Thermal Self-initiation Research Articles

Self-Initiated Butyl Acrylate Polymerizations in Bulk and in Solution Monitored By In-Line Techniques.

High-temperature acrylate polymerizations are technically relevant, but yet not fully understood. In particular the mechanism and the kinetics of the thermal self-initiation is a topic of current research. To obtain more detailed information the conversion dependence of the polymerization rate, rbr, is determined via in-line DSC and FT-NIR spectroscopy for reactions in bulk and in solution at temperatures ranging from 80 to 160 °C. Solution polymerizations revealed that dioxane is associated with the highest rbr, while aromatic solvents result in the lowest values of rbr. Interestingly, rbr for polymerizations in solution with dioxane depends on the actual monomer concentration at a given time in the system, but is not depending on the initial monomer concentration. The overall rate of polymerization in bulk and in solution is well represented by an equation with three or four parameters, respectively, being estimated by multiple linear regression and the temperature as additional parameter.

Synthesis of Ultra-high Molecular Weight SiO2-g-PMMA Particle Brushes

A simple route to synthesize ultra-high molecular weight particle brushes by surface-initiated atom transfer radical polymerization (SI-ATRP) from silica nanoparticles was developed. SiO2-g-PMMA and SiO2-g-PS particle brushes were prepared with different [SiO2–Br]0 concentration of initiating sites on the surface of the nanoparticles. Ultra-high MW (> 106) SiO2-g-PMMA particle brushes with narrow molecular weight distribution (< 1.3) and different grafting densities were synthesized. The grafting density of SiO2-g-PMMA particle brushes decreased with increasing target degree of polymerization. The same conditions were applied to the synthesis of SiO2-g-PS particle brushes. However, due to the lower propagation rate constant of styrene, coupling between SiO2-g-PS particle brushes occurred and also some fraction of unattached homopolystyrene was generated by the thermal self-initiation of styrene, preventing successful synthesis of ultra-high MW SiO2-g-PS particle brushes.

On the Thermal Self-Initiation Reaction of n-Butyl Acrylate in Free-Radical Polymerization

This experimental and theoretical study deals with the thermal spontaneous polymerization of n-butyl acrylate (n-BA). The polymerization was carried out in solution (n-heptane as the solvent) at 200 and 220 °C without adding any conventional initiators. It was studied with the five different n-BA/n-heptane volume ratios: 50/50, 70/30, 80/20, 90/10, and 100/0. Extensive experimental data presented here show significant monomer conversion at all temperatures and concentrations confirming the occurrence of the thermal self-initiation of the monomer. The order, frequency factor, and activation energy of the thermal self-initiation reaction of n-BA were estimated from n-BA conversion, using a macroscopic mechanistic model. The estimated reaction order agrees well with the order obtained via our quantum chemical calculations. Furthermore, the frequency factor and activation energy estimates agree well with the corresponding values that we already reported for bulk polymerization of n-BA.

Tailoring structure formation and mechanical properties of particle brush solids via homopolymer addition.

Recent progress in the area of surface-initiated controlled radical polymerization (SI-CRP) has enabled the synthesis of polymer-grafted colloids with precise control over the architecture of grafted chains. The resulting 'particle brush materials' are of interest both from a fundamental as well as applied perspective because structural frustrations (associated with the tethering of chains to a curved surface) imply a sensitive dependence of the interactions between brush particles on the architecture of surface-tethered chains that offers new opportunities to design hybrid materials with novel functionalities. An important prerequisite for establishing structure-property relations in particle brush materials is to understand the role of homopolymer impurities that form, for example, by thermal self-initiation. This contribution presents a detailed discussion of the role of homopolymer additives on the structure and mechanical properties of particle brush materials. The results suggest that the dissolution of homopolymer fillers follows a two-step mechanism comprised of the initial segregation of homopolymer to the interstitial regions within the array and the subsequent swelling of the particle brush (depending on the respective degree of polymerization of brush and linear chains). Addition of even small amounts of homopolymer is found to significantly increase the fracture toughness of particle brush assembly structures. The increased resistance to failure could enable the synthesis of robust colloidal crystal type materials that can be processed into complex shapes using 'classical' polymer forming techniques such as molding or extrusion.

Poly(α-methylene-γ-valerolactone) 1. Sustainable monomer synthesis and radical polymerization studies

The synthesis of α-methylene-γ-valerolactone from the sustainable γ-valerolactone is described. The radical polymerization of this monomer with varying levels of purity has been studied under both homogeneous and heterogeneous conditions. Solution and heterophase polymerization perform equally well. The heat of polymerization of the monomer has been determined by reaction calorimetry. Properties of poly(α-methylene-γ-valerolactone) dispersions obtained at very different polymerization conditions are presented, including data on thermal self-initiation and copolymerization with butyl acrylate.

Effect of Thermal Self-Initiation on the Synthesis, Composition, and Properties of Particle Brush Materials

Surface-initiated atom transfer radical polymerization (SI-ATRP) has emerged as a powerful tool to synthesize polymer-tethered particles (here called particle brushes) that can self-assemble into hybrid materials with well-defined microstructure, morphology, and enhanced mechanical properties or optical transparency, as compared to binary particle/polymer nanocomposite materials. However, side reactions—such as the thermal self-initiation (TSI) of some monomers, such as styrene, during the polymerization—can result in the formation of varying amounts of homopolystyrene, in addition to particle brushes. The presence of homopolystyrene impurity reduces the predictability of properties and impedes the interpretation of structure–property relations in particle brush materials. This contribution presents a systematic evaluation of the formation of TSI homopolystyrene and its implications on the properties of polystyrene-tethered silica based particle brush materials. Kinetic and molecular weight studies reveal that the fraction of untethered chains generated by TSI depends on reaction conditions and increases with the degree of polymerization of surface-tethered chains. The presence of homopolystyrene results not only in a decrease of the softening temperature but also in an increase of ductility and toughness of particle brush solids. Structural analysis of particle brush assemblies admixed with small particle tracer inclusions suggests that the increase in ductility is related to the preferential segregation of the polystyrene impurity within the corner regions of the Wigner–Seitz cell of the particle brush array.

ICAR ATRP for Estimation of Intrinsic Macro-Activation/Deactivation Arrhenius Parameters under Polymerization Conditions

The potential of Initiators for Continuous Activator Regeneration Atom Transfer Radical Polymerization (ICAR ATRP) to determine reliable Arrhenius parameters for ATRP activation/deactivation of macrospecies is illustrated using styrene as monomer and CuBr2/TPMA (TPMA: tris(2-pyridylmethyl)amine) as deactivator. Regression is based on an extensive set of experimental data limited to conversions below 0.50 to avoid the interference of diffusional limitations on the activation/deactivation process and recorded at temperatures below 90 °C to avoid the influence of thermal self-initiation. Diffusional limitations on termination are accounted for based on literature data. The activation energy for the activation and deactivation reaction involving macrospecies are respectively 29 and 1.7 kJ mol–1. The corresponding pre-exponentional factors are 6.9 × 105 and 1.8 × 107 L mol–1 s–1. At 70 °C, the corresponding rate coefficients amount to 2.2 × 10 and 9.9 × 106 L mol–1 s–1, confirming the relatively high activity of CuBr/TPMA as ATRP catalyst.

Kinetic modeling of miniemulsion nitroxide mediated polymerization of styrene: Effect of particle diameter and nitroxide partitioning up to high conversion

The miniemulsion polymerization of styrene mediated by N-(2-methyl-2-propyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl (SG1) at 396 K is modeled up to high conversion as a function of the targeted chain length (TCL) and particle diameter. Thermal self-initiation and diffusional limitations are explicitly accounted for. The importance of the compartmentalization of nitroxide, initiator and macroradicals and of nitroxide partitioning is assessed using 3-dimensional Smith-Ewart equations. Diffusional limitations on termination are important for higher particle diameters only (>∼70 nm). The influence of diffusional limitations on deactivation, however, can be significant even for intermediate particles diameters (∼40 nm). For a TCL of 300, low particle diameters (<∼20 nm) provide theoretically both a better livingness and control over chain length compared to the bulk case at the expense of a significant reduction of the polymerization rate. For a sufficiently high particle diameter (∼30 nm), a rate acceleration can be obtained accompanied by an improved livingness but with a somewhat reduced control over chain length. For TCLs higher than 300, better overall average polymer properties can be achieved up to particle diameters of ∼ 50 nm. Nitroxide partitioning is shown to lead on average to a limited increase of the polymerization rate without significantly affecting the average polymer properties.

Enhancing the fraction of grafted polystyrene on silica hybrid nanoparticles

Polymer-grafted inorganic nanoparticles are being developed for a diverse array of applications, ranging from drug delivery to multifunctional composites. In many instances, performance of these core-shell hybrids is limited by relatively broad distributions of size and composition, as well as the presence of impurities, such as unattached polymer chains. Herein, further synthetic improvements, and associated characterization techniques, to enhance the fraction of the grafted polystyrene shell on silica hybrid nanoparticles are discussed. We found that during surface-initiated atom transfer radical polymerization (SI-ATRP) from the silica nanoparticles, thermal self-initiation of styrene produces unattached polymer chains. Size exclusion chromatography afforded a facile approach to quantify the mass of the unattached polymer, and provide a substantial refinement to estimates of chain graft density beyond traditionally-used approaches, such as thermogravimetry. This fraction of unattached polymer is still present even after post-polymerization work-up via precipitation and re-dissolution. Removal necessitates additional procedures, such as high speed centrifugation. Selection of a lower polymerization temperature, in concert with a more reactive Cu complex, significantly reduces the amount of unattached polystyrene impurity. The improved polymerization conditions and post-polymerization purification provide more refined polystyrene-grafted silica nanoparticles to clarify structure-property relationships of these core-shell hybrids.

Thermal Self-Initiation in Stable Free-Radical Polymerization of Styrene

Stable free radical polymerization (SFRP) of styrene was carried out using bis(4-bromomethylbenzoyl)peroxide (BBMBPO)/4-hydroxyl-2,2,6,6-tetramethyl-1-piperidinyloxy (HTEMPO) or 4,4′-azobis(4-cyanopentyl)-α-bromoisobutyrate (ABCBIB)/HTEMPO as bimolecular initiating system. It was found that the molecular weights of obtained polystyrenes were controlled by the concentration of HTEMPO. Besides those generated by the initiators, some polystyrene chains were formed via thermal initiation. Polystyrene chains generated by the initiator had initiating groups for atom transfer radical polymerization (ATRP) at their ends, so they grew to higher molecular weights in ATRP of styrene. While the molecular weights of polystyrene chains generated by thermal initiation kept constant in ATRP conditions because of their inert terminals for ATRP. The amount of polystyrenes generated by thermal initiation was investigated quantitatively via gel permeation chromatography technique.

Novel Cationic RAFT-Mediated Polystyrene/Clay Nanocomposites: Synthesis, Characterization, and Thermal Stability

Two novel cationic RAFT agents, PCDBAB and DCTBAB, were anchored onto MMT clay to yield RAFT-MMT clays. The RAFT-MMT clays were then dispersed in styrene where thermal self-initiation polymerization of styrene to give rise to exfoliated PS/clay nanocomposites occurred. The RAFT agents anchored onto the clay layers successfully controlled the polymerization process resulting in controlled molecular masses and narrow polydispersity indices. The nanocomposites prepared showed enhanced thermal stability, which was a function of the clay loading, clay morphology, and slightly on molecular mass.

A Mathematical Model for the Reactive Extrusion of Methyl Methacrylate in a Co-rotating Twin-Screw Extruder

A mathematical model for the reactive extrusion of methyl methacrylate (MMA) is presented. Key variables, such as pressure, temperature, residence time, filling ratio, number-average, and weight-average molecular weights (Mn and Mw) along the extruder length can be calculated using the model. The flow in the extruder is modeled using a simplified approach; however, this approach is versatile enough to include any screw profile, such as right-handed and left-handed elements, and kneading disks. A polymerization kinetic model that considers a mixture (“cocktail”) of initiators is coupled to the flow equations. An effective model for the auto-acceleration effect is used. Second-order thermal self-initiation of MMA is considered in the reaction mechanism. The model can be easily performed on a personal computer, and a wide range of process conditions can be modeled, because of its flexibility.

Quantification of reaction violence and combustion enthalpy of plastic bonded explosive 9501 under strong confinement

The confinement experienced by an explosive during thermal self-initiation can substantially affect performance in terms of deflagration–to–detonation characteristics and explosion/detonation violence. To this end, we have developed an experiment to quantitatively observe enthalpy change and reaction violence in thermally initiated plastic bonded explosive (PBX) 9501. Traditionally, researchers attempt to quantify violence using terminal observations of fragment size, fragment velocity, and through subjective observations. In the work presented here, the explosive was loaded into a heated gun assembly where we subjected a 300 mg charge to a cook-off schedule and a range of static and inertial confinements. Static confinement was controlled using rupture disks calibrated at 34.5 and 138 MPa. The use of 3.15 and 6.3 g projectile masses provided a variation in inertial confinement. This was a regime of strong confinement; a significant fraction of the explosive energy was required to rupture the disk, and the projectile mass was large compared to the charge mass. The state variables pressure and volume were measured in the breech. From these data, we quantified both the reaction enthalpy change and energy release rate of the explosive on a microsecond time scale using a thermodynamic analyisis. We used these values to unambiguously quantify explosion violence as a function of confinement at a fixed cook-off schedule of 190 C for 1 h. P2τ, a measure of critical shock energy required for shock ignition of an adjacent explosive was also computed. We found variations in this confinement regime to have a weak effect on enthalpy change, power, violence and shock energy. Violence was approximately 100 times lower than detonating trinitrotoluene, but the measured shock energy approached the critical shock energy for initiating secondary high explosives.

Kinetic modeling of the chain-end functionality in atom transfer radical polymerization

The evolution of chain-end functionality of polymers synthesized by atom transfer radical polymerization (ATRP) was modeled. By comparing various kinetic models, the effect of specific side reactions was estimated. The slow elimination of hydrobromic acid from the polymer end-groups as well as the thermal self-initiation of the monomer may affect the chain-end functionality. Although the polymerization possesses several characters of a living process (i.e. linear increase of molecular weight versus conversion, low polydispersity index), the final polymer may have a limited functionality. However, polymers with enhanced functionality can be prepared through adjusting certain experimental parameters such as conversion of the monomer or initial concentration of the reactants. Plots of the percentage of chain-end functionality versus [XMtm+1(L)z]0 for the bulk polymerization of styrene at 110 °C: [M]0 = 8.74 mol L−1, [RX]0 = 0.0874 mol L−1, [Mtm(L)z]0 = 0.0874 mol L−1. Simulation plots: ideal model (▪), CLD model (▵), thermal-initiation model (⧫), elimination model (○). The inset shows an expansion of the region 90 to 100% of the y-axis.

DEVELOPMENT OF A KINETIC MODEL FOR INIFERTER CONTROLLED/“LIVING” FREE-RADICAL POLYMERIZATION CONSIDERING DIFFUSION-CONTROLLED EFFECTS 1*

A kinetic model incorporating the effects of diffusion-controlled reactions on INIFERTER (initiator-transfer agent-terminator) thermal free-radical polymerization has been developed. Molecular weight development is done using the method of moments. Diffusion-controlled effects are modeled using free-volume theory. The reactions considered to be diffusion-controlled are: monomer propagation, bimolecular radical termination, dormant termination, chain transfer to monomer, and chain transfer to iniferter. Radical generation by thermal self-initiation is also included in the model. Model predictions indicate that diffusion-controlled propagation reduces the “living” behavior of the system, whereas a diffusion-controlled termination enhances its livingness. The livingness of the system is also enhanced by diffusion-controlled dormant termination. When diffusion-controlled termination is important, our simulations show that the overall effect of the diffusion controlled phenomena in iniferter polymerization is ...

Atom transfer radical polymerization from nanoparticles: a tool for the preparation of well-defined hybrid nanostructures and for understanding the chemistry of controlled/"living" radical polymerizations from surfaces.

Structurally well-defined polymer--nanoparticle hybrids were prepared by modifying the surface of silica nanoparticles with initiators for atom transfer radical polymerization and by using these initiator-modified nanoparticles as macroinitiators. Well-defined polymer chains were grown from the nanoparticle surfaces to yield individual particles composed of a silica core and a well-defined, densely grafted outer polystyrene or poly(methyl methacrylate) layer. In both cases, linear kinetic plots, linear plots of molecular weight (M(n)) versus conversion, increases in hydrodynamic diameter with increasing conversion, and narrow molecular weight distributions (M(w)/M(n)) for the grafted polymer samples were observed. Polymerizations of styrene from smaller (75-nm-diameter) silica nanoparticles exhibited good molecular weight control, while polymerizations of methyl methacrylate (MMA) from the same nanoparticles exhibited good molecular weight control only when a small amount of free initiator was added to the polymerization solution. The difference in polymerization behavior for styrene and MMA was ascribed to the facts that styrene undergoes thermal self-initiation while MMA does not and that termination processes involving freely diffusing chains are faster than those involving surface-bound chains. The polymerizations of both styrene and MMA from larger (300-nm-diameter) silica nanoparticles did not exhibit molecular weight control. This lack of control was ascribed to the very high initial monomer-to-initiator ratio in these polymerizations. Molecular weight control was induced by the addition of a small amount of free initiator to the polymerization but was not induced when 5--15 mol % of deactivator (Cu(II) complex) was added.

Chlorolanthanocene-dialkylmagnesium systems for styrene bulk polymerization and styrene-ethylene block copolymerization

The bulk polymerization of styrene has been investigated at 105°C in the presence of exclusively dialkylmagnesium or combination of chlorolanthanocene and dialkylmagnesium. In the presence of butylethylmagnesium or n,s-dibutylmagnesium, styrene polymerization proceeds via thermal self-initiation, but is accompanied by a reversible transfer to dialkylmagnesiums to yield in turn oligostyrylmagnesium species; the latter are finally hydrolysed to oligostyrenes with Mn = 500–1 500 and Mw/Mn = 2.0–2.8. The analysis of the oligostyrenes by MALDI-TOF mass spectrometry establishes the presence of ethyl and butyl headgroups, consistent with the transfer process. When the dialkylmagnesium is combined with a lanthanocene such as (C5Me5)2NdCl2Li(OEt2)2 (1), an increase in activity is obtained which is ascribed to additional styrene polymerization initiated by in situ generated alkyl(hydride)lanthanocene species. The influence of various reaction parameters on the performance of this system has been investigated. The oligostyrenes (Mn = 500–9 000) produced under optimum conditions have a relatively narrow molar mass distribution (Mw/Mn = 1.20–1.40) which can be explained in terms of an efficient transfer between the chain-growing lanthanide and the oligostyrylmagnesium species. The MALDI-TOF mass spectra of the oligostyrenes produced with various dialkylmagnesium-lanthanocene combinations gives an insight into the initiation mechanism. Finally, the combination of butylethylmagnesium and Cp*2NdCl2Li(OEt2)2 has been used to achieve (styrene-co-ethylene) block copolymers.

Chlorolanthanocene-dialkylmagnesium systems for styrene bulk polymerization and styrene-ethylene block copolymerization

The bulk polymerization of styrene has been investigated at 105°C in the presence of exclusively dialkylmagnesium or combination of chlorolanthanocene and dialkylmagnesium. In the presence of butylethylmagnesium or n,s-dibutylmagnesium, styrene polymerization proceeds via thermal self-initiation, but is accompanied by a reversible transfer to dialkylmagnesiums to yield in turn oligostyrylmagnesium species; the latter are finally hydrolysed to oligostyrenes with M n = 300-1500 and M w / M n = 2.0-2.8. The analysis of the oligostyrenes by MALDI-TOF mass spectrometry establishes the presence of ethyl and butyl headgroups. consistent with the transfer process. When the dialkylmagnesium is combined with a lanthanocene such as (C 5 Me 5 ) 2 NdCl 2 Li(OEt 2 ) 2 (1), an increase in activity is obtained which is ascribed to addinonal styrene polymerization initiated by in situ generated alkylthydride)lanthanocene species. The influence of varions reaction parameters on the performance of this system has been investigated. The oligostyrenes (M m = 500-9000) produced under optimum conditions have a relatively narrow molar mass distribution (M w /M n = 1.20-1.40) which can be explained in terms of an efficient transfer between the chain-growing lanthanide and the oligostyrylmagnesium spectes. The MALDI-TOF mass spectra of the oligostyrenes produced with various dialkyimagnesium-lanthanocene combinations gives an insight into the initiation mechanism. Fanally, the combination of butylethylmagnesium and Cp * 2 NdCl 2 Li(OEt 2 ) 2 has been used to achieve (styrene-co-athylene) block copolymers.

Theoretical Aspects of Controlled Radical Polymerization in a Dispersed Medium

Kinetics of controlled radical polymerization (CRP) is examined in a dispersed medium, when the persistent radical effect operates. For that purpose, nitroxide-mediated CRP is considered in a miniemulsion system with oil-soluble alkoxyamine unimolecular initiator and in the absence of thermal self-initiation. An analytical expression of the rate of irreversible termination is proposed as a function of particle size, and it is compared with the rate of termination in bulk. From these expressions, the evolution with monomer conversion of the concentration of released nitroxide is calculated. For large particles, the polymerization kinetics was found to be similar in bulk and in the miniemulsion. In contrast for small particles, owing to slower terminations, the concentration of released nitroxide was shown to be smaller in the miniemulsion than in bulk, leading thus to a lower proportion of dead chains, a faster polymerization, and a broader molar mass distribution.

Influence of N-oxyls on the free radical autopolymerization of styrene

Through comparative studies on the controlled thermally initiated radical polymerization of styrene in the presence of N-oxyls, the effect of the type of the N-oxyls (2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (OH-TEMPO), 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (oxo-TEMPO) and 4-acetamido-2,2,6,6-tetramethylpiperidine-N-oxyl (acetamido-TEMPO)), as well as the concentration of the N-oxyls (3–20 mmol L–1) are discussed. The radical formation proceeds exclusively through the thermal initiation of styrene, i.e. without using any initiator. It is observed, that the thermal self-initiation of styrene increases the radical yield remarkably with the N-oxyls concentration, independent of the type of the N-oxyl. The concentration-dependent side reactions of substituted N-oxyls are remarkably more pronounced than those of unsubstituted N-oxyl. This leads to the reduction of the overall rate of polymerization (vbr) as compared to TEMPO (vbr = vbr (thermal polymerization)) with respect to the concentration, but independent of the type of substituents. Durch vergleichende Untersuchungen der kontrollierten thermisch initiierten radikalischen Polymerisation von Styrol in Gegenwart eines N-Oxyls wird der Einfluß der Art des N-Oxyls (2,2,6,6-Tetramethylpiperidin-N-oxyl (TEMPO), 4-Hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl (OH-TEMPO), 4-Oxo-2,2,6,6-tetramethylpiperidin-N-oxyl (Oxo-TEMPO), 4-Acetamido-2,2,6,6-tetramethylpiperidin-N-oxyl (Acetamido-TEMPO)) und der Konzentration des N-Oxyls (3–20 mmol L–1) diskutiert. Die Radikalbildung erfolgt ausschließlich durch die thermische Initiierung von Styrol, also ohne Zusatz eines Initiators. Beobachtet wird eine wesentliche Erhöhung der Radikalausbeute der thermischen Selbstinitiierung von Styrol in Abhängigkeit von der Konzentration des N-Oxyls, unabhängig von der Art des N-Oxyls. Die konzentrationsabhängigen Nebenreaktionen substituierter N-Oxyle sind wesentlich ausgeprägter. Das führt zur Reduzierung der Bruttopolymerisationsgeschwindigkeit (vbr) im Vergleich zu TEMPO (vbr = vbr (thermische Polymerisation)) in Abhängigkeit von der Konzentration, aber unabhängig von der Art der Substituenten.