Association in anionic polymerization. Its effect on the polymerization mechanism

A critical analysis is presented of the measurement and interpretation of association phenomena in anionic polymerization in non-polar solvents. In chain initiation, measured differences in reaction order arise because of differences in the way the process is defined. Some also may be caused by the extreme sensitivity of these systems to traces of polar impurities. In chain propagation, differences in measured association numbers of the active centers exist in diene polymerization. These can be caused by problems in the experimental technique. This is illustrated by data on concentrated solution viscosity measurements on linear and regular four-star polystyrenes and polyisoprenes as models for dimeric and tetrameric association.     

Cationic polymerization of formaldehyde

Cationic polymerization of monomeric formaldehyde has been studied at low temperature. It was found that high molecular weight polyoxymethylene with the reduced viscosity higher than 1.0 dl/g was readily obtained when the polymerization of formaldehyde was carried out in saturated hydrocarbons in the presence of LEWIS acids, BRÖNSTED acids, I₂, ICl ICl₃, IBr, sulfur dioxide and binary systems of sulfur dioxide with these compounds at considerably low temperature. The degree of polymerization was effected by the polymerization conditions, especially solvent, polymerization temperature and addition of chain transfer agents such as water, formic acid, methanol or methyl formate.     

Cationic polymerization of glycidol. Polymer structure and polymerization mechanism

Glycidol (2-epoxy-1-propanol) was polymerized under the action of Lewis acids (BF₃OEt_2,, SnCl₄) and protonic acids (CF₃COOH, CF₃SO₃H). Polymers with number-average molecular weights (M?_n) varying from 2500 to 6000 g/mol were obtained. The structure of obtained polymers was characterized by ¹³C and ²⁹Si NMR spectroscopy, and the amount of 1-3 units, 1-4 units and chain branches was determined. The structure of the polymers was related to the polymerization mechanism, especially to the contribution of activated monomer mechanism in chain growth.     

Formation of spherulites during polymerization of lactams

The mechanism of the crystal growth of nylon in the course of alkaline polymerization of lactams was studied in terms of kinetics and morphologies of the resultant crystals. It was found that in the bulk polymerization of α-pyrrolidone, the resultant polymer precipitated in the form of spherulites. Spherulites formed in the early stage of polymerization were swollen, soft balls in which thin lamellar crystallites were radiating along the spherulitic radius. It is considered that the growing chain molecules, when they attained a critical length beyond which they lost the solubility in the system, will precipitate into the form of lamellae in which molecules are aligned perpendicularly to the lamellar surface. Such spherulites matured as the polymerization proceeded. In the early stage of the polymerization, the mutual coupling of the swollen spherulites occurred. In the subsequent stage, the vacant space was occupied by the product of further polymerization, so that the coupled spherulites grew into a sphere again. On the other hand, the propagation of the chains in the individual spherulites made the spherulites compact. It is postulated that the chains grown from the surface of the lamellar nuclei, precipitate in the interstices between the lamellae and a part of these chains will form molecular ties between the lamellae giving rise to compact spherulites. The decrease in the rate of polymerization observed may be attributable to the occlusion of the growing chain ends into the precipitates. The impurity in the system, i.e., water, seems to affect the rate of the monomer addition, but does not the morphology. Spherulites were also grown in the case of nylon 6, the polymerization of ?-caprolactam.     

Kinetics of radical polymerization, 55. The polymerization of p-methylstyrene in solution

Radical polymerization of p-methylstyrene was studied in benzene solution at 50°C using AIBN as the initiator. The rate of inititation was determined at various dilutions; it shows a slight linear dependence on the composition of the polymerization mixture. The polymerization rate is directly proportional to the square root of initiator concentration; however, in contrast to the classical kinetic picture, it is not directly proportional to the monomer concentration. The experimentally measured solvent effect can be interpreted neither by the diffusion theory nor by the EDA theory, it is, however, quantitatively consistent with the theory of hot radicals. The so-called absolute values of the rate constants of chain propagation (k?₂) and chain termination (k₄) were determined by means of the rotating sector method. The value of k₄ was found to be constant within the limits of experimental error, consequently the experimentally observed solvent effect is caused solely by the change in the chain propagation rate constant.     

Solution polymerization of 1,3,5-trioxane and 1,3,5,7-tetroxocane, 1. Rate of polymerization,characterization and melting behaviour of as-polymerized poly(oxymethylene)

A relationship between the rate of polymerization and the structure of the as-polymerized crystals is investigated in the cationic polymerization in solution of 1,3,5-trioxane and 1,3,5,7-tetroxocane. The monomer/solvent reaction systems chosen were: 1,3,5,7-tetroxocane/nitrobenzene (I); 1,3,5,7-tetroxocane/l,2-dichloroethane (II), 1,3,5-trioxane/chlorinated parafin (III), and 1,3,5,7-trioxane/normal paraffin (IV). The poly(oxymethylene), (POM), obtained in each case was characterized as to molecular weight, X-ray cristallinity and by differential scanning calorimetry (DSC). The data from the DSC curves indicated a folded chain structure for systems I and II, where the rate pf polymerization was very high. However in system II at high polymer conversion a fraction of POM appears with a melting behaviour characteristic of an extended chain structure. For systems III and IV, where the rate of polymerization was very low, the DSC curves indicated the presence of two crystal forms. One of these forms shows a clear response to overheating and annealing treatments, which indicates the presence of an extended chain structure. The second form shows ambiguous melting behaviour and by thermal data alone it is impossible to determine its structure.     

Chain transfer polymerization of methyl methacrylate initiated by organolanthanide complexes

A novel chain transfer polymerization mediated by Cp*₂ Sm(III) species and organic acids is described. The chain transfer polymerization involves the reaction of organic acids such as thiols or ketones with an active bond between samarium(III) and the enolate at a living chain end of poly(methyl methacrylate) (PMMA). This chain transfer reaction resulted in termination of the living chain end and the regeneration of the active initiator which would consist of (C₅Me₅)₂Sm(III) and deprotonated organic acids. The chain transfer polymerization were confirmed by turnover numbers (TON). tert‐Butyl thiol exhibited the chain transfer reactivity effectively to control the molecular weight of PMMA without decreasing of the polymer yield and the stereoregularity. As a result of this chain transfer polymerization, thermal and optical properties of the PMMA obtained were improved by the control of chain end groups or by reducing a large amount of the samarium initiator.     

Radical polymerization and copolymerization of some vinyl phosphates

Diethyl vinyl phosphate ( 1 ), diphenyl vinyl phosphate ( 2 ), and phenyl divinyl phosphate ( 3 ) were prepared, and their polymerization and copolymerization behaviors as well as the hydrolysis of the polymers obtained were investigated. The monomers 1 and 2 were homopolymerized to give low molecular weight polymers, probably as a result of high chain transfer reactivity to monomer. Although the monomer 3 gave a cross-linked polymer, this polymerization was found to proceed mainly by a cyclopolymerization mechanism to give a polymer with a five-membered ring unit which was readily hydrolyzed. The alkaline hydrolysis of poly( 1 ) occurred only partly and thus pure polyvinyl alcohol was not obtained. From the copolymerizations of 1 and 2 with vinyl monomers such as vinyl chloride, the reactivities of these monomers were found to be somewhat lower than that of vinyl acetate.     

Study of radical polymerization by means of ESR. I. Homogeneous polymerization of styrene

An extremely sensitive instrument for ESR measurements in liquid media was developed on the basis of a new principle of registration of the ESR signals. By means of this device the spectrum of macroradicals of polymerizing styrene was recorded, and the stationary concentration of growing chains was measured quantitatively. Thus the first direct data for the chain propagation constant of the radical polymerization of styrene were obtained.     

Crystallization of poly(L-proline) in the course of polymerization

The heterogeneous polymerization of L -proline N-carboxy anhydride was carried out in acetonitrile and benzene, using butylamine as the initiator. Polyproline I was formed in acetonitrile and polyproline II in benzene in the course of the polymerization. The conversion attained 99% and 66% after 1 week-polymerization in acetonitrile and benzene, resp. Extended chain crystals were obtained by the polymerization in acetonitrile. On the other hand, poly(L -proline) obtained by the polymerization in benzene gave aggregates into which many growing chain ends were occluded. The differences in the crystallization behaviour during the polymerization between the two different liquids may be due to the conformation of the resultant polymers. Polyproline I exhibits a cross-section per chain of 0.709 nm², whereas polyproline II shows only one of 0,386 nm². The wider the cross-section per chain the more the growing, active chain ends can react with the monomer, giving rise to a higher conversion. In contrast with the case of crystallization of such α-helical chains as poly(L -alanine) during polymerization, hexagonal lamellar crystals were formed.     

Synthesis of semitelechelic poly[N-(2-hydroxypropyl)methacryl-amide] by radical polymerization in the presence of alkyl mercaptans

The free radical polymerization of N-(2-hydroxylpropyl)methacrylamide (HPMA) in the presence of chain transfer agents was studied. Methyl 3-mercaptopropionate (MMP) and 2-mercaptoethylamine (MEA) were used as chain transfer agents and 2,2′-azobisisobutyronitrile (AIBN) was used as the initiator. Semitelechelic (ST) polymers produced under various reaction conditions were analyzed by Matrix Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) and size exclusion chromatography (SEC). The molecular weight of the ST polymers was regulated by the initial molar ratio of chain transfer agent to HPMA. The free radicals formed by the decomposition of AIBN initiated a fraction of polymer chains; such chains were not terminated in a functional group. The amount of these polymer chains depended on the concentration and efficiency of the chain transfer agent, concentration of AIBN, and conversion. Decreasing the concentration of AIBN and increasing the concentration of the chain transfer agent reduced the formation of nonfunctionalized polymer chains.     

Free radical polymerization kinetics of n-lauryl methacrylate

The kinetics of free radical polymerization of n-lauryl methacrylate (dodecyl methacrylate) were studied at 60 and 80°C in bulk and in benzene solution. The analysis of the data showed the kinetic results deviated from classical behaviour and that this deviation could be explained by the chain length dependence of the diffusion controlled termination rate constant, K_t. Primary radical termination is explicitly considered and rejected as being insignificant in this work. Because of its lessened flexibility poly(n-lauryl methacrylate) shows a greater chain length dependence for K_t than does poly(methylmethacrylate).     

New aspects in the anionic polymerization of phenyl glycidyl ether

A kinetic equation describing the anionic polymerization of phenyl glycidyl ether initiated by potassium anions in the presence of 18-crown-6 was derived. Rate constants of growth and chain transfer reactions were calculated. The equilibrium constant of complex formation by the active centre with alcohol being a product of chain transfer reactions was also evaluated.     

Anionic ring-opening polymerization of 2,2-dimethyltrimethylene carbonate

The anionic ring-opening polymerization of 2,2-dimethyltrimethylene carbonate in solution with sec-butyllithium as initiator was found to result under suitable conditions in a bimodal molecular weight distribution, vic. cyclic oligomers and a high-molecular-weight polymer. The concentration of cyclic oligomers approaches that of a ring chain equilibrium. In the kinetically controlled regime high polymer yields were obtained. Optimum reaction conditions for a polymer yield of 85 – 95% are a temperature of ?10°C, an initial monomer concentration of 10 weight-% and toluene as solvent. In THF the ring chain equilibrium is achieved readily. The ceiling temperature was determined to be ≈ 30°C. The homopolymer was characterized by spectroscopic, thermoanalytical and viscosity measurements. A high degree of crystallinity was observed.     

Living cationic polymerization of cyclohexyl vinyl ether

The cationic polymerization of cyclohexyl vinyl ether (CHVE) initiated by α-halogeno ethers in the presence of a Lewis acid activator has been investigated first. In conditions leading to a living polymerization of alkyl vinyl ethers (ethyl, isobutyl, etc.), an extremely fast polymerization, accompanied by chain transfer reactions, is observed with CHVE. In fact, we have shown that the polymerization of this monomer may be directly initiated by the HI and HCl adducts of CHVE, in the absence of any electrophilic activator. However, even in these conditions, the polymerization cannot be controlled. A “living” polymerization of CHVE was finally obtained by addition of ammonium salts (NBu₄X; X = Cl, Br, I) to the systems free of electrophilic activators. The added salts stabilize the α-halogeno ether chain ends and reduce the overall reactivity. Using this procedure, we have synthesized poly(CHVE)s with M?_n's ranging from 6 · 10³ to 4 · 10⁴ g · mol^?1 (in good agreement with the predicted values assuming the formation of one polymer chain per initiator molecule) and narrow molecular weight distributions (M?_w/M?_n < 1,2). Though most of poly(alkyl vinyl ether)s exhibit glass transition temperatures far below 0°C, the glass transition temperature of the poly(CHVE) is close to +50°C, indicating that this monomer can be used as a precursor to rigid poly(vinyl ether) blocks.     

A new chain transfer reaction in the anionic polymerization of 2,3-epoxypropyl phenyl ether and other oxiranes

Carbonyl groups were found to be present in polymers of 2,3-epoxypropyl phenyl ether obtained by anionic polymerization. On the basis of quantitative analyses these groups were assumed to be formed in a reaction involving chain transfer to the monomer which proceeds in a manner different from that reported in the literature. Results of the present study were compared with those referring to the polymerization of methyloxirane, phenyloxirane and some chlorophenyl 2,3-epoxypropyl ethers.     

Gas‐phase polymerization of ethylene using supported metallocene catalysts: Study of polymerization conditions

The influence of polymerization conditions on the gas‐phase polymerization of ethylene with a supported metallocene catalyst (Cp₂ZrCl₂/SiO₂) was investigated. The experiments were carried out in a 1.0‐L semi‐batch autoclave reactor and in a 0.6‐L glass reactor, at pressures up to 100 psi and temperatures up to 57 °C. Anchor and helicoidal agitators were used. Trimethylaluminum (TMA) and methylaluminoxane (MAO) were used as cocatalysts. Hydrogen was used as the chain‐transfer agent. The polymer products were characterized by gel permeation chromatography (GPC) and scanning electron microscopy (SEM). Effects of pre‐contact time of the catalyst and cocatalyst, catalyst injection configuration, agitator design, hydrogen volume, polymerization temperature and pressure were studied. The heterogeneity of polymer particles inside different zones of the reactor was also verified. The results indicated that several of these factors were crucial to obtain well‐controlled gas‐phase polymerization. SEM analyses showed that irregular growth of polymer within the shells often formed polymer particles of irregular shape. Flory's distribution was used to describe the polymerization mechanism for each site type. Interactions between catalyst sites and support surface are considered to broaden these distributions. Several Flory's distributions (4 to 7 different site types) were required to describe the broad and sometimes multimodal molecular weight distributions. Mass and heat transfer resistances may also be important in further broadening these distributions.

SEM – Morphology of polymer particles, polymer system GR5, × 2.0k.     

Radical polymerization of vinyl monomers by captodative substituted dimers

Polymerization of vinyl monomers such as styrene, methyl methacrylate, and vinyl acetate was carried out in the presence of the captodative substituted dimers 1 – 6 at 50 to 110°C. It was found that the dimer initiates the radical polymerization of these monomers, even of vinyl acetate. The initiation ability of the dimers depends on the nature of both captive and dative groups as well as the substituent at a remote position. The kinetic study suggests that in the polymerization initiated by the dimers primary-radical termination and chain transfer play a minor role.     

The mechanism of anionic polymerization of dimethylketene

In the reaction of dimethylketene with GRIGNARD reagent of ether polyketonic type oligomers were formed, but the non-distillable fraction was of polyester type. Therefore it is suggested that the propagation reaction to polyketone proceeds slowly, but those to polyester is a fast one. From the results of fractionations of polymers prepared under various conditions, the polymerization mechanism is explained by three kinds of growing chain ends which are in dynamic equilibrium.     

Solvent effects on free-radical polymerization, 3. Solvent effect on polymerization rate of methyl methacrylate and N-vinyl-2-pyrrolidone

The homopolymerizations of methyl methacrylate (MMA) in N-methyl-2-pyrrolidone (NMP) and of N-vinyl-2-pyrrolidone (NVP) in methyl isobutyrate (MiB) were investigated at 60°C using AIBN as initiator. Reaction rate and dynamic viscosity of the medium were determined as function of monomer dilution. It follows from the estimated monomer exponent n that MMA reacts in NMP faster (n = 0,739) and NVP in MiB slower (n = 1,495) in comparison with reaction rates obtained from the direct proportionality of monomer concentration. The observed deviation was quantitatively separated in partial deviations caused by chain initiation, propagation and termination. It was found that the viscosity effect on chain termination plays a dominant role in the global solvent effect. Chain propagation is influenced by electron donor-acceptor interactions between monomers and solvent.