Showing papers in "Macromolecular Symposia in 1993"

Supramolecular chemistry — Molecular information and the design of supramolecular materials

Abstract: Molecular information expressed through molecular recognition events provides means for directing the spontaneous formation of supramolecular species from complementary components. It may allow the design and engineering of supramolecular materials, in particular of liquid crystalline and of polymeric nature. Thus, supramolecular mesophases have been obtained from molecular recognition-induced association of suitable subunits. The self-assembly of complementary ditopic components generates liquid crystalline “polymers” of supramolecular nature; it takes place by a progressive growth revealed by electron microscopy: from nuclei, to filaments, to tree-like species, to strings and fibers that present helicity induced by the chirality of the subunits. A rich variety of structures and properties may be expected to result from the blending of supramolecular chemistry with polymer chemistry and materials science.

Ring‐opening oligomerization reactions using aluminium complexes of schiff's bases as initiators

Abstract: Bifunctional telechelics with defined structure can be prepared by oligomerization of oxiranes, β-butyrolactone and L-lactide using aluminium Schiff's base complexes as initiators. Chiral initiator (SALCENAlCl) shows a stereoelective character leading to preferential oligomerization of one enantiomer from a racemic monomer mixture. The reaction with β-butyrolactone proceeds through O-alkyl cleavage. Alkoxy Schiff's bases aluminium complexes are used for oligomerization of L-lactide. All the prepared oligomers were fully characterized by IR, elemental analysis, 1H and 13C NMR and GPC.

Ti complex catalysts including thiobisphenoxy group as a ligand for olefin polymerization

Abstract: Titanium complexes were prepared by the reaction of 2,2′-thiobis(6-tert-butyl-4-methylphenol) (TBP) with TiCl4 or Ti(OPri)4. These complexes in combination with methyalumoxane as cocatalyst are highly active towards ethylene and propene, giving polymers having high molecular weights. The polymerization activities for ethylene and propene are comparable to those of Cp2ZrCl2-MAO catalyst. Polypropylene obtained had extremely high molecular weight (Mw>6 million) and low regioregularity (30% of head-to-head and tail-to-tail linkages). Highly syndiotactic polystyrene was obtained with these catalysts with activity up to 27 kg polymer per g Ti and hour. Copolymerization of styrene with ethylene gave highly alternating copolymer with isotactic styrene units. These catalysts are also active toward both conjugated and nonconjugated dienes such as butadiene and 1,5-hexadiene. Polybutadiene had mainly cis-1,4-structure (98%). The structure of poly(1,5-hexadiene) is rather complicated, which is quite different from that prepared with heterogeneous TiCl3 catalysts.

Polymer blends and alloys

Abstract: The state of thermodynamic miscibility of a polymer blend and its resulting phase behavior and mechanical properties is largely determined by the balance between enthalpic and entropic contributions to the free energy of mixing. Recent applications of this thermodynamic consideration are reviewed for a variety of binary and ternary blends of homopolymers and copolymers. Recent advances in predicting polymer blend miscibility and Lower Critical Solution Temperature behavior from liquid heats of mixing data, through use of a modified Guggenheim quasichemical model combined with the Sanchez lattice fluid model are discussed. Qualitative application of partial miscibility ideas to interfacial adhesion in multiphase polymer blends is also presented.

Exchange and free radical grafting reactions in reactive extrusion

Abstract: The potential of two types of reactions for reactive extrusion was evaluated in our laboratory: exchange reactions of copolyesters and free radical grafting of monomers onto polyolefins. Specifically, the alcoholysis of ethylene and vinyl acetate copolymers, the transesterification of ethylene and alkyl acrylate copolymers (EAA) and the aminolysis of alkyl acrylate copolymers were investigated as the exchange reactions while the grafting of maleic anhydride (MA) onto polypropylene (PP) as the free radical reactions. The exchange reactions are very slow without catalysts. Tin derivatives, such as dibutyl tin dilaurate (DBTDL) and dibutyl tin oxide (DBTO), are good catalysts for the alcoholysis and the transesterification, but not for the aminolysis. For the latter, tautomeric compounds, such as 2-pyridone and 2-mercaptopyridone, are efficient. Comparative kinetic studies showed that, for all the three exchange reactions, the reacting medium (solution or the homogeneous melt) does not affect their reaction mechanisms nor their intrinsic rate constants. Additionally, mechanical mixing does not contribute to the overall reactions. As for the free radical grafting of MA, the presence of electron-donating monomers and styrene (ST) in particular was shown to be very effective in improving the grafting yield while minimising the degradation of PP. This was proven to be due to the formation of a charge transfer complex between MA and such a monomer.

Syntheses and models for stereospecific metallocenes

Abstract: Stereospecific, bridged [LL']M(R)+ catalysts (L, L' = substituted-cp, ind, or flu; R = hydrocarbyl) have been prepared by three new synthetic procedures. Propylene polymerizations with our new catalyst synthetic reagents, tris(pentafluorophenyl)boron (F15), magnesium dichloride, and trityl tetrakis(pentafluorophenyl)borate (trityl-F20) are compared with results obtained with the N, N-dimethyl-anilinium-F20 salt and with methylaluminoxane (MAO). Trimethylaluminium (TMA) or triethylaluminium(TEAL) scavenged poisons in the MAO free polymerizations. The site-control mechanism is discussed.

Donor‐ and acceptor‐modified metallocene‐based homogeneous Ziegler‐Natta catalysts

Abstract: The polymerization kinetics of propene polymerization using metallocene/methylaluminoxane (MAO) homogeneous catalysts have been investigated to explore the role of donor/acceptor interactions and to enhance the catalyst productivities. In the case of the non-stereospecific Cp2ZrCl2/MAO model system it has been demonstrated that, in addition to the well known irreversible deactivation, reversible deactivations, which are second order relative to the zirconium active site concentration, account for the decay of the polymerization rate. While MAO injection during polymerization enhances the polymerization rate, zirconocene addition deactivates the catalyst which can be reactivated by injecting additional MAO. A sequence of dynamic equilibria involving the formation of active cationic metallocene intermediates as well as inactive zirconocene species, e.g., zirconocene dimers, is proposed. Lewis base and Lewis acid additives have been added as probes to examine the role of such equilibria in the case of metallocene-based catalyst systems such as MAO-activated Cp2ZrCl2, racemic ethylenebisindenyl zirconium dichloride (EBIZrCl2), and racemic ethylenebis (4,5,6,7-tetrahydroindenyl) zirconium dichloride (EBTHIZrCl2). While the conventional donors such as 2,6-ditert.butyl-4-methylphenol (BHT) and 2,2,6,6-tetramethylpiperidine (TMP) reduce catalyst productivities, even at very low donor/Al molar ratios, increasing propene concentration and addition of trimethylboroxine (TMB) substantially enhance catalyst productivities and affect molecular weights of the polypropylene produced with metallocene/MAO catalysts.

Ethylene polymerization kinetics with heterogeneous Ziegler‐Natta catalysts

Abstract: Ethylene polymerization kinetics was examined using a supported Ti-based catalyst activated by AlEt3. A computer-based technique for deconvoluting the GPC curves into constituent Flory components was developed. The kinetic analysis showed that the catalyst contains five populations of active centers. Each population has different formation and deactivation rates, produces polymers of different molecular weights and is differently affected by hydrogen.

Recent advances in the neodymium catalysed polymerisation of 1,3-dienes

Abstract: The benefits of using a homogeneous neodymium-based catalyst for the industrial “high cis” polymerisation of 1,3-butadiene are underlined. Preformed homogeneous catalysts for the “high cis” polymerisation of 1,3-butadiene based on Nd(carboxylate)3/diisobutylaluminium hydride/tbutyl chloride have been examined. The effects of changing (a) the order of catalyst component addition, (b) the carboxylate component and (c) the halogen component, on catalyst homogeneity, activity and polymer characteristics have been examined.

Hemiisotactic poly(propylene) through propene polymerization with the ipr[3‐mecpflu]zrcl2/mao catalyst system: A kinetic and microstructural analysis

Abstract: Through polymerizing propene by means of the catalyst system iPr[3-MeCpFlu]ZrCl2/MAO it was possible for the first time to generate hemiisotactic poly(propylene); the microstructure of this polymer is characterized by an alternating sequence of monomer units with equal and statistical configuration. iPr[3-MeCpFlu]ZrCl2 is one of the new developed stereorigid ansa-metallocene catalysts which by means of the structure of the bridged η-ligand system are considered to give access to the description and calculation of the behaviour of the stereospecifity. In the present paper the α-olefin polymerization was investigated with the iPr[3-MeCpFlu]ZrCl2/MAO catalyst system by means of kinetics and product analytics; parameter were the [Al]:[Zr] ratio, the monomer concentration, the polymerization temperature, the polarity of the solvent as well as the chain length of the starting α-olefin. The order of the polymerization reaction concerning the propene concentration was found to be 1,3; this broken order indicates a complex reaction. Concerning the dependency of the polymerization rate on the [Al]:[Zr] ratio a bell-shaped curve was found; obviously, at very high [Al]:[Zr] ratios the complex MAO compound inhibits the active zirconocene centre. The microstructure of the poly(propylene)s generated in toluene at 25°C corresponds with the expected hemiisotactic structure. On the contrary, the microstructure of the poly(propylene)s generated in toluene at higher polymerization temperatures and in the polar solvents o-dichlorobenzene and methylene chloride at 25°C shows considerable deviations from a hemiisotactic structure. Under these polymerization conditions the stereospecifity of the iPr[3-MeCpFlu]ZrCl2/MAO catalyst can only be satisfyingly explained with isomerization reactions of the active zirconocene species between two insertion steps. These isomerization reactions without monomer coordination occur preferably starting from the unstereospecific state of the catalyst. This interpretation is confirmed by 1-butene and 1-hexene polymerization experiments.

The role of non‐coordinating anions in homogeneous olefin polymerization

Abstract: We present an overview of homogeneous olefin polymerization catalysts in which chain initiation and propagation involves a metallocenium ion (e.g. Cp2ZrMe+) that can be produced by equilibrium or irreversible CH3− transfer. The Kaminsky catalyst is an example of the former. Complexes of (C6F5)3B with water, alcohols, mercaptans, silanols and oximes are strong acids which can protonate dialkylmetallocenes to produce weakly-coordinated metallocenium salts that are hydrocarbon soluble and which produce high molecular weight poly-1-olefins. Polymer molecular weight is correlated with the steric and electronic effects of the ligands attached to the metal.

Crystallization kinetics in relation to polymer processing

Abstract: Phase distribution of quenched samples has been determined by a deconvolution procedure of WAXS spectra in a wide range of cooling rates. The informations collected together with isothermal and DSC results provide a very wide set of data on the crystallization kinetics of polymers relevant which covers conditions encountered in most polymer processing operations. They have been compared with predictions of a non-isothermal crystallization model assuming two independent and parallel crystallization processes competing during solidification.

Polylactones 27. Anionic polymerization of L‐lactide. Variation of endgroups and synthesis of block copolymers with poly(ethylene oxide)

Abstract: L-Lactide was polymerized in toluene with various alkoxide initiators. These initiators were prepared in situ from potassium t.-butoxide and primary or secondary alcohols such as tetradecanol, diethylene glycol butyl ether, menthol or testosteron. All these alcohols were incorporated as ester endgroups into the polylactide chain. However 1H-NMR spectroscopy proves the existence of more OH-endgroup than ester endgroups. This finding and 10–20% racemization observed for all anionic polymerizations suggest that chain-transfer reactions with the monomer via deprotonation take place. When poly(ethylene glycol) monomethyl ether in combination with KOtBu was used as initiator, twoblock copolymers were obtained. With poly(ethylene glycol) A-B-A-triblock copolymers could be synthesized. The quantitative reaction of the poly(ethylene glycol)s with L-lactide could be proven by both 1H NMR spectroscopy and gelpermeation chromatography. DSC-measurements show that depending on their lengths either the polylactide or the poly(ethylene oxide)blocks can crystallize. Due to partial racemization the melting temperatures (Tm) of the poly(L-lactide) blocks did not exceed 155°C.

Mechanistic aspects of intumescent fire retardant systems

Abstract: The mechanism of intumescence is studied in mixtures of ammonium polyphosphate (APP) with several polycondensates as a part of a systematic study of intumescent fire retardants. It is shown that APP reacts on heating with the polycondensate to form a precursor of the intumescent char which is obtained on further heating. The introduction of the intumescent system in polypropylene does not modify the structure of the char formed on heating. However, the polymer modifies the foaming behaviour of the system. The thermal decomposition of the char occurs with volatilisation of phosphorus moieties and formation of a relatively thermally stable residue. Effects of the thermal behaviour of the char, on fire retardance, are discussed.

Zirconocenium cation catalysis of propene polymerization

Abstract: rac-Ethylenebis(1-η5-indenyl)dimethylzirconium (1) was reacted with triphenylcarbenium tetrakis(pentafluorophenyl)borate (2) to produce in situ the rac-ethylenebis(indenyl)methylzirconium cation (3). This aluminium-free catalyst showed propene polymerization activity (A) and stereoselectivity which both increase with the decrease of polymerization temperature (Tp). At very low Tp, 3 behaved as a “single-site” catalyst. An efficient way to produce such cation is to react ansa-zirconocene dichloride with 2 in the presence of TEA (=triethylaluminium). A superior cationic catalyst was obtained from rac-dimethylsilylenebis(1-η5-indenyl)dichlorozirconium, 2, and TEA, which polymerizes propene at −20°C(−55°C) with activity of 2×109 (3×108) g polypropene per (mol Zr η mol C3H6 η h) to polypropenes which are 93.8% (99.4%) isotactic with melting temperature Tm = 152.6°C (159.9°C) and viscosity-average molecular weight Mv = 1.4×105 (2.2×105). The addition of methylaluminoxane lowers the A of the cationic catalyst especially at low Tp. Rigorously speaking, the cation derived from 1 or 3 behaves as a “single site” catalyst only at very low Tp. The use of TEA significantly and unexpectedly enhances the efficiency of the zirconocenium catalyst system.

Structure‐activity and structure‐selectivity correlations in metallocene‐based catalysts for α‐olefin polymerization

Abstract: Zirconocene complexes carrying substituents at various positions of their ring-bridged ligand frameworks were studied as catalysts for methylalumoxane-activated propene polymerization. Effects of different substitution patterns on catalyst activities and on chain lengths and stereo- and regio-regularities of the polymers produced are correlated with geometrical features of the complexes, e.g. with parameters describing their structural rigidity or the aperture and obliquity of their coordination gaps.

Interfacial interactions in particulate filled thermoplastics: Mechanism, strength, properties

Abstract: Using the results of stress analysis the factors influencing the micromechanical deformations in particulate filled composites were studied. It was shown that interfacial interactions influence slightly shear yielding and crazing, but have a significant effect on dewetting. Interactions can be modified by surface treatment. Types and mechanisms of surface treatment are discussed. A model describing the composition dependence of tensile properties makes possible the quantitative evaluation of interaction. Attention is called to the importance of composite structure; aggregation, orientation of anisotropic particles, nucleating effect of the filler, all can change the properties of the composite.

Mechanistic aspects of group transfer polymerization

Abstract: The experimental evidence supporting the involvement of enolate anions in group transfer polymerization(GTP) is reviewed. The results of silyl group exchange studies between living silyl ketene acetal-ended oligomers under typical GTP conditions are discussed. It is concluded that the observations of significant amounts of silyl group exchange in the presence of polymerizing monomer are not consistent with the originally proposed “associative mechanism” based on the GTP Criterion which precludes intermolecular silyl group exchange.

New mechanistic aspects and structure activity relationships in the allyl nickel complex catalysed butadiene polymerization

Abstract: The present state of knowledge of the mechanism of the allyl nickel complex catalysed stereoselective butadiene polymerization is outlined. On the basis of the reaction model, which has been worked out more thoroughly and comprehensively by our systematic experimental investigations during the last years, the thermodynamic and kinetic control of the catalytic activity and selectivity in dependence on the structure of the allylnickel complexes will be discussed. Finally, it will be shown how the structure of the technical nickel catalyst could be elucidated for the first time and how its catalytic properties can be explained in the framework of the derived structure-activity relationships.

Predictions from the trigger mechanism for Ziegler‐Natta polymerization of α‐olefins

Abstract: The trigger mechanism is a new concept for understanding the polymerization of α-olefins with Ziegler-Natta catalysts. The mechanism is based on the interaction of two monomers in the transition state, where an incoming monomer triggers the insertion of a complexated monomer. The monomer site is never a free site, and is thus protected from attack by Lewis bases in the reaction medium. The stereochemical discrimination is due to the interaction of the two monomers. This interaction determines the orientation of the incoming monomer, and the stereochemistry of the insertion is given when the monomer is complexated. The model predicts a reaction rate order vs. monomer concentration between 1.0 and 2.0, a reduced overall polymerization rate above a given temperature, very complex kinetics for copolymerization, a lower stereochemical discrimination for the first inserted monomer, and a high stereochemical discrimination.

Termination processes in group transfer polymerization

Abstract: Two major termination processes in the GTP of methacrylates and acrylates have been studied. For methacrylates, intramolecular cyclization is the dominant, if not sole, termination reaction. Both cyclization and O/C-silyl isomerization of the chain end are observed for acrylates. Oligomerization experiments were used to study the relative rates of cyclization and propagation and the influence of temperature, catalyst and dp. Under typical GTP conditions for methacrylates, cyclization is at least two orders of magnitude slower than monomer addition.

What is a ‘gel’?

Abstract: Some of the existing definitions of the term ‘gel’ are discussed and shortcomings are identified. A purely phenomenological definition is proposed: a gel is a soft, solid or solid-like material consisting of two or more components one of which is a liquid, present in substantial quantity. ‘Solid-like gels’ are further defined in terms of the dynamic mechanical properties.

Design and preparation of novel particulate and continuous polymeric macroporous media for the separation of biological and synthetic molecules

Abstract: Novel approaches to macroporous polymeric separation media with enhanced pore size distributions, controlled surface chemistries, and excellent separation properties have been explored. In the first of these approaches, size-monodisperse spherical polymeric particles containing both large and small pores are prepared with the use of a variety of polymeric porogens. The choice of polymeric porogen as well as its concentration greatly affects the pore size and pore size distribution of the final beads. The polymer beads obtained by this technique have been tested in size exclusion chromatography and in the separation of proteins in reversed-phase mode. Techniques that allow the specific functionalization of pores according to their sizes have been developed. Thus, the surfaces of the large pores of the monodisperse particles can be made hydrophilic or chiral while the smaller pores remain hydrophobic or achiral. Similarly, a novel polymeric separation medium based on a continuous polymer rod incorporating macropores has been prepared by an in situ polymerization within the confines of the tube of a chromatographic column. For example, non-particulate continuous polymer rods with flow-through pores as well as separation pores have been prepared from styrene and divinylbenzene and used in the very fast (30 s) separation of proteins by reversed-phase chromatography. Alternatively, continuous rods prepared from glycidyl methacrylate and ethylene dimethacrylate can be transformed into materials that are suitable for ion exchange or other chromatographic modes. A characteristic of the continuous polymeric rod media is their permeability which allows the mobile phase to flow through the medium without causing a high back-pressure. The continuous rod-shaped media can also be modified in pore size-selective fashion to introduce different chemistries in the large and the small pores. These new media are advantageous as they are extremely easy to prepare, durable, require no packing, and can be used for a variety of separations.

Nitrogen based flame retardants for nitrogen containing polymers

Abstract: The future aspects of Nitrogen based flame retardants in Nitrogen containing polymers are promising, as they contain no other chemical element than already present in the polymers. Sucessful applications are: * Melamine in Combustion Modified High Resilient Foam * Melamine in B-1 rigid foam * Melamine Cyanurate in Polyamides * Melamine and Melamine salts in intumescent paints * Melamine Cyanurate in thermoplastic Polyurethanes.

Novel synthesis of high molecular weight polymers with extremely low polydispersity by the unique initiation properties of organo‐lanthanide complexes

Abstract: Organolanthanide(III) complexes such as |(C5Me55)2SmH|2 and (C5Me5)2SmMe(THF) were found to initiate the living polymerizations of methyl methacrylate (MMA) to give high molecular weight polymers (Mn > 500 × 103) with extremely low polydispersity (Mw/Mn = 1.04). The syndiotacticity increased up to 95.2% by lowering the temperature to −95°C. The molecular structure of the 1:2 adduct of (C5Me5)2SmH with MMA determined by X-ray method indicates that this intermediate assumes the 8 membered ring conformation where the Sm atom is bound to MMA in an enolate form and the ester of penultimate MMA is coordinated to the metal. Based upon these results, an anionic coordination mechanism has been proposed for the present reaction. Organolanthanide(II) complexes also exhibit high activity and proceed the living polymerizations. Organolanthanide(III) complexes also initiate the living polymerizations of lactones such as ϵ-caprolactone and δ-valerolactone. The stoichiometric reactions indicate that real active species assumes the alkoxylanthanide(III) form.

Recent developments in the field of star-shaped polymers

Abstract: The interest for star-shaped polymers arises from their unique structure and enhanced segment density. This paper will review the different methods of preparation for star-shaped polymers. Two cases of special interest will be considered in greater detail: the core-first method which gives access to end-functionalized star-shaped polymers and the three-step method which leads to the so called hetero-star copolymers with rather specific properties.

Thermodynamics and kinetics of ring‐opening polymerization of cyclic alkylene phosphates

Abstract: Polymerization of cyclic compounds containing phosphorus has been used to prepare models of chains of nucleic acids and teichoic acids. Polymers of high molecular weight (Mn≥105) have been prepared in the past in this laboratory by ring-opening polymerization. These processes have a number of particular features, related to the low ring strain of the involved monomers (the majority of six-membered esters are strainless) and to the presence of the labile exocyclic groups. Results of the studies of these polymerizations contributed to the understanding of general phenomena of ring-opening polymerizations related to these features. In this review paper the thermodynamics of strainless rings is discussed. It is shown that for the same monomer, polymerization can either be exothermic (δHSS 0, δSSS>0). The presence of the labile exocyclic groups introduce a potential possibility of chain transfer. It is shown that in the cyclic, non-strained six-membered monomers kP/ktr is relatively low for cyclic triesters but much higher (≈300) in the polymerization of cyclic amides, since the alcoholate is a better leaving group then the amide group. The importance of these findings for other classes of monomers (cyclic amines, siloxanes, orthoesters) having also labile exocyclic groups is discussed.

Influence of the chemorheology of toughened epoxy matrices on the processing behavior of high performance composites

Abstract: The rheological and kinetic behavior of a new commercial toughened epoxy matrix is characterized through a complete experimental program. The kinetic behavior of the thermosetting resin is described by applying a new empirical model, accounting for autocatalytic and difussion control effects. The rheological model uses the kinetic information and describes the matrix viscosity as a function of temperature and degree of reaction using only three fitting parameters. The complete model is verified through calorimetric and rheological tests performed at different temperatures (isotherms) or heating rates (dynamic tests); or under more complex conditions simulating the processing conditions during autoclave lamination. Finally the chemo-rheological model is considered as a part of a master model describing the complete processing behavior of thermoset matrix composites. The use of the master model in control and optimization algorithms is also illustrated.

Invariant values of kinetic parameters — Evaluation of fire retardancy application to the PP‐APP/PER system

Abstract: A method allowing the determination of the formulation leading to the best fire retardancy properties is presented in the case of intumescent systems. The method consists in the determination of invariant physicochemical parameters deduced from TG analysis: invariant activation energy and invariant preexponentiel factor. The relation between the values given by an evaluation test (LOI) and the invariant activation energy is then discussed.

Fast switching ferroelectric liquid-crystalline polymers

Abstract: The principles of ferroelectricity, especially in liquid crystals (FLC) and polymers, are briefly presented. In a liquid-crystalline side chain polyacrylate electro-optical switching with response times of 200–400 μs was measured. Besides the ferroelectric switching an even shorter electroclinic switching process was detected. The influence of the spacer length and the molecular weight on the ferroelectric properties is discussed. The kind and number of chiral centers and their location in the side group strongly affects the electrooptical properties. In one polymer a change of the optical contrast caused by a sign reversal of the spontaneous polarization was found. In another FLC polymer three switching states (antiferroelectric behaviour) were detected. With coloured and fluorescent FLC polymers, obtained by mixing or by copolymerization new promising applications in electrooptical devices are expected.