Number-average Sequence Length Research Articles

Modeling of Chain Sequence Length and Distribution in Random Copolyesters.

The performances and properties of random copolyesters, including biodegradability, mechanical and thermal properties, transparency, etc., are highly influenced by their chain structures. However, obtaining detailed chain sequence information remains a significant challenge. This study introduces a mathematical model based on a probabilistic approach to determine the sequence length and distribution in random copolyesters. Two types of copolyesters, A1A1BB-A2A2BB, representing poly(butylene adipate-co-terephthalate) (PBAT), and A1A1B1B1-A2B2, using poly(butylene succinate-co-glycolic acid) (PBT-PGA) as an example, are the focus. The predicted sequence lengths of various copolyesters derived from the model are in good agreement with the values reported in the literature. The chain sequence distribution obtained from the model provides better insights into the unique properties of the copolyesters. It is observed that the incorporation of hydroxyl acid units into copolyester chains effectively reduces the sequence length without altering the copolymer composition, offering a strategic approach for enhancing degradation performance while maintaining mechanical properties of random copolyesters. The influence of the number-average sequence length becomes particularly significant when the copolymer composition ranges between 0.7 and 0.9, with a higher copolymer composition required for copolyesters containing hydroxyl acid monomer units. This model represents a powerful tool for researchers, enabling a deeper understanding of the relationship between copolymer composition and its structural characteristics in random copolyesters and facilitating the development of high-performance random copolyesters.

Synthesis and Structure-Property Relationships of Novel High Molecular Weight Fully Biobased 2,5-Thiophenedicarboxylic Acid-Based Polyesters.

High molecular weight fully biobased poly(propylene succinate-co-2,5-thiophenedicarboxylate) (PPSTF) random copolyesters based on the emerging biobased aromatic diacid, 2,5-thiophenedicarboxylic acid (TFDCA), in full composition range were synthesized via melt polycondensation. Their crystallization behavior, thermal-mechanical, gas barrier, and biodegradable properties were systematically investigated. A certain level of comonomer cocrystallization was evidenced by XRD, and PTF units had stronger crystallization competitive capability compared to PS units due to the higher stiffness of TFDCA units. These copolyesters exhibited excellent thermal stability, and mechanical properties can be easily controlled by tuning the varied ratio of flexible to rigid segments. Gas barrier properties were studied from both theoretically calculated and experimental perspectives, and the copolyesters even with 50 mol % PS units still showed superior gas permeation resistance. The selected lipase from Aspergillus oryzae can degrade the copolyesters with up to 60 mol % PTF units. The nonbiodegradable-biodegradable transition was found to occur at the number-average sequence length of aromatic PTF units as low as about 3. Interestingly, when compared with their terephthalic acid-based (TA-based) and 2,5-furandicarboxylic acid-based (FDCA-based) analogues with the same content of aromatic units, the apparent degradation rate constant (k) and half period (t1/2) of PPSTF60 were actually between them. These findings offer much promise for the application of polyesters containing odd-carbon diol monomers in green packaging and other fields.

Effects of chain microstructure on the thermal, mechanical and crystallization behaviors of poly(ε-caprolactone-co-lactide) copolymers: Processable biomaterials with tunable properties

The current research underlines one-pot melt polymerization process for the copolymerization of L-lactide (LA) and ε-caprolactone (CL), where the composition of the comonomers is so varied as to obtain different chain microstructures of the resulting copolymers. The conversion of monomers to caproyl and lactidyl units is determined from 1H NMR (nuclear magnetic resonance) spectroscopy. The number-average sequence lengths of caproyl and lactidyl units as a function of monomer composition is evaluated using 13C NMR spectroscopy. The blocky chain structure in induced in the copolymers with the increasing LA content by virtue of difference of reactivity of LA and CL monomers, in order to develop thermally processable materials possessing mechanical integrity. The chain microstructure governs the thermal and mechanical properties which are reflected by an increase in melting temperature from 111.8 ℃ to 161.8 ℃, and increase in elongation at break from 2.2% to 812%. The copolymers possess a lower degree of crystallinity, where the crystalline LA units serve as physical crosslink points and contribute to the mechanical strength of the copolymers. Further the synthesized copolymers are scalable and thermally processable by conventional injection-molding-cum-extrusion technique to yield flexible products, where the adhesion and multiplication of fibroblast cells on the copolymers indicate their biocompatibility.

Soft, Formstable (Co)Polyester Blend Elastomers.

High crystallization rate and thermomechanical stability make polylactide stereocomplexes effective nanosized physical netpoints. Here, we address the need for soft, form-stable degradable elastomers for medical applications by designing such blends from (co)polyesters, whose mechanical properties are ruled by their nanodimensional architecture and which are applied as single components in implants. By careful controlling of the copolymer composition and sequence structure of poly[(L-lactide)-co-(ε-caprolactone)], it is possible to prepare hyperelastic polymer blends formed through stereocomplexation by adding poly(D-lactide) (PDLA). Low glass transition temperature Tg ≤ 0 °C of the mixed amorphous phase contributes to the low Young’s modulus E. The formation of stereocomplexes is shown in DSC by melting transitions Tm > 190 °C and in WAXS by distinct scattering maxima at 2θ = 12° and 21°. Tensile testing demonstrated that the blends are soft (E = 12–80 MPa) and show an excellent hyperelastic recovery Rrec = 66–85% while having high elongation at break εb up to >1000%. These properties of the blends are attained only when the copolymer has 56–62 wt% lactide content, a weight average molar mass >140 kg·mol−1, and number average lactide sequence length ≥4.8, while the blend is formed with a content of 5–10 wt% of PDLA. The devised strategy to identify a suitable copolymer for stereocomplexation and blend formation is transferable to further polymer systems and will support the development of thermoplastic elastomers suitable for medical applications.

Effect of acrylonitrile sequence distribution on the thermal stabilization reactions and carbon yields of poly(acrylonitrile-co-methyl acrylate)

Polyacrylonitrile (PAN) copolymers are important precursors for making high performance carbon fibers. In this study, a type of inert comonomer, methyl acrylate (MA), was copolymerized with AN, and the effects of AN sequence distribution on the cyclization, oxidation and carbon yield were systematically studied. The sequence distribution of P(AN-co-MA) could be adjusted by altering MA/AN feeding ratio during radical polymerization. The number-average sequence length of AN in the copolymers decreases quickly along with the increase of MA content in PAN. The stabilization and carbonization weight losses were determined by thermogravimetric analysis, and the corresponding chemical changes were detected by Fourier transform infrared spectrums and X-ray photoelectron spectroscopy. The thermal cyclization and oxidation reactions were separated by altering gas environment from nitrogen to air, and were monitored by differential scanning calorimetry. It is concluded that the sequence distribution of AN in P(AN-co-MA) strongly affects reaction activation energies, cyclized structures and carbon yield.

Isothermal crystallization fractionation and fraction characterization of trans-1,4-poly(isoprene-co-butadiene)

Polymer chain structural factors, like chain microstructure, comonomer content in copolymer, dyad sequence distribution, molecular weight (MW) and molecular weight distribution (MWD), etc. have a great influence on the properties and applications of polymer materials. In this paper, chain structural factors and thermal behaviors of novel trans-1,4-poly (isoprene-co-butadiene) copolymers (TBIR-15, TBIR-20, TBIR-40) with various comonomer contents synthesized by TiCl4/MgCl2-Al (i-Bu)3 catalyst system were studied by 13C-NMR, GPC, DSC and WAXD. Eight fractions for each TBIR sample were obtained through isothermal crystallization fractionation of TBIR in dilute solution. It is proved that all fractions in the TBIR samples were high trans-1,4-configuration copolymers (>89 mol%). The TBIR copolymers were mainly composed by G fractions (44∼76 wt%), which cannot be crystallized from the dilute solutions at −20 °C. The G fractions of the TBIR-15 and TBIR-20 were multi-block copolymers composed by short trans-1,4-polyisoprene (TPI) blocks with the number-average sequence length of isoprene units (nIp)∼6. The G fraction of TBIR-40 was random copolymer with the melting temperature (Tm) as low as −3.4 °C and the crystallinity (Xc) as low as 4.4%. Fractions D and E in TBIR-15 and TBIR-20 were multi-block copolymers with long TPI blocks (nIp = 6∼24) and the above two fractions show decreased Tm (29∼38 °C) from imperfect TPI crystals. A special fraction A in TBIR-40 was random copolymer containing a few TPI and trans-1,4-polybutadiene (TPB) blocks, and both TPI and TPB blocks had the ability to crystallize. The Mw for each fraction was in the range of 2–6 × 105 and Mw/Mn was in the range of 2–6. Based on the analysis, the possible chain sequence distribution model for each fraction was proposed. It was expected that this research would provide a further understanding about the TBIR copolymer chain structures and inspire the copolymer synthesis with ideal structure and composition as high performance rubber material.

Semibatch RAFT copolymerization of acrylonitrile and N-isopropylacrylamide: Effect of comonomer distribution on cyclization and thermal stability

Utilizing reversible addition-fragmentation chain transfer (RAFT) polymerization, the composition profile of PAN-based copolymers becomes a tunable parameter in preparing carbon fiber precursors. In this work, poly(acrylonitrile-co-N-isopropylacrylamide) copolymers were prepared via a semibatch process mediated by 2-cyano-2-propyl dodecyl trithiocarbonate (CPDT). The monomer reactivity ratios were determined by the Fineman-Ross (FR), Kelen-Tüdos (KT), and non-linear least squares (NLLS) models. The number average sequence length of acrylonitrile (AN) was found to be highly dependent on the content of N-isopropylacrylamide (NIPAM). NIPAM was introduced at controlled rates into a reaction vessel in attempt to facilitate its distribution in the polymer backbone. These semibatch copolymers were evaluated based on their ring-closing cyclization efficiency, which was characterized by differential scanning calorimetry (DSC), fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). Significantly, DSC exotherms were reduced and cyclization occurred much faster reaching a greater thermal stability when NIPAM was introduced at slow feed rates.

Synthesis and characterization of trans-1,4-butadiene/isoprene copolymers: Determination of sequence distribution and thermal properties

Two series of trans-1,4-poly(butadiene-co-isoprene) copolymers (TBIR) were prepared using the catalyst system TiCl4/MgCl2-Al(i-Bu)3 at different reaction temperatures. All dyad and triads sequence distributions, the number-average sequence length and the sequence concentration of the copolymers were calculated according to 13C-NMR spectra. The influences of temperature and initial molar ratio of butadiene to isoprene (Bd to Ip) on the distribution of the chain segments in the TBIR copolymers were discussed. The correlation of copolymer compositions and thermal properties were also evaluated, which facilitated the understanding of controlling the degree of crystallinity and the transition temperature by changing Bd content and temperature.

Characterization of monomer sequence structure distribution of poly(1-butene)/poly(propylene-co-butene) in-reactor alloys through 13C-NMR

In situ FTIR testing demonstrated that crystallization of novel poly(1-butene)/poly(propylene-co-butene) in-reactor alloys from melt shows thermodynamic stable Form I directly rather than general reported unstable crystal Form II. In order to make clear this phenomenon, the microstructure and monomer sequence distribution of the as-obtained poly(1-butene)/poly(propylene-co-butene) in-reactor alloys was determined by 13C-NMR. The raw alloy was separated by fractional dissolution to five grades. The characterization of raw alloys and different grades demonstrates that propylene monomer unit distributes in a form of isolated or segmented along poly(1-butene) molecular chains. The fractional dissolution of the selected alloy indicates that chain structure changes gradually with fractional solvent. The number average sequence length of propylene and 1-butene unit has been calculated. The number average sequence length and distribution could help us to study the crystallization and transformation clearly. From the results of in situ FTIR and NMR, the random distribution of the propylene unit with certain content in the as-obtained alloys play role to accelerate the crystallization transformation.

Unique microstructure analysis of ethylene-propylene copolymer synthesized using catalytic system based on α-diimine nickel complexes: a comparative study by 13C NMR technique

The microstructure of rubber-like ethylene-propylene copolymer (MN4) produced by a mixed nickel-based system (MN) containing catalysts of dibromo[N,N′-bis(2,6-diisopropylphenyl)-2,3-butanediimine]nickel(II) n1 and dibromo[N,N′-(phenanthrene-9,10-diylidene)bis(2,6-diisopropylaniline)]nickel(II) n2 was determined by 13C NMR technique. Sequences distribution of ethylene (E), propylene (P), EP, inverted propylene and uninterrupted methylene and also methylene number-average sequence lengths for the copolymer (MN4) were estimated. The results obtained from the MN4 EP copolymer were compared with reported copolymers which had been synthesized using constrained geometry catalyst (CGC) and vanadium-based Ziegler-Natta catalyst. The results demonstrated that the MN4 EP copolymer had fewer alternating comonomer sequences than ethylene-propylene elastomers obtained by CGC and vanadium-based (V) catalysts. A large number of the inversion structures (66 %) and high mole percent of sequences containing a long branch (3.2 mol%) were also observed in unique microstructure of the copolymer (MN4).

Correlation of chemical composition and microstructure with properties of poly(ɛ‐caprolactone‐co‐p‐dioxanone) random copolymers

ABSTRACTWith the aim to develop novel biodegradable materials with good flexibility and fast degradation rate, random copolymers of ɛ‐caprolactone (CL) and p‐dioxanone (PDO) with a full range of compositions were synthesized in bulk using stannous octoate as the ring‐opening catalyst. The chemical composition and number average sequence lengths of CL and PDO units determined by 1H‐NMR were used to correlate with various properties of the copolymers. Although both CL and PDO are crystalline components, only one crystalline phase could be present for each copolymer. The low limit of average block length for the copolymers that could crystallize is 3.22 for LCL and 3.43 for LPDO, respectively. The crystallinity and crystalline morphology of the copolymers are dependent on the crystalline component as well as its number average sequence length. Irrespective of composition, all the copolymers have good solubility in chloroform with glass transition temperature much below room temperature, implying good flexibility of the materials. The incorporation of PDO component could significantly increase the water wettability of the copolymer surfaces and thereby accelerate the degradation rate of the materials. In conclusion, flexible biodegradable polymers with adjustable degradation and crystalline properties were acquired by random copolymerization of CL and PDO, which are expected to use in tissue engineering and drug delivery fields. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2978–2986, 2013

Structures and properties of thermoregulated acrylonitrile–methyl acrylate sheet containing microphase change materials

AbstractA series of melt processable poly(acrylonitrile–methyl acrylate) copolymers with a feeding ratio of 85/15 mol% (abbreviated as 85/15 AN/MA below) containing 0–25 wt% of microencapsulated phase change materials (micro‐PCMs) were synthesized by aqueous precipitation polymerization. The number average sequence lengths (NASLs) and tacticity of 85/15 AN/MA were determined using 13C‐NMR. The results showed that the NASL of contiguous AN sequences is 10.00 and the tacticity is atactic with approximately 51% m and 49% r diads (the relative orientation of two adjacent cyanogroup). 85/15 AN/MA containing various contents of micro‐PCMs were processed into sheets in melt condition at 210°C and 20 MPa. The structures and properties of the copolymers and sheets containing micro‐PCMs were studied by using Fourier transform infrared spectroscopy, nuclear magnetic resonance (1H‐NMR), scanning electron microscopy, differential scanning calorimetry, gel permeation chromatography, X‐ray diffraction, and dynamic mechanical analysis. The composition of AN/MA copolymer agreed well with its monomer feed ratio. The melting enthalpy of the sheets containing 25 wt% of micro‐PCMs was approximately 25 J/g, which increased steadily as the micro‐PCMs content increased. The modulus of the sheets decreased with the micro‐PCMs content increased. The glass transition temperature (Tg) of the sheets was in the range of 79.7–81.7°C, which also increased with the micro‐PCMs content increasing. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers

Relationship between Sequence Length and Thermal Properties of Poly(ethylene Terephthalate-Co-4, 4’-Bibenzoate) (PETBB)

Poly(ethylene terephthalate-co-4,4’-bibenzoate)s (PETBBs) with 0-25mol% bibenzoate unit were prepared from terephthalic acid (PTA), 4,4'- bibenzoic acid (BBA) and ethylene glycol (EG). The chemical composition and number-average sequence length of the resultant PETBBs were investigated by 13C-NMR. It was improved that the chemical composition of PETBBs is identical with the feed ratio, and the number-average sequence length of PET segment (LPET) becomes reduced with increasing BBA unit. The molecular weights of PETBBs reaches the high level for practical use. DSC analysis explains that the glass transition temperature (Tg) increases almost linearly and the crystallization becomes difficulty with increasing content of BBA unit. X-ray diffraction also illustrates that both crystallinity and crystal size reduced as more BBA unit is incorporated. Moreover, the relationship between the number-average sequence length and the melting temperature was investigated. The linear correlation between the number-average sequence length (LPET) and the melting temperature was observed.

Synthesis, structure and properties of poly(L-lactide-co-[formula omitted]-caprolactone) statistical copolymers

Four poly(L-lactide-co-ε-caprolactone) (PLCL) copolymers were synthesized at 120, 130, 140 and 150 °C by ring opening polymerization using stannous octoate catalyst at a 2000:1 comonomer:catalyst ratio. Gel permeation chromatography (GPC) and 1H NMR measurements were performed to determine the molecular weight, composition and chain microstructure of copolymers of L-lactide(LA):ε-caprolactone(CL) synthesized using 90:10, 80:20, 75:25 and 70:30 feed ratios. The overall conversion of these PLCL copolymers was in the range of 80%–90% leading to weight average molecular weights (Mw) between 98,500 and 226,000 g mol−1 depending on feed composition and polymerization temperature. At temperatures lower than 140 °C, the incorporation of CL units into polymer chains was incomplete because of the low reactivity of CL, thus at 120 °C the copolymer composition was difficult to control obtaining more LA in the copolymer than the desired, hence the blocky character of PLCL copolymers also increased. At 150 °C the catalyst was less effective and the molecular weights of the copolymers took lower values. A temperature of 140 °C was established as optimal to obtain highest yields and molecular weight. The number average crystallizable lactide sequence lengths (lLA) shifted from 6.5 to 16.7 LA repeat units for PLCL polymerized at 140 °C while the randomness character (R) value shifted from 0.4 for polymerization at 130 °C to 0.6, at 150 °C. Increasing the LA content in the copolymers the glass transition temperature and the crystallizability and melting temperature of PLCLs approached to that of PLLA homopolymer. The aging sensitivity of PLCLs increased with CL content. A double Tg behavior due to phase separation associated to crystallizing LA blocks was observed during aging. The mechanical properties, however, evolved toward the PLLA character when the molar content of LA in PLCL was increased from 66% to 90%, observing a shift from an elastomeric thermoplastic behavior to that of a glassy plastic, reflected by an increase in tensile modulus (from 12.0 to 1343.1 MPa) and a decrease in strain recovery after break (from 93.5% to 25.0%). Small amounts of CL content in the copolymers produced large improvements in their deformability with regard to PLLA. In addition, thermogravimetric analysis demonstrated that PLCLs are more stable to thermal degradation than PLLA and they undergo a more complex degradation mechanism than those of the corresponding homopolymers.

An experimental and theoretical study of the carbazole to carbazole intramolecular energy transfer in N‐vinylcarbazole/vinyl tert‐butyl‐benzoate copolymers of different molar compositions

AbstractFluorescence depolarization and quenching measurements of N‐vinyl carbazole/vinyl tert‐butyl‐benzoate copolymers in dilute solution of several fluid solvents and in a rigid poly(methyl methacrylate) matrix were performed to study the effect of copolymer composition on the intramolecular energy transfer (IET) between carbazole groups (Cz) along the polymer chain. IET efficiency, as the amount of CzCz intramolecular excimer formation, monotonically increases with Cz content up to a number average sequence length of carbazole of ca 12 (Cz molar content ≥ 90%). After this value, IET efficiency remains almost constant. Molecular dynamics simulations on isotactic and syndiotactic copolymer fragments of several monomer compositions were used for obtaining different parameters related to the efficiency for the IET process between Cz units along the polymer chain. Copyright © 2011 Society of Chemical Industry

Copolymerzation of L,L-lactide with ε-caprolactone by using novel zinc L-proline organometallic catalyst

AbstractCopolyesters based on L,L-lactide (LA) and ε-caprolactone (CL) with different LA: ε-CL ratios were synthesized by ring opening polymerization using zinc (L-proline) as a catalyst. Ring opening polymerization results in random P(L, L-lactide-ε-caprolactone) copolymers of molecular weights between 9,800 to 30,000 and block copolymer up to 51,800 Da. 1H NMR spectroscopy analysis was used for calculation of monomer incorporation in the copolymer. FT-IR spectra were used to differentiate between random and block copolymer using carbonyl region splitting. 13C quantitative NMR spectroscopy analysis coupled with information from TGA and DSC allowed determination of the polymer structure. Copolymers (CP-1, CP-2 and CP-3) prepared by ROP are random copolymer (h>1), while CP-4 prepared by ROP (sequential addition of έ-caprolactone and L, L-lactide) has a multi block character (h<1). The LA number-average sequence length (LLA) decreased from 14 to 5 for LA-CL 80:20 to 50:50 copolymers prepared by ROP. The copolymer (CP-4) prepared by sequential addition showed the number-average sequence length (LLA) to be 83.0.

Study on micro-phase separation structure of poly(ethylene terephthalate)-poly(hexylene terephthalate) (PET-PCT) copolymer

AbstractPET-PCT copolyesters were prepared from terephthalic acid (PTA), ethylene glycol (EG) and 1,4-cyclohexane dimethanol (CHDM) through direct esterification and the subsequent polycondensation. The chemical composition and sequence length distribution of the copolyesters were investigated by 1H-NMR and 13C-NMR. The dynamic rheological behavior and dynamic mechanical property were studied by means of ARES-RFS and DMA. The results demonstrated that the obtained PET-PCT copolyesters were random block copolyesters and the content of PCT units incorporated was always greater than the feed and the numberaverage sequence length of each block in the copolyesters was in proportion to its content. The relation between storage modulus (G’) and frequency (freq) deviated from linearity as the frequency (freq) tend to zero (the terminal region effect), and the frequency band where the terminal region effect appeared would shift to higher value with the increasing content of PCT, suggesting the presence of phase separations. Moreover, two glass transition temperatures were observed from DMA curves, which revealed the incompatibility of PET segments and PCT segments, i.e. the micro-phase separation took place in PET-PCT copolyesters.

Kinetic study of atom transfer radical homo- and copolymerization of styrene and methyl methacrylate initiated with trichloromethyl-terminated poly(vinyl acetate) macroinitiator

Atom transfer radical bulk copolymerization of styrene (St) and methyl methacrylate (MMA) was performed in the presence of CuCl/PMDETA as a catalyst system and trichloromethyl-terminated poly(vinyl acetate) telomer as a macroinitiator at 90°C. The overall monomer conversion was followed gravimetrically and the cumulative average copolymer composition at moderate to high conversion was determined by 1H NMR spectroscopy. Reactivity ratios of St and MMA were calculated by the extended Kelen–Tudos (KT) and Mao–Huglin (MH) methods to be rSt=0.605±0.058, rMMA=0.429±0.042 and rSt=0.602±0.043, rMMA=0.430±0.032, respectively, which are in good agreement with those reported for the conventional free-radical copolymerization of St and MMA. The 95% joint confidence limit was used to evaluate accuracy of the estimated reactivity ratios. Results showed that in the controlled/living radical polymerization systems such as ATRP, more reliable reactivity ratios are obtained when copolymer composition at moderate to high conversion is used. Good agreement between the theoretical and experimental composition drifts in the comonomer mixture and copolymer as a function of the overall monomer conversion was observed, indicating the accuracy of reactivity ratios calculated by copolymer composition at the moderate to high conversion. Instantaneous copolymer composition curve and number-average sequence length of comonomers in the copolymer indicated that the copolymerization system tends to produce a random copolymer.

HOMO- AND COPOLYMERIZATION OF ETHYLENE CATALYZED BY ANSA-METALLOCENES WITH DOUBLY HETERO-BRIDGES

Three ansa-metallocenes (Me2C)(Me2Si)Cp2TiCl2(1), [(CH2)5C](Me2Si)Cp2TiCl2(2) and (Me2C)(Me2Si)Cp2ZrCl2(3) with larger dihedral angles and longer distance from metal to the center of Cp planes were synthesized and used as catalysts for ethylene polymerization in the presence of methylaluminoxane (MAO). In the case of ethylene polymerization, compared the feature structures of unbridged metallocenes, singly bridged metallocenes and doubly bridged metallocenes 1, 2, 3, there exhibit the relationship between structural characteristics and the catalytic properties with behavior of polymers. And the view that the dimensions of the short dimethylsilylene and isopropylidene bridges of metallocene 3 do not match the co-ordination requirement of the larger zirconium atom can also explain the result for ethylene polymerization. Catalyzed by complexes 1 and 2 respectively, the copolymerizations of ethylene with 1-hexene, 1-octene, especially the long chain α-olefins (1-tetradecene, 1-hexadecene) were investigated, and showed high incorporations and comonomer effects in almost all the examples. The melting points of the copolymers are approximately 4%–31% below those of the homopolymers, and the enthalpies of fusion decrease greatly due to the lower crystallinity. The microstructures of the copolymer were analyzed with 13C-NMR to determine the comonomer sequence distributions and number–average sequence lengths. Comparison between results for the complexes 1 and 2 with different bridging groups respectively showed that the nature of the bridge has a significant effect on both the incorporation and sequence distribution of the α-olefin. Because of the bridges in ansa-metallocene complexes, the copolymerization behavior was different in using catalyst 1 and catalyst 2.

Determining the composition and microstructure of ethylene–propylene copolymers by pyrolysis–gas chromatography

The average composition and detailed microstructure of copolymers of ethylene and propylene have been studied by pyrolysis–gas chromatography (Py–GC), using a statistical modeling approach to analyze the data. The trimer distribution obtained from Py–GC is used to infer monomer arrangement information, which is quantified in terms of a number-average sequence length for each monomer. These values are used to define the microstructure and to calculate the average composition. Compared with other available techniques, Py–GC provides a simple, quick and reliable approach to study the microstructure and composition of polyolefin copolymers. Details of this Py–GC method are discussed, including an examination of its advantages and disadvantages, and a summary of the qualitative and quantitative analysis aspects of this approach is presented. The combination of a statistical modeling approach with Py–GC to study copolymer composition and microstructure allows one to investigate the complex problem of monomer arrangement in copolymers using a widely available analytical technique. We expect that with further advances in separation technology, especially two-dimensional gas chromatography (GC × GC), research of this type will be become increasingly accurate and reproducible in the near future.