Formation Of Long Chain Branches Research Articles

Recent Advances in Controlled Production of Long-Chain Branched Polyolefins.

Polyolefins, composed of carbon and hydrogen atoms, dominate global polymer production. This stems from the wide range of physical and mechanical properties that various polyolefins can cover. Their versatile properties are largely tuned by chain microstructure, including molar mass distribution, comonomer content, and long-chain branching (LCB). Specifically, LCB imparts unique characteristics, notably enhances processability crucial for downstream applications. Tailoring LCB structural features has encouraged academic and industrial efforts, chronicle in this review from a chemistry standpoint. While encompassing post-reaction modification based traditional methods like peroxide grafting, ionizing beam irradiation, and coupling reactions, the main focus is given to catalyst-centric strategies and innovative polymerization schemes. The advent of single-site catalysts-metallocenes and late transition metals catalysts-amplifies interest in tailored chemical methods, but the progress in LCB formation flourishes via tandem catalytic systems and bimetallic catalysts under controlled reaction conditions. Specifically, the breakthrough in coordinative chain transfer polymerization unveils a novel avenue for controlled LCB synthesis by sequential chain propagation, transfer, liberation, and enchainment. This short review highlights recent approaches for the production of LCB polyolefins that can provide a roadmap crucial for researchers in academia and industry, steering their efforts toward further advancements in the production of tailored polyolefin.

Influence of a Multiple Epoxy Chain Extender on the Rheological Behavior, Crystallization, and Mechanical Properties of Polyglycolic Acid

This study investigated the impact of a multiple epoxy chain extender (ADR) on the rheological behavior, crystallization, and mechanical properties of polyglycolic acid (PGA). Tests of the torque and melt mass flow rate and dynamic rheological analysis were conducted to study the rheological behavior of PGA modified with ADR. The rheological results of the modified PGA showed a significantly increased viscosity and storage modulus with an increase in the ADR amount, which could be attributed to the chain extension/branching reactions between PGA and ADR. It was proved that ADR could be used as an efficient chain extender for tailoring the rheological performance of PGA. The Han plot of the modified PGA showed a transition of viscous behavior to elastic behavior, while the ADR content was increased from 0 to 0.9 phr. The formation of long-chain branches (LCBs) was confirmed via the Cole–Cole plot and weighted relaxation spectrum, wherein the LCBs substantially changed the rheological behavior of the modified PGA. The vGP plots predicted a star-type topological structure for the LCBs. The results of non-isothermal crystallization kinetics suggested that the crystallization of the modified PGA was predominantly homogeneous nucleation and three-dimensional growth. The crystallinity decreased slightly with the increase in the ADR amount. Compared to neat PGA, the modified PGA samples exhibited better tensile and flexural performances.

Tandem coordinative chain transfer polymerization for long chain branched Polyethylene: The role of chain displacement

The controlled introduction of long chain branches (LCBs) in the commonly linear structure of polyolefins has been frequently pursued by researchers to improve their melt processability. To account for that, we introduce a new tandem dual catalytic system for the formation of LCB in ethylene polymerization. Coordinative chain transfer polymerization in the first step produces Al-alkyls via frequent transfer of growing polymer chains from the single-site α-diimine nickel polymerization catalyst to AlEt2Cl chain transfer agent (CTA). Next, the displacement catalyst, Ni(acac)2, converts the dormant alkyl chains into macromers through a β-hydride elimination and retrieves the Al-based CTA. Due to the high comonomer affinity of the polymerization catalyst, the released macromers are incorporated in growing polymer chains. By repeating this reaction cycles, a branch-on-branch structure is formed. Interestingly, a separate cocatalyst is not required, as AlEt2Cl serves as both the cocatalyst and the CTA for the CCTP system. This reaction scheme employs the “nickel-effect” mechanism for the formation of LCB in coordinative polymerization for the first time. The efficiency of the proposed mechanism in the formation of LCBs is confirmed by comprehensive rheological studies and NMR spectroscopy. Moreover, density functional theory (DFT) simulations rationalize the formation of LCBs through the proposed Ni-based displacement reaction.

Crystallinity effect on electron-induced molecular structure transformations in additive-free PLA

Additive-free biodegradable polylactides (PLAs) with different crystallinities from 0 to 13.3% (according to x-ray diffraction, XRD) were used as starting materials modified by the electron beam (EB) with several irradiation doses at 25 °C (below glass transition temperature, <Tg) and 80 °C (>Tg) in nitrogen. The crystallinity effects on radical decay and molecular structure transformation were investigated. After irradiation, the main stable radical remained in PLA is –O–C*(CH3)–CO–. During irradiation, radical reactions were inhibited by high crystallinity. Large amounts of EB-induced crosslinking and long-chain branching (LCB) were introduced by performing irradiation at 80 °C. An increasing crystallinity has been found that restrained crosslinking and LCB due to the poor chain mobility in the crystalline areas. An increase of crystallinity from 0 to 13.3% obviously decreased the saturated crosslink content from 65% to 43% and restrained LCB formation. This study also verified that strict moisture control combined with applying an irradiation temperature above Tg can be a universal method to introduce large amounts of crosslinking and LCB in different grades of additive-free PLA.

Resolving long-chain branch formation in tandem catalytic coordinative chain transfer polymerization of ethylene via thermal analysis

Resolving long-chain branch formation in tandem catalytic coordinative chain transfer polymerization of ethylene via thermal analysis

Melt spinning of PLA/PCL blends modified with electron induced reactive processing

AbstractThe effect of electron induced reactive processing (EIReP) on the properties of biodegradable polylactide/masticated polycaprolactone (PLA/PCL) blends was firstly investigated without introducing any chemical additives. Subsequently, the melt spinnability of EIReP modified PLA/PCL blends was explored by a piston spinning. The EIReP modified PLA/PCL blends showed improved melt strength and elastic behavior. This is due to the formation of long chain branches and the enhanced chain entanglements. The crystallinity of PLA phase increased from 2.4% to 35.7% as the applied dose increased from 0 to 50 kGy. The PLA/PCL blends treated with 12.5 kGy demonstrated 2.4‐fold of impact toughness than that of neat PLA, suggesting a superior interfacial adhesion and chain interactions between two phases. The crystallinity of the PLA/PCL fibers decreased from 33.3% to 17.8% as the applied dose increased from 12.5 kGy to 50 kGy. It is resulted from the hindered chain mobility of branched/grafted structures during fiber formation. The fracture strain of the fibers using a non‐irradiated blend was 232.9% which increased to 311.5% for the 50 kGy irradiated blends. However, the fibers spun from irradiated blends demonstrated similar initial modulus but reduced tenacity compared to fibers spun from non‐irradiated blend material.

Al-alkenyl-induced formation of long-chain branched polyethylene via coordinative tandem insertion and chain-transfer polymerization using (nBuCp)2ZrCl2/MAO systems: An experimental and theoretical study

Recently, we have identified the Al-alkenyl species iBuAl(oct-7-en-1-yl)2 (Al-1) as a promising long-chain branching (LCB) promoter in the ethylene polymerization catalyzed by rac-{EBTHI}ZrCl2-based systems, proposing the incorporation of its alkenyl moiety into the growing polyethylene (PE) chain as a crucial step for LCB formation. The success of the process was attributed to the high copolymerization ability of the selected catalyst. In the present contribution, we aimed at probing the efficiency of Al-1 in combination with catalyst systems based on the unbridged metallocene (nBuCp)2ZrCl2 (Zr-1), namely homogeneous Zr-1/MAO or heterogeneous MAO on silica-supported-Zr-1/TIBAL (supp-Zr-1/TIBAL); in the literature, such systems are claimed to be less efficient copolymerization catalysts compared to bridged systems (i.e. rac-{EBTHI}ZrCl2). In fact, while the supported catalyst produces only purely linear PEs, branched structures were obtained in the presence of the congener homogenous system. Remarkably, the qualitative rheological data suggest that the extent of LCB formation in such samples is comparable to that of the PEs synthesized under similar reaction conditions with the EBTHI/MAO/Al-1 system. This testifies that the Al-alkenyl species is a suitable LCB promoter, even with metallocenes having quite different copolymerization ability (at least in that series). This was rationalized considering the formation of a Zr/Al heterobimetallic species facilitating the Al-1 incorporation via the coordinative tandem insertion and chain-transfer polymerization mechanism, identified in the previous study, and supported by new DFT computations.

Assessing 1,9-Decadiene/Ethylene Copolymerization with Ziegler–Natta Catalyst to Access Long Chain-Branched Polyethylene

1,9-Decadiene/ethylene copolymerization is assessed as a way for Ziegler–Natta catalysts to access long chain-branched polyethylene (PE). A MgCl2/9,9-bis-(methoxymethyl)fluorine/TiCl4 catalyst with triethylaluminium as a cocatalyst is exemplified for the task. 1,9-Decadiene was found to induce a substantial comonomer effect on catalyst activity and continuing decreases of PE molecular weight. Both the double bonds of 1,9-decadiene were poorly reactive during polymerization, of which the polymer chain-attached was even much less reactive than the original one. As a consequence, at decreased feeds, 1,9-decadiene gave small amounts (<0.1 mol %) of pendant vinyl groups to PE without prompting the formation of long-chain branches. Long chain-branching was realized at increased 1,9-decadiene feeds, which was however accompanied by proportional gelation.

Long-Chain Branched Polyethylene via Coordinative Tandem Insertion and Chain-Transfer Polymerization Using rac-{EBTHI}ZrCl2/MAO/Al–alkenyl Combinations: An Experimental and Theoretical Study

In situ synthesis of topologically modified linear polyethylenes (PE) using single-site polymerization catalysis is a challenging task, but it can enable the production of valuable advanced polymer materials with tailored properties. Described herein is an investigation aimed at the efficient generation of long-chain branches (LCB) in linear PEs using Al-alkenyl species, namely, iBuAl(oct-7-en-1-yl)2 (Al-1), in combination with homogeneous rac-{EBTHI}ZrCl2 (Zr-1)/MAO or heterogeneous MAO on silica-supported-rac-{EBTHI}ZrCl2 (supp-Zr-1)/TIBAL catalytic systems. As corroborated by extensive rheological studies and 13C NMR spectroscopy, the Al-alkenyl reagent was found to be quite efficient in the formation of LCB, via a mechanistic pathway involving both insertion and transmetallation reactions. Formation of LCB has been rationalized by density functional theory (DFT) computations carried out on the putative [rac-{EBTHI}Zr-R]+ (R = Me, nPr, and pentyl) cationic species and including a solvent model. Of the three possible isomers of Al/Zr heterobimetallic complexes derived from the cationic species [rac-{EBTHI}Zr-R]+ and Al-1, only one was identified, on kinetic and thermodynamic grounds, as the key intermediate. The DFT study also unveiled that (i) insertion of ethylene into the Zr-alkyl bond of the growing PE chain is accompanied by a reversible decoordination of the Al-vinyl transfer agent (AVTA), (ii) the vinyl 1,2-coordination/insertion of the alkenyl moieties of Al-1 into the Zr-alkyl bond, resulting in the formation of branching, is in direct kinetic competition with the insertion of ethylene, and (iii) the recoordination of the AVTA after either insertion step is thermodynamically favored and mostly responsible for the transmetallation phenomenon.

Synthesis of Long-Chain Branched Polyolefins by Coordinative Chain Transfer Polymerization

Synthesis of long-chain branched polymers has been a crucial concern in the polyolefin industry. In this study, a method to produce long-chain branches (LCBs) in coordinative chain transfer copolymerization (CCTcoP) is suggested. A dialkylzinc compound bearing vinyl groups ((9-decenyl)2Zn) is prepared, which works well not only as a chain transfer agent but also as a comonomer in CCTcoP, resulting in the generation of LCBs. The generation of LCBs is confirmed by gel permeation chromatography studies and through the analysis of rheology data. The formation of LCBs by connecting the two growing polyolefin chains can facilitate the generation of polymers with molecular weights higher than that expected. Owing to the presence of LCBs, considerable shear thinning behavior is observed. Ethylene/1-octene copolymers can be prepared facilely to show almost the same shear thinning behavior with the commercial grade of low-density polyethylene, which is known to have a substantial amount of LCBs.

Trimethylolpropane trimethacrylate functionalized polypropylene/polyhexene-1 blend with enhanced melt strength

ABSTRACTThis study was aimed at investigating the variations in the melt strength of polypropylene as a result of blending with another polymer having long side branches in the presence of multi-functional agent and radical initiator. The formation of long chain branches was confirmed by strain hardening behavior in extensional viscometry and grafting efficiency data obtained from FTIR spectroscopy. strain hardening behavior, rheological parameters and grafting efficiency kept their ever-increasing trends by increasing the concentration of Trimethylol-propane-trimethacrylate (TMPTMA) until 3 phr, and the inefficiency of TMPTMA in branching reactions at concentrations higher than 3 phr is rooted in homo-polymerization of TMPTMA.

Ethylene polymerization to branched thermoplastic elastomers through proper activation of heterogeneous nickel (II) α-diimine complex and thermal drawing process

Ethylene polymerization with heterogeneous nickel (II) α-diimine complex, covalently attached to silica, under various conditions revealed effective control over polymer structure and activity. NMR, WAXD, thermal and mechanical characterization techniques provide the relationship between crystallinity and elastic properties and polymer microstructure. Catalytic activation with high ethylaluminium sesquichloride (EASC) amount (Al/Ni = 1500 and 2000) can improve productivity and induce long-chain branch formation. Increasing temperature, changes polyethylene behavior from properties characteristics of thermoplastics toward elastomers by only ethylene polymerization. Moreover, uniaxial stretching at melting, crystallization and onset of crystallization temperatures showed to have significant impact on crystallization behavior and mechanical properties enhancement. In particular, drawing at Tc resulted in formation of interlamellar crystallites followed by improving stress-at-break up to 200% and crystalline index to 80%.

The effect of ultrasonic extrusion on the structure and properties of BR gum and prepared compounds and vulcanizates

Abstract Ultrasonic extrusion of BR was conducted and processing characteristics were evaluated. Ultrasonic power consumption increased with the increase of ultrasonic amplitude. Die pressure decreased with the increase of ultrasonic amplitude indicating that the imposition of ultrasound can be used to increase extrusion output rate. The effect of ultrasonic amplitude on molecular structure of BR, including molecular weight and gel formation behavior were studied. Depending on the ultrasonic amplitude, structure changes occurred in BR, including a degradation, formation of long-chain branching and gel. Untreated and treated BR were mixed with CB, silica and silica/silane. Bound rubber content and flocculation behavior were studied. Formation of the long-chain branching was shown to increase the bound rubber content and rubber-filler interaction and to reduce the filler-filler interaction and flocculation of BR/silica compound. Both the untreated and treated BR, BR/CB, BR/silica and BR/silica/silane were vulcanized and their crosslink density, gel fraction and mechanical properties were measured. Based on DMA temperature sweep of BR/silica vulcanizate, it was shown that formation of the long-chain branching during ultrasonic treatment reduced the loss tangent at 60 °C, predicting a lower rolling resistance if the rubber used in tires.

Comparing Long‐Chain Branching Mechanisms for Ethylene Polymerization with Metallocenes and Other Single‐Site Catalysts: What Simulated Microstructures Can Teach Us

AbstractThree different long‐chain branch (LCB) formation mechanisms for ethylene polymerization with metallocenes in solution polymerization semi‐batch and continuous stirred‐tank reactors are modeled to predict the microstructure of the resulting polymer. The three mechanisms are terminal branching, C–H bond activation, and intramolecular random incorporation. Selected polymerization parameters are varied to observe how each mechanism affects polymer microstructure. Increasing the ethylene concentration during semi‐batch polymerization reduces the LCB frequency of polymers made with the terminal branching and intramolecular mechanisms, but has no effect on those made with the C–H bond activation mechanism, which disagrees with most previous data published in the literature. The intramolecular mechanism predicts that LCB frequencies hardly depend on polymerization time or ethylene conversion, which also disagrees with the published experimental data for these systems. For continuous polymerization reactors, experimental data relating polydispersity to LCB frequency can be well described with the terminal branching mechanism, but both C–H bond activation and intramolecular models fail to describe this experimental relationship. Therefore, detailed simulations confirm that the terminal branching mechanism is indeed the most likely mechanism for LCB formation when ethylene is polymerized with single‐site coordination catalysts such as metallocenes in solution polymerization reactors.

Synthesis of Polyethylenes with Controlled Branching with α-Diimine Nickel Catalysts and Revisiting Formation of Long-Chain Branching

The synthesis of polyethylenes with precise branching, especially long-chain branching (LCB), using ethylene monomer as a single feedstock is of a significant academic and industrial interest. On the basis of the ortho-aryl effect, a series of α-diimine nickel complexes with monoaryl-substituted anilines has been designed and prepared for the synthesis of the polyethylenes with controlled branching. The introduction of the ortho-aryl on aniline moieties enhanced the branching control ability of the α-diimine nickel catalysts. A different mechanistic model was proposed to interpret the presence of methyl and LCB but absence of other short branches in the obtained polyethylenes. LCB was formed by ethylene insertion into the primary Ni-alkyl species originating from nickel migration to methyl terminal of the growing chain because of restricted ethylene insertion into secondary Ni-alkyl species with an α-ethyl or a bulkier alkyl group.

Kinetics of Long-Chain Branch Formation in Polyethylene

Long-chain branch (LCB) formation was measured in polyethylene homopolymers and 1-hexene–ethylene copolymers made using a variety of metallocene and chromium catalysts while the reaction ethylene concentration was varied. Kinetic analysis of this data indicates a marked difference between LCB formation versus short-chain branch (SCB) formation from the incorporation of 1-hexene comonomer. SCB generation exhibits first-order dependence on [comonomer/ethylene] concentration whereas LCB creation does not, which indicates different insertion pathways for the two. SCB formation follows the expected random intermolecular incorporation, whereas LCB generation is consistent with a different, and more selective, intramolecular mechanism of macromer incorporation. The latter holds that, once it is terminated by an active site, the macromer tends to remain coordinated to the site for a short time before either dissociation or insertion into a successor chain growing on the same site to form a LCB. In this way, struc...

Upcycling of polypropylene—the influence of polyethylene impurities

Long chain branching (LCB)—a well‐known industrial process—is shown as an innovative tool for the treatment of PP post‐consumer waste. The introduction of LCB by reactive extrusion does not only compensate the degradation during product life (e.g., thermally and UV‐induced chain scission), it also improves the melt properties (e.g., melt strength, strain hardening). Thus, not only a re‐cycling process, even a real “up‐cycling” can be achieved. Compared with virgin material, PP from post‐consumer waste contains impurities like other polyolefines (PE‐HD, PE‐LD, PE‐LLD, copolymers), the total removal is economically not viable. Hence, the focus of this work was the influence of PE‐HD on the LCB formation of PP. Based on model mixtures with virgin PP and 10% PE‐HD, it is shown that PE‐HD influences the mechanical properties and gel content of the chemically modified blend but has no detrimental effect on the improved melt properties. POLYM. ENG. SCI., 57:1374–1381, 2017. © 2017 Society of Plastics Engineers

Continuous modification of polypropylene via photoinitiation

A twin screw extruder was used for continuous modification of polypropylene (PP) via UV radiation. Long chain branches (LCBs) were incorporated in the PP backbone to modify its rheological properties. Benzophenones (BPH) as photoinitiator and trimethylolpropane triacrylate (TMPTA) as coagent were utilized during PP photomodification. Radiation was carried out after mixing in the extruder on solid stretched strands with ∼0.3 mm thickness. The effects of photoinitiator concentration, radiation time, and coagent presence were studied via a replicated two-level full factorial design of experiments. It was shown that photomodification of PP can be done continuously. Formation of LCBs in the experimental runs was confirmed via rheological measurements. Gel content of the samples was also measured. It was found that LCBs can be formed in PP with and without TMPTA at certain processing conditions. The amount of gel in the samples prepared with TMPTA was higher; however, the gel content could be controlled by manipulating BPH concentration and radiation time. POLYM. ENG. SCI., 55:2423–2432, 2015. © 2015 Society of Plastics Engineers

Efficiency of curcumin, a natural antioxidant, in the processing stabilization of PE: Concentration effects

The stabilising efficiency of curcumin was studied in polyethylene during processing and under oxygen at high temperature. The effect of the natural antioxidant was investigated at concentrations of 0–1000 ppm in combination with a phosphonite secondary antioxidant (Sandostab P-EPQ) of 1000 and 2000 ppm, respectively. The polymer was homogenized with the additives then processed by six consecutive extrusions taking samples after each processing step. The samples were characterized by FT-IR spectroscopy, melt flow index, colour, and OIT measurements. Compared to the effect of pure phosphorous antioxidant, the melt stability of PE is increased already at 5 ppm curcumin content. The melt as well as the high temperature oxidative stability (OIT) of the polymer are controlled by both types of antioxidants. Curcumin hinders the oxidation of polyethylene and the formation of long chain branches during processing, which can be attributed to the fact that curcumin is not only a hydrogen donor but its unsaturated linear moiety can also scavenge alkyl and oxygen centred macroradicals. Curcumin discolours polyethylene already at small concentrations but the colour fades with increasing number of extrusions.

The Preparation and Characterization of Branching Poly(ethylene terephthalate) and its Foaming Behavior

Chain extension was an effective method for increasing the molecular weight, the melt strength and the foaming property of linear polymer. In this paper, pyromellitic dianhydride (PMDA) was used as chain extender to improve these properties of linear poly (ethylene terephthalate) (PET). The intrinsic viscosity, rheological and thermal characterizations of various PET samples was investigated. The results demonstrated that the increasement of the viscoelasticity at low frequencies was correlated to the raise of the intrinsic viscosity and the formation of long chain branching. These structural changes resulted in the decreasement of the crystallization temperature and melt temperature as well as the increase in the cold crystallization values with the increasing content of PMDA. The cellular morphology and expansion ratio of CEPET foams were also obviously improved by the introduction of PMDA. The expansion ratio of CEPET foam with the PMDA content of 1.0 phr would reach 31.78. In addition, the effect of the chain extension reaction time on the intrinsic viscosity, the rheological behavior, and foaming properties of PET were also studied. The results showed that the intrinsic viscosity, the rheological behavior, and the foamability of CEPET also decreased gradually with increasing chain extension reaction time, which should be attributed to the occurrence of more and more intense thermal degradation.