Kinetics and mechanisms of emulsion polymerization initiated by oil-soluble initiators. IV. Kinetic modeling of unseeded emulsion polymerization of styrene initiated by 2,2′-azobisisobutyronitrile

To explain the kinetic features of particle formation and growth in unseeded emulsion polymerization initiated by oil-soluble initiators, a mathematical kinetic model is proposed, based on the assumption that when initiator radicals or monomer radicals in the water phase enter monomer-solubilized emulsifier micelles, initiate polymerization, and propagate to a chain length which is long enough not to desorb from the micelles, the micelles are regarded to be transformed into polymer particles. It is demonstrated by comparing the experimental results obtained in the emulsion polymerization of styrene initiated by the oil-soluble initiator, 2,2'-azobisisobutyronitrile, with sodium lauryl sulfate as emulsifier that the proposed kinetic model satisfactorily explains the kinetic features such as the effects of initial emulsifier, initiator, and monomer concentrations on both the number of polymer particles produced and the monomer conversion versus time histories. © 1993 John Wiley & Sons, Inc.     

On the main locus of radical formation in emulsion polymerization initiated by oil-soluble initiators

The effect of the monomer/water ratio on the rate of polymerization per polymer particle in both seeded emulsion polymerizations and miniemulsion polymerizations was used in an attempt to elucidate the main locus of radical formation in emulsion polymerization initiated by an oil-soluble initiator (AIBN). It was found that, for the rest of conditions constant, the polymerization rate per polymer particle increased when the monomer/water ratio increased, namely when the amount of initiator dissolved in the aqueous phase per polymer particle decreased. This is an evidence against a dominant aqueous phase formation of radicals. On the other hand, these results are consistent with a mechanism in which the radicals are mainly produced in the oil-phase with significant aqueous phase termination.     

Styrene miniemulsion polymerization initiated by 2,2′-azobisisobutyronitrile

The effects of various parameters on the dodecyl methacrylate (DMA) or stearyl methacrylate (SMA) containing styrene miniemulsion polymerizations were investigated. These parameters include the type of initiators [2,2′-azobisisobutyronitrile (AIBN) vs. sodium persulfate (SPS)], the size of the homogenized monomer droplets, the AIBN concentration, and the SDS concentration. A small quantity of a water-insoluble dye was also incorporated into the polymerization system to study the related particle nucleation mechanisms. The oil-soluble AIBN promotes nucleation in the monomer droplets, whereas homogeneous nucleation predominates in the reaction system with the water-soluble SPS. Homogeneous nucleation, however, cannot be ruled out in the DMA or SMA containing polymerizations with AIBN as the sole initiator. Increasing the level of AIBN or SDS enhances formation of particle nuclei via homogeneous nucleation. The reaction kinetics is primarily controlled by the competitive events of monomer droplet nucleation and homogeneous nucleation. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2537–2550, 1999     

Compartmentalized polymerization initiated by oil-soluble initiators. Calculation of average number of radicals per particle

Expressions for calculating the stationary state distribution of radicals in compartmentalized systems with a constant number of reaction loci containing an oil-soluble initiator are given. Besides pairwise formation of radicals in the particles, desorption and reabsorption, water phase termination, solubility of the initiator in the aqueous phase, and the possibility of formation of a single radical species are taken into consideration. The calculation is based on a probabilistic analysis leading to a third-order recurrence relation solved using confluent, hypergeometric Kummer functions. Some calculated curves illustrating the de-pendence of the average number of radicals per particle on various relevant parameters are included. © 1995 John Wiley & Sons, Inc.     

6,6′-Bisfunctionalized 2,2′-bipyridines as metallo-supramolecular initiators for the living polymerization of oxazolines

[Cu(I) {6,6′-bis(bromomethyl)-2,2′-bipyridine}₂](PF₆) complexes were used as metallo-supramolecular initiators for the polymerization of 2-oxazolines resulting in defined polymers with a central 6,6′-disubstituted 2,2′-bipyridine unit. The living character of the polymerization was demonstrated with the linear relationship between the weight-average molecular weight M̄_w and the [monomer]/[initiator] ratio as well as in the synthesis of block copolymers. The metal ions could be removed resulting in uncomplexed polymers with a free central metal binding unit.     

Solvation effect in thermal decomposition of 2,2′-azoisobutyronitrile

The solvation effect in the thermal decomposition of a radical initiator (AIBN) in monomer–solvent mixtures is discussed. Equations were derived which comprise the initiator decomposition constant as a function of the monomer mole fraction for chosen types of solvation. In addition, equations were deduced presenting the concentrations and partial relative decomposition rates for the solvated initiator species as a function of the monomer mole fraction. The equations obtained were compared to the experimental literature data and possible dependences of decomposition constants on monomer concentration were simulated for various solvated species. The simulated relationships were found to be straight lines, curves of saturated type (possessing a plateau), S-shaped curves, and maximum or minimum curves. © 1995 John Wiley & Sons, Inc.     

Compartmentalized polymerization with oil-soluble initiators. Kinetic effect of single radical formation

The stationary state distribution of radicals in compartmentalized systems initiated by oil-soluble initiators have been calculated for various cases of single radical formation as well as a simultaneous generation of single radicals and pairs of radicals in the particles. The effect of a contribution from radicals produced by initiator dissolved in the aqueous phase has been considered. Desorption and reabsorption of radicals, aqueous phase termination, total rate of radical formation and the water-solubility of the initiator are quantified in terms of dimensionless parameters. The calculations predict that single radicals generated in the particles are kinetically indistinguishable from radicals produced in the aqueous phase over a wide range of variation of the parameters. It is shown that if the rate of generation of single radicals constitutes only about 10 per cent of the overall rate of radical formation in the particles, the former radicals account for the major part of the rate of polymerization. The mechanisms previously proposed to account for the similar kinetic behaviour observed with water-soluble and oil-soluble initiators are discussed. It is concluded that the present calculations support the view that this similarity is mainly due to radicals produced by the water-soluble fraction of the initiator. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2347–2354, 1997     

Atom transfer radical polymerization of styrene initiated by polychloroalkanes and catalyzed by CuCl/2,2′-bipyridine: A kinetic and mechanistic study

A series of polychloroalkanes, known as telogen agents for redox telomerization, were used as initiators for atom transfer radical polymerization (ATRP) of styrene using the heterogeneous CuCl/2,2′-bipyridine catalyst. In copper-catalyzed redox telomerization, the reactivity of RCCl₃-type telogens is strongly influenced by the nature of the R group. In ATRP, the 2,2′-bipyridine ligand levels the activity of the catalytic system in such a way that all 1,1,1-trichloroalkanes are efficient initiators in ATRP, whatever the R group. The nature of this substituent influences the overall rate of polymerization through both the number of active sites per chain and the [Cu (I)]/[Cu (II)] ratio. By the combining of several analytical techniques, it is proved that some polychloroalkanes such as CCl₃CO₂CH₃, CCl₃CF₃, or CCl₄ are bifunctional initiators. Finally, a general mechanism of initiation is proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2933–2947, 1998     

Solid-state polymerization of maleimide by 2,2′-azobisisobutyronitrile

The solid-state polymerization of maleimide by 2,2′-azobisisobutyronitrile was studied over the temperature range 65–85°C. The polymerization was carried out either by heating the solid mixture of maleimide and 2,2′-azobisisobutyronitrile or by heating the two compounds separately in twin ampules. The results of the kinetic study showed that the lifetime of the propagating radicals was short and that the rate of termination was proportional to the mole amount of propagating radicals. The ESR spectrum showed that stable radicals that were not main propagating radicals were in the polymerizing system.     

Dispersion polymerization of acrylamide: Part II. 2,2′‐Azobisisobutyronitrile initiator

Dispersion polymerization of acrylamide in tert‐butyl alcohol (TBA)‐water media (TBA ⩾ 50 vol %) using poly(vinyl methyl ether) (PVME) as the stabilizer and 2,2′‐azobisisobutyronitrile (AIBN) as the initiator at 50°C has been studied. The conversion‐time curve shows autoacceleration taking place from the very early stage of the reaction (measured from 4% conversion level). Molecular weight increases with conversion indicating that the gel effect is operative. This suggests that a major part (if not the whole) of the polymerization occurs in the particle phase. The effects of the concentrations of the stabilizer, the initiator, the monomer, and the solvent composition on particle size have been explained on the basis of particle phase polymerization. The feeding of the particles by the monomer presumably occurs through the solvent channels of the swollen particles. The swelling data of polyacrylamide films in various TBA‐water mixtures are given. The similarity and differences between the AIBN and ammonium persulfate (APS) initiated systems (published earlier by us) have been discussed. In general, particles are more polydisperse and bigger in the former case than in the latter. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 493–499, 1999     

Kinetics of polymerization initiated by peroxides containing heterocyclic groups. I. Kinetics of bulk polymerization of styrene and vinyl acetate initiated by difuroyl peroxide

The rate and degree of bulk polymerization of styrene and vinyl acetate initiated by difuroyl peroxide and, for comparison, by dilauroyl and dibenzoyl peroxides were measured at several temperatures as a function of the initiator concentration. Also the rates of initiation were determined by the inhibition method with Banfield's radicals. The rate of polymerization initiated by difuroyl peroxide appears to be lower than could be expected from the rate of initiation determined by the inhibition method and from the decomposition of difuroyl peroxide. In the case of polymerization of vinyl acetate there are significant deviations from the proportionality between R_p and the square root of the initiator concentration, which follows from the conventional kinetic scheme. The degrees of polymerization are also low, and the plots of P _n^?1 versus R_p are not linear. These deviations can be accounted for by postulating a retardation effect of the furan cycle and chain transfer to difuroyl peroxide.     

Synthesis of micron-sized polymeric particles in soap-free emulsion polymerization using oil-soluble initiators and electrolytes

Soap-free emulsion polymerization of styrene using oil-soluble initiators and electrolytes was investigated to synthesize micron-sized polystyrene particles. It was clear that an oil-soluble initiator, such as AIBN, worked like a water-soluble initiator in soap-free emulsion polymerization of styrene to prepare monodispersed particles with negative charges, probably because of the polarization of the electron-attractive functional groups decomposed from the initiators and the pi electron cloud of benzene in a styrene monomer. The addition of an electrolyte enabled secondary particles to effectively promote hetero-coagulation for particle growth by reduction of an electrical double layer and prevention of self-growth. Changing the concentration and type of electrolyte enabled us to control the size up to 12 μm in soap-free emulsion polymerization of styrene using AIBN. Conventionally, organic solvents and surfactants have been used to prepare micron-sized polymeric particles, but this method enabled the synthesis of micron-sized polymeric particles in water using electrolytes without surfactants.     

Design of effective systems for controlled radical polymerization of styrene: Application of 4,4′-dimethyl and 5,5′-dimethyl 2,2′-bipyridine copper(II) complexes

[Cu(II)(4,4′-dimethyl-2,2′-bipyridine)₃](PF₆)₂ ( 2 ) and [Cu(II) (5,5′-dimethyl-2,2′-bipyridine)₃](PF₆)₂ ( 4 ) were used together with aluminium isopropoxide and (1-bromoethyl)benzene in the controlled radical polymerization of styrene resulting in polystyrenes with predetermined molecular weight and narrow molecular weight distribution. The received polymers are colorless with a content of copper lower than 210 ppm. The substitution pattern at the bipyridine ligands has a distinct influence on the polymerization. The rate of polymerization of styrene using 2 /[(CH₃)₂CHO]₃Al/C₆H₅CH(CH₃)Br is two times larger than utilizing 4 /[(CH₃)₂CHO]₃Al/C₆H₅CH(CH₃)Br.     

苯乙烯阴离子本体聚合引发剂缔合及其机理的研究

分别以正丁基锂和叔丁基锂为引发剂,采用自制管式流动反应装置,对较高温度下苯乙烯阴离子本体聚合动力学进行了研究.证实了正丁基锂主要以六元缔合结构形式引发聚合,并导致超分子团聚体的形成,从而使进一步的聚合因单体扩散受阻而受到限制,并伴随聚合转化率停滞平台（SCP）的产生.随后由于前期聚合累积的能量,使超分子结构完全解离.聚合温度越高,SCP持续时间越短.结果还表明,在正丁基锂引发剂中,存在一个以六元缔合结构为基础形成的更大的缔合体结构.原子力显微镜照片显示,超分子结构的直径分别为20～30nm和50～60nm.此外,在阴离子聚合过程中活性种的缔合结构只决定于初始引发剂的分子结构,而不同活性种缔合结构对阴离子聚合的链增长存在很大影响,从而解释了采用不同结构的锂系引发剂引发苯乙烯单体聚合时聚合速率存在巨大差异的原因.