Controlled Polymerization of Multivinyl Monomers: Formation of Cyclized/Knotted Single‐Chain Polymer Architectures

Seventy years ago, Flory and Stockmayer predicted that the polymerization of multivinyl monomers (MVMs) would inevitably lead to insoluble cross‐linked gel networks. Since then, the use of MVMs has largely been limited to as cross‐linking agents. More recently, however, polymerization strategies such as reversible deactivation radical polymerization (RDRP) have paved the way for the exploration of new possibilities in terms of both polymer architectures and functional capabilities. This Minireview provides historical context to the problem of polymerizing MVMs, before highlighting how RDRP has led to the formation of new cyclized/knotted polymer structures. Although the potential of such cyclized/knot polymer architectures is far from being fulfilled, some emerging applications are discussed.     

3D single cyclized polymer chain structure from controlled polymerization of multi-vinyl monomers: beyond Flory-Stockmayer theory

Controlled/living radical polymerization (CRP) is a widely used technique that allows the synthesis of defined polymer architectures through precise control of molecular weights and distributions. However, the architectures of polymers prepared by the CRP techniques are limited to linear, cross-linked, and branched/dendritic structures. Here, we report the preparation of a new 3D single cyclized polymer chain structure from an in situ deactivation enhanced atom transfer radical polymerization of multivinyl monomers (MVMs), which are conventionally used for the production of branched/cross-linked polymeric materials as defined by P. Flory and W. Stockmayer nearly 70 years ago. We provide new evidence to demonstrate that it is possible to kinetically control both the macromolecular architecture and the critical gelling point in the polymerization of MVMs, suggesting the classical Flory-Stockmayer mean field theory should be supplemented with a new kinetic theory based on the space and instantaneous growth boundary concept.     

Can Flory-Stockmayer theory be applied to predict conventional free radical polymerization of multivinyl monomers? A study via Monte Carlo simulations

The conventional free radical polymerization (FRP) of multivinyl monomers (MVMs) inevitably leads to gelation even at low monomer conversion resulting in difficulties to control and monitor the reaction process. Flory and Stockmayer (F-S theory) studied it based on two fundamental assumptions: (1) independent and equivalent vinyl groups; (2) no intramolecular cyclization. However, until now its applicability to FRP of MVMs (especially regarding the extent of intramolecular cyclization) is still controversial. In this paper, Monte Carlo simulations are used to study FRP of divinyl monomers by two kinetic models: with/without cyclization models. The results of the simulations are compared with the calculated gel points based on F-S theory and the experimental data. It is found that the intramolecular cyclization has a negligible impact on the polymerization process and the gel point before gelation, which are in agreement with the prediction by F-S theory, but the effect becomes significant above the gel points.     

Rational Design of Biomolecules/Polymer Hybrids by Reversible Deactivation Radical Polymerization (RDRP) for Biomedical Applications

Hybrids, produced by hybridization of proteins, peptides, DNA, and other new biomolecules with polymers, often have unique functional properties. These properties, such as biocompatibility, stability and specificity, lead to various smart biomaterials. This review mainly introduces biomolecule-polymer hybrid materials by reversible deactivation radical polymerization(RDRP), emphasizing reverse addition-fragmentation chain transfer(RAFT) polymerization, and nitroxide mediated polymerization(NMP). It includes the methods of RDRP to improve the biocompatibility of biomedical materials and organisms by surface modification. The key to the current synthesis of biomolecule-polymer hybrids is to control polymerization. Besides, this review describes several different kinds of biomolecule-polymer hybrid materials and their applications in the biomedical field. These progresses provide ideas for the investigation of biodegradable and highly bioactive biomedical soft tissue materials. The research hotspots of nanotechnology in biomedical fields are controlled drug release materials and gene therapy carrier materials. Research showed that RDRP method could improve the therapeutic effect and reduce the dosage and side effects of the drug.Specifically, by means of RDRP, the original materials can be modified to develop intelligent polymer materials as membrane materials with selective permeability and surface modification.     

Molar Mass Dispersity Control by Iodine-mediated Reversible-deactivation Radical Polymerization

Dispersity(D)of polymers has a great effect on the properties of polymeric materials,and therefore how to control D is very important but still a huge challenge in polymer synthesis,especially for reversible-deactivation radical polymerization(RDRP)strategy.Herein,we successfully developed a novel strategy to adjust D of polymers by visible light-controlled reversible complexation mediated living radical polymerization(RCMP)and combination of single-electron transfer-degenerative chain transfer living radical polymerization(SET-DTLRP)at room temperature.In RCMP system,2-iodo-2-methylpropionitrile(CP-I)and ethyl 2-iodo-2-phenylacetate(EIPA)were used as alkyl iodide initiators,by using methyl methacrylate(MMA)as the model monomer and n-butylacrylate(BA)as the end-capping reagent to regulate D of polymers.Subsequently,we successfully prepared the block copolymer PMMA-b-PBA with adjustable D by reactivating the polymer end-chains via SET-DTLRP in the presence of copper wire,fully demonstrating that it is a promising strategy that can keep the"living"feature of polymers while regulating their molar mass dispersities easily.     

Copper‐Mediated Polymerization without External Deoxygenation or Oxygen Scavengers

As a method for overcoming the challenge of rigorous deoxygenation in copper‐mediated controlled radical polymerization processes [e.g., atom‐transfer radical polymerization (ATRP)], reported here is a simple Cu⁰‐RDRP (RDRP=reversible deactivation radical polymerization) system in the absence of external additives (e.g., reducing agents, enzymes etc.). By simply adjusting the headspace of the reaction vessel, a wide range of monomers, namely acrylates, methacrylates, acrylamides, and styrene, can be polymerized in a controlled manner to yield polymers with low dispersities, near‐quantitative conversions, and high end‐group fidelity. Significantly, this approach is scalable (ca. 125 g), tolerant to elevated temperatures, compatible with both organic and aqueous media, and does not rely on external stimuli which may limit the monomer pool. The robustness and versatility of this methodology is further demonstrated by the applicability to other copper‐mediated techniques, including conventional ATRP and light‐mediated approaches.     

PREFACE

正We are delighted to present this special themed topic of Chinese Journal of Polymer Science(CJPS) devoted to the recent advances in reversible deactivation radical polymerization(RDRP). RDRP has been widely recognized as one of the most important synthetic methods for polymers, allowing access to well-defined polymeric materials with predictable molecular weight, controlled dispersity, and tailor-made architecture. Since its discovery,     

Using Kinetic Modeling and Experimental Data to Evaluate Mechanisms in PET-RAFT

Reversible deactivation radical polymerization (RDRP) techniques have become important tools for polymer chemists because they control the structure and are tolerant to functionality. Photoinduced polymerizations have seen a growing interest due to their mild conditions, as well as spatial and temporal control over the polymerization. Among these techniques, photoinduced electron/energy transfer reversible addition–fragmentation chain transfer polymerization (PET-RAFT) is one of the most widely investigated. While PET-RAFT is seen as an increasingly useful tool, there is still much to understand about the mechanism of this process. In particular, there are ongoing questions regarding the kinetic contribution of electron versus energy transfer. In order to better understand the mechanism, this work aims to use kinetic modeling along with experimental data to help determine the likelihood of the proposed mechanisms for the PET-RAFT process using the trithiocarbonate-mediated polymerization of methyl acrylate with fac-tris[2-phenylpyridinato-C²,N]iridium(III) as a photocatalyst. Simulation data show that electron transfer without a corresponding reduction pathway cannot explain the experimental kinetics, while energy transfer offers a good fit to experimental data. © 2019 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 139–144     

Stereospecific reversible deactivation radical polymerization of biomass‐based acrylates for precise control of tacticity and molecular weight

Reversible deactivation radical polymerization (RDRP) of biomass‐based acrylates, (S )‐ and (R )‐2‐isopropyl‐5‐methylene‐1,3‐dioxolan‐4‐ones (S‐MiPDO and R‐MiPDO), was successfully performed to produce a well‐defined polymer with simultaneously controlled tacticity and molecular weight, and low dispersity (? < 1.3). In particular, meso triad (mm ) of the polymer was continuously controlled as designed from 28.1% to about 100% by changing the molar ratio of S‐MiPDO/R‐MiPDO in feed. In kinetic studies, the rate of RDRP was strongly influenced by the stereostructures of the propagating radical, and it was much lower in isospecific RDRP than atactic one in reversible chain transfer catalyzed polymerization (RTCP) in contrast to atom transfer radical polymerization (ATRP) where the rate would not change regardless of the tacticity. Increase of molecular weight and low ? of the polymer were also observed in reversible addition‐fragmentation chain transfer (RAFT) polymerization of MiPDO. In addition, block copolymers including stereoblock copolymers were feasibly synthesized by RTCP of styrene and methyl methacrylate using poly(MiPDO) prepolymer. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 445–456     

Stimuli-responsive binary mixed polymer brushes and free-standing films by LbL-SIP

We report a facile approach to preparing binary mixed polymer brushes and free-standing films by combining the layer-by-layer and surface-initiated polymerization (LbL-SIP) techniques. Specifically, the grafting of mixed polymer brushes of poly(n-isopropylacrylamide) and polystyrene (pNIPAM-pSt) onto LbL-macroinitiator-modified planar substrates is described. Atom transfer radical polymerization (ATRP) and free radical polymerization (FRP) techniques were employed for the syntheses of pNIPAM and pSt, respectively, yielding pNIPAM-pSt mixed polymer brushes. The composition of the two polymers was controlled by varying the number of macroinitiator layers deposited on the substrate (i.e., LbL layers = 4, 8, 12, 16, and 20); consequently, mixed brushes of different thicknesses and composition ratios were obtained. Moreover, the switching behavior of the LbL-mixed brush films as a function of solvent and temperature was demonstrated and evaluated by water contact angle and atomic force microscopy (AFM) experiments. It was found that both the solvent and temperature stimuli responses were a function of the mixed brush composition and thickness ratio where the dominant component played a larger role in the response behavior. Furthermore, the ability to obtain free-standing films was exploited. The LbL technique provided the macroinitiator density variation necessary for the preparation of stable free-standing mixed brush films. Specifically, the free-standing films exhibited the rigidity to withstand changes in the solvent and temperature environment and at the same time were flexible enough to respond accordingly to external stimuli.     

Synthesis of block copolymers consisting of liquid crystalline and amorphous segments by living/controlled radical polymerization

Living polymerization is most often observed in systems where the growing species are ions. In such systems the chain ends do not react to each other due to elestrostactic repulsion, but only to monomers allowing, this way, the control in structure of the formed polymer. Free radicals, which are the growing species in the radical polymerization, easely undergo combination and prevent a living radical polymerization. Thus, a great challenge to polymer science was in meeting a system that offered to the radical polymerization a radical stabillization alike in ionic polymerizations. At the same time, the radicals should undergo rapid propagation and should not be able to initiate new chains, in a controlled reaction. Some succesfull techniques of living/controlled radical polymerization, such as stable free radical polymerization (SFRP), mediated by nitroxide, INIFERTER and atom transfer polymerization (ATRP) will be overviewed here, as well as their application to the synthesis of liquid crystalline polymers.     

State‐of‐the‐Art and Progress in Method of Moments for the Model‐Based Reversible‐Deactivation Radical Polymerization

Reversible‐deactivation radical polymerization (RDRP) techniques have received lots of interest for the past 20 years, not only owing to their simple, mild reaction conditions and broad applicability, but also their accessibility to produce polymeric materials with well‐defined structures. Modeling is widely applied to optimize the polymerization conditions and processes. In addition, there are numerous literatures on the kinetic and reactor models for RDRP processes, which show the accessibility on polymerization kinetics insight, process optimization, and controlling over chain microstructure with predetermined molecular weight and low dispersity, copolymer composition distribution, and sequence distribution. This review highlights the facility of the method of moments in the modeling field and presents a summary of the present state‐of‐the‐art and future perspectives focusing on the model‐based RDRP processes based on the method of moments. Summary on the current status and challenges is discussed briefly.

     

Stochastic Simulation of Controlled Radical Polymerization Forming Dendritic Hyperbranched Polymers

Stochastic simulation of the formation process of hyperbranched polymers (HBPs) based on the reversible deactivation radical polymerization (RDRP) using a branch-inducing monomer, evolmer, has been carried out. The simulation program successfully reproduced the change of dispersities (Đs) during the polymerization process. Furthermore, the simulation suggested that the observed Đs (=1.5–2) are due to the distribution of the number of branches instead of undesired side reactions, and that the branch structures are well controlled. In addition, the analysis of the polymer structure reveals that the majority of HBPs have structures close to the ideal one. The simulation also suggested the slight dependence of branch density on molecular weight, which was experimentally confirmed by synthesizing HBPs with an evolmer having phenyl group.     

Prediction and Experimental Characterization of the Molecular Architecture of FRP and ATRP Synthesized Polyacrylate Networks

Summary: This work reports experimental and modeling studies concerning the conventional (FRP) and atom transfer radical polymerization (ATRP) of acrylate/diacrylate monomers. In the framework of a recently developed general approach, kinetic models including crosslinking reactions and branching by chain transfer to polymer are discussed for FRP and ATRP polymerization systems. Besides molecular weight distribution (MWD), fairly good predictions of the z-average radius of gyration could be obtained for these non-linear polymers. A set of experiments was performed at 1 L scale in a batch reactor using n-butyl acrylate (BA) or methyl acrylate (MA) as monovinyl monomers and 1,6-Hexanediol diacrylate (HDDA) or bisphenol A ethoxylate diacrylate (BEDA) as crosslinkers. In FRP experiments, AIBN was used as initiator and ATRP polymerizations were initiated by ethyl 2-bromopropionate (EBrP) and mediated by CuBr using PMDETA (N,N,N′,N″,N″-pentamethyldiethylenetriamine) as ligant. Polymerizations were carried out in solution at 60 °C with different dilutions using toluene and DMF as solvents. Products formed at different polymerization times were analyzed by SEC/RI/MALLS yielding average MW, MWD, z-average radius of gyration and monomer conversion. Important differences in the molecular architecture of the synthesized FRP and ATRP highly branched polyacrylates have been identified. Comparisons of experimental results with predictions have put into evidence the important effect of intramolecular cyclizations at all dilutions, even with ATRP polymerizations.     

聚醋酸乙烯酯的光引发可控合成——介绍一个高分子化学综合实验

本实验开展了醋酸乙烯酯的可逆加成-断裂链转移自由基聚合(RAFT),分别运用偶氮二异丁腈(AIBN)引发和可见光引发两种方式进行了聚合反应,运用核磁共振和凝胶色谱等多种手段对所得聚合物结构进行了表征与分析。通过比较AIBN引发与光引发所获得聚合物端基结构的异同,加深了学生对RAFT聚合方法原理的理解。同时,运用该方法实现了聚合物两端端基结构的高度功能化,深入体会聚合物合成设计概念。本综合实验教学不仅通过对比法加深了学生对实验原理和专业知识的理解,提升了学生创新研究能力,而且训练了学生的实验操作技能、大型仪器使用能力和结果分析能力,提升了综合素质。     

Novel biodegradable adaptive hydrogels: controlled synthesis and full characterization of the amphiphilic co-networks

Adaptive and amphiphilic poly(N,N-dimethylamino-2-ethyl methacrylate-graft-poly[epsilon-caprolactone]) co-networks (netP(DMAEMA-g-PCL)) were synthesized from a combination of controlled polymerization techniques. Firstly, PCL cross-linkers were produced by ring-opening polymerization (ROP) of epsilon-CL initiated by 1,4-butane-diol and catalyzed by tin(II) 2-ethylhexanoate ([Sn(Oct)2]), followed by the quantitative esterification reaction of terminal hydroxyl end-groups with methacrylic anhydride. Then, PCL cross-linkers were copolymerized to DMAEMA monomers by atom-transfer radical polymerization (ATRP) in THF at 60 degrees C using CuBr complexed by 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) and 2-ethyl isobutyrylbromide (EiBBr) as catalytic complex and initiator, respectively. A comprehensive study of gel formation was carried out by employing dynamic light scattering (DLS) to determine the gel point as a function of several parameters and to characterize the viscous solutions obtained before the gel point was reached. The evolution of the mean diameters was compared to a model previously developed by Fukuda and these attest to the living formation of the polymer co-network. Furthermore, we also demonstrated the reliability of ATRP for producing well-defined and homogeneous polymer co-networks by the smaller deviation from Flory's theory in terms of cross-linking density. For sake of clarity, the impact of polymerization techniques over the final structure and, therefore, properties was highlighted by comparing two samples of similar composition, but that were produced by either ATRP or thermal-initiated free-radical polymerization (FRP).     

Initiator-Activation Strategy-Enabled Organocatalyzed Reversible-Deactivation Radical Polymerization Driven by Light

Organocatalyzed reversible-deactivation radical polymerizations (RDRPs) are attractive for many applications. Here, we developed photoredox-mediated RDRP by activating (hetero)aryl sulfonyl chloride (ArSO₂Cl) initiators with pyridines and designing a novel bis(phenothiazine)arene catalyst. The in situ formed sulfonyl pyridinium intermediates effectively promote controlled chain-growth from ArSO₂Cl, enabling access to various well-defined polymers with high initiation efficiencies and controlled dispersities under mild conditions. This versatile method allows “ON/OFF” temporal control, chain-extension, facile synthesis of different polymer brushes via organocatalyzed grafting reactions from linear chains. Time-resolved fluorescence decay studies and calculations support the reaction mechanism. This work provides a transition-metal-free RDRP to tailor polymers with readily available aromatic initiators, and will promote the design of polymerization leveraged from photoredox catalysis.     

Enzymatic Cascade Catalysis for the Synthesis of Multiblock and Ultrahigh‐Molecular‐Weight Polymers with Oxygen Tolerance

Synthesis of well‐defined multiblock and ultrahigh‐molecular‐weight (UHMW) polymers has been a perceived challenge for reversible‐deactivation radical polymerization (RDRP). An even more formidable task is to synthesize these extreme polymers in the presence of oxygen. A novel methodology involving enzymatic cascade catalysis is developed for the unprecedented synthesis of multiblock polymers in open vessels with direct exposure to air and UHMW polymers in closed vessels without prior degassing. The success of this methodology relies on the extraordinary deoxygenation capability of pyranose oxidase (P2Ox) and the mild yet efficient radical generation by horseradish peroxidase (HRP). The facile and green synthesis of multiblock and UHMW polymers using biorenewable enzymes under environmentally benign and scalable conditions provides a new pathway for developing advanced polymer materials.     

Bioactive Peptide Brush Polymers via Photoinduced Reversible‐Deactivation Radical Polymerization

Harnessing metal‐free photoinduced reversible‐deactivation radical polymerization (photo‐RDRP) in organic and aqueous phases, we report a synthetic approach to enzyme‐responsive and pro‐apoptotic peptide brush polymers. Thermolysin‐responsive peptide‐based polymeric amphiphiles assembled into spherical micellar nanoparticles that undergo a morphology transition to worm‐like micelles upon enzyme‐triggered cleavage of coronal peptide sidechains. Moreover, pro‐apoptotic polypeptide brushes show enhanced cell uptake over individual peptide chains of the same sequence, resulting in a significant increase in cytotoxicity to cancer cells. Critically, increased grafting density of pro‐apoptotic peptides on brush polymers correlates with increased uptake efficiency and concurrently, cytotoxicity. The mild synthetic conditions afforded by photo‐RDRP, make it possible to access well‐defined peptide‐based polymer bioconjugate structures with tunable bioactivity.     

Covalently attached graft polymer monolayer on organic polymeric substrate via confined surface inhibition reaction

A grafting technique was proposed for the preparation of polymer monolayer on polymeric substrate. On the basis of our recent work on polymer‐supported inhibitor (PSI), hydroquinone (HQ) was first implanted onto polypropylene (PP) surface through UV‐initiated grafting. The resulting immobilized HQ was used as PSI for the thermal‐induced free radical polymerization (FRP) of acrylic acid (AA). The inhibition mechanism was similar to that of free HQ molecule, that is, polymer chain‐carrying radical or peroxy radical could be deactivated by abstracting hydrogen atom from hydroxyl group of immobilized HQ, and the resulting oxyradical (semiquinone radical) combined with another active chain free radical. According to this mechanism, a devised redox initiator consisting of sodium hydrogen sulfite and ammonium persulfate was used to initiate FRP of AA in water at low temperature (50 °C). High crystalline biaxial oriented PP film with HQ immobilized was deliberately laid in this system as a radical trap to capture poly(acrylic acid) (PAA) short chain radical. Through X‐ray photoelectron spectra (XPS) analysis it was found that the atom ratio of C_HQ (carbon in HQ) to C_COOH (carbon in COOH) decreased with prolonging polymerization time and became stable after about 30 min. The formed PAA short chain on the surface showed a distribution of monolayer, and the saturated thickness was calculated as 5–7 Å. The degree of polymerization of graft chain in PAA monolayer was estimated as 15–20 through three different models. Relating to surface coverage being 100% in ideal densely packed PAA monolayer, real monolayer surface coverage in such reaction system was estimated as 12.3–18.5%. This method was expected to give us a general approach for constructing kinds of graft polymer monolayer on polymeric substrate, because the involved chemistry was only common inhibition reaction between immobilized inhibitor (HQ) and FRP system in solution (herein redox initiating system of AA). We named this grafting chemistry as confined surface inhibition reaction. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 745–755, 2007