Dielectric properties of stereoregular poly(methyl methacrylates)

To obtain more useful information about the effect of the degree of stereoregularity on the motion of the polymer chain, the dielectric and dilatometric measurements were made for a series of stereoregular poly(methyl methacrylates) (PMMA). The α- and β-absorptions were observed in each sample, of which the dielectric behaviors of the α-process are discussed. The temperature dependence of the relaxation time of the α-process was sufficiently represented by the WLF equation and the resulting values of the parameters f_g and B in the modified WLF equation were found smaller for isotactic-rich PMMA than those values for syndiotactic PMMA. It may be deduced from these results that the chain mobility of the isotactic PMMA is larger than that of the syndiotactic. The dielectric increment of the α-process in the isotactic PMMA is much larger than that in the syndiotactic PMMA, increasing rapidly with temperature, and taking its maximum in the temperature range of 55 to 60°C. The dielectric transition was clearly observed in the case of isotactic-rich PMMA.     

Adsorption of stereoregular poly(methyl methacrylates) on γ-alumina: Spectroscopic analysis

ABSTRACTS: Three poly(methyl methacrylates) (PMMA) with a racemic fraction ranging from 0.25 to 0.91 have been adsorbed from a chloroform solution on γ-alumina and studied by diffuse reflectance infrared spectroscopy (DRIFT) and solid-state ¹³C NMR spectroscopy. From DRIFT spectra, it is seen that the fraction of carbonyl groups bonded to the surface goes from 0.29 for s-PMMA to 0.41 for i-PMMA, but there is a larger amount of s-PMMA retained on the surface (at any given solution concentration). NMR spectroscopy indicates, from shifts of the methylene and α-methyl carbon peaks (due to the γ-gauche effect), that the adsorption is accompanied by changes in conformation, with an increase in the number of trans conformers, particularly with i-PMMA. These results indicate that s-PMMA adsorbs on γ-alumina in a brush-like configuration, with a relatively small number of groups attached to the surface, and tails and loops sticking out of the surface, whereas i-PMMA adsorbs in a sheet-like configuration, with a greater number of interacting groups. In both cases, the adsorption is accompanied with an increase in the number of trans conformers as compared to the bulk conformation. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2985–2995, 1999     

NMR spectroscopic,osmometric, and viscometric study of association of syndiotactic poly(methyl methacrylate) in dilute solutions

Association of syndiotactic (s) poly(methyl methacrylates) (PMMAs) in toluene, benzene, and acetonitrile has been studied by NMR spectroscopy, osmometry, and viscometry. It was found that in solutions of highly syndiotactic PMMA in toluene, most of the polymer exists in the associated state. Association takes place by means of stereospecific intermolecular and intramolecular interactions of long parallel syndiotactic sequences. The minimum length of syndiotactic sequences necessary for association of s-PMMA in toluene is 7–8 monomer units. Viscosity data indicate that in dilute solutions the associated structures appear as large formations which do not have the character of compact particles. The formation of the aggregates of PMMA is a relatively slow process. A model of the structure of the aggregates of s-PMMA in toluene is proposed. As compared with that in toluene, the tendency to aggregation in benzene and acetonitrile is negligible. Integral parameters currently used for the characterization of polymer-solvent interaction, like “good” or θ solvent, are not suitable for predicting the tendency of s-PMMA to aggregation. The formation of aggregates evidently has some connection with the effect of the solvent upon the conformational structure of syndiotactic sequences.     

Branched poly(methyl methacrylates) modified by C60 fullerenes

The poly(methyl methacrylates) of branched structure with a covalently bonded fullerene were synthesized by three-dimensional radical polymerization of methyl methacrylate with triethylene glycol dimethacrylate or allyl methacrylate in toluene containing C₆₀. The kinetics of copolymerization of methyl methacrylate with multifunctional co-monomers in the absence of fullerene is compared with that in its presence. The physicochemical characteristics and thermal stability of the obtained copolymers are also compared. The electron spin resonance (ESR) method was applied to study the kinetics of accumulation of the fullerene radicals in the course of the (co)polymerization of methyl methacrylate.     

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.     

Glass transition temperature and architecture of branched poly(methyl methacrylates)

It has been shown that, for a number of MMA-based copolymers prepared through the copolymerization of MMA with a bifunctional comonomer in the presence of a chain-transfer agent, an increase in the concentration of the comonomer (the branching agent) is accompanied by a rise in the molecular mass of the copolymers and a reduction in their glass transition temperature T _g by 30–40°C. This phenomenon provides evidence for the presence of branches in the copolymers and the incorporation of the chain-transfer agent into the branches. The value of T _g linearly declines with an increase in the between the concentration of the branching agent and the molecular mass of the copolymer, that is, with an increase in the number of branches (and end groups) in a macromolecule. A correlation between the glass transition temperature and the rheological and diffusion-sorption characteristics of the branched copolymers has been established. The number of end groups in the copolymers under study has been calculated in accordance with the previously advanced model for the formation of branched macromolecules. The experimental dependences of T _g on the content of the branching agent, the number of end groups, and the molecular mass of the copolymers confirm this model.     

Isotactic and stereoblock poly(methyl methacrylates) produced by butylmagnesium initiators

The use of tert-, sec-, iso- and normal-butylmagnesium bromides and chlorides in toluene, tetrahydrofuran and mixed solvents to prepare Bernoullian, stereoblock and isotactic poly(methyl methacrylate). PMMA, and the factors controlling the stereochemistry are described. By careful control of solvent composition, highly isotactic polymer can be prepared with all the butylmagnesium bromides, but none of the corresponding dibutylmagnesiums. The halide content does not influence stereospecificity when it is in excess of the Grignard stoichiometry but it influences the mol. wt distribution and retards the polymerization. The mol. wt distributions are usually polymodal and dependent on the same factors that control stereospecificity. The effect of THF concentration on stereospecificity operates in a manner quite distinct from its effect on complex concentration and rate. The concentration of residual THF in toluene-rich solution determines the type of initiation and propagating species that prevail. Structures are proposed for those responsible for stereospecific initiation and propagation. It is concluded that tert-butylmagnesium bromide is the most reliable and robust initiator for preparing isotactic PMMA.     

Ordered structures of syndiotactic poly(methyl methacrylates) studied by a combination of infrared,Raman, and NMR spectroscopy.

The C ? O stretching vibrations in Raman and infrared spectra of poly(methyl methacrylate) (PMMA) samples of various stereoregularity in solutions and in the solid state were measured. It was found that for ordered structures of syndiotactic (s) PMMA, transition dipole coupling of ester groups in ordered s-PMMA sequences leads to a splitting of the C ? O stretching vibration into three components with wavenumbers equal in the solid state and in solution, and of different activity in Raman and infrared spectra. A comparison of the time dependences of self-aggregation of s-PMMA as measured by NMR and infrared spectroscopy indicates that the first phase of the aggregation process is detected by both methods. NMR analysis of the effect of the degree of stereoregularity on the self-aggregation of s-PMMA shows that the prerequisite for the generation of a stable ordered structure is the mutual interaction of two s-sequences of a minimum length of 9 monomer units. The experimental data obtained lead to the conclusion that, in self-aggregated s-PMMA, some of the ester groups are in close contact, with the interacting s-sequences probably forming a double helix.     

Light-scattering study of the aggregation of syndiotactic poly(methyl methacrylate) in solution.

Syndiotactic poly(methyl methacrylate (s-PMMA) may undergo aggregation in n-butyl chloride (n-BuCl) at temperatures below the theta temperature. The aggregation behavior of the s-PMMA with weight-average molecular weight M(w) =6.06 x 10(5) g mol(-1) was studied by a combination of static and dynamic laser-light-scattering experiments. A solution of concentration 1.12 x 10(-4) g mL(-1) was quenched from 50 degrees C (above the theta temperature in n-BuCl, 35 degrees C to 12 degrees C, and the aggregation process was measured over 60 h. The time dependence of M(w) the root-mean-square z-average radius of gyration < R(g) >, and the average hydrodynamic radius were used to monitor the growth of the aggregates, with the result M(w) approximately < R(g) > d(f) (where d(f) = 1.98 +/- 0.02), which implies the formation of a fractal aggregate. The observed fractal dimension, d(f), is close to that expected for a reaction-limited cluster aggregation for which d(f) = 2.1. In addition, atomic force microscopy was used to image the aggregates.     

Ordered structures of stereoregular poly(methyl meth-acrylates) in solutions studied by spectroscopic methods

Three groups of problems concerning ordered structures in solutions of stereoregular poly(methyl meth-acrylates) (PMMA) were studied by spectroscopic methods: (i) Self-aggregation of syndiotactic (s) PMMA in solution was followed by means of NMR and IR spectroscopy; it has been established that double helices are formed during the initial stage of aggregation, and activation and equilibrium thermodynamic parameters (ΔH, ΔS) which characterize the transition coil–double helix in s-PMMA were determined. (ii) NMR spectroscopy has demonstrated that association in solutions of atactic PMMA proceeds via interactions of stereocomplex type (interaction between m-diads and r-tetrads). (iii) It has been found that ¹³C CP/MAS NMR spectra of crystalline forms of PMMA (prepared from aggregated solutions by solvent evaporation) conform to the double-helix structure of these systems.     

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.     

Dynamic light scattering studies of atactic and syndiotactic poly(methyl methacrylate)s in dilute theta solution

The present article considers the coil‐to‐globule transition behavior of atactic and syndiotactic poly(methyl methacrylates), (PMMA) in their theta solvent, n‐butyl chloride (nBuCl). Changes in Rh in these polymers with temperature in dilute theta solutions were investigated by dynamic light scattering. The hydrodynamic size of atactic PMMA (a‐PMMA‐1) in nBuCl (M_w: 2.55 × 10⁶ g/mol) decreases to 61% of that in the unperturbed state at 13.0°C. Atactic PMMA (a‐PMMA‐2) with higher molecular weight (M_w: 3.3 × 10⁶ g/mol) shows higher contraction in the same theta solvent (α_η = R_h(T)/R_h (θ) = 0.44) at a lower temperature, 7.25°C. Although syndiotactic PMMA (s‐PMMA) has lower molecular weight than that of atactic samples (M_w: 1.2 × 10⁶), a comparable chain collapse was observed (α_η = 0.63) at 9.0°C. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2253–2260, 1999     

Microstructural study of poly(methyl methacrylate-co-n-propyl acrylate) by 13C NMR

Analysis of carbonyl and β-CH₂ signals in the 100?MHz ¹³C NMR spectra of poly(methyl methacrylate-co-n-propyl acrylate) (PMMA/nPrA), provided distribution of configurational-compositional sequences for a series of the copolymer samples of different composition at pentad level for carbonyl signal and hexad level for the backbone methylene carbons. Computer simulation of the spectra based on incremental calculation of the chemical shifts for individual sequences provided very good agreement with the experimental spectra.     

Synthesis of stereoregular poly(alkyl malolactonates)

Optically pure methyl, ethyl, isopropyl, and benzyl (R)-malolactonate were prepared from (S)-(-)-malic acid and were polymerized in the bulk with tetraethylammonium benzoate as the initiator to yield high-molecular-weight, crystalline polymers. The optical purity of methyl and benzyl malolactonate was determined by ¹H NMR spectroscopy of the β-lactone complexed with a chiral europium shift reagent. Enantiomeric excesses of 100% were found (the experimental error was 3%). Optically active poly(β-malic acid) was obtained from optically active poly[benzyl (S)-malate] by catalytic hydrogenolysis of the pendent benzyl esters. Ethyl and benzyl (R)-malolactonate were also copolymerized, and the benzyl esters of the resulting copolymer were converted into carboxylic acid units by hydrogenolysis.     

Preparation and (13C)NMR spectroscopy of stereoregular poly(methacrylic acid)

Several procedures for synthesis of stereoregular poly(methacrylic acid) have been examined and the polymer characterized by (¹³C)NMR. Using d⁶ DMSO as solvent for spectroscopy gives better spectra than those previously obtained using aqueous solutions and stereochemical splittings can be resolved in the methyl signals. Free-radical polymerization in toluene solution is a Bernouilli process giving mainly heterotactic/syndiotactic polymer. Polymers produced with free-radical initiation in aqueous solution have a higher, and pH dependent, content of syndiotactic triads. A previously described procedure for producing regular polymers by hydrolysis of poly(trimethylsilyl methacrylate) requires modification to produce isotactic contents of above 90% and does not give truly syndiotactic polymer. In contrast, polymerization with γ-radiation can produce polymers with close to 90% of syndiotactic triads.     

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.