Chain transfer to solvent in the radical polymerization of N‐isopropylacrylamide

Chain transfer to solvent has been investigated in the conventional radical polymerization and nitroxide‐mediated radical polymerization (NMP) of N‐isopropylacrylamide (NIPAM) in N,N‐dimethylformamide (DMF) at 120 °C. The extent of chain transfer to DMF can significantly impact the maximum attainable molecular weight in both systems. Based on a theoretical treatment, it has been shown that the same value of chain transfer to solvent constant, C_tr,S, in DMF at 120 °C (within experimental error) can account for experimental molecular weight data for both conventional radical polymerization and NMP under conditions where chain transfer to solvent is a significant end‐forming event. In NMP (and other controlled/living radical polymerization systems), chain transfer to solvent is manifested as the number‐average molecular weight (M_n) going through a maximum value with increasing monomer conversion. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

The Quenched Instationary Polymerization System, 5. The Peak Broadness as an Invariant Quantity of the Number,Molar Mass and Hyper Distributions

The total number, molar mass and hyper distributions generated by quenched instationary polymerization techniques are dominated by the radical chain length distribution (RCLD) whereas the contribution from the polymer chain length distribution (PCLD) is in most cases negligible. For the determination of the rate constant of propagation (k_p) the location of different extraordinary points of the distribution curves is determined by the use of the first and second derivatives. For the number, molar mass and hyper distributions these points are related in an unambiguous way to k_p[M]t_x and can be used to extract k_p. The choice of t_x (duration of the dark period, or an initiation period, or the sum of different periods) depends on the experimental conditions (δ‐pulse, incomplete pre‐effect, combination of periods differing in initiation extent) and is essential for the proper determination of k_p. The broadness of appearing peaks (introduced as the difference between two successive points of inflections) turned out to remain the same irrespective which type of distribution curve was analyzed. Analytical expressions for the peak broadness were derived for different types of quenched instationary polymerization conditions. For δ‐pulse initiation the broadness of the Poisson peak depends simply on the number of propagation steps that occurred whereas for non‐δ‐pulse initiation conditions the peak broadness is governed by the corresponding duration of the initiation period.     

Chain Transfer and Efficiency of End‐Group Introduction in Free Radical Polymerization of Methyl Methacrylate in the Presence of Poly(methyl methacrylate) Macromonomer

Summary: Experimental and modeling studies of addition–fragmentation chain transfer (AFCT) during radical polymerization of methyl methacrylate in the presence of poly(methyl methacrylate) macromonomer with 2‐carbomethoxy‐2‐propenyl ω‐ends (PMMA‐CO₂Me) at 60 °C are reported. The results revealed that AFCT involving PMMA‐CO₂Me formed in situ during methyl methacrylate polymerization has a negligible effect on the molecular weight distribution.

     

Simulation of Molecular Weight Distributions Obtained by Pulsed Laser Polymerization (PLP): New Analytical Expressions Including Intramolecular Chain Transfer to the Polymer

A new approach for the simulation of PLP (pulsed laser polymerization) is presented. This approach allows one to obtain new analytical solutions for different polymerization schemes, including either chain transfer to the monomer or intramolecular chain transfer to the polymer. The first results of the simulation of PLP experiments on n‐butyl acrylate at 20 °C and ambient pressure are presented.

MWDs simulated for PLP of n‐butyl acrylate, in bulk at 20 °C and ambient pressure using three models: the model with intramolecular chain transfer to the polymer (solid line), the model with chain transfer to monomer (dashed line), and the classical model (dotted line).     

Accelerated iterative strategy for the divergent synthesis of dendritic macromolecules using a combination of living radical polymerization and an irreversible terminator multifunctional initiator

Our laboratory has reported the elaboration of an iterative strategy for the synthesis of dendritic macromolecules from conventional monomers. This synthetic method involves a combination of self‐regulated metal‐catalyzed living radical polymerization initiated from arenesulfonyl chlorides and an irreversible terminator multifunctional initiator (TERMINI). The previous TERMINI, (1,1‐dimethylethyl)[[1‐[3,5‐bis(S‐phenyl‐4‐N,N′ diethylthiocarbamate)phenyl]ethenyl]oxy]dimethylsilane, was prepared in nine reaction steps. The replacement of the previous TERMINI with one that requires only three steps for its synthesis, diethylthiocarbamic acid S‐{3‐[1‐(tert‐butyl‐dimethyl‐silanyloxy)‐vinyl]‐5‐diethylcarbamoylsulfanyl‐phenyl} ester, and the use of the more reactive Cu₂S/2,2′‐bipyridine rather than the Cu₂O/2,2′‐bipyridine self‐regulated catalyst have generated an accelerated method for the synthesis of dendritic macromolecules. This method provides rational design strategies for the synthesis of dendritic macromolecules with different compaction by the use of a single monomer. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4894–4906, 2005     

Evolution of the termination rate coefficient in styrene polymerization

Styrene bulk polymerization was conducted at 70 °C with a high initiator concentration, and this ensured that the dominant chain‐stopping mechanism was the combination of free radicals. The evolution of the molecular weight distribution (MWD) of the polymer was measured via the periodic removal of samples during the course of the reaction and their analysis with gel permeation chromatography. The overall termination rate coefficient was independent of the conversion in the dilute regime, as observed from cumulative MWDs. In the middle of the conversion range, the observed trend was compatible with a translational‐diffusion‐controlled mechanism for the termination step. A bimodal distribution of the molecular weights was also found at high conversions and could be explained in terms of an increase in the free‐radical concentration and a very low termination rate coefficient. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 178–187, 2005     

Monte Carlo Simulation of Controlled/Living Radical Polymerization in Emulsified Systems

A new MC simulation method is proposed for the controlled/living radical polymerization in a dispersed medium, assuming an ideal miniemulsion system. This tool is used to consider the effects of particle size on the polymerization rates and the molecular weight distributions. For NMP, the polymerization kinetics are basically governed by two conflicting factors, (i) the confined space effect that promotes the coupling reaction between a radical and a trapping agent and (ii) the isolation effect of radicals into different particles that suppresses the overall frequency of bimolecular termination. For RAFT polymerization, a significant rate enhancement by reducing the particle size could be observed only for the systems with fast fragmentation of adduct radicals.

     

Predicting MWD and Branching Distribution of Terminally Branched Polymers Undergoing Random Scission

A new semi‐analytical approach to model simultaneous chain scission and branching is described that assumes the separation of the scission and the branching problem. The required properties of the linear segments or primary polymers forming the branched architectures are found by a kinetic model. The general rules for the construction of branched architectures from populations of linear segments then lead to an analytical expression for the branching distribution and a semi‐analytical expression for the bivariate length/branching distribution. The method is applied to the scission of an initially branched polymer and subsequent terminal branching on scission points and activated terminal double bonds. Exact agreement is found with Monte Carlo sampling results.

     

Kinetics of Stable Free Radical Mediated Polymerization inside Submicron Particles

Controlled/living radical polymerization systems in which the active period is extremely small, ϕ_A ≪ 1, such as the cases of stable free radical mediated polymerization (or nitroxide mediated polymerization) and atom transfer radical polymerization, are considered theoretically. The polymerization rate, R_p, for such systems increases by lowering the trapping agent concentration [X]. When the polymerization is conducted inside small particles, R_p decreases with D below the diameter D_p,SMC at which a single molecule concentration (SMC) is equal to [X]_bulk. On the other hand, when the average number of trapping agents in a particle is smaller than about 10, the fluctuation of n_X among particles is significant, which leads to a larger R_p than in the cases where all particles contain the same n_X. Because of the effects of SMC and fluctuation, R_p may show an acceleration window, D_p,SMC < D_p < D_p,Fluct where R_p is slightly larger than that in bulk.

     

Kinetics of dispersion polymerization: Effect of medium composition

The effect of the medium composition (monomer and solvent) on the kinetics of dispersion polymerization of methyl methacrylate (MMA) was studied via reaction calorimetry. It was found that increasing the monomer concentration increased the reaction rate; the exponent of the dependency of the initial reaction rate on the MMA concentration was found to be 0.93. Narrow particle size distributions were achieved at the lower monomer concentrations (0.24–0.81 mol/L) and a minimum size (2.45 μm) was found at an intermediate concentration (0.44 mol/L). The average molecular weight of the PMMA increased and the molecular weight distribution broadened with increasing monomer concentration. During a dispersion polymerization, the MMA concentration was found to decrease linearly with conversion in both phases, whereas the ratio of concentrations in the particles and continuous phase ([M]_p/[M]_c) remained constant (0.47) with partitioning favoring the continuous phase. The average number of free radicals per particle in MMA dispersion polymerization was estimated to be high from the nucleation stage onward (>5000). The increasing rate during the first ～ 40% conversion was primarily caused by the increasing volume of the polymer particle phase. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3638–3647, 2008     

Dispersion copolymerization of methyl methacrylate and n-butyl acrylate

In the dispersion copolymerization of methyl methacrylate (MMA) and n-butyl acrylate (BA), the particle size increases with an increasing MMA fraction in the comonomer. The power dependence of the particle size on the initiator concentration also increases with an increasing MMA concentration. Similar to what can be found in the homopolymerizations, two populations can be observed in the molecular weight distributions of the copolymers. Core–shell structured particles with a poly(methyl methacrylate)-rich core and a poly(n-butyl acrylate)-rich shell result from the copolymerizations because of the significantly different reactivity ratios. The reaction rates of the dispersion copolymerization are lower than those of the homopolymerization of BA and close to or lower than those of the homopolymerization of MMA, depending on the ratio of the monomers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2105–2112, 2007     

Monte Carlo simulation of diepoxides and monoepoxides cured with amines

The diepoxide–monoepoxide–diamine curing systems are investigated with a Monte Carlo simulation. The dependence of the molecular weight distribution (MWD), gel fraction, and cycle rank of the polymers on the differences in the epoxy reactivities and the contents of the monoepoxide as a reactive diluent are discussed. Before gelation, the MWD of the curing systems with a lower content of the monoepoxide is broader than the MWD of the curing systems with a higher content, and it leads to a lower critical conversion. The gel fraction and cycle rank of the polymers decrease with an increasing amount of the diluent. Even fully cured, the system with a 0.6 epoxy molar fraction of the monoepoxide still has a large fraction of sol, about 49%. Although the various reactivities of the monoepoxide result in different ways of forming gels during curing, the final gel fractions are always near 100% as long as the epoxy molar fraction of the diluent is no more than 0.2. The profiles of the molecular weights of the polymers calculated by the model are in agreement with the experimental data. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1857–1868, 2002     

Determination of Propagation Rate Coefficient of Acrylates by Pulsed‐Laser Polymerization in the Presence of Intramolecular Chain Transfer to Polymer

Unusual difficulties are faced in the determination of propagation rate coefficients (k_p) of alkyl acrylates by pulsed‐laser polymerization (PLP). When the backbiting is the predominant chain transfer event, the apparent k_p of acrylates determined in PLP experiments for different frequencies should range between k_p (propagation rate coefficient of the secondary radicals) at high frequency and k at low frequency. The k value could be expressed from kinetic parameters: , where k_fp is the backbiting rate coefficient, k_p2 is the propagation rate coefficient of mid‐chain radicals, and [M] is the monomer concentration.

Apparent propagation rate coefficients determined for different frequencies by simulating the PLP of n‐butyl acrylate at 20 °C. Horizontal full lines show the values of k_p and k.     

Ligand‐free SET‐DTLRP of MMA at room temperature

An iodine‐based initiator, 2‐iodo‐2‐methylpropionitrile (CPI), was utilized for the single‐electron transfer and degenerative chain transfer mediated living radical polymerization (SET‐DTLRP) of methyl methacrylate (MMA) in the absence of ligand, at ambient temperature. The CPI‐initiated ligand‐free polymerizations manifested reasonable control over molecular weights with relatively narrow distributions (M_w/M_n ≤ 1.35). The living nature of the polymers was further confirmed by successful chain extension reaction and ¹H NMR with high chain‐end fidelity (～96%). Screening of the available solvents suggested that the controllability of this polymerization was highly dependent on the kind of solvents, wherein dimethyl sulfoxide was a better solvent for a controlled molecular weight. The proposed ligand‐free SET‐DTLRP initiated by CPI was intriguing since it would dramatically decrease the concentration of Cu(0) ions both in polymerization system and resultant polymer, and provided a more economical and eco‐friendly reversible‐deactivation radical polymerization technique. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

RAFT‐Polymerization of Styrene up to High Pressure: Rate Enhancement and Improved Control

Summary: Application of high pressure, up to 2 500 bar, in cumyl dithiobenzoate‐mediated styrene reversible addition fragmentation chain transfer (RAFT) polymerizations was found to be extremely advantageous with respect to both rate and control of polymerization. The overall rate of polymerization could be increased by a factor of approximately 3 with, e.g., at 23% conversion, concomitantly reducing the polydispersity indices from 1.35 to 1.10. No significant effect of increased pressure on the rate retardation effect was found.

SEC curves of polystyrene samples with identical peak molecular weights, generated by CDB‐mediated styrene bulk polymerization at 70 °C at 1 and at 2 000 bar.     

Modeling the reversible addition–fragmentation transfer polymerization process

A kinetic model has been developed for reversible addition–fragmentation transfer (RAFT) polymerization with the method of moments. The model predicts the monomer conversion, number‐average molecular weight, and polydispersity of the molecular weight distribution. It also provides detailed information about the development of various types of chain species during polymerization, including propagating radical chains, adduct radical chains, dormant chains, and three types of dead chains. The effects of the RAFT agent concentration and the rate constants of the initiator decomposition, radical addition, fragmentation, disproportionation, and recombination termination of propagating radicals and cross‐termination between propagating and adduct radicals on the kinetics and polymer chain properties are examined with the model. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1553–1566, 2003     

Change and Convergence of Polymer Distribution During Nonrandom Degradation

The CLD development during nonrandom degradation is investigated, assuming the rate of bond scission depends on the chain length and the position of the bond within the chain. As the degradation proceeds, the polydispersity index usually approaches a fixed value pertinent to the degradation mechanism, independent of the initial distribution. The exact limiting values are presented for several cases. These values may be useful to estimate the degradation mechanisms. For example, if the limiting PDI is smaller than 2, the bonds in larger chains may be easier to break than shorter ones, and if it is smaller than 4/3, the size effect is not enough and the breakage may tend to occur in the middle of the chain.

     

Fundamentals of RAFT Miniemulsion Polymerization Kinetics

Summary: The polymerization rate of RAFT-mediated miniemulsion polymerization, in which the time fraction of active radical ϕ_A is larger than a few percent, basically increases with reducing the particle size. For smaller particle sizes, however, the statistical variation of monomer concentration among particles may slow down the polymerization rate. The rate retardation by increasing the RAFT concentration occurs with or without the intermediate termination in a zero-one system. According to the present theoretical investigation, smaller particles are advantageous in implementing a faster polymerization rate, a narrower MWD, and a smaller number of dead polymer chains.     

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.