Stereoregularity of poly(methyl acrylate)

The stereoregularity of poly(methyl acrylate) and poly(methyl acrylate-αd) was determined from the NMR spectra. A method of quantitative determination of stereoregularity of poly(methyl acrylate) proposed in this paper is based on the fact that in the 100 Mc./sec. NMR spectrum the absorption peaks due to methylene protons in syndiotactic configurations overlap absorptions due to only one of two methylene protons in isotactic configurations. The stereostructure of poly(methy1 acrylates) polymerized with anionic catalysts such as Grignard reagents, n-butyllithium, and LiAlH₄ is generally richer in isotactic diads than in syndiotactic diads. For example, poly(methyl acrylate) polymerized with phenylmagnesium bromide as catalyst at ?20°C. consists of 99% isotactic and 1% syndiotactic diads. In radical polymerization, the isotacticity of poly(methyl acrylate) is independent of polymerization temperature. Poly(methyl acrylates) polymerized with a Ziegler-Natta catalyst consisting of Al(C₂H₅)₂Cl and VCl₄ have configurations similar to those polymerized by radical initiators. The stereoregularity of poly(methyl acrylate-α-d) resembled that of poly(methyl acrylate) polymerized under the same conditions.     

Stereoregularity of radically polymerized poly(alkyl acrylates) and poly(trimethylsilyl acrylates) and the preparation of syndiotactic poly(methyl acrylate)

Nuclear magnetic resonance (NMR) spectroscopy was used to determine the stereoregularity of radically polymerized poly(ethyl acrylates), poly(trimethylsilyl acrylates), and poly(isopropyl acrylate-α,β-d₂). The ethyl acrylate polymers consisted of a random configuration having about 50% of isotactic diads, and their stereoregularities were independent of the polymerization temperature (40 to ?78°C). Poly(trimethylsilyl acrylates) and poly(isopropyl acrylate-α,β-d₂) prepared at low temperatures had a syndiotactic configuration. Syndiotactic poly(methyl acrylate) was derived from syndiotactic poly(trimethylsilyl acrylate). For poly(methyl acrylate), an approximate estimation of the stereoregularity by infrared spectroscopy was proposed.     

Stereoregularity of poly(vinyl ether)

α-Methylvinyl isobutyl and methyl ethers were polymerized cationically and the structure of the polymers was studied by NMR. Poly(α-methylvinyl methyl ether) polymerized with iodine or ferric chloride as catalyst was found to be almost atactic, whereas poly(α-methylvinyl isobutyl ether) polymerized in toluene with BF₃OEt₂ or AlEt₂Cl as catalyst was found to be isotactic. In both cases, the addition of polar solvent resulted in the increase of syndiotactic structure as is the case with polymerization of alkyl vinyl ether. tert-Butyl vinyl ether was polymerized, and the polymer was converted into poly(vinyl acetate), the structure of which was studied by NMR. A nearly linear relationship between the optical density ratio D₇₂₂/D₇₃₆ in poly(tert-butyl vinyl ether) and the isotacticity of the converted poly(vinyl acetate) was observed.     

Characteristic ratio of poly(tetrahydrofurfuryl acrylate) and poly(2-ethylbutyl acrylate)

The synthesis, characteristic ratio C_∞ and glass transition temperature (T_g) of poly(tetrahydrofurfuryl acrylate) (PTHFA) and of poly(2-ethylbutyl acrylate) (P2EBA) are reported. P2EBA has slightly lower flexibility (C_∞ = 9.2) than PTHFA (C_∞ = 8.6), mainly because of the higher bulkiness of its side group and the closer proximity to the main chain. The C_∞ results compared with the corresponding polymethacrylates show an increase in flexibility due to the absence of the α-methyl group. Comparison with poly(methyl acrylate) clearly shows the influence of the bulkiness of the side group on the chain flexibility. The lower T_g of P2EBA than that of PTHFA may be explained by the higher flexibility of the 2-ethylbutyl side group. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1589–1592, 1997     

Stereoregularity of poly(2-vinylpyridine) and poly-(4-vinylpyridine)

In order to determine the stereoregularity of poly(4-vinylpyridine), 4-vinylpyridine-β,β-d₂ was synthesized from 4-acetylpyridine. The ¹H-NMR spectra of the deuterated and nondeuterated polymers were measured and analyzed. From the ¹H-NMR spectra of poly(4-vinylpyridine-β,β-d₂), triad tacticity can be obtained, while the ¹H-NMR spectra of nondeuterated poly(4-vinylpyridine) give the fraction of isotactic triad. The ¹³C-NMR spectra of poly(4-vinylpyridine) were also observed, and the spectra of C₄ carbon of polymers were assigned by the pentad tacticities. The fraction of isotactic triad of poly(2-vinylpyridine) and poly(4-vinylpyridine) obtained under various polymerization conditions were determined. The radical polymerization and anionic polymerizations with phenylmagnesium bromide and n-butyllithium as catalysts of 4-vinylpyridine gave atactic polymers.     

Calorimetric study of free-radical polymerized poly(p-biphenyl acrylate) and poly(p-cyclohexylphenyl acrylate)

DSC traces and specific heat data for poly(p-diphenyl acrylate) (PPBA) and poly(p-cyclohexylphenyl acrylate) (PPCPA) obtained by radical polymerization are reported. The results indicate the existence of a definite ordered phase and of a reversible firstorder solid–liquid transition in both polymers although x-ray diffraction studies showed that they are not crystalline in the conventional sense. The extent of the ordered phase present in each polymer is calculated, and the problems involved in such determination by thermal measurements are discussed. On the basis of the experimental results reported here in conjunction with the x-ray data, models are proposed for the morphology of these polymers.     

Thermal degradation of poly(ethyl methacrylate) and its copolymer with poly(ethyl acrylate)

The thermal degradation behaviour of poly(ethyl methacrylate) homopolymers and poly(ethyl methacrylate) and poly(ethyl acrylate) copolymers synthesized by using the benzoyl peroxide-di-methyl aniline redox pair at different temperatures (18–35C) was investigated. Contrary to some reports in the literature, the thermal degradation of PEMA was observed to take place in multi steps. These are assigned to be loss of labile end groups, side chain scission, anhydride formation and main chain degradation steps. Dominating chemical formations at the end of these steps were characterized by FTIR spectroscopy.The homopolymer samples synthesized at 18C showed a greater thermal stability against degradation. Copolymerization with small amounts of ethyl acrylate was observed to impart thermal stability to PEMA by stabilizing mainly the end groups against degradations.     

Solution properties of poly(methyl acrylate) and (1:1) poly(styrene-co-methyl acrylate)

Poly(methyl acrylate) (PMA) and 1:1 poly(styrene-co-methyl acrylate) (PSMA) were prepared by solution and bulk polymerization, respectively. The copolymer was analyzed with NMR to ascertain its composition and microstructure. The solution properties of unfractionated PMA and fractionated PSMA in ethyl acetate were investigated by light-scattering and viscosity techniques at 35°C. Narrow composition heterogeneity as revealed from the light-scattering measurements in different solvents justified the use of a single solvent for the copolymer characterization. The equations relating the limiting viscosity number to molecular weight, the molecular dimension to molecular weight, etc., were found for homopolymer and copolymers in ethyl acetate at 35°C. In the evaluation of the Flory constant K for the unperturbed state by methods based on Flory-Fox-Schaefgen, Kurata-Stockmayer, and Stockmayer-Fixman expressions, only the first method gave a value for PMA in ethyl acetate, consistent with that obtained in other solvents, whereas similar values of K were obtained by the three methods for PSMA in ethyl acetate. The studies indicate reduced thermodynamic interaction for PSMA–ethyl acetate compared to PMA–ethyl acetate, but increased steric effect in the copolymer compared with the homopolymer.     

Miscibility of poly(vinyl chloride) with poly(methyl-co-hexyl acrylate) and poly(methyl-co-2 ethyl hexyl acrylate)

The miscibility and phase behavior in blends of PVC with poly(methyl-co-hexyl acrylate)[MHA] and poly(methyl-co-2 ethyl hexyl acrylate)[MEH] were studied. It was found that PVC is miscible with MHA copolymers having a HA volume fraction from 0.30 to 0.92 and MEH copolymers having an EH volume fraction from 0.30 to 0.83 at 100°C. By applying the mean field theory to the phase diagrams of these blend systems, segmental interaction parameters which represent the binary interaction between different monomer units were estimated. The calculated values reflect the fact that the miscibility window observed for PVC/MHA and PVC/MEH blend systems was attributed to the effect of repulsion between different monomer units within the copolymer. To investigate the effect of specific interaction on the miscibility for these blend systems, an attempt was also made to describe the blend interaction parameter as a function of polar group concentration in the acrylate copolymer. The blend interaction parameter values exhibit a u-shaped curve as a function of the weight fraction of C?O group in the copolymer, and the lowest blend interaction parameter value appears at about 0.24 C?O weight fraction.     

Photolysis of poly(ethyl acrylate) and poly(n–butyl acrylate) in vacuo

Thin films of poly(ethyl acrylate) and poly(n-butyl acrylate) were decomposed in vacuo by means of a high pressure Hg lamp, and the rate of development of volatile products was measured. The main gaseous products were CO, CO₂, and the alcohol, aldehyde, alkane, and formate derived from the respective ester groups. In addition poly(ethyl acrylate) evolved acetal as well as ethyl propionate, while n-butyl valerate was evolved from poly(n-butyl acrylate) only after prolonged exposure. All products and the principal features of the decomposition are discussed.     

Stereoregularity of poly(methyl α-chloroacrylate)

Triad and tetrad tacticities of poly(methyl α-chloroacrylate) and poly(methyl α-chloroacrylate-β-d₁) were determined by nuclear magnetic resonance (NMR) spectroscopy. Methyl α-chloroacrylate-β-d₁ and its polymer were first synthesized. Isotactic poly(methyl α-chloroacrylate) was prepared with ethylmagnesium chloride-benzal-acetophenone in combination as catalyst. The syndiotacticity of radically polymerized polymers increased with decreasing polymerization temperature. For radical polymerization, enthalpy and entropy differences between isotactic and syndiotactic additions were calculated to give ΔH ? ΔH = 850 cal/mole and ΔS ? ΔS = 0.93 eu. The stereoregularity of the polymer prepared with phenylmagnesium bromide catalyst was analyzed in fairly good agreement with first-order Markov statistics, while polymerization with fluorenyllithium seems predominantly to proceed by a mechanism similar to free-radical mechanism. Stereoregularity-controlling power for individual substituents is briefly discussed.     

Nanodomains in a hydrophilic-hydrophobic IPN based on poly(2-hydroxyethyl acrylate) and poly(ethyl acrylate)

The morphology of a series of hydrogels based on the interpenetration of poly(2-hydroxyethyl acrylate) and poly(ethyl acrylate) has been studied through transmission electron microscopy, TEM, atomic force microscopy, AFM, and dynamic-mechanical spectroscopy, DMA. For the TEM analysis phosphotungstic acid, PTA, was used as alternative selective staining agent to those commonly used. The good agreement between TEM and AFM images allowed us to confirm that the PTA technique is a very powerful tool for TEM analysis of these hydrogel systems. All the results show that the IPNs presented phase-separation with two kinds of microdomains: those preferentially with a hydrophilic nature and those with preferentially a hydrophobic one, of sizes that range from 30 nm to 100 nm. Each one of these domains is composed by smaller nanodomains of alternating hydrophobic and hydrophilic component ranging between 6 and 10 nm sizes, those preferentially with a hydrophilic nature having a larger proportion of hydrophilic nanodomains. The AFM images of the IPN with the highest PHEA mass fraction, x_PHEA = 0.75, suggest that the hydrophilic phase is co-continuous in the material. A disperse hydrophilic phase is found when the PHEA mass fraction is reduced up to x_PHEA = 0.38. This phase-separation is explained in terms of some characteristic parameters of the networks such as the mesh size and the number of units between cross-links. The morphology found makes the systems very attractive for cell adhesion substrates and for matrices of scaffolds in soft tissue engineering.     

The crystalline character of atactic poly(p-biphenyl acrylate) and poly(p-cyclohexylphenyl acrylate) in their dynamic mechanical response

Dynamic mechanical properties have been determined in atactic poly(p-biphenyl acrylate) (PPBA) and poly(p-cyclohexylphenyl acrylate) (PPCPA) in the temperature range from 80 to 540°K at frequencies in the range 10³–10⁴ Hz. The general behavior of the dynamic elastic modulus as a function of temperature shows a transition region from the glassy state at about 390°K for both polymers, a plastic region extending over a temperature interval of about 100°K, and another transition to the melt situated at 540 and 480°K for PPBA and PPCPA, respectively. The experimental data show that the mechanical behavior of both polymers strongly resembles that of crystalline polymers. The loss spectrum of PPBA shows the presence of several important maxima: one corresponding to the melting point, characterized by a very rapid increase of losses with increasing temperature (α′ relaxation), one in the glass-temperature range, characterized by a rather broad peak (α′ relaxation), and others below T_g, associated with secondary relaxation effects. The analysis of the different transitions and relaxations indicates that some of these processes can be ascribed to motions taking place in the ordered regions of the polymer. PPCPA shows a similar loss pattern; however, owing to the lower melting point the α maximum is partially submerged in the α′ relaxation associated with the melting process. Of particular interest is the γ process in the glassy state of this polymer, caused by the chair–chair transition of the cyclohexyl rings. The limited intensity of this relaxation as compared with that of most polymers containing cyclohexyl side groups, has been interpreted as due to the high ΔF associated with such a transition for cyclohexyl rings linked to phenylene groups. This leads to some interesting conclusions about the conformation of the side groups in PPCPA.     

Pyrolysis of poly(2-propylheptyl acrylate)

The thermal destruction processes of poly(2-propylheptyl acrylate) take place at the range of temperature 250–950 °C was investigated using pyrolysis–gas chromatography. Knowledge of the types and amounts of pyrolysis products will provide important information about the thermal degradation of homopolymer poly(2-propylheptyl acrylate) and the mechanisms involved. Unsaturated monomers 2-propylheptyl acrylate and 2-propylheptyl methacrylate, according to by-product alkyl alcohol 2-propylheptylalcohol, alkene 2-propylheptene-1, carbon dioxide, carbon monoxide, methane, and ethane were formed during thermal degradation of poly(2-propylheptyl acrylate).     

Stereoregularity of poly(2-vinylpyridine) determined by 1H-NMR and 13C-NMR spectroscopy

In order to determine the stereoregularity of poly(2-vinylpyridine), 2-vinylpyridine-β,β-d₂ was synthesized. The ¹H-NMR spectra of the deuterated polymer in D₂SO₄ and o-dichlorobenzene solutions showed three peaks, which were assigned to triad tacticities. Since the absorptions of heterotactic and syndiotactic triads of methine protons overlap those of methylene protons in nondeuterated polymers, only isotactic triad intensities can be obtained from the ¹H-NMR spectra of nondeuterated poly(2-vinylpyridine). The ¹³C-NMR spectra of poly(2-vinylpyridine) were obtained in methanol and sulfuric acid solutions. In methanol solution the absorption was split into three groups, which cannot be explained by triads, and in sulfuric acid solution several peaks were observed. These splittings may be due to pentad tacticity. The results show that poly(2-vinylpyridine) obtained by radical polymerization is an atactic polymer.     

Stereoregularity of fractionally crystallized poly(propylene oxide) samples by 13C-NMR spectroscopy

Several stereoregular poly(propylene oxide) (PPO) samples were synthesized employing either trimethyl aluminum hydrolyzates or Pruitt–Baggett type catalysts. Dilute isooctane solutions of these samples were fractionally crystallized by stepwise cooling from 333 K down to 273 K. The fractions which dissolve at 273 K were noncrystallizable and were further fractionated by GFC. The isolated fractions were characterized by ¹³C-NMR, DSC, and viscometry techniques. Both structural (head-to-head and tail-to-tail linkages) and steric (syndiotactic dyads) irregularities were found in otherwise isotactic chains and their abundance apparently increased as the melting point and the chain length of the fractions decreased.     

Dynamics of adsorbed poly(methyl acrylate) and poly(methyl methacrylate) on silica

Deuterium NMR and modulated differential scanning calorimetry (MDSC) were used to probe the behavior of ultrathin adsorbed poly(methyl acrylate) (PMA). The spectra for the bulk methyl-labeled PMA-d₃ were consistent with the motions of the polymer segments being spatially homogeneous. For the polymers adsorbed on silica, multicomponent line shapes were observed. The segmental mobility of the surface polymers increased with increased adsorbed amounts. In contrast to the behavior of the polymers in bulk, the adsorbed lower-molecular-mass PMA-d₃ was less mobile than the adsorbed high-molecular-mass polymer. The presence of a polymer overlayer was sufficient to suppress the enhanced mobility of the more-mobile segments of the adsorbed (inner) polymer. MDSC studies on adsorbed poly(methyl methacrylate) showed that the glass-transition temperature of the thin polymer films increased and broadened compared to the behavior of the polymer in bulk. The presence of a motional gradient with the less-mobile segments near the solid-polymer interface and the more-mobile segments near the polymer-air interface was consistent with the experimental observations.