Effect of leaving group in dithiocarbamates on mediating melt radical reaction during preparing long chain branched polypropylene

The effectiveness of dithiocarbamates in mediating melt radical reaction during preparing long chain branched polypropylene (LCB-PP) was studied. ¹H NMR, FTIR, GPC-TALLS and rheological measurements were used to characterize the structure of the resultant LCB-PP. The results showed that adding dithiocarbamates could help forming more long chain branches on PP, and furthermore the chemical structure of the leaving group (R) of dithiocarbamates played a key role in adjusting melt reaction of PP in the presence of peroxide and multifunctional monomer. When the R formed a stable radical (compared to PP macroradicals) after leaving from the dithiocarbamate, such as allyl or benzyl radicals, the presence of dithiocarbamates could reduce the chain scission of PP due to the formation of dormant state species. In this case, the polymerization degree of grafted monomer was also reduced comparing with conventional radical polymerization under the same conditions. Thus more multifunctional monomer could react with PP macroradical to form more branching points and then more uniformly distributed LCB structure on the PP backbone. The reaction mechanism was investigated through model reactions.     

Influence of the chemical structure of dithiocarbamates with different R groups on the reversible addition‐fragmentation chain transfer polymerization

Four dithiocarbamates, carbazole‐9‐carbodithioic acid benzyl ester (R1), carbazole‐9‐carbodithioic acid naphthalen‐1‐ylmethyl ester (R2), 2‐(carbazole‐9‐carbothioylsulfanyl)‐2‐methyl‐propionic acid ethyl ester (R3), and (carbazole‐9‐carbothioylsulfanyl)‐phenyl‐acetic acid methyl ester (R4), were synthesized and used to the reversible addition‐fragmentation chain transfer (RAFT) polymerizations of styrene (St), methyl methacrylate (MMA), and methyl acrylate (MA), respectively. The influence of chemical structure of dithiocarbamates with different R groups on the RAFT polymerizations was investigated. The results showed that the four RAFT agents were effective RAFT agents for the polymerizations of styrene or MA, and that the polymerizations were well‐controlled with the characteristics of controlled/“living” polymerization. The polymerization rate of styrene with thermal initiation was markedly influenced by the chemical structures of the group R in dithiocarbamates, and decreased in the order of R3 > R2 > R4 > R1. For the polymerization of MA, the efficiency of RAFT agents was in the following order: R2–R3 > R1 > R4. However, they were not efficient enough to control the polymerization of MMA. The obtained polystyrene (PSt) with carbazole group labeled strongly absorbed UV light at 294 nm and emitted fluorescent light in N,N‐dimethyl formamide (DMF). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 982–988, 2007     

Inhibition of cholesterol biosynthesis by organosulfur compounds derived from garlic

The study was undertaken to test the inhibitory potential on cholesterogenesis of organosulfur compounds derived from garlic. The primary rat hepatocytes maintained in Dulbecco's modified Eagle's medium were treated with [2-¹⁴C]-acetate as substrate for cholesterol synthesis in the presence or absence of test compounds at 0.05 to 4.0 mmol/L. Eleven watersoluble and six lipid-soluble compounds of garlic were tested. Among water-soluble compounds,S-allyl cysteine (SAC),S-ethyl cysteine (SEC), andS-propyl cysteine (SPC) inhibited [2-¹⁴C]acetate incorporation into cholesterol in a concentration-dependnet manner, achieving 42 to 55% maximal inhibition. γ-Glutamyl-S-allyl cysteine, γ-glutamyl-S-methyl cysteine, and γ-glutamyl-S-propyl cysteine were less potent, exerting only 16 to 29% maximal inhibitions. Alliin,S-allyl-N-acetyl cysteine,S-allylsulfonyl alanine, andS-methyl cysteine had no effect on cholesterol synthesis. Of the lipid-soluble compounds, diallyl disulfide (DADS), diallyl trisulfide (DATS), and dipropyl disulfide (DPDS) depressed cholesterol synthesis by 10 to 25% at low concentrations (0.5 mmol/L), and abolished the synthesis at high concentrations (1.0 mmol/L). Diallyl sulfide, dipropyl sulfide, and methyl allyl sulfide slightly inhibited [2-¹⁴C]acetate incorporation into cholesterol only at high concentrations. The complete depression of cholesterol synthesis by DADS, DATS, and DPDS was associated with cytotoxicity as indicated by marked increase in cellular LDH release. There was no apparent increase in LDH secretion by water-soluble compounds exceptS-allyl mercaptocysteine, which also abolished cholesterol synthesis. Judging from maximal inhibition and IC₅₀ (concentration required for 50% of maximal inhibition), SAC, SEC, and SPC are equally potent in inhibiting cholesterol synthesis.     

Preparation and characterization of long chain branched polypropylene mediated by different heteroaromatic ring derivatives

Three different heteroaromatic ring derivatives (coagents) were used to adjust the melt radical reaction of polypropylene (PP). From the results of high temperature ¹H NMR and high-temperature size-exclusion chromatography (HT-SEC), all the coagents could quickly convert the tertiary PP macroradicals into resonance stabilized macroradicals and effectively restrain the β-scission of PP macroradicals. Long chain branched structures, without significant bimodal distribution of molecular weight, could be introduced into coagents-functionalized PP samples. Effect of chemical structures of heteroaromatic rings and electron-attracting groups on the molecular structures and melt properties of modified PP samples was studied. The possible reactions of coagents-functionalized PP samples in different theoretical molar ratio of coagent to alkoxy radical were also proposed according to the results of ¹H NMR, torque curves, rheological measurements and HT-SEC coupled with laser light scattering detector.     

Controlling melt reactions during preparing long chain branched polypropylene using copper N,N-dimethyldithiocarbamate

Effect of copper N,N-dimethyldithiocarbamate (CDD) on melt reactions during preparing long chain branched polypropylenes (LCB-PP) via free radical grafting was studied. The structure and rheological properties of the modified PPs were characterized. The results showed that CDD could efficiently control two side reactions, i.e. degradation of PP backbone and homopolymerization of multifunctional monomer (trimethylol propane triacrylate (TMPTA)) in the presence of peroxide. Meanwhile the addition of CDD also increased the efficiency of forming LCB structure. The reaction between CDD and active free radicals (carbon centered and alkoxy species) led to forming in situ dithiocarbamate radicals, which cannot attack PP backbone and are weaker initiator for TMPTA. The resultant dithiocarbamate radicals could react with the PP macroradicals and the acrylic radicals reversibly, which prolong the life time of PP macroradical and increase the reaction probability between macroradicals. The obtained LCB-PP showed high melt strength.     

Reversible addition‐fragmentation chain transfer (RAFT) polymerization of styrene using novel heterocycle‐containing chain transfer agents

BACKGROUND: Controlled/‘living’ radical polymerization is a new and robust method to synthesize polymers with predetermined molecular weight, narrow polydispersity and tailored architecture. Several methods have been developed but reversible addition‐fragmentation chain transfer (RAFT) has several advantages over the other methods. It has been reported that the effectiveness of RAFT agents depends strongly on the nature of the Z and R groups. RESULTS: Three new dithiocarbamates, namely (2‐ethoxy carbonyl)‐prop‐2‐yl‐pyrrole‐1‐carbodithioate (CTA‐A), (1‐phenyl ethyl)‐pyrazole‐1‐carbodithioate (CTA‐B) and (2‐ethoxy carbonyl)‐prop‐2‐yl‐pyrazole‐1‐carbodithioate (CTA‐C), were synthesized for studying the effect of the Z and R group of a chain transfer agent on the RAFT polymerization of styrene, initiated by 2,2′‐azobisisobutyronitrile. Well‐controlled molecular weight with narrow polydispersity (1.10–1.46) was achieved. The increase in molecular weight with conversion is linear and follows first‐order kinetics. CONCLUSION: The detailed kinetic results show that the structure of the activating (Z) group of dithiocarbamates has significant effects on the reactivity of dithiocarbamates towards the polymerization of styrene. In the homopolymerization of styrene it was found that, from the polydispersity index of polystyrenes obtained and the kinetic results, the pyrazole‐based dithiocarbamates (CTA‐B and CTA‐C) are very effective compared to the pyrrole‐based dithiocarbamate (CTA‐A). All the polymerizations show controlled living characters. Copyright © 2007 Society of Chemical Industry     

RAFT polymerization of N-vinylcaprolactam and effects of the end group on the thermal response of poly(N-vinylcaprolactam)

The reversible addition–fragmentation chain transfer (RAFT) radical polymerization of N-vinylcaprolactam (NVCL) was performed using either S-benzyl-S-(benzyl propionate) trithiocarbonate (CTA 1) or N, N-diethyl-S-(α,α′- dimethyl-α″-acetic acid) dithiocarbamate (CTA 2) as a chain transfer agent (CTA). The polymerizations were controlled processes that yielded polymers with high conversion (>60%), controlled molecular weights that were close to the theoretical values and a narrow molecular weight distribution (minimal value: 1.13). The cloud point temperatures of poly(N-vinylcaprolactam) (PNVCL) were measured by turbidimetry and shifted to lower temperature and concentrations as the hydrophobicity of the end groups and the molecular weights of the polymers increased.     

Aluminated hierarchical silicalite-2 particles: Catalyst with remarkably increased lifetime for methanol to hydrocarbons

Aluminated hierarchical silicalite-2 with both large particle size and high external surface area was prepared by aluminating silicalite-2 under the protection of TBA⁺ cations in a mild alkaline solution. The textural and acidic properties of thus formed material (Z11-meso) were characterized by SEM, TEM, XRD, N₂ adsorption, FTIR, NH₃-TPD, and ²⁷Al NMR spectroscopy. The resulting hierarchical Z11-meso contained mainly tetrahedral coordinated aluminum species with low Brønsted/Lewis ratio (B/L) while preserved the zeolitic structure. Owning to the high external surface area (S_ext), hierarchical porous structures and low B/L ratio, Z11-meso outperformed the microporous ZSM-11 counterparts in methanol to hydrocarbons in terms of both activity and stability.     

Morpholine‐based RAFT agents for the reversible deactivation radical polymerization of vinyl acetate and N‐vinylimidazole

Reversible addition–fragmentation chain transfer (RAFT) polymerization of less‐activated monomers in a controlled fashion is challenging due to the high reactivity and instability of the propagating radicals. We have designed dithiocarbamate‐based RAFT agents with morpholine as activating ‘Z’ group and benzyl, ethyl(1‐ethanoate)yl, ethyl(2‐propanoate)yl and cyanomethyl as ‘R’ leaving groups and investigated them for the reversible deactivation radical polymerization of vinyl acetate (VAc) and N‐vinylimidazole (N‐VIm). RAFT polymerization of VAc and N‐VIm at 70 °C using azobisisobutyronitrile as a free radical initiator proceeded in a controlled fashion as demonstrated by a linear increase in molar mass with conversion. Interestingly, the polymerization of VAc followed fast kinetics (approx. 60 min) with good to moderate control affording high‐molar‐mass poly(VAc) polymers. Furthermore, the synthesized chain transfer agents were able to polymerize N‐VIm under controlled conditions. The morpholine RAFT agents bearing cyanomethyl and ethyl(2‐propanoate)yl leaving groups showed better control of the polymerization of VAc and N‐VIm compared to the others. © 2020 Society of Chemical Industry     

The nucleation effect of N,N′-bis(benzoyl) alkyl diacid dihydrazides on crystallization of biodegradable poly(l-lactic acid)

Five N,N??-bis(benzoyl) alkyl diacid dihydrazides were synthesized from benzoyl hydrazine and alkyl diacyl dichloride which were derived from alkyl diacid via acylation. PLLA/N,N??-bis(benzoyl) alkyl diacid dihydrazide samples were prepared by melt blending and hot-press forming process. The nucleation effect of N,N??-bis(benzoyl) alkyl diacid dihydrazide on crystallization of biodegradable poly(l-lactic acid) (PLLA) was investigated using differential scanning calorimetry (DSC) and vicat softening analysis. The results showed that five N,N??-bis(benzoyl) alkyl diacid dihydrazides acted as powerful nucleating agent for PLLA; with incorporation of N,N??-bis(benzoyl) alkyl diacid dihydrazide, the crystallization peak became sharper and shifted to higher temperature as the degree of supercooling decreased at a cooling rate of 1?°C/min from melt. The nucleation activities of five N,N??-bis(benzoyl) alkyl diacid dihydrazides were quantitatively determined. It is shown that N,N??-bis(benzoyl) suberic acid dihydrazide has higher nucleating activity than the other N,N??-bis(benzoyl) alkyl diacid dihydrazides. In the presence of N,N??-bis(benzoyl) alkyl diacid dihydrazide, the melting behavior of PLLA is affected significantly. In addition, the thermal stability of PLLA/0.8?% N,N??-bis(benzoyl) alkyl diacid dihydrazide is tested and reported. Compared to the neat PLLA, the onset degradation temperature of PLLA/0.8?% N,N??-bis(benzoyl) alkyl diacid dihydrazide samples has been decreased significantly.     

Antioxidation and mechanism of phosphites including the free phenolic hydroxyl group in polypropylene

Inhibiting the degradation of polypropylene (PP) in melt processing and usage is an important issue in the plastic industry. It is becoming more and more urgent to increase the antioxidation of phosphites alone while maintaining the water resistance. In this study, one phosphite antioxidant, named bis‐2,2′‐methyl‐4,6‐di‐tert‐butylphenyl phosphite (BM46TBPP), which contains a water‐resistant inner ring and a free phenolic hydroxyl group together, was synthesized. Then, antioxidation in PP was characterized with multiple extrusions and oxidation induction times (OITs). Finally, the hydrogen‐donating ability of this antioxidant was tested with 2,2‐diphenyl‐1‐picrylhydrazyl radical colorimetry to explain the antioxidation mechanism. The results show that the phosphite BM46TBPP displays better antioxidation than tris(2,4‐di‐tert‐butylphenyl) phosphite (Irgafos 168) in melt processing and OIT testing. Furthermore, the priority of this antioxidant was more obvious when it was used in the presence of oxygen, so the antioxidant even made the PP stabilized by it alone show a longer OIT value than the PP stabilized by the complex system including Irgafos 168 and 2,4‐di‐tert‐butylphenol because there was a free phenolic hydroxyl group in the BM46TBPP antioxidant molecule and the hydroxyl group made the antioxidant show intermolecular synergistic antioxidation through hydrogen donation to radicals. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44696.     

Effect of oxygen functional groups of reduced graphene oxide on the mechanical and thermal properties of polypropylene nanocomposites

Reduced graphene oxide (rGO) with various surface structures was prepared by reducing graphene oxide (GO) with hydrazine hydrate (N₂H₄), sodium borohydride (NaBH₄) and l ‐ascorbic acid, respectively. The resulting rGO were used to fabricate rGO/polypropylene (PP) nanocomposites by a melt‐blending method. The surface structure of rGO as well as multifunctional properties of rGO/PP nanocomposites were thoroughly investigated. It was shown that rGO with highest C/O ratio could be obtained by reducing GO with N₂H₄. The crystallization behaviors, tensile strength, thermal conductivity and thermal stability of rGO/PP nanocomposites were significantly improved with the increase of C/O ratio of rGO. For example, with only 1 phr (parts per hundred PP) rGO reduced by N₂H₄, the degree of crystallinity, tensile strength, maximum heat decomposition temperature and thermal conductivity of PP nanocomposite were increased by 6.2%, 20.5%, 48.0 °C and 54.5%, respectively, compared with those of pure PP. Moreover, the thermal degradation kinetics indicated that the decomposition activation energy of rGO/PP nanocomposites could be enhanced by adding rGO with higher C/O ratio. © 2018 Society of Chemical Industry     

Isomerization of olefins and substituted olefins catalyzed by nickel complexes

The catalytic activity of several Ni (diphosphine)₂/acid homogeneous systems (acid = HCN, CF₃COOH, CCl₃COOH, and H₂SO₄) in the double bond and cistrans isomerizations of olefins has been tested with the following compounds: pentene-1, cis-pentene-2, trans-pentene-2, allyl benzene, allyl cyanide, allyl alcohol. The catalytic efficiency of the Ni(diphosphine)₂/acid systems depends markedly on the nature both of the diphosphine and of the acid employed. When the diphosphine is 1,4-bis(diphenylphosphino)butane and acid = HCN a particularly efficient catalytic system is formed in situ and only in the case of the allyl cyanide is the isomerization limited. During the isomerization of pentene-1, the stereoselectivity in the formation of the cis-2 and trans-2 isomers is quite dependent on the nature of the cocatalyst. It is proposed that a σ-alkyl mechanism is operating in the $\text{Ni(O)}\text{CF}{}_3\text{COOH}$ system and a π-allyl one in the case of $\text{Ni(O)}\text{HCN}$.     

Reaction and formation mechanism of Fe-Si3N4 composite prepared by flash combustion synthesis

In this paper, in order to improve the performance of Fe–Si₃N₄ composite synthesized by flash combustion, the detailed nitridation mechanism and formation process were discussed. In the process of high temperature nitriding, ξ phase rapidly melted to form Fe-Si melt and the cycle of rapid surface nitriding → rupture of nitridation shells → new melt exposing and nitriding occurred continually on the surface of Fe-Si melt. Since ions have different activities on the surface and internal, Fe ions near the surface migrated inwards and Si ions inside moved out; meanwhile, the Fe-Si melt kept shrinking. The nitriding reaction of the Fe-Si melt finished till the overall activity a_Si approached 0, leaving the atom ratio of [Fe] to [Si] at 3:1. During the falling of the formed Si₃N₄ and Fe₃Si melt in the N₂ flow, the surface of Si₃N₄ was oxidized to form a SiO₂ film. The nitridation product fell into the product pool, loosely stacking and adhering together by the SiO₂ film. α-Si₃N₄ dissolved and precipitated to form β-Si₃N₄ crystals, and the β-Si₃N₄ crystals kept growing to form radioactive elongated crystals. As the temperature decreased, the Fe₃Si melt cooled down; the Si-N-O melt, α-Si₃N₄ and the roots of elongated β-Si₃N₄ crystals formed the dense areas.     

A structural evidence for the preferential coordination of the primary amide group versus the unionised carboxyl group: synthesis,molecular and crystal structure,and properties of [Cu(HADA)2], a new copper(II) bis-chelate (H2ADA=N-(2-carbamoylmethyl)iminodiacetic acid)

Bis(N-carboxymethyl-N-(2-carbamoylmethyl)glycinato)copper(II) has been synthesised by reaction of Cu₂(CO₃)(OH)₂ and N-(2-carbamoylmethyl)-iminodiacetic acid (H₂ADA) in water. The new compound [Cu(HADA)₂] (1) has been characterised by thermal, spectral, magnetic and X-ray diffraction methods. Its crystal structure was solved to a final value of R1=0.036. The crystal consists in a 3D hydrogen bonded network builds up by centro-symmetric molecules where the Cu(II) atom exhibits an elongated octahedral coordination. The ligand HADA⁻ adopts a fac-NO + O(apical amido) tridentate chelating conformation [Cu1–N1(amino)=2.063(2), Cu1–O31(carboxylato)=1.917(2), Cu1–O11(amido)=2.380(2) Å] and has a free N-carboxymethyl arm. A structural comparison between 1 and bis(N,N-bis(carboxymethyl)glycinato)copper(II) dihydrate (compound 2) reveals that the amide primary group is preferred to the unionised carboxyl group to form a Cu(II)–N, O-glycinamide-like chelate ring instead of a Cu(II)–N, O-glycine-like one.     

Effect of blend ratio on bulk properties and matrix-fibril morphology of polypropylene/nylon 6 polyblend fibers

Ternary blends of polypropylene (PP), nylon 6 (N6) and polypropylene grafted with maleic anhydride (PP/N6/PP-g-MAH) as compatibilizer with up to 50 wt% of N6 were investigated. PP-g-MAH content was varied from 2.5 to 10%. Blends of the two polymers PP/N6 (80/20) without the compatibilizer were also prepared using an internal batch mixer and studied. The ternary blends showed different rheological properties at low and high shear rates. The difference depended on the amount of N6 dispersed phase. Co-continuous morphology was observed for the blend containing 50% N6. This blend also exhibited higher viscosity at low shear rate and lower viscosity at high shear rates than the value calculated by the simple rule of mixture. At higher shear rates, viscosity was lower than that given by the rule of mixture for all blend ratios. An increase in viscosity was observed in the 80/20 PP/N6 blend after the concentration of the interfacial agent (PP-g-MAH) was increased. Polyblends containing up to 30% N6 could be successfully melt spun into fibers. DSC results showed that dispersed and matrix phases in the fiber maintained crystallinity comparable to or better than the corresponding values found in the neat fibers. The dispersed phase was found to contain fibrils. By using SEM and LSCM analyses we were able to show that the N6 droplets coalesced during melt spinning which led to the development of fibrillar morphology.     

Investigation on polypropylene and polyamide-6 alloys/montmorillonite nanocomposites

Propropylene (PP) and polyamide-6 (PA6) alloys nanocomposites were prepared using melt intercalation technique by blending PP and PA6 while used organophilic montmorillonite (OMT). The melt intercalation of PP and PA6 alloys was carried out in the presence of a compatibilizer such as maleic anhydride-g-polypropylene (MAPP). Their structures were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and High Resolution Electronic Microscope (HREM). It was found different blend sequences have influence on the dispersibility of OMT and self-assembled structures of OMT appeared in PP and PA6 alloys. The crystallization behavior and crystal structure of PP and PA6 alloys/montmorillonite nanocomposites were investigated by X-ray diffraction. It showed that the blend sequences have influence on crystal structure and a higher cooling rate results in increasing of γ-crystalline phase. Flammability properties are characterized by Cone Calorimeter, which show an unusual phenomenon.     

Polypropylene-grafted nanoparticles as a promising strategy for boosting physical properties of polypropylene-based nanocomposites

SiO₂ nanoparticles grafted to terminally hydroxylated polypropylene (PP-g-SiO₂) with different molecular weights were melt mixed with PP to prepare a series of PP/PP-g-SiO₂ nanocomposites. PP/PP-g-SiO₂ offered several advantages over pristine PP and PP/unmodified SiO₂ such as highly uniform dispersion up to 10 wt.-%, +200–400% faster crystallization and +30% increments for both the Young's modulus and the tensile strength without largely sacrificing the melt viscosity of PP. We concluded that grafted chains act as crystallization nuclei and co-crystallize with matrix chains to make PP-g-SiO₂ nanoparticles as a physical cross-linker between lamellae, while the linkage disappears in melt and grafted chains minimize the cohesive attraction between nanoparticles.     

Epitaxial crystallization of poly(butylene adipate) on highly oriented isotactic polypropylene thin film

The crystallization behaviors of PBA on highly oriented iPP substrate both from solution and isotropic melt were studied by means of optical microscopy, AFM, X-ray and electron diffractions. The results clearly indicate the occurrence of heteroepitaxy of PBA on the iPP substrate in its β-form with both molecular chains ±50° apart from. This is based on the existence of perfect matching between the interchain distance of β-PBA along [100] direction and the distance of the out-sticking methyl side group arrays along the [101] direction of the (010) iPP plane. Electron diffraction pattern further confirms that the (010) lattice plane of the β-PBA is in contact with the iPP substrate.     

Preparation and structure of an enantiomeric water-bridged dinuclear indium complex containing two homochiral N atoms and its performance as an initiator in polymerization of rac-lactide

A novel dinuclear indium complex (1) containing a sole water bridge was prepared from the reaction of InCl₃ with an unsymmetric N₂O₂-ligand and the molecular structure of the complex 1 was determined. The X-ray crystallography data show that the dinuclear complex possesses two homochiral N atoms, namely, N(R)N(R) and N(S)N(S) enantiomers. Complex 1 was used as initiator for ring-opening polymerization of rac-lactide, giving heterotactic-rich polylactide (P_r = 0.63–0.69) with narrow polydispersities (1.13–1.31).