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.     

A new grafting monomer for synthesizing long chain branched polypropylene through melt radical reaction

A grafting monomer (p-(3-butenyl)styrene, BS)) containing two carbon–carbon double-bonds with different reactivity was used in melt radical reaction to prepare long chain branched polypropylene (LCB-PP). High-temperature size-exclusion (HT-SEC) coupled with triple detectors, rheometer and NMR were used to characterize the microstructure of the resultant LCB-PP. BS showed double functions in mediating radical reaction, i.e. stabilizing macroradicals and promoting branching reaction. Compared to styrene and divinylbenzene, using BS as a grafting monomer led to the formation of more uniformly distributed LCB structure on the PP backbone. Thus the resultant sample showed strain hardening in extensional flow and had high melt strength.     

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.     

Combined effects between activating group Z and leaving group R in dithiocarbamates for controlling degradation and branching reactions of polypropylene

Four dithiocarbamates with different chemical structures (S-Propyl N,N-dipropyldithiocarbamate, S-allyl N,N-dipropyldithiocarbamate, S-Propyl N-pyrrolocarbodithioate and S-allyl N-pyrrolocarbodithioate) were used to adjust the melt radical reaction of polypropylene (PP). The combined effects between activating group Z (N,N-dipropyl and N-pyrrolo) and leaving group R(allyl and propyl) in the dithiocarbamates on melt reaction of PP were studied in the presence of peroxide and trimethylolpropane triacrylate monomer. The results from ¹H NMR, FTIR, GPC, rheological measurements and model experiments showed that chemical structures of activating group Z and leaving group R played a key role in the molecular structures and melt properties of modified PP samples. The presence of S-allyl N-pyrrolocarbodithioate with stronger activating group N-pyrrolo and easier leaving group allyl led to the formation of long chain branching structure with more branched points (comb-like topology). The possible reactions in the above systems were also discussed.     

Fluorescent polyolefins by free radical post-reactor modification with functional nitroxides

The 4-(1-naphthoate)-2,2,6,6-tetramethylpiperidine-1-oxyl (NfO-TEMPO) has been synthesized and successfully grafted in the melt onto a random poly(ethylene-co-1-ottene) copolymer. Functionalized polyolefins have been prepared by coupling reaction between NfO-TEMPO free radicals and macroradicals which have been formed by H-abstraction induced by the presence of a peroxide. In order to deepen insight into the functionalization mechanism, the reaction has been investigated by Electron Paramagnetic Resonance (EPR). EPR spectra collected during the reaction run, have evidenced the decrease of TEMPO signal as a consequence of temperature increasing. This decrease has been attributed to the formation of a covalent bond between macroradicals and nitroxide free radical. The resulting functionalized polyolefins, PO-g-(NfO-TEMPO), have been characterized by FT-IR and ¹H NMR which has allowed to evaluate their functionalization degree, whereas UV–Vis and fluorescence spectroscopy have been used to investigate their optical properties. The comparison with low molecular weight model compounds, has allowed to state that our methodology can be conveniently adopted to prepare fluorescent polyolefins where the optical properties of the chromophore has been completely transferred to polymer backbone both in solution and in the condensed phase.     

Peroxy-initiated chain degradation,crosslinking, and grafting in PP–PE blends

Polyolefines are frequently functionalized with polar monomers via peroxide-initiated grafting that starts at macroradicals. However, polyolefine macroradicals also undergo undesired secondary reactions. Polypropylene (PP) is degraded, while polyethylene (PE) is crosslinked. Mechanistically, PP radicals are split by β scission while PE radicals recombine to clusters. If these opposed tendencies can be balanced in PP–PE blends was investigated in this study. In principle, coupling of PP and PE radicals can lead to graft copolymers PE-g-PP. But the chances for graft reactions are good only in homogenous PP–PE blend melts. It is difficult to decide whether these blends are, in fact, in the melt one-phase or two-phase systems because molten PP and PE are too similar in all respects. PP–PE blends were processed with peroxide in the melt and, for comparison, also in solution. According to viscosity and gel permeation chromatography and differential scanning calorimetry results, graft reactions occurred only in the (presumably homogeneous) blend solutions in which degradation was subdued and crosslinking prevented. In PP–PE blend melts, on the contrary, the two polymers reacted fairly independently of each other. PP was degraded, and PE crosslinked. Apparently, these blend melts, although transparent, are two-phase systems. © 1998 John Wiley & Sons, Inc. J Appl Polm Sci 68: 2019–2028, 1998     

Light-induced crosslinking polymerization of a novel N-substituted bis-maleimide monomer

The photoinduced polymerization of a N-alkyl substituted bis-maleimide (MI) has been studied by real-time infrared spectroscopy. The polymerization of the neat monomer proceeds rapidly and extensively because of both its great absorbance in the UV-range, and the presence on the alkyl chain of easily abstractable hydrogen atoms, which are needed to produce the initiating radicals. Such photoinitiator-free resin was found to be less sensitive to oxygen inhibition than typical UV-curable acrylate resins, the peroxyl radicals formed being capable to propagate the chain reaction. This N-substituted bis-maleimide was also successfully used as monomeric photoinitiator to induce the crosslinking polymerization of acrylate monomers through the free radicals formed upon its photolysis.     

Functionalization of polyolefins with maleic anhydride in melt state through ultrasonic initiation

The functionalization reaction of high-density polyethylene (HDPE), linearly low-density polyethylene (LLDPE), polypropylene (PP) and EPDM rubber with maleic anhydride (MAH) in melt state through ultrasonic initiation was studied. The effect of ultrasonic intensity on the percentage of grafting, viscosity-average molecular weight, melt flow rate and gel content of the functionalized products were investigated by means of chemical titration, Fourier-transform infra-red spectroscopy (FT-IR), intrinsic viscosity and melt flow rate, etc. The molecular structures of the functionalized products prepared via ultrasonic initiation and via peroxide initiation were characterized by ¹H NMR spectroscopy. The results show that the functionalization reaction of HDPE, LLDPE and EPDM with MAH can be realized by ultrasonic initiation. This reaction mainly consists of the chain scission under ultrasonic irradiation, the end chain reaction of the produced macroradicals with MAH, and the terminated reaction of the produced succinyl radicals with the macroradicals or H* radicals through recombination or dismutation. The functionalized product through ultrasonic initiation mainly consists of the products containing an anhydride ring attached to the chain terminus. And the products prepared through peroxide initiation mainly contain an anhydride ring grafted on the side chain.     

Synthesis and characterization of highly functionalized polymers based on N,N,N′,N′-tetraalkyl-4,4′-diaminostilbene and maleic anhydride

Novel, highly functionalized rod-like copolymers have been synthesized by alternating copolymerization of N,N,N′,N′-tetraalkyl-4,4′-diaminostilbenes (TDASs) with maleic anhydride. These unique copolymers have been characterized by SEC, DSC and TGA. The solubility of these copolymers in organic solvents is strongly dependent on the length of the alkyl chains on the amino groups and the solubility in aqueous media is pH dependent. Light scattering studies indicate that the conformation of the polymer backbone does not change upon increasing temperature or introducing charges to the amino groups. High chain rigidity is further corroborated by the high T_g of the polymers. There is no observed glass transition below 280 °C. The light scattering and thermal results are indicative of a rod-like backbone structure.     

Novel chemical-bonded polymerizable sulfur-containing photoinitiators comprising the structure of planar N-phenylmaleimide and benzophenone for photopolymerization

As a continuation of the research on chemical-bonded photoinitiators comprising the structure of planar N-phenylmaleimide (NPMI) and benzophenone (BP), three novel polymerizable sulfur-containing photoinitiator MTPBP, CMTPBP and BMTPBP were synthesized by introducing NPMI group into benzophenone (BP). BP was selected as the reference to evaluate their photoefficiency. These novel photoinitiators possess greatly red-shifted UV maximal absorption, and their fluorescence emission varies. Three representative types of different functionality monomers, methyl methacrylate (MMA), 1,6-hexanediol diacrylate (HDDA) and trimethylolpropane triacrylate (TMPTA), initiated by the three novel photoinitiators were studied through dilatometer and photo-DSC using unsaturated tertiary amine N,N-dimethylaminoethyl methacrylate (DMAEMA) as the coinitiator. The results show surprising high efficiency of these chemical-bonded photoinitiators towards different monomers in contrast to BP, and they can initiate photopolymerization without the coinitiator because of the photolysis at C-S bond. The results also verify that the higher viscosity of monomers and the larger molecular size of the photoinitiators may restrict the bimolecular H-abstraction reaction.     

Characterization of high melt strength propylene/1-butene copolymer synthesized by in situ heat induction melt reaction

Propylene/1-butene copolymer powders were produced through bulk copolymerization of propylene with 1-butene in a 12 m³ polymerization reactor. High melt strength polypropylene (HMSPP) was synthesized by in situ heat induction melt reaction, in which pure propylene/1-butene copolymer powders without any additives were used as a basic resin and trimethylolpropane triacrylate (TMPTA) as a crosslinking agent. The structure and properties of the resultant HMSPP were characterized by means of various measurements. The content of TMPTA strongly influenced the melt strength and melt flow rate (MFR) of HMSPP. With increasing the content of TMPTA, the melt strength of HMSPP increased, and the MFR reduced. In addition, owing to the existence of crosslinking structure, thermal stability and tensile strength of HMSPP were improved compared with pristine propylene/1-butene copolymer. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The rheological,thermostable, and mechanical properties of polypropylene/fullerene C60 nanocomposites with improved interfacial interaction

Polypropylene (PP)/C₆₀ nanocomposites with improved interfacial interaction were prepared via in situ melt radical reaction. It was found that the relaxation time of PP/C₆₀ nanocomposites containing peroxide increased due to the reaction between C₆₀ and PP macroradicals and the formation of long chain branched or crosslinking structure. Thermogravimetric analysis (TGA) results showed that the thermal stability of PP/C₆₀ nanocomposites was enhanced. The initial decomposition temperatures and activation energy of PP/C₆₀ nancomposites were strongly influenced by the content of unreacted C₆₀. Because of the improved interfacial interaction, PP/C₆₀ nanocomposites containing peroxide showed obvious increase in tensile strength, Young' modulus, flexural strength, flexural modulus, and impact strength, compared to PP/C₆₀ nanocomposites (without peroxide) containing the same content of C₆₀. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

A study of the interaction of mechanically generated polypropylene radicals with phenolic antioxidant in the absence of oxygen using ESR spectroscopy

Polypropylene (PP) was mechanically degraded in the absence and in the presence of a phenolic antioxidant, 2,6-di-t-butyl-p-cresol. The interaction of macroradicals of PP with the antioxidant was studied by ESR spectroscopy in the temperature region between 77 and 303 K and at antioxidant concentrations of 0.1, 0.5, and 1.5 wt % in the absence of oxygen. In the absence of the antioxidant, the end macroradicals of PP are formed which decay with increase in temperature. In the presence of antioxidant both macroradicals of PP and phenoxy radicals are generated. The PP macroradicals are very stable up to the temperature close to T_g of the polymer. Beyond this temperature they decay via bimolecular mechanism—according to the second-order kinetics. The rate constants for PP macroradicals decay were determined, and the apparent activation energies for the regions of slow and fast decay were calculated.     

Studies of UV crosslinked poly(N-vinylpyrrolidone) hydrogels by FTIR, Raman and solid-state NMR spectroscopies

Poly(N-vinylpyrrolidone) (PVP) hydrogels have become increasingly important materials for pharmaceutical and biomedical applications. UV-light initiated oxidative crosslinking of PVP represents a novel method for producing PVP based hydrogel materials. However, the mechanism of the gelation by this approach is poorly understood. In this study, the reaction mechanism for the crosslinking process is investigated by FTIR, Raman, and solid-state CP/MAS NMR techniques. Both FTIR and Raman spectra indicate that in the process of free radical oxidative crosslinking, the pyrrolidone ring is partially transformed into a succinimide ring. Solid-state NMR data have confirmed this change, and provided evidence that stable intermediates of 4-hydroperoxy-pyrrolidone (PVP-OOH) and its accompanied 4-hydroxy-pyrrolidone (PVP-OH) are formed. The pyrrolidone hydroperoxide intermediate can account for the efficient crosslinking, producing a sufficient level of macroradicals to form stable hydrogels.     

Structure and properties of high melt strength polypropylene prepared by combined method of blending and crosslinking

High melt strength polypropylene (HMSPP) was prepared by in situ heat induction reaction, in which pure polypropylene (PP) powders without any additives was used as basic resin, and low density polyethylene (LDPE) and trimethylolpropane triacrylate (TMPTA) were added as blending resin and as crosslinking agent, respectively. Microstructure of the obtained HMSPP (PP/LDPE/TMPTA blends) was characterized by FTIR, Wide‐angle X‐ray diffraction (WAXD), and testing of gel content. The effect of LDPE content on melt strength and melt flow rate of HMSPP were investigated. When the content of LDPE was 40 wt %, the melt strength of the HMSPP was above 16 CN, which was much higher than those of pure PP powder (2.6 CN) and PP/LDPE blends without TMPTA (6.1 CN). Moreover, thermal behavior and mechanical properties of the HMSPP were also investigated. The results showed that the thermal stability and impact strength of HMSPP were greatly improved. In addition, HMSPP possessed good processing performance and good foaming properties. The foams produced by HMSPP showed uniform, closed, and independent cells. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Poly(N-isopropylacrylamide) hydrogels with improved shrinking kinetics by RAFT polymerization

Functional poly(N-isopropylacrylamide) (PNIPAM) hydrogels were prepared by reversible addition fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) in the presence of N,N-methylenebisacylamide (BIS) as a cross-linker and 4-cyanopentanoic acid dithiobenzoate as chain transfer reagent (CTA). The swelling behaviors were investigated and the hydrogels by RAFT polymerization (RAFT gels) showed accelerated shrinking kinetics and higher swelling ratio comparing with conventional hydrogel (CG). It could be attributed to the presence of dangling chains mainly caused by CTA, which could retard the crosslinking reaction rate greatly. Another CTA, 3-(trithiocarbonyl) propanoic acid, was adopted to further investigate the effect of CTA. It showed the similar effect except the different accelerated degree to the shrinking kinetics. Furthermore, the living character of the RAFT process was used to polymerize a new batch of monomer (NIPAM) from functional RAFT gels to introduce grafted structure. The PNIPAM-g-PNIPAM hydrogels indicted further accelerated shrinking kinetics than functional backbone hydrogels.     

Supercritical carbon dioxide-assisted reactive extrusion for preparation long-chain branching polypropylene and its rheology

An environmental benign process, which uses supercritical carbon dioxide (ScCO₂) as a processing aid, is developed in this work to prepare long chain branching polypropylene (LCB-PP). Results from the oscillatory shear rheology, melt elongational behavior and Fourier transformed infrared spectroscopy (FTIR) show that long chains have been linked as branches to the original linear PP chains using scCO₂-assisted reactive extrusion in the presence of cumene hydroperoxide and 1,6-hexanediol diacrylate. Compared to the initial linear PP, the branched samples show higher storage modulus (G′) at low frequency, distinct strain hardening of elongational viscosity, lower melt flow rate, increased crystallization temperature and improvement of the melt strength. ScCO₂ can improve the branching efficiency of modified PPs. The elastic response, melt strength and strain hardening parameter of the modified PPs increase with increasing scCO₂ concentration, which is ascribed to scCO₂ acting as a plasticizer for reducing PP viscosity and a carrier for active chemical species.     

The preparation and rheology characterization of long chain branching polypropylene

In order to study the rheological behavior of long chain branching (LCB) polypropylene (PP), linear polypropylene was modified by melt grafting reaction in the presence of 2,5-dimethyl-2,5(tert-butylperoxy) hexane peroxide and pentaerythritol triacrylate (PETA) in mixer. The transient torque curves and Fourier transformed infrared spectroscopy (FTIR) results indicated that macroradical recombination reactions took place and PETA had been grafted onto PP backbone. Various rheological plots including viscosity curve, storage modulus, loss angle, Han plot, Cole-Cole plot were used to distinguish LCB PP from linear PP. On the other hand, to quantify the LCB level in modified PPs, a new method was suggested on the basis of macromolecular dynamics models. The results showed that the level of LCB was in the range of 0.025-0.38/10⁴ C . Moreover, the length of the branched chains and the content of the branched component increase with PETA concentration. Furthermore, the LCB efficiency of monomer can also be calculated, less than 20% of grafting monomers was used to form branch structure.     

挤出共混中的高剪切应力对PP-HDPE-滑石粉共混材料力学性能的影响

采用提高双螺杆挤出机螺杆转速的方法,研究了双螺杆挤出机的高剪切应力作用对聚丙烯(PP)、PP-滑石粉和PP-高密度聚乙烯(HDPE)-滑石粉共混材料力学性能的影响.结果表明:双螺杆挤出机的高螺杆转速、高剪切应力作用,可促进材料中滑石粉颗粒的均匀分散、促进HDPE均匀分散于PP基体的乙丙共聚物的相态之中,可引起PP或HDPE的断链反应、引起HDPE大分子自由基与PP基体中的EPR相以及与滑石粉表面偶联助剂之间的结合反应,引起共混材料缺口冲击强度的显著增大;少量极性烯类单体的加入和较低的挤出反应温度条件(180℃)有利于形成的大分子自由基与极性烯类单体的接枝、嵌段反应、界面间的偶联结合反应及其原位增粘作用,并引起PP-HDPE-滑石粉共混材料缺口冲击强度的进一步增大.     

Functional modification of poly(ethylene terephthalate) with an allyl monomer: Chemistry and structure characterization

Radical grafting of biocidal precursor monomer 3-allyl-5,5-dimethylhydantoin (ADMH) onto poly(ethylene terephthalate) (PET) fabric was investigated. Based on SEM pictures, it was suggested that benzoyl peroxide could generate macromolecular radicals on PET when it was delivered to the areas properly. The macroradicals were in a form of benzoyl structure which can react with ADMH to form a short chain graft. It seems the reaction occurred at the end of polymers or defect areas with terephthalate groups at the end. After being converted to N-chloramide structure, the poly(ADMH)-g-PET fabric demonstrates total kill of 10⁵-10⁶ CFU/mL Escherichia coli at a contact time of 2 h.