Chain Microstructure Research Articles

Simultaneous Deconvolution of MWD and CCD of Ethylene/1‐Olefin Copolymers Using Genetic Algorithm

SummaryMultiple‐site‐type catalytic systems produce ethylene/1‐olefin copolymers with broad molecular weight distribution (MWD) and chemical composition distribution (CCD) because each active site type produces chains with distinct chain microstructures. Simultaneous deconvolution of the MWD and CCD can be used to identify the number of active site types and chain microstructures produced on each active site type by performing parameter estimation to minimize the sum of the squares of differences between experimental and model data. However, conventional optimization algorithms often rely on the adequate initial guess. In this work, a genetic algorithm, which is a stochastic optimization search heuristic that mimics the natural evolution process, was implemented during the simultaneous deconvolution. The proposed approach was validated with simulated data of model ethylene/1‐butene copolymers produced using a system with three active site types.

Modulation of the Microstructure of Polyethylene from Ethylene Polymerization Mediated by α-Diimine Ni Complex/MAO via Chain Transfer to ZnEt2 and Temperature

Temperature and chain transfer agents (CTA) are two primary factors that influence the microstructure of the resultant polymer. This contribution mainly studied the effects of polymerization temperature and concentration of diethyl zinc (ZnEt2) on ethylene polymerization and chain microstructure of resultant polyethylene (PE). The polymerization temperature showed a notable influence on the catalytic activity and the molecular weight of PE. It was observed that raising polymerization temperature from 4°C to 36°C resulted in a gradual decrease of the catalytic activity and a dramatic reduction of the molecular weight. The molecular weight of the prepared PE also decreased regularly with the increase of the concentration of ZnEt2 while the polydispersity did not broaden. The branching structure of PE prepared in the presence of varying concentrations of ZnEt2 was investigated via 13C-NMR spectroscopy. The addition of ZnEt2 had a moderate influence on the total branching degree and the branching distribution of PE. A possible mechanism is proposed for the influence of ZnEt2 on the microstructure of resultant PE.

Combination of TREF, high-temperature HPLC, FTIR and HPer DSC for the comprehensive analysis of complex polypropylene copolymers

A novel, powerful analytical technique, preparative temperature rising elution fractionation (prep TREF)/high-temperature (HT)-HPLC/Fourier transform infrared spectroscopy (FTIR)/high-performance differential scanning calorimetry (HPer DSC)), has been introduced to study the correlation between the polymer chain microstructure and the thermal behaviour of various components in a complex impact polypropylene copolymer (IPC). For the comprehensive analysis of this complex material, in a first step, prep TREF is used to produce less complex but still heterogeneous fractions. These chemically heterogeneous fractions are completely separated by using a highly selective chromatographic separation method--high-temperature solvent gradient HPLC. The detailed structural and thermal analysis of the HPLC fractions was conducted by offline coupling of HT-HPLC with FTIR spectroscopy and a novel DSC method--HPer DSC. Three chemically different components were identified in the mid-elution temperature TREF fractions. For the first component, identified as isotactic polypropylene homopolymer by FTIR, the macromolecular chain length is found to be an important factor affecting the melting and crystallisation behaviour. The second component relates to ethylene-propylene copolymer molecules with varying ethylene monomer distributions and propylene tacticity distributions. For the polyethylene component (last eluting component in all semi-crystalline TREF fractions), it was found that branching produced defects in the long crystallisable ethylene sequences that affected the thermal properties. The different species exhibit distinctively different melting and crystallisation behaviour, as documented by HPer DSC. Using this novel approach of hyphenated techniques, the chain structure and melting and crystallisation behaviour of different components in a complex copolymer were investigated systematically.

Effects of repeat unit sequence distribution and residual catalyst on thermal degradation of poly(l-lactide/ε-caprolactone) statistical copolymers

To evaluate the effects of residual catalyst and chain microstructural features on the thermal decomposition of poly(l-lactide/ε-caprolactone) (PLCL), poly(l-lactide) (PLLA) and poly(ε-caprolactone) (PCL) homopolymers and their copolymers were synthesized by ring opening polymerization (ROP) using stannous octoate (SnOct2) or bismuth (III) subsalicylate (BiSS) as catalyst. Thermogravimetric analysis (TGA) confirmed that ε-caprolactone repeat units are more thermally resistant than lactide units, both by comparison of the two homopolymers behavior and also by a detailed thermogravimetric analysis of PLCL copolymers of well resolved chain microstructures. It was noticed a large influence of metal-catalyzed depolymerization in all polymers, yet more significant for tin. In this regard the homopolymers having a higher residual metal content displayed onsets of thermal degradation at lower temperature values, e.g. 233 °C and 287 °C for PLLA and PCL containing ∼4500 ppm of Sn vs. 271 °C and 345 °C for PLLA and PCL containing ∼50 ppm. The type and content of the catalysts were revealed responsible of the final distribution of sequences obtained during the PLCL copolymers synthesis. 1H NMR spectroscopy revealed a higher randomness character (R→1) and shorter lactide average sequence lengths (lLA) for PLCL copolymers when they were BiSS-initiated; these copolymers degraded homogeneously almost entirely in a single stage and presented higher resistance to thermal degradation. In contrast, the more blocky PLCLs, containing >250 ppm of residual Sn, were found more sensitive to thermal degradation and degraded heterogeneously presenting two different peaks in the Differential TG curve. The 1H NMR analysis of a blocky PLCL sample (66% of LA, R∼0.5 and ∼500 ppm of Sn) further demonstrated that LA-rich sequences degrade at lower temperatures and, at a later stage, those of CL. It was also proved that the mechanism of this first stage of weight loss, occurring at lower temperatures, is highly influenced by the lLA, since increased rates of weight loss and heavier weight losses were associated to higher lLA values.

Unravelling the contribution of chain microstructure in the mechanism of the syndiotactic polypropylene pyrolysis

The pyrolysis of a highly syndiotactic and high-molecular weight polypropylene (sPP) has been thoroughly studied through preparative TGA experiments, which were performed up to final temperatures between 262 °C and 427 °C. The 13C NMR characterization of the samples shows a two-stage process clearly evidenced by changes in the chain microstructure. In a first period, up to a 22% weight loss, the molecular weight collapses and only vinylidene end-capped chains are produced. In a second stage, over 22% weight loss, a wider variety of groups, including both terminal vinylen and isobutenyl functions as well as n-propyl tails, are also detected. Regardless the type, all these new species are formed at the expense of propylene units belonging to rrrr pentads.The evolution of the Eact predicted by means of analytical TGA allows us to associate the lowest Eact value found with the changes taking place during the very beginning of the process (up to a 3% weight loss), i.e. the molecular weight collapse and a some evidence of configurational stereo-specificity. As regards the further Eact build-up, it can be rationalized in terms of the ability of chains to be rolled in the most stable GGTT conformation, as it is supported by complementary FTIR and DSC characterization of the samples.

Selective Reduction of PHA Biopolyesters and Their Synthetic Analogues to Corresponding PHA Oligodiols Proved by Structural Studies

A highly selective method is described for controlling the degradation of polyhydroxyalkanoates, PHA, via a reduction reaction that uses lithium borohydride. Using this method, oligo(hydroxyalkanoate)diols derived from a poly(3-hydroxybutyrate-co-4-hydroxybutyrate) biopolyester [poly(3HB-co-4HB)] and from synthetic atactic poly[(R,S)-3-hydroxybutyrate] (a-PHB) were obtained. The structural characterization of the oligo(hydroxyalkanoate)diols was conducted using NMR and ESI-mass spectrometry analyses, which confirmed that oligomers that were terminated by two hydroxyl end groups were formed. The reduction of the ester groups occurred in a statistical way regardless of the chemical structure of the comonomer units or of the microstructure of the polyester chain. The presented method can be used to synthesize various PHA oligodiols that are potentially useful in the further synthesis of tailor-made biodegradable materials.

Synthesis and characterization of poly (l-lactide/ε-caprolactone) statistical copolymers with well resolved chain microstructures

In view of the improved properties of elastomeric copolymers with a random distribution of sequences (randomness coefficient (R) → 1), several synthesis of statistical poly(l-lactide/ε-caprolactone) (PLCL) copolymers at a 75:25 mass feed ratio were conducted evaluating the polymerization conditions (temperatures, times of reaction and comonomer/catalyst (M/C) molar ratios). The mechanisms of reaction of both catalyst employed, SnOct2 and BiSS, differed in the coordination and activation of the monomers leading to different chain microstructures. BiSS was found to be less prone to react with the monomer with higher reactivity, i. e., lactide (LA), than SnOct2, and consequently, the desired random PLCLs (R > 0.85) were obtained. In contrast, the more reactive SnOct2 exhibits a tendency to favor blocky sequences (R < 0.6) that provide more capability to crystallize. The random BiSS-PLCLs showed crystallinity with lower melting temperature (Tm) than moderately blocky SnOct2-copolymers, being both influenced by the average lactide block length (lLA) obtained, a parameter affected by the LA content and R. The Tm values increase from 82 to 125 °C when lLA only rises from 3.3 to 5.4, whereas the corresponding Tm of the SnOct2-PLCLs is in the range of 148–163 °C despite the wide range of lLA (5.4–23.5). Also indicative of the high sensitivity to microstructural chain aspects of the copolymers, the crystallinity degree (xc) of the PLCLs grows in a logarithmic manner shifting quickly from 2.0% to around 20.0% when lLA increases from 3.3 to 5.0. Finally, the adhesion and proliferation of adipose derived stem cells (ADSCs) on some representative PLCLs were successfully performed with no signs of cytotoxicity despite in some cases of the presence of a slight residual amount of toxic tin.

The Structure of Acrylonitrile-Butadiene Rubbers: Paramagnetic-Probe Study

Chain microstructures and supramolecular structures of nitrile-butadiene rubbers are studied by ESR spectroscopy. On the temperature dependences of the rotational mobility (correlation time τc) of paramagnetic probe 2,2,6,6-tetramethylpiperidine-1-oxyl, relaxation transitions are observed. It is shown that there is a correlation between the Arrhenius parameters of rotational mobility of radical and the copolymer composition and that different brands of rubbers differ in microstructur e and supramolecular structure.

Controlled synthesis of styrene-n-butyl acrylate copolymers with various chain microstructures mediated by dibenzyl trithiocarbonate

Copolymers of styrene and n-butyl acrylate are synthesized via pseudoliving radical copolymerization mediated by dibenzyl trithiocarbonate as a reversible addition fragmentation chain transfer agent. In a wide range of monomer feed compositions, narrowly dispersed compositionally uniform copolymers of desired molecular masses are formed, and their gradient structures are controlled by the compositions of the initial monomer feeds. The effects of the compositions, the chain microstructures, and the compositional homogeneity of the copolymers on their glass-transition temperatures are studied.

Heterogeneous Ziegler-Natta, metallocene, and post-metallocene catalysis: Successes and challenges in industrial application

Abstract

Facile Syntheses of Cucurbit[6]uril-Anchored Polymers and Their Noncovalent Modification

The general strategy for facile synthesis of Cucurbit[6]uril- (CB[6]-) anchored polymers without the functionalization of CB[6] was presented. The acrylamide as a typical monomer was used to synthesize a series of CB[6]-anchored polyacrylamides (CB[6]-PAM) using potassium persulfateas as initiator and oxidant. The CB[6]-PAM samples were characterized by 1H NMR, 13C NMR, HMQC, FTIR, and TGA. It was found that the composition and chain microstructure of CB[6]-PAM polymers could be tunable by changing the content of potassium persulfate, CB[6] and acrylamide. In addition, CB[6]-PAM could be assembled into nanosized vesicles, which were confirmed by TEM, AFM, and SEM measurements. By taking advantage of the exceptional binding affinity of the CB[6], the CB[6]-PAM could be modified with butyl amine hydrochloride. The result makes the CB[6]-anchored polymer potentially useful in many applications. Furthermore, this synthetic approach could be extend to CB[6]-anchored polymers with two different chains. As a typical example, CB[6]-anchored poly(4-vinylbenzylamine hydrochloride salt) and polyacrylamide was synthesized successfully.

The Influence of Chain Microstructure of Biodegradable Copolyesters Obtained with Low-Toxic Zirconium Initiator toIn VitroBiocompatibility

Because of the wide use of biodegradable materials in tissue engineering, it is necessary to obtain biocompatible polymers with different mechanical and physical properties as well as degradation ratio. Novel co- and terpolymers of various composition and chain microstructure have been developed and applied for cell culture. The aim of this study was to evaluate the adhesion and proliferation of human chondrocytes to four biodegradable copolymers: lactide-coglycolide, lactide-co-ε-caprolactone, lactide-co-trimethylene carbonate, glycolide-co-ε-caprolactone, and one terpolymer glycolide-colactide-co-ε-caprolactone synthesized with the use of zirconium acetylacetonate as a nontoxic initiator. Chain microstructure of the copolymers was analyzed by means of 1H and 13C NMR spectroscopy and surface properties by AFM technique. Cell adhesion and proliferation were determined by CyQUANT Cell Proliferation Assay Kit. After 4 h the chondrocyte adhesion on the surface of studied materials was comparable to standard TCPS. Cell proliferation occurred on all the substrates; however, among the studied polymers poly(L-lactide-coglycolide) 85 : 15 that characterized the most blocky structure best supported cell growth. Chondrocytes retained the cell membrane integrity evaluated by the LDH release assay. As can be summarized from the results of the study, all the studied polymers are well tolerated by the cells that make them appropriate for human chondrocytes growth.

Microstructure and intercalation dynamics of polymer chains in layered sheets

The layered polymer nanocomposites have attracted great interest from scientists due to their unique properties. However, the formation mechanism of these nanocomposites in the intercalation process is not fully understood yet. In this work, we simulate the intercalation processing of polymer chains into two layered sheets. By systematically tuning the polymer–sheet interaction, temperature, chain length, organic surfactant modification and interlayer distance, we found that the intermediate polymer–sheet interaction, low polymer molecular weight and large interlayer distance would enhance the intercalation kinetics. However, the presence of surfactants promotes the intercalation process of chains into layered sheets only when the interaction between polymer and surfactants is strong enough. In the intercalation process, the polymer chain first elongates and then contracts. To uncover the mechanism of aggregation and formation of layered sheets–polymer network, we examine the formation of bridge chains between layered sheets. Generally, the polymer chain does not adsorb totally on one single layer, but crosses over from one layer to another. Moreover, the number of bridge chains monotonically decreases with the increase of the polymer–sheet interaction and interlayer distance, but increases with the polymer chain length. It is expected that the structural and dynamic behavior of polymer chains intercalated into the layered sheets could deepen our understanding of polymer nanocomposites filled with layered sheets.

Novel Poly(L-lactide-co-ε-caprolactone) Matrices Obtained with the Use of Zr[Acac]4as Nontoxic Initiator for Long-Term Release of Immunosuppressive Drugs

Slowly degradable copolymers of L-lactide and ε-caprolactone can provide long-term delivery and may be interesting as alternative release systems of cyclosporine A (CyA) and rapamycin (sirolimus), in which available dosage forms cause a lot of side effects. The aim of this study was to obtain slowly degradable matrices containing immunosuppressive drug from PLACL initiated by nontoxic Zr[Acac]4. Three kinds of poly(L-lactide-co-ε-caprolactone) (PLACL) matrices with different copolymer chain microstructure were used to compare the release process of cyclosporine A and rapamycine. The influence of copolymer chain microstructure on drug release rate and profile was also analyzed. The determined parameters could be used to tailor drug release by synthesis of demanded polymeric drug carrier. The studied copolymers were characterized at the beginning and during the degradation process of the polymeric matrices by NMR spectroscopy, GPC (gel permeation chromatography), and DSC (differential scanning calorimetry). Different drug release profiles have been observed from each kind of copolymer. The correlation between drug release process and changes of copolymer microstructure during degradation process was noticed. It was determined that different copolymer composition (e.g., lower amount of caprolactone units) does not have to influence the drug release, but even small changes in copolymer randomness affect this process.

Chain Microstructure, Crystallization, and Morphology of Olefinic Blocky Copolymers

AbstractThe chain microstructure, crystallization, and morphology of three olefinic blocky copolymers (OBCs) are compared. The weight percentage of the hard block and the octene content in it are calculated from the 13C NMR spectra with a two‐site first‐order Markovian model. The lengths of the hard blocks are compared from the melting and crystallization behaviors. The remarkable difference between the crystallinity measured by DSC, and wide‐angle X‐ray diffraction (WAXD) indicates the presence of partially ordered phases. Small angle X‐ray scattering (SAXS) shows that the partially ordered phases in OBC‐A are mainly located at the interface between the crystalline and amorphous phases, but exist as separated microdomains in OBC‐B. As the hard block becomes shorter, there are also more, separated partially ordered phases in OBC‐C. Spherulites are observed in all OBCs by POM and the size of the spherulites decreases in the order: OBC‐A &gt; OBC‐B &gt; OBC‐C. TEM shows that spherulites are poorly developed in a thin film of OBC‐B.

High dielectric performance of tactic polynorbornene derivatives synthesized by ring‐opening metathesis polymerization

AbstractFunctional polynorbornenes (PNBEs) containing pyrrolidine moiety and bis(trifluoromethyl)biphenyl side group were synthesized via ring‐opening metathesis polymerization (ROMP), and the microstructure of polymer chain was characterized by NMR spectroscopy. Poly(N‐3,5‐bis(trifluoromethyl)biphenyl‐norbornene‐pyrrolidine) (PTNP) and poly(N‐phenyl‐norbornene‐pyrrolidine) (PPNP) are supposed to have practically trans double bonds and adopt isotactic syn conformation, whereas poly(N‐3,5‐bis(trifluoromethyl)biphenyl‐norbornene‐dicarboximide) (PTNDI) has both trans and cis double bonds and atactic microstructure. PTNP, PTNDI, and PPNP have much different dielectric constants of 20, 7, and 3, respectively, which is attributed to both the polar 3,5‐bis(trifluoromethyl)biphenyl group and the stereoregular chain structure. The existence of rigid pyrrolidine moiety has a positive contribution to form the tactic polymer chain during ROMP. Polymers are highly thermal stable up to ∼300 °C. Having good dielectric properties and thermal stability, these functional PNBEs are expected as the potential dielectric material in thin film capacitors. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Butadiene Polymerization Catalyzed by Tri(aryloxo)aluminum Adduct of Cobalt Acetate

Tris(2-phenylphenoxo)aluminum () exists as a dimeric form in toluene. When toluene-insoluble anhydrous cobalt acetate is treated with tris(2-phenylphenoxo)aluminum in toluene, the toluene-soluble adduct is formed. The 2-phenylphenoxo ligand in the adduct can be replaced with another aryloxo ligand to give (aryloxo) (aryloxo = 2-methylphenoxo, 2-isopropylphenoxo, 4-methylphenoxo, 4-isopropylphenoxo, or 4-tert-butylphenoxo). These complexes are active for butadiene polymerization without gel formation when activated with an equivalent amount of for 2 h. The highest activity, 175 kg/mol-Co (turnover number, 3200) was achieved with at for 2 h. The microstructure of the polymer chains is mostly trans-1,4-configuration (70-75%) with the remaining being 1,2-vinyl. The cis-1,4-configuration observed by IR is minimal (1-5%). By replacing the 2-phenylpheoxo with a 4-alkylphenoxo ligand, the amount of 1,4-configuration slightly increases, resulting in increase in the endothermic melting signal at in the DSC curve. The molecular weights of the polymers are high (: 300000~800000) with a fairly narrow molecular weight distribution (, 2.0-2.7).

Effects of chain microstructures and derived crystallization capability on hydrolytic degradation of poly(l-lactide/ε-caprolactone) copolymers

Hydrolytic degradation of bioabsorbable (co)polyesters is affected by a great number of factors, such as chemical composition, hydrophilicity, pH of the medium, morphology of the sample, initial distribution of molecular weights, etc. In this study it is demonstrated the importance of the amorphous/crystalline character, controlled by the repeat unit sequence distribution in chain microstructure of crystallizable lactide building block copolymers. Three statistical poly(l-lactide/ε-caprolactone) copolymers were degraded in phosphate buffered saline (PBS) at 37 °C for a period up to 14 weeks. PLCL74b and PLCL74r, presenting similar copolymer composition (∼74% of lactide) but a different randomness character (R = 0.46 vs. 0.96) reflected in their lactide sequence length distribution (lLA) (8.16 vs. 4.01), displayed a completely dissimilar behavior during the course of degradation. The blocky PLCL74b showed a high crystallization capability, reaching a value of 51.0 Jg−1 of melting enthalpy (ΔHm) at the end of the study, whereas random PLCL74r presented a ΔHm = 33.7 Jg−1. On the contrary, PLCL62r, having a similar randomness character to PLCL74r, a ∼62% of lactide content and the lowest lLA (2.55), showed a ΔHm = 10.9 Jg−1. As a consequence, PLCL74b exhibited the slowest degradation rate (half degradation time (t1/2) = 31.5 days), while PLCL62r was the less resistant to hydrolytic degradation with a t1/2 = 18.2 days. This is due to the larger hindrance the water finds to penetrate the crystalline domains and consequently, to the higher resistance of crystalline domains to hydrolytic degradation. Apart from slowing down the degradation, the development of crystalline domains caused deterioration in the mechanical properties of the studied copolymers, which make them unworkable as the degradation process progressed.

Controlled Chain‐Scission of Polybutadiene by the Schwartz Hydrozirconation

Controlled chain-scission of polybutadiene (PB), polyisoprene, and poly(styrene-co-butadiene), induced by bis(cyclopentadienyl) zirconium hydrochloride (Cp(2)ZrHCl), was revealed at room temperature. The chain-scission reaction of linear PB was studied by means of GPC, NMR spectroscopy, and MALDI-TOF-MS. It was confirmed that the molecular weights of degraded products were quasi-quantitatively controlled by Cp(2)ZrHCl loading, irrespective of the starting PB, whereas the microstructure of PB chains was crucial to the scission reaction. The hydrozirconation of model molecules indicated that the existence of an internal double bond in compounds with multiple double bonds was essential for chain cleavage. The chain-cleavage mechanism was proposed to involve hydrozirconation of internal double bonds in PB chains and β-alkyl elimination. Furthermore, metallocene-catalyzed chain-scission by a chain-transfer reaction was developed. It is believed that the reported chain scission offers a promising pathway for end-group functionalization by chain cleavage and presents a new application of Schwartz's reagent.

Unsaturated Polyphosphoesters via Acyclic Diene Metathesis Polymerization

A powerful methodology for the synthesis of unsaturated polyphosphoesters (UPPEs) relying on acyclic diene metathesis (ADMET) polymerization is described. A facile two-step protocol gives fast access to structurally versatile and potentially degradable and biocompatible materials. For the first time this allows to control the microstructure of either backbone and/or side chains of polyphosphoesters. Four different monomers are polymerized under different conditions to yield UPPEs with molecular weights ranging from 7000 up to 50 000 g mol–1 with reasonable polydispersity. The unsaturated and saturated polyphosphoesters are characterized in detail with various techniques. A major benefit of the UPPEs is revealed by differential scanning calorimetry proving the control over thermal properties with tunable melting or glass transition temperatures for the different saturated and unsaturated PPEs which is an important feature for future applications. This unique combination of the benefits of metathesis polyme...