Mechanistic transformations involving living and controlled/living polymerization methods

Chiral Acetal-Based Stereo-Controlled Degradable Polymer Synthesis

The precise synthesis of chiral polymers remains a significant challenge in polymer chemistry, particularly for applications in advanced biomedical and electronic materials. The development of degradable polymers is important for eco-friendly and advanced materials. Here, we introduce a stereo-controlled degradable polymer via cascade enyne metathesis polymerization and enantioselective acetal synthesis through Pd-catalyzed asymmetric hydroamination. This approach allows for the creation of chiral acetal-based polymers with controlled stereochemistry and degradability, highlighting their potential for use in drug delivery and electronic applications. This concept article reviews the background, development, and potential applications of these stereo-controlled degradable polymers.

A Comparative Clinical Performance of Polymethyl Methacrylate (PMMA) and Urethane Dimethacrylate (UDMA) Materials in Provisional Fixed Prostheses

AIM

This study aimed to compare the clinical outcomes of polymethyl methacrylate (PMMA) and urethane dimethacrylate (UDMA) as materials for fixed provisional restorations, focusing on handling properties, chair time, and periodontal outcomes, due to their prevalent use in dental practice.

MATERIALS AND METHODS

A comparative clinical study was conducted with 150 patients at the Department of Fixed Prosthodontics, Damascus University, Damascus, Syria. Patients undergoing prosthetic treatments with crowns and bridges received two fixed provisional restorations using a direct approach. The first, made immediately after abutment preparation, used PMMA. The second, created post-clinical try-in of the final restoration, utilized UDMA. Both restorations were maintained for one week. We assessed chair time, handling properties via the Visual Analog Scale (VAS), and periodontal health using the Plaque Index and Gingival Index. The Kolmogorov-Smirnov test was used to assess data normality. Differences between the two groups in the outcome variables were analyzed using the Mann-Whitney U test. The level of significance was set at (P < 0.05).

RESULTS

The handling properties of chemically activated PMMA resin were superior to those of light-activated UDMA resin. However, UDMA resin outperformed in terms of chair time and periodontal outcomes. The mean chair time was 9.45 ± 1.01 minutes for PMMA and 4.40 ± 0.77 minutes for UDMA. Mild gingivitis or plaque accumulation was observed in 57.3% of PMMA restorations and 44.0% of UDMA restorations. Moderate gingivitis and plaque accumulation were noted in 42.7% of PMMA restorations, while 56.0% of UDMA restorations showed no plaque accumulation or gingival inflammation.

CONCLUSION

Chemically activated PMMA resin offers excellent handling properties, whereas light-activated UDMA resin provides advantages in chair time and periodontal health, making it a preferable choice for provisional restorations. Limitations and future research: The study was limited to short-term outcomes and did not assess the long-term durability of the restorations or their aesthetic impact on patient satisfaction. Further studies are recommended to evaluate the long-term performance of these materials, their cost-effectiveness, and their aesthetic outcomes to provide a more comprehensive understanding of their clinical utility.

Nanogels: Smart tools to enlarge the therapeutic window of gene therapy

Gene therapy can potentially treat a great number of diseases, from cancer to rare genetic disorders. Very recently, the development and emergency approval of nucleic acid-based COVID-19 vaccines confirmed its strength and versatility. However, gene therapy encounters limitations due to the lack of suitable carriers to vectorize therapeutic genetic material inside target cells. Nanogels are highly hydrated nano-size crosslinked polymeric networks that have been used in many biomedical applications, from drug delivery to tissue engineering and diagnostics. Due to their easy production, tunability, and swelling properties they have called the attention as promising vectors for gene delivery. In this review, nanogels are discussed as vectors for nucleic acid delivery aiming to enlarge gene therapy's therapeutic window. Recent works highlighting the optimization of inherent transfection efficiency and biocompatibility are reviewed here. The importance of the monomer choice, along with the internal structure, surface decoration, and responsive features are outlined for the different transfection modalities. The possible sources of toxicological endpoints in nanogels are analyzed, and the strategies to limit them are compared. Finally, perspectives are discussed to identify the remining challenges for the nanogels before their translation to the market as transfection agents.

Sulfonated Block Copolymers: Synthesis, Chemical Modification, Self-Assembly Morphologies, and Recent Applications

Scientific research based on the self-assembly behavior of block copolymers (BCs) comprising charged-neutral segments has emerged as a novel strategy mainly looking for the optimization of efficiency in the generation and storage of electrical energy. The sulfonation reaction re- presents one of the most commonly employed methodologies by scientific investigations to reach the desired amphiphilic character, leading to enough ion concentration to modify and control the entire self-assembly behavior of the BCs. Recently, several works have studied and exploited these changes, inducing improvement on the mechanical properties, ionic conduction capabilities, colloidal solubility, interface activity, and stabilization of dispersed particles, among others. This review aims to present a description of recent works focused on obtaining amphiphilic block copolymers, specifically those that were synthesized by a living/controlled polymerization method and that have introduced the amphiphilic character by the sulfonation of one of the segments. Additionally, relevant works that have evidenced morphological and/or structural changes regarding the pristine BC as a result of the chemical modification are discussed. Finally, several emerging practical applications are analyzed to highlight the main drawbacks and challenges that should be addressed to overcome the development and understanding of these complex systems.

Syndiotactic Poly(4-methyl-1-pentene)-Based Stereoregular Diblock Copolymers: Synthesis and Self-Assembly Studies

Syndiotactic poly(4-methyl-1-pentene) (sP4M1P)-based stereoregular diblock copolymers, namely sP4M1P-b-polystyrene and sP4M1P-b-polymethylmethacrylate, were prepared from an α-bromoester-capped sP4M1P macroinitiator, which was chain extended with styrene and methyl methacrylate, respectively, via the atom transfer radical polymerization reaction. The α-bromoester-capped sP4M1P was generated by the esterification of hydroxyl-capped sP4M1P with α-bromoisobutyryl bromide. The hydroxyl-capped sP4M1P was synthesized by inducing a selective chain transfer reaction to aluminum during the syndiospecific polymerization of 4-methyl-1-pentene in the presence of a syndiospecific metallocene catalyst. As stereoregular diblock copolymers are difficult to prepare using existing methods, the current study offers an effective process for the preparation of sP4M1P-based stereoregular diblock copolymers. These copolymers were found to have well-defined architectures and they can undergo molecular self-assembly into ordered nanostructures, as evidenced by small-angle X-ray scattering analyses.

Block Copolymer Synthesis by a Sequential Addition Strategy from the Organocatalytic Group Transfer Polymerization of Methyl Methacrylate to the Ring-Opening Polymerization of Lactide

Sequential block copolymerization involving comonomers belonging to different classes, e.g., a vinyl-type monomer and a heterocycle, is a challenging task in macromolecular chemistry, as corresponding propagating species do not interconvert easily from one to the other by crossover reactions. Here, it is first evidenced that 1-methoxy 2-methyl 1-trimethylsilyloxypropene (MTS), i.e., a silyl ketene acetal (SKA)-containing initiator, can be used in presence of the P₄ -t-Bu phosphazene organic base to control the ring-opening polymerization (ROP) of racemic lactide (rac-LA). The elementary reaction, which rapidly transforms SKA groups into propagating alkoxides, can be leveraged to directly synthesize well-defined poly(methyl methacrylate)-b-polylactide (PMMA-b-PLA) block copolymers. This is achieved using P₄ -t-Bu as the single organic catalyst and MTS as the initiator for the group transfer polymerization (GTP) of methyl methacrylate (MMA), followed by the ROP of rac-LA. Both polymerization methods are implemented under selective and controlled/living conditions at room temperature in THF. This sequential addition strategy further expands the scope of organic catalysis of polymerizations for macromolecular engineering of block copolymers involving propagating species of disparate reactivity. This article is protected by copyright. All rights reserved.

Photoinduced Controlled/Living Polymerizations

The application of photochemistry in polymer synthesis is of interest due to the unique possibilities offered compared to thermochemistry, including topological and temporal control, rapid polymerization, sustainable low-energy processes, and environmentally benign features leading to established and emerging applications in adhesives, coatings, adaptive manufacturing, etc. In particular, the utilization of photochemistry in controlled/living polymerizations often offers the capability for precise control over the macromolecular structure and chain length in addition to the associated advantages of photochemistry. Herein, the latest developments in photocontrolled living radical and cationic polymerizations and their combinations for application in polymer syntheses are discussed. This Review summarizes and highlights recent studies in the emerging area of photoinduced controlled/living polymerizations. A discussion of mechanistic details highlights differences as well as parallels between different systems for different polymerization methods and monomer applicability.

Synthesis of Block Copolymers by Mechanistic Transformation from Reversible Complexation Mediated Living Radical Polymerization to the Photoinduced Radical Oxidation/Addition/Deactivation Process

A versatile strategy for the fabrication of block copolymers by the combination of two discrete living polymerization techniques─reversible complexation mediated living radical polymerization (RCMP) and photoinduced radical oxidation addition deactivation (PROAD) processes─is reported. First, RCMP is conducted to yield poly(methyl methacrylate) with iodide end groups (PMMA-I). In the following step, PMMA-I is used as macroinitiator for living PROAD cationic polymerization of isobutyl vinyl ether. Successful formation of the block copolymers is confirmed by 1H NMR, FT-IR, GPC, and DSC investigations.

Recent Advances in the Synthesis of Complex Macromolecular Architectures Based on Poly(N-vinyl pyrrolidone) and the RAFT Polymerization Technique

Recent advances in the controlled RAFT polymerization of complex macromolecular architectures based on poly(N-vinyl pyrrolidone), PNVP, are summarized in this review article. Special interest is given to the synthesis of statistical copolymers, block copolymers, and star polymers and copolymers, along with graft copolymers and more complex architectures. In all cases, PNVP is produced via RAFT techniques, whereas other polymerization methods can be employed in combination with RAFT to provide the desired final products. The advantages and limitations of the synthetic methodologies are discussed in detail.

The challenges of controlling polymer synthesis at the molecular and macromolecular level

In this Perspective, we outline advances and challenges in controlling the structure of polymers at various size regimes in the context of structural features such as molecular weight distribution, end groups, architecture, composition and sequence.

Recent advances in externally controlled ring-opening polymerisations

Switchable catalysis is a powerful tool in the polymer chemist's toolbox as it allows on demand access to a variety of polymer architectures. Switchable catalysts operate by the generation of a species which is chemically distinct in behaviour and structure to the precursor. This difference in catalytic activity has been exploited to allow spatiotemporal control over polymerisations in the synthesis of (co)polymers. Although switchable methodologies have been applied to other polymerisation mechanisms for quite some time, for ring opening polymerisation (ROP) reactions it is a relatively young area of research. Despite its infancy, the field is accelerating rapidly. Here, we review recent developments for selected external stimuli for ROP, including redox chemistry, light, allosteric and mechanical control. Furthermore, a brief review on switch catalysis involving exogeneous gases will also be provided, although this area differs from traditional switchable catalysis techniques. An outlook on the future of switchable catalysis is also provided.

Miktoarm Star Copolymers Prepared by Transformation from Enhanced Spin Capturing Polymerization to Nitroxide-Mediated Polymerization (ESCP-Ŧ-NMP) toward Nanomaterials

Reversible-deactivation radical polymerization (RDRP) serves as a powerful tool nowadays for the preparations of unique linear and non-linear macromolecules. In this study, enhanced spin capturing polymerizations (ESCPs) of styrene (St) and tert-butyl acrylate (tBA) monomers were, respectively, conducted in the presence of difunctional (1Z,1'Z)-1,1'-(1,4-phenylene) bis (N-tert-butylmethanimine oxide) (PBBN) nitrone. Four-arm (PSt)4 and (PtBA)4 star macroinitiators (MIs) can be afforded. By correspondingly switching the second monomer (i.e., tBA and St), miktoarm star copolymers (μ-stars) of (PSt)2-μ-(PtBA-b-PSt)2 and (PtBA)2-μ-(PSt-b-PtBA)2) were thus obtained. We further conducted hydrolysis of the PtBA segments to PAA (i.e., poly(acrylic acid)) in μ-stars to afford amphiphilic μ-stars of (PSt)2-μ-(PAA-b-PSt)2 and (PAA)2-μ-(PSt-b-PAA)2. We investigated each polymerization step and characterized the obtained two sets of "sequence-isomeric" μ-stars by FT-IR, 1H NMR, differential scanning calorimeter (DSC), and thermogravimetric analysis (TGA). Interestingly, we identified their physical property differences in the case of amphiphilic μ-stars by water contact angle (WCA) and atomic force microscopy (AFM) measurements. We thus proposed two microstructures caused by the difference of polymer chain sequences. Through this polymerization transformation (Ŧ) approach (i.e., ESCP-Ŧ-NMP), we demonstrated an interesting and facile strategy for the preparations of μ-stars with adjustable/switchable interior and exterior polymer structures toward the preparations of various nanomaterials.

Merging of cationic RAFT and radical RAFT polymerizations with ring-opening polymerizations for the synthesis of asymmetric ABCD type tetrablock copolymers in one pot

A new bifunctional and switchable RAFT agent and a mechanism switching strategy were proposed to control the cationic RAFT polymerization, radical RAFT polymerization and ring-opening polymerization of vinyl and cyclic ester monomers and to produce block copolymers.

One-step synthesis of poly(methacrylate)-b-polyester via “one organocatalyst, two polymerizations”

In a “one organocatalyst, two polymerizations” system, triarylsulfonium hexafluorophosphate salt could spontaneously catalyze photo-ATRP and ROP. A well-defined PTMC-b-PMMA block copolymer was successfully synthesized in one-step.

Macromolecular Engineering by Applying Concurrent Reactions with ATRP

Modern polymeric material design often involves precise tailoring of molecular/supramolecular structures which is also called macromolecular engineering. The available tools for molecular structure tailoring are controlled/living polymerization methods, click chemistry, supramolecular polymerization, self-assembly, among others. When polymeric materials with complex molecular architectures are targeted, it usually takes several steps of reactions to obtain the aimed product. Concurrent polymerization methods, i.e., two or more reaction mechanisms, steps, or procedures take place simultaneously instead of sequentially, can significantly reduce the complexity of the reaction procedure or provide special molecular architectures that would be otherwise very difficult to synthesize. Atom transfer radical polymerization, ATRP, has been widely applied in concurrent polymerization reactions and resulted in improved efficiency in macromolecular engineering. This perspective summarizes reported studies employing concurrent polymerization methods with ATRP as one of the reaction components and highlights future research directions in this area.

Electrical monitoring of photoisomerization of block copolymers intercalated into graphene sheets

Insulating polymers have received little attention in electronic applications. Here, we synthesize a photoresponsive, amphiphilic block copolymer (PEO-b-PVBO) and further control the chain growth of the block segment (PVBO) to obtain different degrees of polymerization (DPs). The benzylidene oxazolone moiety in PEO-b-PVBO facilitated chain-conformational changes due to photoisomerization under visible/ultraviolet (UV) light illumination. Intercalation of the photoresponsive but electrically insulating PEO-b-PVBO into graphene sheets enabled electrical monitoring of the conformational change of the block copolymer at the molecular level. The current change at the microampere level was proportional to the DP of PVBO, demonstrating that the PEO-b-PVBO-intercalated graphene nanohybrid (PGNH) can be used in UV sensors. Additionally, discrete signals at the nanoampere level were separated from the first derivative of the time-dependent current using the fast Fourier transform (FFT). Analysis of the harmonic frequencies using the FFT revealed that the PGNH afforded sawtooth-type current flow mediated by Coulomb blockade oscillation.

Block copolymers are electrically insulating and therefore characterization with electrical or electrochemical methods is not possible. Here, the authors demonstrate electrical monitoring of the photoisomerization transition in a benzylidene oxazolone block co-polymer intercalated into graphene sheets.

Effects of various Cu(0), Fe(0), and proanthocyanidin reducing agents on Fe(iii)-catalysed ATRP for the synthesis of PMMA block copolymers and their self-assembly behaviours

Well-defined PMMA, PMMA-b-PBzMA and PMMA-b-PBMA polymers were obtained via green Fe-ATRP with the aid of proanthocyanidins. Interestingly, microphase separation was observed in PMMA-b-PBMA polymer with upper critical ordering temperature behaviour.

Photoinduced free radical promoted cationic polymerization 40 years after its discovery

Free radical promoted cationic photopolymerization has been described with its historical background, main principles and usage in polymer synthesis.

Controlled Radical Homopolymerization of Representative Cationically Polymerizable Vinyl Ethers

Facile direct radical homopolymerization of vinyl ethers without a hydroxy group was achieved up to near full conversion. This polymerization was conducted in water suspension in the presence of lithium hydroxide using a thermally triggered azo-initiator of dimethyl 2,2'-azobis(2-methylpropionate). In the polymerization system, appropriate hydrogen bonding and cation-π interactions under basic conditions are keys to the successful direct radical homopolymerization. The hydrogen bonding between water and vinyl ether oxygen reduces the reactivity of the growing radical, thus suppressing unfavorable side reactions such as β-scission. In addition, Li⁺ interacts with the oxygen and the vinyl group of vinyl ethers. The vinyl ether tends to be "activated" and the polymerization can be facilitated. Based on the results of free radical polymerization of vinyl ethers, controlled polymerization was also accomplished using the appropriate dithiocarbamate RAFT agent in view of the solubilities of the radical leaving group.

Controlled Synthesis of Block Copolymers by Mechanistic Transformation from Atom Transfer Radical Polymerization to Iniferter Process

A straightforward transformation protocol combining two distinct living polymerization methods for the controlled synthesis of block copolymers is described. In the first step, bromo-terminated poly(methyl methacrylate) is prepared by atom transfer radical polymerization (ATRP). Then, a bromide end group is substituted with a triphenylmethyl (trityl) functionality under visible light irradiation using dimanganese decacarbonyl (Mn₂ (CO)₁₀ ) photochemistry. The resulting polymers with trityl end groups are used as macroiniferter for the polymerization of styrene and tert-butyl acrylate (tBA) to yield desired block copolymers with narrow molecular weight distribution. Moreover, the amphiphilic copolymers with acrylic acid functionalities are obtained by the hydrolyzation of poly(tert-butyl acrylate) containing block copolymers with trifluoroacetic acid.

Integration of metal-free ring-opening metathesis polymerization and organocatalyzed ring-opening polymerization through a bifunctional initiator

We demonstrate the first integration of metal-free ROMP and organocatalyzed ROP to provide entirely metal-free syntheses of block copolymers.

Thiol-yne Click Polymerization

Thiol-yne click polymerization (TYCP) is one of the most significant synthetic techniques for artificial polymers, due to its simplicity, efficiency, and functionality tolerance. In nature, it is a classic nucleophilic addition reaction and a step-growth polymerization, which can be initiated or accelerated in the presence of free-radicals, amines, and transition metals, respectively. Its rate is greatly influenced by the structures (i.e., their electrophilicity and steric hindrance) of the used thiols and/or alkynes. With aliphatic monomers being used as feeding materials, the topological architectures (such as linear, branching, and cross-linked network, etc.) and available functional groups (such as hydroxyl, carboxyl, amino, and epoxy groups, and so on) can be facilely tailored via altering the chemical structure and feeding order. In contrast, for aromatic monomers, mono-addition occurs only during the process of thiol-yne click reaction, leading exclusively to linear poly(vinyl thioether)s. These sulfur-containing polymers synthesized by TYCP are promising to be widely utilized as high refractive index materials, photovoltaic materials, drug-delivery vehicles, biomaterials, and hybrid materials, etc.

Block Copolymers by Mechanistic Transformation from PROAD to Iniferter Process

A facile strategy for synthesizing block copolymers by the combination of two different living polymerization techniques, namely, photoinduced radical oxidation/addition/deactivation (PROAD) and iniferter processes is described. In the first step, PROAD polymerization of isobutyl vinyl ether using bromotriphenylmethane, dimanganese decacarbonyl (Mn₂ (CO)₁₀ ), and diphenyliodonium bromide (Ph₂ I⁺ Br^- ) is carried out to yield polymers with triphenylmethyl (trityl) end groups. These prepolymers are used as macroiniferters in thermally induced free radical polymerization of vinyl monomers such as methyl methacrylate, tert-butyl acrylate, and styrene, resulting in the formation of corresponding block copolymers free from homopolymers. The precursor polymer and final block copolymers are characterized by ¹ H NMR, FT-IR, GPC, and DSC analyses.