Synthesis and characterization of star-like microgels by one-pot free radical polymerization

A one-pot free radical polymerization process was used to prepare methyl acrylate/ethylene glycol dimethacrylate (MA/EGDMA) and methyl methacrylate/ethylene glycol dimethacrylate (MMA/EGDMA) polymers. The role of monomer and crosslinker reactivity ratios in producing different network structures was demonstrated. While both systems produced branched polymers that exhibited low intrinsic viscosities with little variation across a wide range of molecular weights, the star-like microgels formed between a less reactive monomer (MA) with a more reactive crosslinker (EGDMA) gave lower bulk solution viscosities than the more statistical polymers formed between similarly reactive monomers and crosslinkers (MMA and EGDMA). This paper presented a simple and cost-effective synthetic route for the production of polymers with high molecular weight and low viscosity with considerable potential for industrial-scale processing.     

Core-shell polyacrylate and polystyrene-block-polyacrylate stars

The polymerization of p-(iodomethyl)styrene (PIMS) yields well-defined branched polymers with reactive iodomethyl groups. The branched poly[p-(iodomethyl)styrene] was used as the transfer agent in the iodine mediated radical polymerization of vinyl monomers. The polymerization proceeds in a controlled way and yields polystyrene and poly(t-butyl acrylate) star polymers with reactive groups at the end of their arms. Polymers so obtained were also used to prepare stars with block copolymer arms: polystyrene-block-poly(t-butyl acrylate). The characterization of star structures was performed by NMR and gel permeation chromatography with absolute molar mass detection (MALLS). Preliminary characterization of the thermal properties of these novel materials is reported.     

Controlled radical copolymerization of multivinyl crosslinkers: a robust route to functional branched macromolecules

Controlled radical polymerization (CRP) has both revolutionized the synthesis of linear polymers and enabled unprecedented topological complexity. While the synthesis of many polymeric architectures requires careful planning and specialized precursors, branched macromolecules such as segmented hyperbranched polymers (SHPs), knotted polymers, core‐crosslinked stars (CCSs), and more can be synthesized through the copolymerization of vinyl monomers and divinyl crosslinkers in only a few steps. In the nearly two decades since its discovery, this strategy has helped elucidate the fundamental polymerization behavior of crosslinkers and also yielded a variety of functional and stimuli‐responsive materials. The purpose of this mini‐review is to therefore overview critical fundamental aspects of CRP of crosslinkers and materials derived therefrom. The process by which both SHPs and CCS polymers are synthesized, the effect of key reaction parameters and intriguing recent advances are described with the intent of both educating new researchers and inspiring new directions in this area. © 2020 Society of Industrial Chemistry     

One photon-two radical-electron transfer photoinitiators. 2-(o-, m-, or p-Methoxypyridine)-p-pyrrolidinestyrilium methyl sulfates as photoinitiators of free radical polymerization

In this paper, the novel hemicyanine dye-borate pair, e.g. 2-((o, m, or p)-methoxypyridine)-p-pyrrolidinestyrilium methyl sulfates (MeOSp)-tetramethylammonium n-butyltriphenyl borate (TBAB) were evaluated and employed as the photoinitiating pairs of multiacrylate monomer polymerization. The kinetic studies clearly demonstrate that the modification of the dye structure by the exchange of the N-alkyl group on N-alkoxy one in pyridinium moiety causes a marked increase in the efficiency of photoinitiation of 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA) polymerization.     

Organic/inorganic hybrid composites prepared by polymerization compounding and controlled free radical polymerization

A new method to produce highly filled and well dispersed polymer solid composites using controlled free radical polymerization has been developed. Grafting of polymers onto ultrafine silica was done in bulk polymerization at 120 °C in presence of N-tert-butyl-1-diethylphosphono2,2-dimethyl propyl nitroxide (DEPN) as a nitroxide stable free radical. Optimum conditions for tert-butyl hydroperoxide grafting onto fumed silica were first determined. The percentage of grafting, the architecture of grafted polymers, the length of chains, and the polydispersity index can be controlled at will using this approach. The effect of the number of grafted polymer chains combined with its molecular weight on the processing of these materials was investigated. The syntheses performed in this work gave grafting percentages of polymers and copolymers ranging from 12 to 88 wt%. All ‘synthesized’ composites gave stable suspensions in toluene and tetrahydrofuran.     

Controlled/living radical polymerization of styrene catalyzed by cobaltocene

Controlled/living radical polymerization of styrene has been achieved by atom transfer radical polymerization (ATRP) catalyzed by cobaltocene (PDI = 1.27-1.41). The effects of the initiators, temperatures and solvents were studied. The end group of PS-Br was characterized by ¹H NMR. Block copolymerization proved that the polymer end is still living and the PMMA-b-PSt block copolymer was synthesized.     

A new approach to controlled/living radical polymerization by DPE method

Controlled free radical polymerizations of methyl methacrylate and styrene in bulk by 1,1-diphenylethene (DPE) were demonstrated in a two-step process, preheating treatment of initiators followed by a living polymerization of monomers. Over the course of polymerization, continuous growing of polymers with unimodal molecular weight distribution and a relatively small polydispersity index (around 1.5 even in the range of Mn ∼ 10⁵ g/mol) on GPC diagrams was observed. In our previous study, the DPE controlled radical polymerization with constant molecular weight throughout the polymerization was caused by the intrinsically low reactivation rate constant (k₂) of DPE capped dormant chains. To raise the reaction temperature in order to increase k₂, a continuous molecular weight growing but broader or bimodal molecular weight distribution was obtained if the living polymerization was conducted in a one-step process. In this work, a two-step polymerization process was proposed. In the first step, the initiator 2,2′-azobisisobutyronitrile (AIBN), control agent DPE, and small amount of monomer were mixed and heated for a specific time period. Then a living polymerization of monomers was conducted in the second step of polymerization. This two-step new approach had minimized the imperfections of the DPE system; thus the polymerization showed better living characters and revealed its enhanced control abilities.     

Kinetics of nitroxide-controlled radical polymerization during the non-stationary state

The activation-deactivation equilibrium of nitroxide-controlled radical polymerization of styrene at 123 °C was investigated. For this purpose the reaction solution was examined time dependently during the initial phase of the polymerization by using an SEC column combination providing a very good separation of the low-molecular weight species. By time-dependent measurement of the alkoxyamine concentration the activation rate of the alkoxyamines PhEt-TIPNO (N-tert-butyl-N-(2-methyl-1-phenyl-propyl)-O-(1-phenyl-ethyl)-hydroxylamine) k_act = 3.2 × 10⁻³ s⁻¹ and PhEt-BIPNO (N-tert-butyl-N-(1-isopropyl-2-methyl-propyl)-O-(1-phenyl-ethyl)-hydroxylamine) k_act = 6.4 × 10⁻³ s⁻¹ can be determined directly.Considering the Persistent Radical Effect theory, the measurement of the free nitroxide concentration allows to determine the pseudo-equilibrium constant of dissociation/combination between dormant and active species for polystyryl-TIPNO and polystyryl-BIPNO, K = 7.5 × 10⁻⁹ mol/L and 1.08 × 10⁻⁸ mol/L, respectively.     

Controlled atom transfer radical polymerization of MMA onto the surface of high-density functionalized graphene oxide

We report on the grafting of poly(methyl methacrylate) (PMMA) onto the surface of high-density functionalized graphene oxides (GO) through controlled radical polymerization (CRP). To increase the density of surface grafting, GO was first diazotized (DGO), followed by esterification with 2-bromoisobutyryl bromide, which resulted in an atom transfer radical polymerization (ATRP) initiator-functionalized DGO-Br. The functionalized DGO-Br was characterized by X-ray photoelectron spectroscopy (XPS), Raman, and XRD patterns. PMMA chains were then grafted onto the DGO-Br surface through a ‘grafting from’ technique using ATRP. Gel permeation chromatography (GPC) results revealed that polymerization of methyl methacrylate (MMA) follows CRP. Thermal studies show that the resulting graphene-PMMA nanocomposites have higher thermal stability and glass transition temperatures (T_g) than those of pristine PMMA.     

Synthesis of new free‐radical initiators for polymerization

Synthesis and properties of new initiating systems formed from commercially available ketones, glycols, and hydrogen peroxide (60%) are presented. In preparation of free radical initiators methyl ethyl ketone, which was oxidized by hydrogen peroxide, was used. Reaction was carried out in etanodiol or 1,4‐butanodiol as diluent. The obtained initiators with cobalt octoate as a promotor were applied for crosslinking of the commercially available unsaturated polyester resin. Properties of the resin were compared with those obtained while it was hardened by the typical curing system containing methyl ethyl ketone hydroperoxide and cobalt octoate. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2238–2243, 2003     

Synthesis and characterization of branched polymers by a free radical approach using a novel ‘macroiniferter’

Free radical bulk polymerization of styrene and methyl methacrylate (MMA) was carried out using a novel ‘macroiniferter’ which resulted in branched polymers with relatively narrow molecular weight distribution. This approach involving the novel macroiniferter; poly[3-(t-butylperoxy)propyl disulfide] (PBPPDS) that has side chain peroxide groups and main chain disulfide linkages was developed to prepare soluble branched polymers as well as to control the extent of branching in vinyl polymers synthesized via a free radical route. The synthesis, characterization and thermal degradation studies of PBPPDS are reported here for the first time. The resulting polystyrene (PS) and poly(methyl methacrylate) (PMMA) polymers were characterized using gel permeation chromatography (GPC), intrinsic viscosity [η] measurements and the degree of branching was studied by the determination of g′ factor.     

Intermolecular radical addition of alkoxyamines onto olefins: An easy access to advanced macromolecular architectures precursors

Novel “second generation” alkoxyamines, derived from N-(2-methylpropyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl (so-called SG1) as initiators for nitroxide-mediated polymerization (NMP) were synthesized by intermolecular radical 1,2-addition (IRA) of a high dissociation rate constant alkoxyamine (BlocBuilder^®, also called MAMA-SG1) onto various activated olefins, such as n-butyl acrylate, acrylic acid, dimethylacrylamide, 2-hydroxyethylacrylate and styrene. The potential of this radical addition was further applied to the synthesis of multifunctional alkoxyamines as precursors for complex macromolecular architectures, namely 3- and 4-arm star polymers. For this, tri- and tetra-acrylates were synthesized by reaction of acryloyl chloride with the 1,1,1-tris(hydroxymethyl)ethane and pentaerythritol, respectively, in the presence of triethylamine. The addition of MAMA-SG1 onto these olefins led to the tri- and tetra-functional SG1-based alkoxyamines which were further used to prepare polystyrene stars of controlled molecular weights and polydispersity values not exceeding 2. The individual arms were recovered by hydrolysis of the ester groups of the star core originating from the alkoxyamine initiator under basic conditions. The decreasing molecular weight determined by GPC during hydrolysis demonstrated the star architecture of the polymers.     

Synthesis of photo- and pH-responsive composite nanoparticles using a two-step controlled radical polymerization method

We present a versatile synthetic method for photo- and pH-sensitive composite nanoparticles using a combined use of reversible addition-fragmentation chain transfer (RAFT) and atom transfer radical polymerization (ATRP). Crosslinked nanoparticles of a random copolymer composed of methyl methacrylate (MMA), 4-vinylbenzyl chloride (VBC) and divinylbenzene (DVB) were first synthesized using RAFT miniemulsion polymerization in aqueous solution. This was followed by solvent exchange through extraction and dialysis that allowed the nanoparticles to be transferred to and highly swollen in an organic solvent (anisole or THF). By dissolving a monomer of a stimuli-responsive polymer in the solution, subsequent ATRP grafting polymerization could be initiated by halide groups on the swollen nanoparticle, resulting in larger composite nanoparticles. Photosensitive poly(1-pyrenylmethyl methacrylate) (PPyMA) and pH-sensitive poly(dimethylaminoethyl methacrylate) (PDMAEMA) were incorporated in the composite nanoparticles, and their photo- and pH-responsive behaviors were investigated. With this method, monomers soluble in organic solvents can be used in conjunction with emulsion polymerization in aqueous solution to design functional composite nanoparticles.     

Thermosensitive copolymer synthesized by controlled living radical polymerization: Phase behavior of diblock copolymers of poly(N‐isopropyl acrylamide) families

In this study, we prepared a series of thermosensitive polymers with low polydispersity index (PDI) values by nitroxide‐mediated controlled radical polymerization (NMRP) with 2,2,6,6‐tetramethyl‐1‐piperdinyloxy nitroxide (TEMPO) as a stable nitroxide‐free radical. Poly(N‐isopropyl acrylamide) (PNIPAAm)‐block‐poly(N‐tert‐butyl acrylamide) (PNTBA) was successfully synthesized, first, through polymerization with N‐isopropyl acrylamide to obtain the reactive polymer PNIPAAm‐TEMPO and, second, through polymerization by the addition of N‐tert‐butyl acrylamide (NTBA). The added molar fraction of NTBA during the second polymerization was adjusted accordingly to obtain the final polymerization product, a thermosensitive polymer (PNIPAAm‐block‐PNTBA), which had a targeted lower critical solution temperature (LCST). The result shows that the synthesis method used in this study effectively controlled the formation of the polymer to obtain a low PDI. The thermosensitive block copolymer, PNIPAAm‐b‐PNTBA (molar ratio = 9:1), with LCSTs in the range 27.7–39.8°C, was obtained through controlled living radical polymerization with PNIPAAm–TEMPO. Specifically, the 5 wt % aqueous solution of PNIPAAm‐b‐PNTBA (molar ratio = 9:1) had an LCST of 37.4°C; this was close to body temperature, 37°C. The 5 wt % aqueous solution of PNIPAAm‐b‐PNTBA (molar ratio = 9:1) showed potential for use in biomedical applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43224.     

Molecularly imprinted polymers via living radical polymerization: Relating increased structural homogeneity to improved template binding parameters

This work examined imprinted polymer networks prepared via controlled/living radical polymerization (LRP) and conventional radical polymerization (CRP) on chain growth, network formation, and efficiency of producing molecularly imprinted, macromolecular memory sites for the template molecule, diclofenac sodium. LRP extended the reaction-controlled regime of the polymerization reaction and formed more homogeneous polymer chains and networks with smaller mesh sizes. In addition, LRP negated the effect of the template on polymer chain growth resulting in polymers with a more consistent PDI independent of template concentration in the pre-polymerization solution. Improved network homogeneity within imprinted poly(HEMA-co-DEAEM-co-PEG200DMA) networks prepared via LRP resulted in a 38% increase in template binding affinity and 43% increase in the template binding over imprinted networks prepared via CRP and a 97% increase in affinity and 130% increase in capacity over non-imprinted networks prepared by LRP. By varying certain parameters, it was possible to create imprinted networks with even higher template binding affinities (155% over non-imprinted) and capacities (261% over non-imprinted). This work is the first to conclusively demonstrate that the observed improvement in binding parameters in weakly crosslinked, imprinted polymer networks could be explained by the more uniform molecular weight evolution associated with the LRP mechanism and the longer lifetime of an active polymer chain relative to the total polymerization time, which allowed for the formation of a more homogenous imprinted polymer network.     

Preparation and characterization of polyaniline N‐grafted with poly(ethyl acrylate) synthesized via atom transfer radical polymerization

Polyaniline (PANI) N‐grafted with poly(ethyl acrylate) (PEA) was synthesized by the grafting of bromo‐terminated poly (ethyl acrylate) (PEA‐Br) onto the leucoemeraldine form of PANI. PEA‐Br was synthesized by the atom transfer radical polymerization of ethyl acrylate in the presence of methyl‐2‐bromopropionate and copper(I) chloride/bipyridine as the initiator and catalyst systems, respectively. The leucoemeraldine form of PANI was deprotonated by butyl lithium and then reacted with PEA‐Br to prepare PEA‐g‐PANI graft copolymers containing different amounts of PEA via an N‐grafting reaction. The graft copolymers were characterized by Fourier transform infrared spectroscopy, elemental analysis, and thermogravimetric analysis. Solubility testing showed that the solubility of PANI in chloroform was increased by the grafting of PEA onto PANI. The morphology of the PEA‐g‐PANI graft copolymer films was observed by scanning electron microscopy to be homogeneous. The electrical conductivity of the graft copolymers was measured by the four‐probe method. The results show that the conductivity of the PANI decreased significantly with increasing grafting density of PEA onto the PANI backbone up to 7 wt % and then remained almost constant with further increases in the grafting percentage of PEA. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effects of molecular weight on liquid-crystalline behavior of a mesogen-jacketed liquid crystal polymer synthesized by atom transfer radical polymerization

The synthesis of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS) with different molecular weight and low polydispersity was achieved by atom transfer radical polymerization in methoxybenzene solution using 1-bromoethylbenzene as an initiator and CuBr/sparteine complex as a catalyst. The concentration of the living centers throughout the polymerization was found to be constant. The liquid-crystalline behavior of the polymers with M_n ranging from 3800 to 17,400 g/mol was studied using DSC and POM. Only the polymers with M_n beyond 10,200 g/mol formed a liquid-crystalline phase, which was quite stable with a high clearing point (higher than the decomposition temperature of the polymer).     

The strength of multi-scale modeling to unveil the complexity of radical polymerization

The strength of multi-scale modeling to support the fundamental understanding and design of radical polymerization processes is illustrated, considering both controlled and free radical polymerization (CRP/FRP) in non-dispersed (bulk/solution) and dispersed (suspension/emulsion) media. At the molecular scale, the importance of joint experimental and theoretical studies is highlighted. At the micro-scale, the concept of apparent rate coefficients is elaborated to account for the possible influence of diffusional limitations on the local reaction rates. At the meso-scale, the key characteristics to fundamentally describe the evolution of the particle size distribution are covered and the possible interaction with the micro- and macro-scale is discussed. At the macro-scale, the main mathematical tools to assess the relevance of mixing and temperature gradients are provided. Several examples on CRP and FRP processes are included to showcase the modeling capabilities for each scale, focusing both on laboratory and industrial reactors.     

原子转移自由基聚合合成支化聚丙烯腈

以二乙烯苯为支化单体,α-溴代异丁酸叔丁酯为引发剂,CuBr和2,2'-联吡啶为催化体系,利用本体和溶液原子转移自由基聚合合成了支化聚丙烯腈.采用核磁共振谱仪、凝胶渗透色谱仪和多角度激光光散射仪等测试了聚合物结构、相对分子质量及其分布.用无水乙酸钠对支化聚丙烯腈进行末端改性,得到了在硫氰酸钠水溶液中性能稳定、可长期保存的支化聚丙烯腈,而改性对聚合物的耐热性能没有影响.     

Nitroxide-mediated radical polymerization of styrene: Experimental evidence of chain transfer to monomer

Chain transfer to monomer during nitroxide-mediated radical polymerization of styrene has been investigated for 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) and N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide (SG1) mediated polymerizations at 125 and 110 °C, respectively. A novel technique employing a fluorescence-labelled polystyrene-TEMPO macroinitiator enabled separate detection of the total chain distribution and the distribution of chains containing the original macroinitiator, thus directly confirming the presence of chains not containing macroinitiator. Chain transfer to monomer results in a low molecular weight tail, which can be very much pronounced, in particular in the number distributions. Quantitative analysis of the total number of chains in both the TEMPO and the SG1 systems, correcting for the contribution of thermal initiation of styrene, yielded chain transfer to monomer constants in agreement with the literature.