Locus Of Polymerization Research Articles

Gradient Copolymer Synthesis through Self-Assembly

Polymerization-induced self-assembly (PISA) is typically performed to produce polymer nanoparticles featuring specific assembly morphologies. Herein, we demonstrate the use of PISA as a synthetic tool to direct gradient copolymer synthesis. Specifically, we leverage hydrophobicity-induced reaction selectivity and the rate acceleration typically associated with polymer compartmentalization upon assembly during PISA to bias reaction selectivity. In the chain extension of a poly(ethylene glycol) macrochain transfer agent, the selectivity of diacetone acrylamide (DAAm) and N,N-dimethylacrylamide (DMA), two monomers with near-identical reactivity in water, can be modulated in situ such that DAAm is preferentially incorporated over DMA upon self-assembly. By increasing the feed ratio of DAAm, monomer differentiation can be further biased toward DAAm due to the locus of polymerization becoming increasingly hydrophobic. This change in selectivity affords the autonomous generation of DAAm-DMA gradient sequences, otherwise inaccessible without outside intervention. Finally, a mild hydrolysis protocol can then be employed to harvest DAAm-DMA sequences, yielding compositionally unique gradient copolymers.

Monomer Concentration in Polymer Particles in Emulsion Polymerization

AbstractThe rate of an emulsion polymerization depends on the monomer concentration at the locus of polymerization in the polymer particles. This note suggests a shortcut method of specifying monomer concentration for new monomers, or monomers for which thermodynamic swelling data is unavailable. While the monomer concentration in a polymer particle in moles per liter varies widely, when described as volume fraction monomer, the variance is much smaller. In fact, for ten monomers for which experimental values are available, the volume fraction monomer in a polymer particle in equilibrium with an aqueous phase saturated with monomer of that polymer is 0.67 ± 0.04. Hence, if no other value is available, one could assume a monomer volume fraction monomer of 0.67 with a good degree of accuracy.

RAFT Emulsion Polymerization for (Multi)block Copolymer Synthesis: Overcoming the Constraints of Monomer Order

Synthesis of (multi)block copolymers using reversible addition–fragmentation chain transfer (RAFT) polymerization generally suffers from the limitation that the order of the blocks must be considered. Herein, syntheses of block copolymers by RAFT polymerization using trithiocarbonate RAFT species are conducted as solution, miniemulsion, and emulsion polymerizations to demonstrate that the issue of monomer order for styrene and butyl methacrylate can be largely overcome in emulsion polymerization under carefully chosen conditions. The presence of monomer droplets in emulsion polymerization—in addition to polymer particles that constitute the locus of polymerization—leads to a reduction in the ratio of RAFT end groups to monomer at the locus of polymerization. Consequently, fragmentation of the RAFT adduct radical in the “backward” (“wrong”) direction is associated with fewer monomer additions, thus minimizing the impact of this undesired kinetic event. It is demonstrated that RAFT emulsion polymerization can be exploited to prepare an alternating pentablock copolymer composed of methacrylates (with 10 mol % styrene) and styrene without consideration of monomer order, thereby significantly broadening the scope of RAFT polymerization for multiblock copolymer synthesis.

Cationic polymerization of styrene using iron-containing ionic liquid catalysts in an aqueous dispersed medium

This work aims to study the cationic miniemulsion polymerization of styrene catalyzed by iron-containing imidazolium-based ionic liquids. The polystyrenes had very high number-average molar mass around 1300 kg mol-1 at 85 °C, molar-mass dispersity close to 2.0 and glass transition temperature higher than 102 °C with average particle diameter that remained practically unchanged during the reaction, indicating that the monomer droplets correspond to the polymerization locus. First-order kinetics up to a limit conversion, along with the increase in molar mass as the temperature decreases, styrene polymerization at low temperatures and catalyst inability to polymerize monomers that react exclusively via free radical and/or anionic polymerization, indicate the cationic nature of polymerization. 1H-NMR and 13C-NMR spectra suggested the formation of polystyrene, allowing for tacticity distribution quantification: 10% isotactic, 20% atactic and 70% syndiotactic configurations. TEM micrographs confirmed the formation of spherical polymer nanoparticles and the presence of catalysts in the polymer matrix.

Preparation of Cross-Linked Monodisperse Poly(acrylic acid) Particles by Precipitation Polymerization.

Water-soluble polymers such as poly(acrylic acid) (PAA) are widely applied in dispersants, thickeners, flocculants, and superabsorbent polymers. Here, submicrometer-sized monodisperse PAA particles were successfully prepared by precipitation polymerization in acetonitrile at 55 °C with 2,2'-azobis(2,4-dimethylvaleronitrile) as an initiator. In a medium with a high dielectric constant, the particles were stabilized by electrostatic repulsion without a colloidal stabilizer during the polymerization. Moreover, the obtained PAA particles were spontaneously cross-linked in the absence of a cross-linker. The degree of the cross-linking was strongly dependent on the amount of water (500-10,000 ppm) in the polymerization medium. The PAA particles swelled more with acrylic acid with an increase in the amount of water. Thus, in the case of high water content, the particles would become the main polymerization loci, which in turn results in the cross-linking structures owing to the α-hydrogen abstraction of PAA chains during the polymerization.

Cross-linking of double oil-in-water-in-oil emulsions: A new way for fragrance encapsulation with tunable sustained release

This paper describes a new route to synthesize polymeric capsules with interesting and diverse diffusive and mechanical properties. Such capsules were obtained by free radical cross-linking copolymerization of the intermediate water phase of a double oil-in-water-in-oil (O/W/O) emulsion. Such a strategy allows separating the locus of polymerization from the fragrance- containing compartment. Indeed fragrance molecules are known to be reactive. The encapsulated species, also corresponding to the innermost oil, was a model fragrance composed of more than 10 various molecules usually used in perfumery. In a first step, the double emulsion containing a hydrophilic monomer, either Methacrylamide or Oligo(ethyleneglycol) methacrylate, and a hydrophilic cross-linker, either Tetra(ethyleneglycol) diacrylate or N,N-Methylene-bis-Acrylamide, was formulated. The influence of the cross-linker agent on the feasibility of the double emulsion was examined. Then, in a second part, the polymerization was performed. Four different systems of capsules were developed and compared in terms of diffusive and mechanical properties. We show that changing the monomer /cross-linker couple allowed modulating the capsule properties.

In Situ Modification of Polyisoprene by Organo-Nanoclay during Emulsion Polymerization for Reinforcing Natural Rubber Thin Films.

Nanoclay-modified polyisoprene latexes were prepared and then used as a reinforcing component in natural rubber (NR) thin films. Starve-fed emulsion (SFE) polymerization gives a higher conversion than the batch emulsion (BE), while the gel and coagulation contents from both systems are comparable. This is attributed to the SFE that provides a smaller average polymer particle size which in turn results in a greater polymerization locus, promoting the reaction rate. The addition of organo-nanoclay during synthesizing polyisoprene significantly lessens the polymerization efficiency because the nanoclay has a potential to suppress nucleation process of the reaction. It also intervenes the stabilizing efficiency of the surfactant—SDS or sodium dodecyl sulfate, giving enlarged average sizes of the polymer particles suspended in the latexes. TEM images show that nanoclay particles are attached on and/or inserted in the polymer particles. XRD and thermal (differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)) analyses were employed to assess the d-spacing of nanoclay structure in NR nanocomposite films, respectively. Based on the overall results, 5 wt% of nanoclay relative to the monomer content utilized to alter the polyisoprene during emulsion polymerization is an optimum amount since the silicate plates of nanoclay in the composite exhibit the largest d-spacing which maximizes the extent of immobilized polymer constituent, giving the highest mechanical properties to the films. The excessive amounts of nanoclay used, i.e., 7 and 10 wt% relative to the monomer content, reduce the reinforcing power because of the re-agglomeration effect.

Partitioning effect of nitrogen catalyst into polymerizing particles on dispersion reversible chain transfer catalyzed polymerization (dispersionRTCP) of methyl methacrylate in supercritical carbon dioxide and organic solvents

Reversible chain transfer catalyzed polymerization (RTCP) in dispersion polymerization system (dispersion RTCP) of methyl methacrylate (MMA) was performed with N-iodosuccimide (NIS) as a nitrogen catalyst in supercritical carbon dioxide (scCO2). The solubility of NIS in scCO2 can be controlled by tuning the pressure, and this led to promote NIS partitioning into polymerizing particles. As a result, the molecular weight distribution control was successfully improved by decreasing the NIS solubility in the medium by tuning the scCO2 at a low pressure of 20 MPa. On the other hand, at the same NIS concentration, a solution RTCP of MMA in toluene as a homogeneous polymerization system did not proceed with a controlled/living manner. The importance of NIS partitioning into the polymerizing particles was also confirmed in hexane as well as scCO2 medium. From these results, it was clarified that the NIS catalyst partitioning into the polymerizing particles as main polymerization loci is a key factor to control the molecular weight distribution in the dispersion RTCP of MMA in scCO2. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 613–620

Effect of Chain Transfer to Polymer in Conventional and Living Emulsion Polymerization Process

Emulsion polymerization process provides a unique polymerization locus that has a confined tiny space with a higher polymer concentration, compared with the corresponding bulk polymerization, especially for the ab initio emulsion polymerization. Assuming the ideal polymerization kinetics and a constant polymer/monomer ratio, the effect of such a unique reaction environment is explored for both conventional and living free-radical polymerization (FRP), which involves chain transfer to the polymer, forming polymers with long-chain branches. Monte Carlo simulation is applied to investigate detailed branched polymer architecture, including the mean-square radius of gyration of each polymer molecule, <s2>0. The conventional FRP shows a very broad molecular weight distribution (MWD), with the high molecular weight region conforming to the power law distribution. The MWD is much broader than the random branched polymers, having the same primary chain length distribution. The expected <s2>0 for a given MW is much smaller than the random branched polymers. On the other hand, the living FRP shows a much narrower MWD compared with the corresponding random branched polymers. Interestingly, the expected <s2>0 for a given MW is essentially the same as that for the random branched polymers. Emulsion polymerization process affects branched polymer architecture quite differently for the conventional and living FRP.

Supercritical continuous precipitation polymerization of acrylic acid in a droplet-based millifluidic device

In this work, a high pressure droplet-based millifluidic device was developed to achieve the continuous precipitation polymerization of acrylic acid in a green reaction medium: a supercritical mixture of 10 wt% ethanol in CO2 (15/19.5 MPa and 75 °C). To avoid clogging the tubular reactor of 762 µm ID, a segmented flow of an aqueous solution of monomer dispersed in the supercritical mixture (containing the initiator) was generated. During the reaction, the droplets were used as reservoirs which enable the continuous transfer of the monomer to the CO2-rich phase. This work shows that a large range of molar masses of poly(acrylic acid) (from 20,000 to 120,000 g.mol−1) with low polydispersity index (from 1.35 to 1.70) can be obtained. Due to the rapid transfer of the monomer compared to its conversion and the resulting low monomer concentration in the reactive medium, the molar mass distributions are not dependent on the residence time but rather on the initial monomer and initiator concentrations only. Hence in the investigated experimental conditions, the continuous reactive extraction process helps to achieve the precipitation polymerization without clogging the tubular reactor and in mastering the molar mass distribution. The effect of pressure influencing the molar mass distribution of poly(acrylic acid) as well as the competition between two possible loci of polymerization are also discussed.

One-pot synthesis of non-spherical hollow latex polymeric particles via seeded emulsion polymerization

Non-spherical (ns) hollow latex (HL) particle with a void on one side of a polymer particle surrounded by a double-layered shell was fabricated via the one-pot seeded emulsion polymerization (SEP). Start from the ns seed formed by swelling styrene (St) or methyl methacrylate (MMA) mixed with azobisisobutyronitrile into a crosslinked spherical poly(styrene/divinyl benzene/acrylic acid) P(St/DVB/AA) particle and then polymerized. The ns seed particle having two lobes of crosslinked P(St/DVB/AA) and polystyrene (PS) protrusion obtained was then swelled with MMA/DVB/AA monomers (MMA:DVB = 1:2). After polymerization, the obtained ns HL particle with a single void on the PS side was generated without the seed removal. The key success factors are the locus of polymerization at the seed surface and the fast/efficient phase separation between PS seed and shell polymers. The double-layered shell composing of PS inner shell and P(DVB/MMA/AA) outer shell prevented the collapse of hollow structure. The ns HL particles (0.79 μm in diameter and 1.04 μm in over length) as a white pigment in the coating had a hiding power better than ns solid particles (ASTM D2803-96a, 2003).

RAFT Inverse Microemulsion Polymerization: Effects of Monomer Solubility and Different Types of Initiators

AbstractThe effects of monomer solubility and different types of initiators are for the first time reported for a reversible addition–fragmentation chain transfer (RAFT) inverse microemulsion polymerization system. 2‐(dimethylamino)ethyl methacrylate is selected as monomer due to its solubility in several solvents. A nonionic surfactant, cyclohexane, and a trithiocarbonyl RAFT chain transfer agent (CTA) are also used as main components. The reactions are performed adjusting the dispersed aqueous phase with selected pH values (5, 7, and 10), using an oil‐soluble or a water‐soluble initiator. In this microemulsion system, the RAFT process is especially influenced by the mono­mer content in the dispersed aqueous phase, directly related to the final pH. It is suggested that monomer diffusion and changes in the monomer/CTA ratio at the polymerization loci are the primary reasons for the different behaviors observed, specially those related to the molar mass properties. image

Hollow core-shell particles via NR latex seeded emulsion polymerization

Abstract Natural rubber (NR)-based hollow latex (HL) particles have been prepared by seeded emulsion polymerization of methyl methacrylate/divinyl benzene/acrylic acid (MMA/DVB/AA) monomers on NR seed. By using tert-butyl hydroperoxide/tetraethylene pentamine (t-BuHP/TEPA) as a redox initiator, the locus of polymerization was localized at the surface of the NR particle. After swelling with monomers, DVB in NR particle gradually moved outward to compensate the co-polymerization. Phase separation between the NR seed and the polymeric shell occurred and a void was subsequently formed at a MMA/DVB molar ratio of 2.7/1. The void cavity was enlarged when the monomer to seed (M/S) weight ratio was increased to 4/1. Without the necessity of any residual core removal, polydisperse NR-based hollow nanocomposites (298 ± 58 nm) possessing NR-P(MMA/DVB/AA) double-layered shell and a single void (155 ± 37 nm) were obtained. SEM images revealed an increase in surface roughness of the HL particle with increasing M/S weight ratios. The large specific surface area of these NR-based hollow nanocomposites (114.6 m2/g) with a 3.5 nm pore diameter indicates that the polymeric shell is mesoporous. The presence of carboxyl groups, as indicated by negative zeta potentials, can further facilitate surface functionalization. The large void cavity of these NR-based HL particles makes them particularly suited as delivery vehicle systems.

Effect of Small Reaction Locus in Free-Radical Polymerization: Conventional and Reversible-Deactivation Radical Polymerization.

When the size of a polymerization locus is smaller than a few hundred nanometers, such as in miniemulsion polymerization, each locus may contain no more than one key-component molecule, and the concentration may become much larger than the corresponding bulk polymerization, leading to a significantly different rate of polymerization. By focusing attention on the component having the lowest concentration within the species involved in the polymerization rate expression, a simple formula can predict the particle diameter below which the polymerization rate changes significantly from the bulk polymerization. The key component in the conventional free-radical polymerization is the active radical and the polymerization rate becomes larger than the corresponding bulk polymerization when the particle size is smaller than the predicted diameter. The key component in reversible-addition-fragmentation chain-transfer (RAFT) polymerization is the intermediate species, and it can be used to predict the particle diameter below which the polymerization rate starts to increase. On the other hand, the key component is the trapping agent in stable-radical-mediated polymerization (SRMP) and atom-transfer radical polymerization (ATRP), and the polymerization rate decreases as the particle size becomes smaller than the predicted diameter.

Metallocene-catalyzed ethylene−α-olefin isomeric copolymerization: A perspective from hydrodynamic boundary layer mass transfer and design of MAO anion

This study reports a novel conceptual framework that can be easily experimented to evaluate the effects of hydrodynamic boundary layer mass transfer, methylaluminoxane (MAO) anion design, and comonomer steric hindrance on metallocene-catalyzed ethylene polymerization. This approach was illustrated by conducting homo- and isomeric copolymerization of ethylene with 1-hexene and 4-methyl-1-pentene in the presence of bis(n-butylcyclopentadienyl) zirconium dichloride (nBuCp)2ZrCl2, using (i) MAO anion 1 (unsupported [MAOCl2]−) and pseudo-homogeneous reference polymerization, and (ii) MAO anion 2 (supported Si−O−[MAOCl2]−) and in-situ heterogeneous polymerization. The measured polymer morphology, catalyst productivity, molecular weight distribution, and inter-chain composition distribution were related to the locus of polymerization, comonomer effect, in-situ chain transfer process, and micromixing effect, respectively. The peak melting and crystallization temperatures and %crystallinity were mathematically correlated to the parameters of microstructural composition distributions, melt fractionation temperatures, and average lamellar thickness. These relations showed to be insightful. The comonomer-induced enchainment defects and the eventual partial disruption of the crystal lattice were successfully modeled using Flory and Gibbs–Thompson equations. The present methodology can also be applied to study ethylene−α-olefin copolymerization, performed using MAO-activated non-metallocene precatalysts.

Terpolymerization of Styrenic Photoresist Polymers: Effect of RAFT Polymerization on the Compositional Heterogeneity

The heterogeneity of the chemical structure of photoresist polymers, both within and between the polymer chains, is believed to contribute to the phenomenon of line edge roughness in high-resolution lithography. The copolymerization and terpolymerization of three important photoresist monomers—4-acetoxystyrene (AOST), styrene (Sty), and tert-butyl acrylate (tBA)—are examined in detail in this work. In particular, the effect of the use of a reversible addition–fragmentation chain transfer (RAFT) agent in these reactions is reported. The pairwise copolymerization reactivity ratios for the RAFT polymerizations are close to but different from those measured under the same conditions but in the absence of the RAFT agent. The differences are suggested to be related to the effect of the RAFT agent on the environment of the locus of polymerization. The course of the terpolymerization reactions can be well predicted from the copolymer reactivity ratios and the classical Alfrey–Goldfinger expression, indicating that the reactions in this system conform to terminal model of reactivity. Calculations of the expected sequence distributions as a function of composition and conversion clearly demonstrate that the polymers prepared under RAFT conditions are significantly more homogeneous than their counterparts prepared using conventional free radical polymerization. The significance of these results for the performance of the RAFT terpolymers in photoresist formulations is highlighted.

Exploring Self-Condensing Vinyl Polymerization of Inimers in Microemulsion To Regulate the Structures of Hyperbranched Polymers

A synthetic method was successfully developed to produce structurally defined hyperbranched polymers using confined micellar space in microemulsion to regulate atom transfer radical polymerization (ATRP) of inimers. Systematic exploration of experimental variables, including five methacrylate-based inimer species, two ATRP ligands, and varied amounts of inimers and catalysts, produced a series of hyperbranched polymers that encompassed a broad range of molecular weights (Mn = 194–1301 kg/mol), high degrees of branching (DB = 0.26–0.41), and narrow molecular weight distribution (Mw/Mn = 1.1–1.7). The ATRP of inimers in the microemulsion media showed a fast polymerization rate with quantitative conversion of methacrylate groups within 0.5 h. At high conversion, there was essentially one hyperbranched polymer per discrete latex particle, whose dimension (hydrodynamic diameter Dh = 10.95–20.13 nm in water) and uniformity directly determined the molecular weight and polydispersity of the hyperbranched polymer. The DB of hyperbranched polymers was quantitatively determined using inverse gated 13C NMR spectroscopy, and its value was affected by several parameters, all related to the effective amount of copper catalysts in the polymerization loci for dynamic ATRP exchange reactions. The use of inimers and ligands that showed high copper complex solubility and a high feed ratio of copper to inimer could increase the concentration of copper catalyst in the discrete particles and consequently the DB value. Within the investigation, the polymerization of inimer 3 using 4,4′-dinonyl-2,2′-dipyridyl (dNbpy) as ligand produced hyperbranched polymers with the highest DB = 0.41 due to the high solubility of Cu(II)/(dNbpy)2 in inimer 3. When acetal group as a linker was incorporated into the inimer, the produced hyperbranched polymers exhibited complete degradation in acidic environment, indicating potential utility in biomedical applications.

Revisit to the self-assembled hybrid acrylate/silica core-shell structured particles in the presence of unmodified silica particles

Polyacrylate/silica nanocomposite particles are synthesized by soap-free emulsion polymerization using negatively charged silica particles, where the anionic initiator, potassium peroxydisulfate (KPS), is used. The influence of the content of hydrophilic silica particles on the composite particle growth and conversion is investigated. Nucleation process of unmodified silica-stabilized emulsion polymerization is studied by characterizing the structures of composite particles forming at different polymerization stages with transmittance electron microscope (TEM) and laser light scattering (DLS). The results show that the presence of silica particles on latex particles reinforces the barrier to radical absorption and retards the polymerization rate and therefore the growth of the latex particles. However, the composite particle size does not decrease much when the content of silica sol is increased to more than 5wt%. A mechanism is proposed to explain the formation of the core-shell structured particles when an electrostatic repulsive force between the silica and polyacrylate particles is present. The polyacrylate precursors forming in the early stage of the soap-free emulsion polymerization are unstable and tend to aggregate and absorb silica particles to lower their interfacial free energy, thus forming the composite particles. In contrast to the monomer droplets dispersed in the sole acrylic soap free emulsion polymerization, the monomer droplets dispersed in the aqueous system containing silica sol can work as polymerization loci due to their relatively small size and the large quantity (or the large specific surface areas), hence enhancing the ability to capture radicals.

Mechanism of particle formation in radical emulsion copolymerization of styrene with α-tert-butoxy-ω-vinylbenzyl-polyglycidol macromonomer

In the batch emulsion copolymerization of styrene and α-tert-butoxy-ω-vinylbenzyl-polyglycidol macromonomer, carried out at macromonomer concentrations exceeding the critical micelle concentration (cmc), particles are formed by a two-step coagulative nucleation mechanism. This mechanism leaves its mark on morphology of particle interface, rate of polymerization and on molecular weight distribution of the obtained polymer. AFM studies revealed that the interface of particles is composed of objects with dimensions close to dimensions of the primary particles. Compartmentalization of styrene in the macromonomer micelles leads to the higher initial rate of styrene conversion than in the similar macromonomer free homopolymerization of styrene. The initial polymerization in the monomer-swollen macromonomer micelles, similar to the microemulsion polymerization, is responsible for the formation of the highest molecular weight component. In the mature particles there are two different polymerization loci: the interfacial layer and the core. This leads to bimodal molecular weight distribution of the formed polymer.

Synthesis and characterization of TiO2 nanoparticles chemically coated with zwitterionic polymer brushes

This study introduces a synthetic method to graft zwitterionic poly (2-methacryroyloxyethyl phosphoryl choline) brushes onto TiO2 nanoparticles with thickness of a few nanometers. The grafting of zwitterionic polymer brushes was characterized by electron microscope measurements, zeta-potential analysis, and thermo-gravimetric analysis. The technique we employed to synthesize those hybrid nanoparticles was a modified seeded emulsion polymerization in which the polymerization loci were changed in the stage of radical initiation and chain propagation. We have found that the polymerization loci should be on the surface for achieving the effective grafting of polymer brushes. From the suspension rheology studies, we have observed that the suspension of our hybrid nanoparticles was dominated by the attractive force as well as by packing volume effect. This unique rheological behavior is expected to be useful for understanding the inter-particle interactions in complex formulations.