architecture is attained via atom transfer radical polymerization (ATRP) of N-hydroxyethyl acrylamide (HEAA), in a one-pot process in the presence of Y(OTf)3 . The effect of temperature, ratio of [Y(OTf)3 ]/[HEAA], and ATRP procedure on the tacticity and degree of control over the polymerization is investigated in detail. Under optimal conditions, using photo ATRP and 15% Y(OTf)3, the content of meso dyads (m) can be increased from 42% to 80% in a homopolymer with a dispersity D=1.22. Well-defined stereoblock copolymers, atactic- b -isotactic poly(HEAA), with D=1.27, are obtained by adding Y(OTf)3 at a specific conversion, initially started without Y(OTf)3 .

Iron(III) Salalen Complexes for the Polymerisation of Lactide

Herein, we report the preparation and characterisation of iron(III) salalen complexes, with variation of ligand substituents and backbone investigated. Six new complexes were prepared and characterised by elemental analysis, mass spectrometry and X‐ray crystallography. These complexes have been applied for the ring opening polymerisation (ROP) of rac‐lactide in propylene oxide. Fe(1)Cl was found to have a moderate isotactic preference (Pm = 0.75–0.80) and demonstrated good molecular weight control in solution (Đ = 1.02–1.18). Fe(2–7)Cl were also active for ROP and activities could be related to ligand structure.

Polymer Synthesis with More Than One Form of Living Polymerization Method.

Controlled radical polymerization (CRP) or controlled/living radical polymerization has revolutionized the polymer industry as a tool for the preparation of a wide variety of polymers. This process enables the preparation of polymers with good control of molecular weight, narrow polydispersity, and a range of architectures including block and graft copolymers, star polymers, and other functional polymers. The mechanistic transformation reaction provides a great opportunity to tune chemical and physical properties of copolymers. It can be applied to combine different homopolymers using post-modification techniques or by the use of a dual initiator, allowing the combination of mechanistically distinct polymerization reactions. This review will cover CRP transformations including the synthesis of block copolymers with both linear structures (AB, ABA, (AB) n , multiblocks, etc.) and branched macromolecular architectures (graft, miktoarm star, and dendritic-like), which are obtained by a combination of more than one form of living polymerization reaction.

Organocatalytic Synthesis and Evaluation of Polycarbonate Pendant Polymer:β-Cyclodextrin-Based Nucleic Acid Delivery Vectors

A family of mPEG-b-polycarbonate (mPEG-PC) diblock pendant polymers were synthesized from trimethylene carbonate and other cyclic carbonate monomers bearing hydrophobic guest ligands via organocatalytic ring-opening polymerization using 1,4,5-triazabicyclo[4.4.0]dec-5-ene catalyst or 1,8-diazabicyclo[5.4.0]undec-7-ene/thiourea cocatalyst. Diblock copolymers composed of a methoxypoly(ethylene oxide) (mPEG) block and a polycarbonate block containing either homopolymer or mixed polycarbonates (PC) were prepared by homopolymerization or copolymerization of the cyclic carbonate monomers in the presence of mPEG2000 or mPEG5000 initiator to give materials having a tunable pendant group density along the polycarbonate backbone. Polycarbonate blocks targeting the 2.4–10 kDa range were prepared with good molecular weight control and modest polydispersities (averaging ∼1.3). Complexation of plasmid DNA with β-cyclodextrin–polyethylenimine2.5 kDa produced nanoparticle cores that were then coated with the mPEG–PC dibl...

Synthesis and characteristics of novel azo-based diblock copolymers and their self-assembly behavior via solvents and thermal annealing

AbstractA series of azo-based diblock copolymers (DBCs) with various compositions were successfully synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization in anisole with PCAEMA-CTA (macro-CTA), DOPAM (new acrylamide monomer) and AIBN (initiator). Kinetic studies on diblock copolymerization manifested a controlled/living manner with good molecular weight control. Structures and properties of monomers and DBCs were determined by 1H nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC) and gel permeation chromatography (GPC). Liquid crystalline (LC) phases and morphological properties were investigated using optical polarizing microscope (OPM), atomic force microscopy (AFM), scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS). Experimental results demonstrated that the prepared PCAEMA-CTA and DBCs possessed low polydispersity index (≤1.37). All DBCs revealed sharp endothermic transition peaks corresponding to the smectic-to-nematic phase. DBCs with high azo contents showed batonnet textures of the smectic phase whereas DBCs of low azo segments displayed threaded textures of the nematic phase. DBC with 49 wt% of azo side-chains generated a lamellar compared to DBCs with low azo block (≤41 wt%) or non-azo block (≤38 wt%) which produced hexagonal-type nanostructures. In addition, all DBCs exhibited reversible trans-cis photoisomerization behavior under UV irradiation and dark storage at different intervals of time.

Atom transfer radical polymerization of methyl methacrylate using copper-based homogeneous and heterogeneous catalysts

This study aimed at polymerization of methyl methacrylate with novel catalysts in the atom transfer radical polymerization (ATRP) condition at 90 °C. This was accomplished using CuBr/N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (CuBr–AEAPTMS) as a homogeneous catalyst and one time with CuBr@AEAPTMS/SBA-15 as a heterogeneous catalyst. Catalysts were characterized using TGA, FT-IR, and UV–Vis spectroscopy. The structural analysis of the polymer was carried out by 13C NMR spectroscopy and GPC. Three characteristic parts of polymer produced by ATRP method including the initiator, monomer units, and end group was shown in 13C NMR spectra. In addition, the presence of C–Br unit showed that the polymerization process is alive. The 1H NMR analysis was used for kinetic investigation of methyl methacrylate polymerization with homogeneous and heterogeneous catalysts that showed high monomer conversion (98 and 90% after 35 min, respectively) and good control of molecular weight with a dispersity (Ð = 1.5–1.7). In addition, the plot of ln ([monomer]0/[monomer] t ) versus time gave linear relationships indicating a constant concentration of the propagating species throughout the polymerization. Finally, the results of the polymerization using heterogeneous catalyst compared with homogeneous catalyst revealed that it was according to ATRP method.

The Kinetics of n‐Butyl Acrylate Radical Polymerization Revealed in a Single Experiment by Real Time On‐line Mass Spectrometry Monitoring

The high‐temperature trithiocarbonate‐mediated reversible addition–fragmentation chain transfer (RAFT) polymerization of n‐butyl acrylate is studied using a high‐resolution mass spectrometric on‐line monitoring method. Therefore, an Orbitrap mass spectrometer, ideally suited for polymer analysis, is coupled to a continuous flow microreactor. Via adjustment of flow rates in the reactor, time sweep experiments can be carried out to allow monitoring a whole polymerization in a single experiment. The n‐butyl acrylate polymerization is monitored in a temperature interval between 100 and 190 °C for reaction times between 1 and 10 minutes. In all cases, full monomer conversions are reached and even at the highest temperatures still relatively good molecular weight control and reasonably low dispersities are obtained. End group analysis from mass spectrometry reveals, however, that the desired RAFT product is already at the lower temperatures less abundant than some side products. The herein described on‐line monitoring technique gives access to quasi‐continuous data acquisition, and is found to be very robust, and low in scattering when compared to classical batch sampling techniques. image

Metal-Free Atom Transfer Radical Polymerization of Methyl Methacrylate with ppm Level of Organic Photocatalyst.

It is well known that the recently developed photoinduced metal-free atom transfer radical polymerization (ATRP) has been considered as a promising methodology to completely eliminate transition metal residue in polymers. However, a serious problem needs to be improved, namely, large amount of organic photocatalysts should be used to keep the controllability over molecular weights and molecular weight distributions. In this work, a novel photocatalyst 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) with strong excited state reduction potential is successfully used to mediate a metal-free ATRP of methyl methacrylate just with parts per million (ppm) level usage under irradiation of blue light emitting diode at room temperature, using ethyl α-bromophenyl-acetate as a typical initiator with high initiator efficiency. The polymerization kinetic study, multiple controlled "on-off" light switching cycle regulation, and chain extension experiment confirm the "living"/controlled features of this promising photoinduced metal-free ATRP system with good molecular weight control in the presence of ppm level photocatalyst 4CzIPN.

Single-Site Heterogeneous Catalysts for Olefin Polymerization Enabled by Cation Exchange in a Metal-Organic Framework.

The manufacture of advanced polyolefins has been critically enabled by the development of single-site heterogeneous catalysts. Metal-organic frameworks (MOFs) show great potential as heterogeneous catalysts that may be designed and tuned on the molecular level. In this work, exchange of zinc ions in Zn5Cl4(BTDD)3, H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin) (MFU-4l) with reactive metals serves to establish a general platform for selective olefin polymerization in a high surface area solid promising for industrial catalysis. Characterization of polyethylene produced by these materials demonstrates both molecular and morphological control. Notably, reactivity approaches single-site catalysis, as evidenced by low polydispersity indices, and good molecular weight control. We further show that these new catalysts copolymerize ethylene and propylene. Uniform growth of the polymer around the catalyst particles provides a mechanism for controlling the polymer morphology, a relevant metric for continuous flow processes.

Highly stereoselective bimetallic complexes for lactide and ε-caprolactone polymerization

A series of Schiff-base compounds containing bimetallic ligands and two aluminum centers have been synthesized and characterized by 1H, 13C NMR and elemental analysis. These compounds could be successfully used as catalysts for the polymerization of rac-lactide and e-caprolactone. The polylactides (PLAs) prepared with these compounds were highly isotactic enriched (Pm = 0.97). The nature and steric hindrance of the ligands coordinated to the central metal ions remarkably influenced the polymer properties. Kinetic studies revealed that the polymerization was first order with respect to each of these compounds and lactide/caprolactone monomers. All the polymerizations were living, with good molecular weight control and relatively narrow molar mass distributions.

Synthesis of amphiphilic homopolymers with high chain end functionality by SET–LRP

ABSTRACTSingle electron transfer‐living radical polymerization (SET–LRP) of two amphiphilic acrylates, 2‐methoxyethyl acrylate up to [M]0/[I]0 = 1,000 and di(ethylene glycol) 2‐ethylhexyl ether acrylate up to [M]0/[I]0 = 200, is accomplished with good control of molecular weight and molecular weight distribution in 2,2,2‐trifluoroethanol at 25 °C using hydrazine activated Cu(0) wire as catalyst, methyl 2‐bromopropionate as initiator, and Me6‐TREN as ligand. The chain end functionality of the resulting polymers has been analyzed by MALDI‐TOF spectrometry to demonstrate the synthesis of perfect or near‐perfect chain‐end functional amphiphilic homopolymers. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 294–303