Radical polymerization of methyl methacrylate in the presence of stereoregular poly(methyl methacrylate). VI. Kinetic template effects at higher conversion

The kinetics of radical template polymerization of methyl methacrylate along isotactic poly(methyl methacrylate) as polymer template was studied. It appeared that in addition to a rate increase at low conversions, a second rate enhancement can take place at higher conversion, depending on the nature of the solvent and temperature. While the initial template effect was previously mainly ascribed to hindered segmental motion of propagation chain radicals, the second rate effect is thought to be caused by hindered translational motion of growing chains in a physical network built up during polymerization. The observed second effect is strong in the weakly complexing and network-promoting solvent toluene, but it is less pronounced in the strongly complexing solvent dimethylformamide. In acetone, also a strongly complexing solvent, the formed stereocomplexes crystallize at low conversions. Due to this crystallization the propagating chains become immobilized, which also results in a strong second rate enhancement. The observed effects are dependent on temperature, but hardly depend on template molecular weight. The influence of the template is further demonstrated by varying its concentration and by the disappearance of the rate enhancement at higher conversion, when the templates become fully covered.     

Radical polymerization of methyl methacrylate in the presence of stereoregular poly(methyl methacrylate). III. Influence of temperature

Methyl methacrylate (MMA) was polymerized by radical initiation at 0, 25, 50, 75 and 100°C in DMF in the presence of preformed isotactic PMMA (iMA) or preformed syndiotactic PMMA (sMA) with different M?_v and also without preformed PMMA (“blank” polymerizations). From the tacticities of the formed polymers it is concluded that blank polymerization does not conform to simple Bernoulli statistics, but follows at least first-order Markov statistics. The formation of long syndiotactic sequences in the presence of iMA and long isotactic sequences in the presence of sMA denotes still higher-order Markov statistics. The stereoregulating action is improved by higher M?_v of the preformed polymer (matrix) and lower reaction temperature. These influences can be explained by assuming an equilibrium between polymer growth on the matrix and in the “free” solution. For polymerizations in the presence of iMA or sMA below 300°K, the differences in activation enthalpies (ΔH_s/i? – ΔH_i/s?) are practically equal to that for the blank polymerization, ca. 900 cal/mole, whereas the differences in activation entropies (ΔS_s/i? – ΔS_i/s?) differ considerably. (ΔS_s/i? – ΔS_i/s?) values are highly negative in the presence of iMA and highly positive in the presence of sMA. From these results it is concluded that the isotactic and syndiotactic polymer matrices exert a steric influence on the monomer addition process, thus promoting so-called stereospecific replica polymerization.     

Radical polymerization of methyl methacrylate in the presence of stereoregular poly(methyl methacrylate). II. Syndiotactic PMMA as matrix

Methyl methacrylate (MMA) was polymerized by radical initiation at 25°C in DMF in the presence of preformed syndiotactic PMMA (sMA) with about 90% syndiotactic triads and of different M?_v, viz., sMA-1, 1.6 × 10⁵; sMA-2, 3.0 × 10⁵; and sMA-3, 8.7 × 10⁵. The MMA:sMA ratio was 6:1. The collected polymers were separated into two fractions by extraction with boiling acetone and characterized by 60 MHz NMR. In all cases isotactic PMMA (i-PMMA) was produced, especially in the initial reaction stages, which associated with the syndiotactic substrate to form acetone-insoluble 1:2 i/s-stereocomplexes. The isotacticity decreased with conversion and was highest in the presence of sMA-3. Characterization of the acetone-soluble fractions indicated that i,s-stereoblock polymers were also produced. From these results it is concluded that this reaction can be considered a stereospecific replica polymerization, the driving force being the strong tendency of i-PMMA and s-PMMA to associate. With sMA of M?_v below about 1.2 × 10⁵, no i-PMMA is formed; in other words, no replica polymerization occurs. For polymerizations in the presence of sMA-2, the critical M?_v of propagating chains, with has to be exceeded before stereoassociation is strong enough to effectuate replica polymerization, has been estimated to be 0.6 × 10⁵.     

Radical polymerization of methyl methacrylate in the presence of stereoregular poly(methyl methacrylate). IV. Influence of solvent type

Methyl methacrylate (MMA) was polymerized by radical initiation at 25°C or 35°C in various solvents in the presence of stereoregular poly(methyl methacrylate) (PMMA). The occurrence of stereospecific replica polymerization appeared to be related to the capability of stereoassociation of isotactic and syndiotactic PMMA. The solvents can be roughly divided into three types. Type A solvents are polar solvents, which promote stereoassociation resulting in gelation and precipitation. Examples are dimethylformamide, dimethyl sulfoxide, and acetone. Type B solvents are nonpolar aromatic solvents like benzene and toluene, wherein stereoassociation is weaker but still leads to gelation. Type C solvents are very good solvents, in which stereoassociation does not occur. Chloroform and dichloromethane belong to this class. In solvents of type A as well as type B, polymerization in the presence of i-PMMA as a polymer matrix was syndiospecific. However, in the presence of s-PMMA as a polymer matrix the polymerization was isospecific only in type A solvents. The syndiotactic or isotactic triad contents of the polymer formed could be as high as ca. 90% at low conversions. In solvents of type C, polymerization in the presence of stereoregular PMMA proceeds according to a normal radical mechanism. Syndiotacticity was always less than 70%. Stereocomplexes formed in situ during replica polymerization were partly crystalline as detected by x-ray diffraction. The highest crystallinity was detected in those formed in type A solvents.     

Radical polymerization of methyl methacrylate in the presence of isotactic poly(methyl methacrylate). VIII. Influence of template concentration

The influence of template concentration on the radical polymerization of methyl methacrylate along isotactic poly(methyl methacrylate) template was studied. The polymerizations were carried out on three template polymers with different molar masses in dimethylformamide at ?5°C. The initial polymerization rate increased linearly with template concentration until the distribution of template chain segments became homogeneous. At that critical concentration a strong increase in the polymerization rate was observed, whereas still higher template concentrations had only a slight effect on the polymerization rate. The polymerizations were stopped when the weight ratio of formed polymer and template was equal to one. The viscometrically determined molar mass of the formed polymers showed a remarkable behavior in the low template concentration region. It was obviously related to the molar mass of the template polymer and was lower than the molar mass found for blank polymerization. This decrease in molar mass was most pronounced in the case of the lowest template molar mass. It is suggested that nondegradative chain transfer occurring near a template chain end is responsible for this decrease. An increase in the molar mass occurred at the critical concentration, similarly to the change of polymerization rate. However, at still higher template concentrations, where template coils started to overlap each other, the molar mass of the formed polymers increased further. The growing chains could leap from one template chain to another and attain a greater chain length than the blank polymerizate.     

Radical polymerization of methyl methacrylate in the presence of isotactic poly(methyl metacrylate). VII. Determination of the rate constants for template polymerization

The separate rate constants k_p and k_t for propagation and termination of radical template polymerization of methyl methacrylate along isotactic poly(methyl methacrylate) as a polymer template have been determined. The polymerizations were carried out in the strongly complexing solvent dimethylformamide at 5°C. For the evaluation of k/k_t from stationary kinetic experiments, the rates of initiation were determined by employing a scavenger method. The nonstationary experiments yielding k_p/k_t were performed by means of the rotating sector technique. As the template rate effects increased with decreasing initiator concentration, the rotating sector curves were corrected for variation in light intensity. It appeared that the radical lifetime increases from 8.4 sec for normal or blank polymerization to 64 sec for template polymerization. The calculated values of k_p are 26.6 and 5.9 l./mole-sec and of k_t 140 × 10⁴ and 1.7 × 10⁴ l./mole-sec for blank and template polymerization, respectively. The changes in k_p and k_t, due to the presence of template polymer, are explained in terms of an extra loss of activation entropy in the stereoselective propagation step and a strong hindrance of segmental diffusion for the termination reaction of the chains growing along the polymer template.     

Radical polymerization of methyl methacrylate in the presence of isotactic poly(methyl methacrylate)

Methyl methacrylate (MMA) was polymerized by radical initiation at 25°C in DMF in the presence of preformed isotactic PMMA (iMA) with about 90% isotactic triads and different M?_v's, viz., iMA-1: 7.2 × 10⁵; iMA-2, 5.0 × 10⁵; iMA-3, 3.5 × 10⁵; iMA-4, 1.25 × 10⁵; and iMA-5, 1.15 × 10⁵. The MMA:iMA ratio was 6:1. The collected polymers were separated into two fractions by extraction with boiling acetone and characterized by 60 MHz NMR. It is found that the M?_v of the polymer formed ran parallel to the M?_v of iMA. In all cases syndiotactic PMMA (s-PMMA) was produced which associated with the isotactic substrate to form acetone-insoluble stereocomplexes. The syndiotactic polymers probably consist of long syndiotactic and heterotactic sequences. The syndiotacticity decreased with conversion and was generally highest in the presence of iMA-1. With iMA-1 even the formation of some additional i-PMMA (in the acetone-insolubles) was indicated, especially in the later stages of the polymerization. Characterization of the acetone-soluble fractions indicated that i,s-stereoblock polymers were also produced, of which the persistence ratios ρ increased with the M?_v of iMA. From these results it is concluded that this reaction differs from the conventional radical polymerization and can be considered a stereospecific replica polymerization, the driving force being the strong tendency of i- and s-PMMA to associate. The formation of i,s-stereoblock polymers and additional i-PMMA indicates that s-PMMA in its turn can also act as a polymer matrix.     

Radical polymerization of methyl methacrylate in the presence of low-molecular-weight poly(methyl methacrylate)

The kinetics of the bulk radical polymerization of methyl methacrylate and the structure and properties (physicomechanical and thermomechanical, as well as diffusion and sorption) of the polymers were examined in relation to the amount of low-molecular-weight poly(methyl methacrylate) added.     

Crosslinking free-radical polymerization of dimethacrylates in the presence of branched poly(methyl methacrylate)

The kinetics of polymerization of ethylene glycol and 1,6-hexanediol dimethacrylates in the presence of branched and linear PMMAs, 1-decanethiol, and methylphenyl sulfide has been studied by isothermal calorimetry. The sol-gel analysis of 1,6-hexanediol polydimethacrylates prepared in the absence and presence of the branched PMMA is performed. The effect of the branched PMMA on the diffusion-sorption, mechanical, thermomechanical, and optical properties of polydimethacrylates is investigated. It has been established that, in the crosslinking free-radical polymerization of dimethacrylates, the branched PMMA serves both as a reactive macromonomer and a chemically inert additive—polymer filler.     

Kinetics of the complex-radical polymerization of methyl methacrylate in the presence of initiating metallocene systems

The kinetics of methyl methacrylate polymerization in the presence of benzoyl peroxide + metallocene (ferrocene, titanocene dichloride, and zirconocene dichloride) initiating systems is considered, and the effects of the nature and amount of metallocene in the system are reported. The polymerization is assumed to be a complex-radical process. The structure of the complex-radical sites of chain propagation and a scheme of their formation are deduced from quantum-chemical calculations.     

Rheological properties of silica dispersions stabilized by stereoregular poly(methyl methacrylate)

The adsorption of stereoregular polymers and its effect on the conformation and dynamics of the polymer at interfaces are only poorly understood. 1H NMR has revealed a lowering of the peaks assigned to isotactic sequences whatever the PMMA tacticity, which provides evidence of stereospecific adsorption of the isotactic segments on silica. Entropic factors are therefore assumed to control the configuration of the adsorbed layer. Tacticity-dependent rheological behavior is revealed by dynamic investigations carried out on silica dispersions. The driving forces likely to induce the stereoselective adsorption and tacticity-dependent rheology of suspensions are discussed.     

Kinetics of emulsion polymerization of methyl methacrylate

The kinetics of the emulsion polymerization of methyl methacrylate at 50°C have been studied in seeded systems using both chemical initiation and γ-radiolysis initiation. Both steady-state rates and (for γ-radiolysis) the relaxation from the steady state were observed. The average number of free radicals per particle was quite high (e.g., ～0.7 for 10^?3 mol dm^?3 S₂O²₈ initiator). The data are quantitatively interpreted using a generalized Smith–Ewart–Harkins model, allowing for free radical entry, exit, biomolecular termination within the latex particles, and aqueous phase hetero-termination and re-entry. From this treatment, there results (i) the dependence of the termination rate coefficient (k_t) on the weight fraction of polymer (w_p), (ii) lower bounds for the dependence of the entry rate coefficient on initiator concentration, and (iii) the conclusion that most exited free radicals undergo subsequent re-entry into particles rather than hetero-termination. The results for k_t(w_p) are consistent with diffusion control at temperatures below the glass transition point. Comparisons are presented of the behavior of methyl methacrylate, butyl methacrylate, and styrene in emulsion polymerization systems.     

Radical polymerization of methyl methacrylate in the presence of Fe(III) and Co(III) porphyrins

The free-radical polymerization of methyl methacrylate in the presence of chlorine-containing complexes of Fe(III) with 5,10,15,20-tetrakis(3′,5′-di-tert-butylphenyl)porphyrin and 5,10,15,20-tetrakis(3′-butoxyphenyl)porphyrin, as well as in the presence of the acetate complex of Co(III) 5,10,15,20-tetrakis(3′,5′-di-tert-butylphenyl)porphyrin, has been investigated. The kinetic features of the process and the molecular mass characteristics of polymers are studied, and a feasible polymerization mechanism is proposed.     

Interpenetrating polymer networks from castor oil-based polyurethanes and poly(methyl methacrylate). V

Castor oil is reacted with hexamethylene diisocyanate under different experimental conditions varying the NCO/OH ratio to yield liquid prepolyurethanes (PPU's). All these polyurethanes were interpenetrated with methyl methacrylate (MMA) and a crosslinker EGDM by radical polymerization initiated by benzoyl peroxide. The novel PPU/MMA interpenetrating polymer networks (IPN's) were obtained as tough films by transfer molding. The characterization of these IPN's includes resistance to chemical reagents, thermal behavior (DSC, TGA), and the mechanical properties, namely, tensile strength, modulus of elasticity, elongation at break (%), and hardness. The morphological behavior (SEM) and dielectrical properties such as electrical conductivity, dielectric constant (ε′), dielectric loss (ε″), and loss tangent (tan δ) were estimated.     

Light scattering studies of stereocomplex formation of stereoregular poly(methyl methacrylate) in solutions

The structure and stereocomplex formation of multi-stereoblock poly(methyl methacrylates) in three different solvents, acetone, tetrahydrofuran (THF) and chloroform, corresponding to strongly-, weakly- and non-complexing solvent, respectively, were investigated by a combination of static and dynamic laser light scattering. Our results revealed that the stereocomplex was caused by weak interactions, and could be melted at higher temperatures. In THF, the intermolecular and intramolecular interactions could be clearly separated at lower temperatures, and the structure of aggregated chains was linear. In acetone, a more compact structure was obtained, which was corroborated by the fact that the stereocomplex had a higher melting temperature than in THF.     

Methacrylic acid and methyl methacrylate oligomers adsorbed to porous isotactic poly(methyl methacrylate) ultrathin films and mechanistic studies of living template polymerization

Methacrylic acid (MAA), methyl methacrylate (MMA), methacrylamide, and oligomers of MAA and MMA were selected as a model of active radical species in living template polymerization using stereocomplex formation. The adsorption behaviors of the aforementioned model compounds were examined toward porous isotactic‐(it‐) poly(methyl methacrylate) (PMMA) ultrathin films on a quartz crystal microbalance, which was prepared by the extracting of syndiotactic‐(st‐) poly(methacrylic acid) (PMAA) from it‐PMMA/st‐PMAA stereocomplexes. The apparent predominant adsorption of oligomers to monomers was observed in both PMAA and PMMA oligomers, suggesting that the mechanism of template polymerization follows the pick up mechanism. Although vinyl monomers were not incorporated into the porous it‐PMMA ultrathin film, both PMMA and PMAA oligomers were adsorbed at the initial stages. However, adsorbed amounts were limited to about 5 and 15% at 0.1 mol L^?1, respectively, which are much smaller values than corresponding st‐polymers. The results imply that radical coupling reaction is prevented during template polymerization to support the resulting living polymerization. ATR‐IR spectral patterns of oligomer complexes and it‐PMMA slightly changed in both cases, suggesting complex formation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5879–5886, 2008