Induced circular dichroism of stereoregular vinyl polymers

Stereoregular vinyl polymers, poly(2-vinyl pyridine)s (P2VPs), were synthesized to examine the tacticity effect on the induced circular dichroism (ICD) via association with chiral acids. The ICD was found to be strongly dependent on the isotacticity of the P2VPs and the acidity of chiral acid in addition to its bulkiness.     

Oligomers of stereoregular polymers

The embryonic state of the ionic methyl methacrylate (MMA) and of trihaloacetaldehyde polymerization was studied. The composition of the oligomer mixtures, the end groups and the stereochemistry of individual compounds of the oligomers was investigated. Depending upon the initiating systems the oligomerization of methyl methacrylate can give either isotactic or syndiotactic oligomers. Trihaloacetaldehye oligomerization is highly dependent on the ceiling temperature of polymerization. The oligomerization of fluoral and chloral is compared with a more detailed study of the bromal polymerization and even more interestingly with the cooligomerization of chloral and bromal.     

Structural models for some crystalline and liquid crystalline polymers

Structural models for the crystalline and liquid crystalline smectic phase are presented for polymethylsiloxanes with mesogenic side groups and for some crystalline and lyotropic liquid crystalline cholesteric phases of cellulose derivatives. Optical and X-ray methods have been applied in these investigations.     

Photoinduced optical anisotropy in films of photochromic liquid crystalline polymers

The light-induced modification of the optical properties of photochromic liquid crystalline (side-group) polymers (LCPs) containing azobenzene moieties was studied. Films of such polymers were irradiated with unpolarized and linearly polarized light. Unpolarized irradiation results in a modification of the order parameter, whereas the director orientation remains constant. The light-induced disturbance of the supramolecular order is strongly dependent on the structure of the polymer. A correlation with the enthalpic stability of the liquid crystalline phases is given. Linearly polarized irradiation causes a modification of the order parameter and a reorientation of the side-groups towards a direction perpendicular to the electric vector of the actinic light. This reorientation process is caused by an angular-dependent photoselection within the steady state of the photoisomerization of the azobenzene units. The amount and kinetic of the reorientation differ considerably as a function of certain structural features of the polymers, such as the content of photochromic moieties and the lengths of the spacer chains between the polymeric backbone and the rod-like moieties. It is shown that under certain circumstances the angular-dependent photoselection process may cause a reorientation of the non-photochromic moieties by a cooperative process. The result is a light-induced rotation of the optical axis of the LCP and thus an efficient modification of the birefringent and dichroic properties. This effect can be used in optical data storage.     

Time-dependent nonlinear optical properties of pyroelectric liquid crystalline polymers

Highly oriented pyroelectric liquid-crystalline polymers were prepared by photopolymerization under the influence of a static electric field from binary mixtures of two acrylate monomers exhibiting chiral smectic C mesomorphism. Both monomers contained nitro groups to yield second order nonlinear optical properties (second harmonic generation) and one of the monomers had two functional groups to yield a crosslinked polymer. The room temperature second order nonlinear susceptibility of the polymers showed during the first two hours a 10 % decrease after which it remained constant during the next 48 days. At elevated temperatures there was a significant difference in the nonlinear optical properties over time between crosslinked and uncrosslinked polymers. The uncrosslinked polymer showed a pronounced loss of second order nonlinear optical activity with time at ≥38°C. The crosslinked polymer showed a much smaller and basically a temperature independent decrease rate in the second order nonlinear optical properties at all the ageing temperatures (23-130°C). Both the loss in mesogen order parameter, very evident for the uncrosslinked polymer, and conformational changes occurring within the mesogens (β mechanism), may account for the observations made.     

X-ray diffraction studies on liquid crystalline order in vinyl polymers with mesogenic side-groups

X-ray studies on some vinyl polymers with azobenzene sidegroups were performed. A model of layered structure rearrangement resulting from the substitution of the methacrylate main chain for the acrylate is proposed. It has been shown that the perfection of a layered structure is reduced with shortening of the flexible tail of the mesogenic group. Improvement of LC order with increase of temperature was revealed.     

The impact of the discovery of stereoregular polymers in macromolecular science

The paper recalls some aspects of the fascinating history of the discovery by Giulio Natta and his coworkers of the stereospecific polymerization of α-olefins and of the determination of the structure of synthetic stereoregular polymers. The most relevant recent advances in the comprehension of the mechanisms regulating their synthesis with heterogeneous supported catalysts and with the newly discovered homogeneous metallocene-based catalysts are briefly outlined.     

Synthesis and electro-optical properties of some ferroelectric liquid crystalline polymers

The synthesis and characterization of several series of ferroelectric side-chain polymers were presented. Differential scanning calorimetry, optical polarizing microscopy and X-ray diffraction measurement were used to determine their thermal transitions and analyze their anisotropic textures. The influence of polymer backbones, flexible spacers, mesogenic cores as well as chiral end groups on the mesomorphic properties of the polymers was discussed. Furthermore, the dielectric relaxation properties of the obtained polymers were also evaluated.     

Energetic calculations of the chain conformation of isotactic polymers of olefins in the crystalline state

Energetic calculations have been carried out on a correlated series of isotactic aliphatic polymers (polypropylene (PP), poly-α-butene (PB), poly-3-methylbutene (P3MB), poly-(S)-3-methylpentene-1 (P(S)3MP) and isotactic polystyrene (PS). The possible variation of the CCC bond angles and of all the internal rotation angles (no group of atoms being taken as a unit) was considered.The possibility to predict the experimental helical parameters without any previous assumption is discussed. The differences in the shape of the minima for polymers with aliphatic and aromatic branched chains have been critically evaluated.For the P(S)3MP the results have shown how the asymmetric configuration of the side group influences the chain conformation of the polymer.     

The role of molecular mechanics in the prediction of the chain conformation of polymers in the crystalline state: Syndiotactic polymers

Molecular mechanics methods have been used in order to find the conformations of various syndiotactic polymers in crystals. Three different classes of polymers have been examined: i) polyolefins, such as poly(propylene), polystyrene, poly(1‐butene) and poly(1,2‐butadiene); ii) polydienes, such as cis‐1,4‐poly(1,3‐pentadiene); iii) alternating copolymers of carbon monoxide with styrene or styrene derivatives. The presence of conformational polymorphism in some of the studied polymers is predicted and explained by maps and minimizations of the conformational energy. The calculated internal parameters and chain axis repeats of all the considered polymers result in very good agreement with X‐ray experimental data reported in literature. The role of intramolecular nonbonded interactions in determining the conformations of the polymer chains is thoroughly discussed.     

Linear polymers of some vinyl monomers containing a terminal acetylenic group

The synthesis and polymerization of representative acrylic-type esters containing a terminal acetylene group, CH₂?C(R)COO(CHR′)_m? C?CH, where R and R′ are H and CH₃ and m = 1 or 2, by anionic initiation to linear polymers are described. In contrast, crosslinked polymers were formed when radical and cationic initiators were used. Crosslinked polymers were also obtained with organolithium compounds but not with sodium naphthalene and sodium benzalaniline; this observation is discussed and compared to the behavior of the acetylenic acrylic esters which do not contain a terminal acetylenic hydrogen. The unpolymerized acetylenic bonds in the resulting linear polymers were shown to be present by infrared spectroscopic methods and by the following post-reactions of these bonds: (1) the heat- and radical-initiated crosslinking of the polymers through the acetylenic bonds; (2) the post-bromination of the acetylenic bonds; and (3) the reaction of decaborane with the acetylenic bonds. The anionic copolymerization of acrylonitrile and styrene with these acetylenic monomers were performed and compared to the copolymerizations with 1-acryloxy-2-butyne and 1-methacryloxy-2-butyne. Dibromination of the linear polymers affords self-extinguishing polymers, while decaboronation yields soluble polymers which do not soften up to 300°C. The linear polymers may be classified as “self-reactive” polymers which yield thermosetting polymers.     

Computation of the electronic density of states distributions of stereoregular polymers

A new algorithm for the computation of the density of states distributions of stereoregular polymers is presented. It is related to the Gilat–Raubenheimer technique for three dimensional systems. Its application to polyethylene is briefly described.     

Electron density and related properties in stereoregular polymers and biopolymers

Formulas fully exploiting the periodicity symmetry of the electron density of polymers are deduced. They are shown to lead to efficient algorithms for the evaluation of Coulomb and exchange interactions in those systems.     

Synthesis and optical properties of some novel arylene-alkynylene polymers

The synthesis and the optical properties of the novel aryleneethynylenes ( 7 , 8 ) and arylenebutadiynylenes ( 10 , 11 ) using either a Heck-type or a Hay-type coupling are reported. The route reported here also makes meta-linked arylene-alkynylenes accessible. The number-average degree of polymerization of the obtained polymers is between 13 and 60. Some of the materials show promising optical properties (photoluminescence).