Synthesis of poly(1.4.-benzamide) in solutions of randomly coiled polymers

The polycondensation of p-aminobenzoic acid by the use of triphenylphospine and hexachloroethane is described. One way to obtain a mixed solution of rigid rod and flexible coil polymers is the polycondensation of the rigid chains in a solution of a flexible polymer matrix. Results on matrix polycondensation of poly(p-benzamide) in solutions of poly(methyl methacrylate), poly(methyl methacrylate-co-styrene), polystyrene and polyacrylonitrile are reported. It is shown that there exists an interrelation between the phase behavior of the mixed polymer solutions and the influence of the matrix polymer on the synthesis of poly(1.4.-benzamide). The ternary phase diagrams of the rigid rod/flexible coil polymer solutions were determined.     

Theory of the cooperative transition between two ordered conformations of poly(L-proline). I. Phenomenological theory.

The states of three residues are correlated in a nearest-neighbor Ising model matrix treatment of a one-dimensional phase transition, in which nucleation is assumed to differ at each end of a regular sequence (asymmetric nucleation). The correlation of the states of three residues requires a 4 X 4 matrix, which cannot be reduced in size because of the asymmetric nature of the nucleation. Also, because of the asymmetry, at least four independent parameters for a homopolymer (rather than the two usually encountered in the helix-coil transition), ant at least five for a specific-sequence copolymer, are required to describe the transition behavior. The most important current interest in such a treatment (for a homopolymer) is its applicability to the poly(L-proline) form I in equilibrium form II interconversion.The earlier treatment of Schwarz, using the nearest-neighbor Ising model (with correlation of only two residues), is identical with the above treatment, and requires only a 2 X 2 matrix which greatly simplifies numerical computations, which are presented in the next two papers of this series. However, the 4 X 4 matrix treatment is required in order to make the asymmetric nature of the nucleation explicit and physically understandable, for a homopolymer; for a specific-sequence copolymer, such as a protein, it is essential in order to show how the asymmetric nature of helix nucleation differs from one amino acid to another.     

Low-frequency dielectric behavior of hydrated poly(L-proline)II

The dielectric behavior of poly(L -proline) II has been studied by the dc transient method and a contactless pendulum technique. The comparison of measurements performed with and without contacting electrodes shows that the static and low-frequency dielectric behavior are due to bulk phenomena. Both mechanisms are strongly dependent upon the water content of the sample. At low hydration, the static conduction may be attributed to an electronic process. The observed low-frequency relaxation can be characterized by a relaxation time which increases with the water content. This behavior can be explained by an increase in stiffness of the poly(L -proline) chain: two adjacent carbonyl groups of the chain may be hydrogen bonded by two water molecules.     

The conformational effect of form I helical poly(L-proline) and form II helical poly(L-proline) to the complex formation with helical poly(L-glutamic acid) through hydrogen bonding

The complex formation between helical poly-L-glutamic acid (PLGA) and helical poly-L-proline (PLP) was studied in a methanol-water (2 : 1) cosolvent and a propanol-water cosolvent (9 : 1). Reduced viscosity, circular dichroism, pH, and molar absorptivity were measured. The experimental results exhibit that the interpolymer complex was formed between helical PLGA and helical PLP through hydrogen bonding. When the complex was formed the unit mole ratio of PLGA : PLP(II) is 2 : 1 and PLGA : PLP(I) is 1.5 : 1, the ability of complex formation of PLP (II) with PLGA is better than that of PLP(I). On complexation the conformations of PLGA and PLP change and this change is more enhanced in the PLGA-PLP(II) than the PLGA-PLP(I) complex; its cause is studied. © 1993 John Wiley & Sons, Inc.     

Theory of the cooperative transition between two ordered conformations of poly(L-proline). III. Molecular theory in the presence of solvent.

Phenomenological theories of the form I in equilibrium to form II interconversion in poly(L-proline) have been presented by Schwarz (using the parameters s, sigma, beta', and beta' in a 2 X 2 matrix formulation) and by the present authors (using the parameters s, sigma, betaC, and betaN in a 4 X 4 matrix formulation). In addition, a molecular theory was developed to compute s, sigma, beta', and beta' under vacuum. In this paper, we take into account the effect of solvent on the parameters s, sigma, beta', and beta' of the isothermal poly(L-proline) form I in equilibrium to form II interconversion. The growth parameter is sensitive to the binding of solvent molecules to the peptide CO groups, but the nucleation parameters sigma, beta', and beta' are not affected by this type of solvent effect. The calculated values of s and sigma under vacuum are in good agreement with the corresponding values derived from experimental data. By combining the theoretical values of s, sigma, beta', and beta' under vacuum with experimentally determined equilibrium constants for the binding of alcohols to the peptide CO groups (which differ in magnitude for form I and form II), it was possible to reproduce the experimental tranistion curves satisfactorily. Alternatively, the binding constants for alcohols, obtained by combining our theoretically computed parameters under vacuum with experimental equilibrium transition curves, are in a satisfactory agreement with those evaluated independently by infrared spectral measurements of the binding of alcohols to the peptide CO groups. It is pointed out that significant errors may arise in analyzing experimental data if short chains are included with long chains in the determination of s, sigma, beta', and beta' from the equilibrium transition curves. The transition of poly(L-proline) from form II to form I when n-butyl alcohol is added to a solution of the polymer in benzyl alcohol is brought about by the slight difference in the binding free energies of both alcohols to the carbonyl groups of form II. The different binding affinities of two alcohols, ROH, to form II may arise from (a) the different hydrogen-bond strength between the alcohol and the proline carbonyl group, and (b) possible differences in nonbonded and electrostatic interactions between the R group and the binding-site environment of the proline carbonyl group. The greater binding affinity of form II (compared to form I) for given alcohol is attributed to the more open and extended conformation of form II.     

Theory of the cooperative transition between two ordered conformations of poly(L-proline). II. Molecular theory in the absence of solvent.

Phenomenological theories of the transition between helical form I (cis peptide bond) and helical form II (trans peptide bond) of poly(-L-proline), which is a typical order in equilibrium order transition, have been presented by Schwarz (using the parameters s, sigma, beta', and beta' in a 2 X 2 matrix formulation) and by the present authors (using the parameters s, sigma betaC, and betaN in a 4 X 4 matrix formulation). A molecular theory of the same transition has been formulated to account for the phenomenological parameters. The statistical weights of regular helical sequences with and without junctions between the two forms were computed from empirical potential energy functions. Two puckering conformations of the pyrrolidine ring, i.e., with the Cgamma atom down and up, were allowed, and the free energy was computed for chains with four types of puckering, viz., regular down, regular up, random A, and random B, in the latter two of which the up and down puckerings were randomly distributed. The random A and random B chains have higher energy than those with regular down or up puckering, in both forms I and II. From both an energetical and a free energetical point of view, form I is more stable than form II under vacuum at room temperature. The dependence of the relative stabilities of form I and form II under vacuum on chain length was examined from both an energy and free energy point of view. The four parameters, s, sigma, beta', and beta', which describe the transitions in Schwarz's theory, were calculated from the statistical weights of various types of sequences. It was found that the thermally induced transition between form I and II under vacuum occurs with the pyrrolidine rings remaining in the down conformation. The calculated values of s suggest that form I is more stable than form II in the regular down chain, while form II is more stable than form I in the regular up chain under vacuum at room temperature. The calculated values of sigma for regular down and regular up pyrrolidine ring puckering are in good agreement with experimental observations, whereas those for random A and random B puckering are much smaller than the experimental values. A theory for the effect of solvent on the parameters s, sigma, beta', and beta' (at constant temperature) is developed, and the computations involving solvent effects are described in the next paper.     

Interpolymer complex formation between helical poly(L-proline) and random-coil poly(methacrylic acid) through hydrogen bonding

Interpolymer complex formation between poly(L -proline) (PLP) with helical structure and poly(methacrylic acid) (PMAA) with random-coil structure through hydrogen bonding in aqueous medium has been studied by several experimental techniques, e.g., viscometry, turbidimetry, potentiometry, conductometry, scanning electron microscopy, and x-ray diffraction methods. The decreases in reduced viscosity of the solution on addition of an increasing quantity of PLP to a constant amount of PMAA reveals the formation of a complex between PLP and PMAA. The minimum in reduced viscosity at a unit-mole ratio [PLP]/[PMAA] = 1.0 suggests a 1 : 1 complex formation. A distinct change in the curves for turbidity, pH, and conductance versus [PLP]/[PMAA] supports this conclusion. A scanning electron micrograph for the 1 : 1 PLP–PMAA complexes shows that the PLP/PMAA complex has the shape of entangled long fibers. An x-ray diffraction pattern for the PLP/PMAA complexes gives no diffraction patterns which appear in pure PLP, indicating the destruction of the helical structure of PLP due to the interpolymer complexation. Mixtures of PMAA with poly(γ-hydroxy-L -proline) (PHLP) which has a similar conformation as PLP, but involves intra- or intermolecular hydrogen bonds, has also been investigated by vicometry measurements. The reduced viscosity of a solution of the mixed polymers increases with increasing [PHLP], indicating no complex formation. All the results reveal that the magnitude and the nature of the forces acting in the polymers play an important role in interpolymer complexation.     

Polymerization and conformational studies of poly(trans-3-ethyl-D and L-proline)

Poly(L -trans-3-ethylproline), L -PT3EP, and poly(D -trans-3-ethylproline), D -PT3EP, were prepared by ring opening polymerization of the corresponding N-carboxyanhydrides (NCA) using triethylamine as an initiator. Using circular dichroism spectroscopy, it was shown that the incorporation of an ethyl group at the 3 position of the pyrrolidine ring caused a noticeable change in the conformational behavior of the polymer in solution. The ethyl group limited to some extent rotation of the polymer chain around the C_? ? CO bond and prevented the mutarotation between the two forms found in poly-L -proline polymers.     

The observation of cis residues in poly(L-proline) in aqueous solution.

Proton Fourier transform NMR spectra at 270 MHz were obtained for three different molecular weight samples of poly(L-proline) in D2O. A weak, but clearly discernable, resonance at 4.3 ppm was observed in each case with an integrated intensity about 2-3% of the Calpha trans proton resonance. Based on the Torchia-Bovey assignment this resonance is attributed to the Calpha cis proton. The presence of cis residues, even in this small concentration, necessitates a reexamination of the conformational properties of this polymer. Definite conclusions cannot be reached from spectra obtained in organic solvents because of the much smaller separation between the cis and trans resonances.     

Monte Carlo calculations on polypeptide chains. VIII. Distribution functions for the end-to-end distance and radius of gyration for hard-sphere models of randomly coiling poly(glycine) and poly(L-alanine).

A Monte Carlo study of the distribution functions for the end-to-end distance and radius of gyration for hard-sphere models of poly(glycine) and poly(L-alanine) random coils has been conducted in the chain-length range n = 3 to 100 monomer units for both unperturbed chains and chains perturbed by long-range interactions (excluded volume effects). The distribution functions for the radius of gyration in all cases have been very precisely calculated, those for the perturbed end-to-end distance less precisely, and those for the unperturbed end-to-end distance least precisely. Empirical distribution functions of the form W(p) = ap-b exp(-cp-d) for the reduced end-to-end distance p = r/"r-2"-one-half and a similar form for the reduced radius of gyration could be least-squares fit to the Monte Carlo data. The expansion factors alpha-r and alpha-s were calculated vs. chain length and were used to test various versions of the two-parameter theory of the excluded volume effect. To be consistent with the chain-length dependence of alpha-r and alpha-s as determined by the Monte Carlo calculations, each of these theories required two different binary cluster integrals, a beta-r based on alpha-r and a beta-s based on alpha-s, both of which were strongly chain-length dependent. Both of these results suggest that the two-parameter theory is not applicable to the models used in this study. It was also found that, except for very short chain lengths, plots of ln alphs-r vs. ln n were linear, and thus that alpha-r could be estimated for long chain lengths. Comparison of these estimates with the experimental data on four polypeptide chains in one-earth solvents that the hard-sphere models used in this study yield expansion factors that do not seriously overestimate the magnitude of the excluded volume effect.     

Simulations on the reinforcement of poly(dimethylsiloxane) elastomers by randomly distributed filler particles

Monte Carlo computer simulations were carried out on filled networks of poly(dimethylsiloxane) (PDMS), which were modeled as composites of crosslinked chains and randomly arranged spherical filler particles. The primary concern of the investigation was the effect of the excluded volume of these particles on the elastomeric properties of the polymers. Calculations were carried out for PDMS chains with different molecular masses between crosslinks, and for filler particles with different sizes and at various volume percentages. Distributions of end-to-end vectors for both unfilled and filled networks were obtained using Monte Carlo simulations based on rotational isomeric state (RIS) theory. More extended configurations, with a higher end-to-end distance, were observed for networks filled with smaller particles. The nominal stress f* and the modulus or reduced nominal stress [f*] were calculated from the distributions of end-to-end vectors using the Mark-Curro approach. Relatively small filler particles were found to increase the non-Gaussian behavior and to increase the normalized moduli above the reference value of unity. Temperature effects on the stress were also investigated. © 1996 John Wiley & Sons, Inc.     

Estimation of the characteristic ratio of poly[2-(triphenylmethoxy)ethyl methacrylate] from hydrodynamic properties of its dilute solutions

Properties of poly[2-(triphenylmethoxy)ethyl methacrylate] (PTEMA) molecules in solutions (under O-conditions and in “good” solvents) were investigated by the light scattering, sedimentation, viscometric and by phase equilibria methods. The O-temperature (47°) was determined for the system mesitylene-PTEMA. Unusual correlations of hydrodynamic data with molecular weight are discussed and slight branching of polymer chains is accepted as a probable explanation. Data for mesitylene solutions under O-conditions are used to estimate the characteristic ratio of PTEMA molecules ($\text{R}{}_0{}^2\text{/nl}{}^2\text{= 19}$).     

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     

Synthesis and conformation of poly(L-azetidine-2-carboxylic acid-L-proline) and poly([L-proline]3-L-azetidine-2-carboxylic acid)

The synthesis, characterization, and conformational assessment of poL y(L -Aze-L -Pro) and poly[(L -Pro)₃-L -Aze] are reported. The polymers were prepared by using the pentachlorophenol active ester as the polymerizable tetrapeptide derivatives. The copolymer, poly(L -Aze-L -Pro), assumes a Form II helix in polar solvents, and is converted into a form I-like helix at a critical solvent composition of ethanol to trifluoroethanol. The CD spectrum of this Form I-like conformation of poly(L -Aze-L -Pro) is similar to that of poly(trans-5-isopropyl-L -proline), indicating that the rigid four-membered ring at the alternating position can lock in the structure by a mechanism similar to that of a bulky substituent at the trans-5-position of proline. The helix conformation of this copolymer was unfolded in a 0.2M CaCl₂ aqueous solution. In contrast to poly(L -Aze-L -Pro), the copolymer of poly[(L -Pro)₃-L -Aze] contains both cis and trans peptide bond geometry when dissolved in a 90:10 ETOH-H₂O mixture. The conversion of the mixed conformation of poly[(L -Pro)₃-L -Aze)] into a polyproline Form II-like structure occurred in highly polar solvent environments such as water.     

Conformational characteristics of poly(vinylpyrrolidone). solvent-dependence of the chain dimensions

Conformational energies as function of rotational angles over two consecutive skeletal bonds for both meso and racemic diads of poly(Nvinyl-2-pyrrolidone) have been computed. The results of these calculations were used to formulate a statistical model that was then employed to calculate the unperturbed dimensions of this polymer. The conformational energies are sensitive to the Coulombic interactions, which are governed by the dielectric constant, of the solvent, and to the size of the solvent molecules. Consequently, the calculated values of the polymeric chain dimensions are strongly dependent on the nature of the solvent, as it was experimentally found before.