Photoinduced Controlled Radical Polymerization

Well‐controlled radical polymerization of methyl methacrylate can be achieved by in situ photochemical generation of copper (I) complex from air‐stable copper (II) species without using any reducing agent at room temperature. The living character of this polymerization was confirmed by both the linear tendency of molecular weight evolution with conversion and a chain extension experiment.

     

Revisiting Chain Transfer to Polymer and Branching in Controlled Radical Polymerization of Butyl Acrylate

Acrylic monomers undergo chain transfer to polymer during polymerization leading to branched and even gelled polymers. It has been experimentally demonstrated that the extent of branching is higher for conventional free radical polymerization than for controlled radical polymerization (ATRP, RAFT, NMP) and this has been qualitatively explained in terms of the differences in the concentrations of highly reactive short‐chain radicals between controlled and conventional radical polymerizations. Contrary to this explanation, in this work, it is quantitatively demonstrated that the short transient lifetime of the radicals, i.e., the time between activation and deactivation of the radical in controlled radical polymerization, is the cause for the low level of branching in these polymerizations.

     

Frontal Polymerization: Polymerization Induced Destabilization of Peracrylates

Frontal photopolymerization has attracted much attention in the last decade as it allows the curing of thick films. Unfortunately, the use of peroxides, which feature appropriate storage stability, also requires inappropriately high initiation temperatures. Here, a new approach involving a copolymerisation‐induced destabilization of (meth)acrylate‐based peroxides that allows lower front temperatures is presented. The increasing degree of branching next to the carbonyl group lowers the decomposition temperature by at least 20 °C. In classical monomer formulations, sufficient storage stability is confirmed.

     

Studies on the Self-condensing Vinyl Living Radical Polymerization of a Novel Acrylate Inimer

IntroductionDendrimers represent a class of macro-molecules with perfectly and regularly branchedstructures.However,the synthesis ofdendrimers isnot trivial and requires multistep synthesis,theircommercial development has been limited only to afew structures[1— 3 ] . Hyperbranched macro-molecules,which posses less perfectly branchedstructures,have some similar properties to those ofdendrimers,but they can be prepared in a singlestep and one- pot reaction,so many macromolecularresearchershavef…     

Continuous Controlled Radical Polymerization of Methyl Acrylate in a Copper Tubular Reactor

The use of copper tubing as both the reactor and as a catalyst source is demonstrated for continuous controlled radical polymerization of methyl acrylate at ambient temperature and at low solvent content of 30%. The high surface area provided by the copper walls mediates the reaction via the single electron transfer–living radical polymerization (SET‐LRP) mechanism. The polymerizations proceeded quickly, reaching 67% conversion at a residence time of 16 min. Ligand concentration could also be reduced without a sharp drop in polymerization rate, demonstrating the potential for decreased raw material and post‐process purification costs. Chain extension experiments conducted using synthesized polymer showed high livingness. The combination of living polymer produced at high polymerization rates at ambient temperature and low volatile organic solvent content demonstrate the potential of a copper reactor for scale up of SET‐LRP.

     

Radical and cationic photopolymerization: New pyrylium and thiopyrylium salt‐based photoinitiating systems

New thiopyrylium and pyrylium salt‐based photoinitiating systems for visible light induced free radical polymerization (FRP) or free radical promoted cationic polymerization (FRPCP) under visible lights are presented. The reaction mechanisms are investigated by laser flash photolysis and the structure/reactivity trend is discussed. The abilities of two different classes of coinitiators are investigated (thiols/disulfides and silanes). In FRP, upon irradiation with a xenon lamp (λ > 390 nm), the (thio)pyrylium salts in combination with thiols or disulfides lead to very high polymerization rates, compared to the reference eosin Y/methyldiethanolamine system. In FRPCP, silanes are found much better coinitiators: a high efficiency of the photopolymerization under air is noted. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7369–7375, 2008     

Dimethyl Dihydroacridines as Photocatalysts in Organocatalyzed Atom Transfer Radical Polymerization of Acrylate Monomers

Development of photocatalysts (PCs) with diverse properties has been essential in the advancement of organocatalyzed atom transfer radical polymerization (O‐ATRP). Dimethyl dihydroacridines are presented here as a new family of organic PCs, for the first time enabling controlled polymerization of challenging acrylate monomers by O‐ATRP. Structure–property relationships for seven PCs are established, demonstrating tunable photochemical and electrochemical properties, and accessing a strongly oxidizing ²PC^.+ intermediate for efficient deactivation. In O‐ATRP, the combination of PC, implementation of continuous‐flow reactors, and promotion of deactivation through addition of LiBr are critical to producing well‐defined acrylate polymers with dispersities as low as 1.12. The utility of this approach is established through demonstration of the oxygen‐tolerance of the system and application to diverse acrylate monomers, including the synthesis of well‐defined di‐ and triblock copolymers.     

Thioketone‐Mediated Polymerization of Butyl Acrylate: Controlling Free‐Radical Polymerization via a Dormant Radical Species

It is demonstrated by experiment and simulation that the commercially available thioketone 4,4‐bis(dimethylamino)thiobenzophenone is capable of controlling AIBN‐initiated bulk butyl acrylate polymerization at 80 °C. On the basis of molecular weight data and from monomer conversion versus time curves, the associated rate parameters are estimated. The addition rate coefficient, k_ad, for the reaction of a propagating chain with the thioketone is close to 10⁶ L · mol⁻¹ · s⁻¹ and the fragmentation rate coefficient, k_frag, is around 10⁻² s⁻¹ giving rise to large equilibrium constants in the order of 10⁸ L · mol⁻¹. Furthermore, cross‐ and self‐termination of the dormant radical species are identified to be operational.

     

Phenacyl Ethyl Carbazolium as a Long Wavelength Photoinitiator for Free Radical Polymerization

A new phenacyl‐type photoinitiator based on ethyl carbazole as a long wavelength photo­initiator is developed for free radical polymerization. Phenacyl ethyl carbazolium hexafluoroantimonate (PECH) photoinitiator is synthesized in a two‐step, one‐pot manner by quaternizing ethyl carbazole with phenacyl bromide and subsequent ion exchange reaction with potassium hexafluoroantimonate. Under irradiation, PECH tends to undergo homolytic bond cleavage bringing about initiating free radicals. However, as evidenced by cyclic voltammetry and real‐time photobleaching studies, formation of initiating cationic species is highly unlikely as the photochemically formed charged carbazole units tend to couple.

     

Optically Active Helical Vinylterphenyl Polymers: Chiral Teleinduction in Radical Polymerization and Tunable Stereomutation

Helical vinyl aromatic polymers are emerging as interesting chiral materials due to their dynamic tailorability, synthetic simplicity, and outstanding chemical and physical stabilities. This Personal Account discusses long‐range chirality transfer in the radical polymerization of vinylterphenyl monomers and tunable stereomutation of the resultant polymers. It begins with a general introduction to the design, synthesis, and characterization of helical poly{(+)‐2,5‐bis[4′‐((S)‐2‐methylbutyloxy)phenyl]styrene}, the first one of this series of polymers. Then, long‐range chirality transfer during radical polymerization of terphenyl‐based vinyl monomers is explained. After that, the chiroptical property control of the resultant polymers by means of the transition from kinetically controlled conformation to thermodynamically controlled conformation and external stimulus is described. This Personal Account concludes by discussing the advantages and disadvantages of the strategy of using vinylterphenyls to obtain optically active helical polymers and providing a short outlook, especially emphasizing the importance of tacticity on the chiroptical properties of polymers.     

Effective Utilization of NIR Wavelengths for Photo‐Controlled Polymerization: Penetration Through Thick Barriers and Parallel Solar Syntheses

This contribution details an efficient and controlled photopolymerization regulated by far‐red (λ=680 nm) and NIR (λ=780 and 850 nm) light in the presence of aluminium phthalocyanine and aluminium naphthalocyanine. Initiating radicals are generated by photosensitization of peroxides affording an effective strategy that provides controlled polymerization of a variety of monomers with excellent living characteristics. Critically, long wavelength irradiation provides penetration through thick barriers, affording unprecedented rates of controlled polymerization that can open new and exciting applications. Furthermore, a more optimized approach to performing solar syntheses is presented. By combining the narrow Q‐bands of these photocatalysts with others possessing complementary absorptions, layered, independent polymerizations and organic transformations may be performed in parallel under a single broadband emission source, such as sunlight.     

Evaluation of the Chain Length Distribution in Free‐Radical Polymerization, 1. Bulk Polymerization

A method for the direct computation of the chain length distribution in a bulk polymerization is developed, based on the discretization procedure introduced by Kumar and Ramkrishna (Chem. Eng. Sci. 1996 , 51, 1311) in the context of particle size distribution. The overall distribution of chain lengths is partitioned into a finite number of classes which are supposed to be concentrated at some appropriate pivotal chain lengths. Several of the involved reactions lead to the formation of chain whose length differs from the pivotal values. Rules have been introduced in order to share chains between two contiguous classes, which have been designed so as to preserve two well‐defined properties of the distribution, such as, for example, two of its moments. The method has been applied to a polymerization system including propagation, bimolecular terminations and two different chain branching mechanisms: chain transfer to polymer and crosslinking. In addition, complex systems such as one with chain length‐dependent kinetic constants or a two‐dimensional distribution of chain length and number of branches have been considered.     

Calculations of Monomer Conversion and Radical Concentration in Reversible Addition‐Fragmentation Chain Transfer Radical Polymerization

Simple expressions are derived for the development of monomer conversion, as well as propagating radical, adduct radical, dormant chain, and dead chain concentrations in reverse addition‐fragmentation transfer polymerization (RAFT). The relations for the profiles of propagating radical concentration and conversion versus time are derived and depend on group parameters of rate constants and chemical recipe. The analytical equations are verified against numerical solutions of the mass‐balance differential equations. This derivation involves the steady‐state hypothesis for radical and RAFT agent concentrations. The errors introduced by these assumptions are negligible when the fragmentation rate constant, k_f, is higher than 10 s⁻¹ or when the cross‐termination rate constant, k_ct, is higher than 10⁵ L · mol⁻¹ s⁻¹.

Calculated concentration profiles (points: numerical, lines: analytical) of propagating radical R, adduct radical A, dormant T, and dead D (= P + P′) chains.     

CFD Analysis of Gas–Particle Heat Transfer in Gas‐Phase Olefin Polymerizations

Computational fluid dynamics (CFD) is used to study the gas–particle heat transfer in gas‐phase olefin polymerizations. Particularly, the effects of particle rotation on the gas–particle heat transfer coefficient and internal particle temperatures are evaluated, showing that particle rotation can exert a significant impact on observed temperature profiles, so that this effect should not be neglected during detailed CFD process simulations. As a consequence, particle rotation can lead to particle cooling and development of spherical gradient symmetry, validating the use of simpler modeling schemes that are based on reaction–diffusion in symmetrical spherical geometry.

     

Studying the Fundamentals of Radical Polymerization Using ESR in Combination with Controlled Radical Polymerization Methods

Electron spin resonance (ESR) spectroscopy can contribute to understanding both the kinetics and mechanism of radical polymerizations. A series of oligo/poly(meth)acrylates were prepared by atom transfer radical polymerization (ATRP) and purified to provide well defined radical precursors. Model radicals, with given chain lengths, were generated by reaction of the terminal halogens with an organotin compound and the radicals were observed by ESR spectroscopy. This combination of ESR with ATRPs ability to prepare well defined radical precursors provided significant new information on the properties of radicals in radical polymerizations. ESR spectra of the model radicals generated from tert-butyl methacrylate precursors, with various chain lengths, showed clear chain length dependent changes and a possibility of differentiating between the chain lengths of observed propagating radicals by ESR. The ESR spectrum of each dimeric, trimeric, tetrameric, and pentameric tert-butyl acrylate model radicals, observed at various temperatures, provided clear experimental evidence of a 1,5-hydrogen shift.     

Photoinitiation mechanism of free radical photopolymerization in the presence of cyclic acetals and related compounds

The behavior of six cyclic acetals and related compounds in the photoinitiation step of a radical photopolymerization was investigated. As shown by the photopolymerization kinetic data obtained from FTIR spectroscopy, most of them are efficient coinitiators in the presence of benzophenone (BP) with efficiencies close to a reference amine coinitiator (ethyl dimethylaminobenzoate, EDB). Laser flash photolysis and ESR spin trapping technique were used to study the photochemical mechanisms of the production of initiating radicals and explain the differences in reactivity. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Free‐Radical Polymerization with Long‐Chain Branching and Scission in a Continuous Stirred‐Tank Reactor

Free‐radical polymerization that involves the polymer transfer reactions leading to both long‐chain branching and scission, as in the cases of high‐pressure olefin polymerization, is considered. In CSTR, the residence time distribution is broad and the primary polymer chain, whose residence time is large, is subjected to polymer transfer reaction for a longer time, leading to a larger number of branching and scission points. The distributions of both branching and scission density are much broader in a CSTR than in a batch, or equivalently, a PFR. The radius of gyration for larger sized polymers formed in a CSTR tends to be much smaller than that for randomly branched polymers.

     

Tetraacylgermanes: Highly Efficient Photoinitiators for Visible‐Light‐Induced Free‐Radical Polymerization

In this contribution a convenient synthetic method to obtain tetraacylgermanes Ge[C(O)R]₄ (R=mesityl ( 1 a ), phenyl ( 1 b )), a previously unknown class of highly efficient Ge‐based photoinitiators, is described. Tetraacylgermanes are easily accessible via a one‐pot synthetic protocol in >85 % yield, as confirmed by NMR spectroscopy, mass spectrometry, and X‐ray crystallography. The efficiency of 1 a , b as photoinitiators is demonstrated in photobleaching (UV/Vis), time‐resolved EPR (CIDEP), and NMR/CIDNP investigations as well as by photo‐DSC studies. Remarkably, the tetraacylgermanes exceed the performance of currently known long‐wavelength visible‐light photoinitiators for free‐radical polymerization     

Network Formation by Free Radical Polymerization in Isothermal Continuous Flow Stirred Reactors

The authors apply the method of moments to the study of network formation in continuous flow stirred reactors when chain transfer to polymer and coupling are present in the reaction scheme. This approach leads to analytical solutions for the various moments involved. The authors start by assuming that the rate of coupling is proportional to the length of dead chains, which allow them to review and extend previous work in this area. This is followed by similar derivations when a coupling agent is present and the rate of coupling is proportional to the number of coupling groups that such agent leaves in dead polymer molecules, demonstrating that higher values of second order moments can be reached at lower levels of unreacted coupling agent.