Study of compositional distribution in a styrene–methyl acrylate copolymer by means of density-gradient centrifugation

Several samples of a copolymer were examined by means of equilibrium centrifugation in a density gradient. The results for the samples without compositional distribution (homogeneous polymers) were used to determine the relation between the chemical composition and the distance from the center of rotation. For one of the homogeneous polymers a quantitative analysis of the schlieren curve was made in order to show the kind of accuracy that can be achieved. For the inhomogeneous polymer it was found that the schlieren curve could, in principle, be described quantitatively by the assumption that fluctuations in composition are independent of fluctuations in molecular weight, but on this basis the average square of the fluctuations in composition is more than four times smaller than the value derived from chromatographic data. A more satisfactory explanation of the experimental results is provided by assuming a linear correlation between average molecular weight and composition, but it is not claimed that this is necessarily the only model that explains the data.     

Direct rotating-sector studies of the systems: Styrene–methyl methacrylate and styrene–methyl acrylate

The rotating-sector technique was applied directly in a study of two copolymerization systems: styrene–methyl methacrylate and styrene–methyl acrylate. The two coupled rate expressions which describe the change in radical concentrations for two-component polymerizations degenerate into a single expression identical in form to the radical expression for a homopolymerization when the ratio of the radical concentrations under intermittent illumination is assumed constant and equal to the ratio under steady illumination. Numerical solutions of the complete rate expression by use of constants from the literature confirm that this assumption is valid for a rotating-sector experiment. The overall lifetimes of these two-component systems were defined and measured experimentally as a function of monomer composition and then compared with lifetimes calculated by using literature rate constants. The agreement was satisfactory. The direct application of the technique to the two-component system provides an independent experiment which for some systems seems to be more sensitive to the value of the cross-termination constant than the usual steady-state method.     

Atmospheric ozonolysis study of methyl acrylate and methyl 3-methyl acrylate

Detailed reaction mechanisms of methyl acrylate (MA) and methyl 3-methyl acrylate (M3MA) with ozone have been investigated using quantum chemistry calculations based on the CCSD(T)/6-31G(d)+CF//B3LYP/6-31+G(d,p) level of theory. Possible reaction channels and products have been presented and discussed. The temperature-dependent and pressure-independent overall and site-specific rate constants are calculated by employing multichannel RRKM theory. The obtained overall rate constants (in cm³ molecule^?1 s^?1) based on the CCSD(T)+CF energies can be described as: $ k_{{({\text{MA}} + {\text{O}}_{ 3} )}} = 3. 7 4 \times 10^{ - 1 3} { \exp }\,( - 3 7 4 6. 1 2/T) $ and $ k_{{({\text{M3MA}} + {\text{O}}_{ 3} )}} = 5. 1 2 \times 10^{ - 1 3} { \exp }( - 3 3 5 4. 8 7/T) $ at 200–400 K and 760 Torr. Under atmospheric conditions, the dominant products of ozonolysis of MA and M3MA are methyl glyoxylate and formaldehyde, methyl glyoxylate and acetaldehyde, respectively. The results of theoretical study are in good agreement with the available experimental measurements. Researches on branching ratios and atmospheric lifetimes also have been obtained as complement to the experimental results.     

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 silicone–acrylate copolymer latexes and their film properties

Emulsion copolymerization of silicones (octomethyl tetracyclosiloxane, D₄ and methacryloxypropyl trimethoxy silane, MATS) and acrylics was carried out using three different polymerization processes: semicontinuous monomer emulsion addition, batch and initiator solution addition. Results showed that only the semicontinuous process led to a stable latex with monodisperse particles. Various polymerizations were carried out through this process with varying silicone/acrylic ratios and MATS content. Films were obtained from different latexes: their properties are significantly influenced by silicone and MATS contents in copolymers.     

A redox-initiated xylan–poly(sodium acrylate) graft copolymer

An aspen 4-O-methylglucuronoxylan was grafted with poly(sodium acrylate) (PSA) in 3.4N NaOH at 25–30°C. with the use of a persulfate–thiosulfate redox initiator system. The formation of a true graft copolymer was indicated by fractional precipitation and light-scattering studies, physical mixtures of the two pure polymers being used as references. A grafted fraction was isolated, containing no ungrafted xylan but possibly some PSA homopolymer, which contained 96.5% PSA and 3.5% xylan, or an average of 3.2 PSA chains of M?_n approximately 90,500 per xylan chain of M?_n approximately 10,500.     

Stereochemistry of alternating methacryloyl-L-valine methyl ester–maleic anhydride copolymer

The stereochemical composition of an alternating methacryloyl-L -valine methyl estermaleic anhydride (L-MAVM/MAn) copolymer which was prepared by the photocopolymerization of L-MAVM and MAn in dioxane at 25°C without initiator was investigated by proton magnetic resonance spectroscopy. The resonance of the ester methyl protons of the L-MAVM unit appeared as three split peaks at 3.65, 3.76, and 3.82 ppm, which could be assigned to those of the coisotactic (di-threo-tri-isotactic), coheterotactic and cosyndiotactic triads, respectively. The triad cotacticity determined according to these assignments indicated that the copolymer was composed predominantly of di-threo-tri-isotactic triad. Similarly PMR spectroscopic investigation of the acryloyl-L -valine methyl ester–maleic anhydride (L-AVM/MAn) copolymer with a 1:1 molar ratio of the monomers showed that the main tactic fraction of the copolymer was also di-threo-tri-isotactic one. Nevertheless, the circular dichroic investigation of the L-AVM/MAn copolymer before and after hydrolysis denied asymmetric induction into the polymer main chain. These results suggest that the propagation step in the photocopolymerization of L-MAVM or L-AVM with MAn proceeds by the trans-trans opening of the complexmer composed of a 1:1 molar ratio of L-MAVM or L-AVM and MAn.     

Alternating copolymerization of polar vinyl monomers in the presence of zinc chloride. III. NMR study of methyl methacrylate–styrene copolymer

The copolymer composition curve of the methyl methacrylate–styrene copolymer obtained by the copolymerization in the presence of ZnCl₂ has more alternating tendency than that of ordinary methyl methacrylate–styrene copolymer obtained by radical copolymerization. The fine structure of the copolymer was examined by NMR, and the mechanism of the propagation step of the copolymerization in the presence of ZnCl₂, which was proposed in the first report of this series, was verified.     

Analysis of β‐CH2 signals in the 13C NMR spectra of the methyl methacrylate–ethyl acrylate copolymer as a tool for microstructure determination

The incremental method of the chemical shift calculation in the ¹³C NMR spectra of the methyl methacrylate–ethyl acrylate copolymer, PMMA/EA, has been applied to the β‐CH₂ carbons. A positive simulation of the DEPT sub‐spectrum shows that it is possible to determine in this way the distribution of configurational‐compositional tetrads providing a tool for microstructure analysis of acrylic copolymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2147–2155, 2000     

Kinetics of cyclization reaction in poly(acrylonitrile/methyl acrylate/dimethyl itaconate) copolymer determined by a thermal analysis

The PAN/MA/DMI (poly(acrylonitrile/methyl acrylate/dimethyl itaconate)) copolymer was synthesized for being used as a carbon fiber precursor. Its high melt-spinnability at 175–210 °C has been demonstrated elsewhere but the cyclization kinetics was thoroughly investigated by DSC analysis herein. The isothermal analysis based on the Kamal equation demonstrates that at a given temperature, the cyclization rate constant k of PAN/MA/DMI system is five to seven times that of a regular PAN/MA system. The activation energy E_a of the PAN/MA/DMI system, owing to the acid-catalysis by the DMI compound, is approximately 30 kJ/mol lower than that of a regular PAN/MA system. An isothermal Avrami analysis confirmed the above finding. Furthermore, based on the obtained nucleation index n, DMI may induce intermolecular cyclization which can rapidly stabilize the fiber shape of PAN/MA/DMI precursor. A nonisothermal analysis, obtained from the Ozawa's method, revealed the same tendency as did the isothermal results, but the difference between the average activation energies E_a obtained with and without DMI was reduced from 30 to 25 kJ/mol. Finally, a cyclization mechanism that involves the formation of a cyclic anhydride was proposed.     

Monomer reactivity ratios for acrylonitrile–methyl acrylate free‐radical copolymerization

Nonlinear monomer reactivity ratios for the homogeneous free‐radical copolymerization of acrylonitrile and methyl acrylate were determined from ¹H NMR and real‐time Fourier transform infrared (FTIR) analyses. All ¹H NMR data were obtained on polymers isolated at low conversions (<10%), whereas the FTIR data were collected in situ. The copolymerizations were conducted in N,N‐dimethylformamide at 62 °C and were initiated with azobisisobutyronitrile. The real‐time FTIR technique allowed for many data points to be collected for each feed composition, which enabled the calculation of copolymer compositions (dM₁/dM₂) with better accuracy. Monomer reactivity ratios were estimated with the Mayo–Lewis method and then were refined via a nonlinear least‐squares analysis first suggested by Mortimer and Tidwell. Thus, monomer reactivity ratios at the 95% confidence level were determined to be 1.29 ± 0.2 and 0.96 ± 0.2 for acrylonitrile and methyl acrylate, respectively, which were valid under the specific system conditions (i.e., solvent and temperature) studied. The results are useful for the development of acrylonitrile (<90%) and methyl acrylate, melt‐processable copolymer fibers and films, including precursors for carbon fibers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2994–3001, 2004     

Addition–fragmentation chain transfer: Molecular weight distributions and retardation in the system methyl methacrylate/methyl α‐(bromomethyl)acrylate

A detailed investigation of addition–fragmentation chain transfer (AFCT) in the free‐radical polymerization of methyl methacrylate (MMA) in the presence of methyl α‐(bromomethyl)acrylate (MBMA) was carried out to elucidate mechanistic details with efficient macromonomer synthesis as an underlying goal. Advanced modeling techniques were used in connection with the experimental work. Curve fitting of simulated and experimental molecular weight distributions with respect to the rate coefficient for addition of propagating radicals to MBMA (k_add) over 60–120 °C resulted in E_add = 21.7 kJ mol^?1 and A_add = 2.18 × 10⁶ M^?1 s^?1 and a very weak temperature dependence of the chain‐transfer constant (E_add ≈ E_p). The rate coefficient for fragmentation of adduct radicals at 60 °C was estimated as k_f ≈ 39 s^?1 on the basis of experimental data of the MMA conversion and the concentration of 2‐carbomethoxy‐2‐propenyl end groups. The approach developed is generic and can be applied to any AFCT system in which copolymerization does not occur and in which the resulting unsaturated end groups do not undergo further reactions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2640–2650, 2004     

Pulsed NMR study of network formation in the course of bulk polymerization of methyl acrylate.

The proton spin-spin relaxation time (T2) during the bulk polymerization of methyl acrylate was measured as a function of the reaction time at various temperatures. Three kinds of T2 (T2L (long), T2S (short) and T2M (intermediate)) were obtained as the polymerization proceeded. The fraction of T2S (FS) increased sigmoidally at a certain reaction time, while that of T2L (FL) decreased reciprocally. The former corresponded to the amount of a polymer whose molecular weight was sufficiently high enough to cause a tight entanglement that produced a transient network structure; the latter reflected a decrease in the mixture of the monomer and the low molecular weight of the polymer. T2M is considered to arise from a relatively mobile region of the entanglement. The relationship between the fractions of T2S + T2M and the polymer yield was found to be linear, which led us to monitor the polymer yield in real time during the polymerization in a non-distractive manner. 13C DD (dipolar decoupling)/MAS (magic angle spinning) NMR spectra were also measured to monitor the polymerization process in terms of the molecular motions between the main chain and the side chain in the formation of a network structure. The 13C DD/MAS NMR spectra show that the side chain motion became restricted as well as the main chain when the "Trommosdorff effect" (gel effect) was observed, and a part of the monomers were trapped in the network structure.     

Mode of termination in the copolymerization of vinyl acetate–isobutyl methacrylate and methyl methacrylate–methyl acrylate at 60°C

The mode of termination in the vinyl acetate–isobutyl methacrylate (VA–IBMA) and methyl methacrylate–methyl acrylate (MMA–MA) copolymerization systems has been investigated at 60°C. by using the dye-interaction technique for functional endgroup estimation. The results show that pairs of poly(vinyl acetate) radicals interact almost exclusively through a disproportionation mechanism. In the homopolymerization of methyl methacrylate and methyl acrylate, about 1.16 and 1.22 carboxyl-containing endgroups per polymer molecule have been estimated, which shows the predominance of disproportionation over combination in these termination reactions. In poly(isobutyl methacrylate) about 1.55 tagged initiator fragments per chain indicate that 29% of the total radicals terminate through the disproportionation mechanism. Cross termination in the (VA–IBMA) copolymerization system occurs almost entirely through combination for monomer feeds richer in isobutyl methacrylate content while for the MMA–MA system, combination is more important at intermediate monomer feed ratios. These results have been discussed in the light of different explanations for the reaction mechanism.