81 research outputs found
Challenges in Chain-Shuttling Polymerization: Novel Ethylene-norbornene Copolymers and vs. Commercial Ethylene-1-octene Copolymers,
No full textEthylene–co–norbornene copolymerization in the presence of a chain transfer agent
No full textEthylene (E)-co-norbornene (N) copolymerizations were conducted by using two ansa-metallocenes, isopropyliden(η5-cyclopentadienyl)(η5-indenyl)zirconium dichloride (1) and isopropyliden(η5-3-methylcyclopentadienyl)(η5-fluorenyl)zirconium dichloride (2), activated with dimethylanilinium tetrakis(pentafluorophenyl)borate, in presence of TIBA and variable amounts of Zn(Et)2 as a potential chain transfer agent (CTA). The study evaluated the effect of CTA on microstructure, molecular weight, chain end group and structural, thermal and mechanical properties of poly(E-co-N)s. The analysis of the data revealed that in all copolymerization reactions with metallocenes 1 and 2 in presence of diethyl zinc chain transfer to zinc alkyl occurs along with the typical chain transfers of E-co-N polymerization. The chain transfer to zinc alkyl is more efficient at low N content in copolymers. The two catalysts show different behavior in copolymerization at high N content: molar masses of copolymers produced by 2 are more affected by CTA concentration, that is, the chain shortening with increasing CTA content is greater in copolymers obtained with catalyst 2. The analysis of mechanical properties revealed that CTA affects the polymer chain structure. An increase of CTA caused a change of ductility, rigidity, and mechanical strength; chains produced in presence of Zn(Et)2 are, at least in part, characterized by segments produced as a result of exchange between different metal centers, possibly jointed through ethylene units
Copolimeri sindiotattici propene-1-olefina: analisi della microstruttura mediante spettroscopia 2D NMR.
No full textThe Role of CNTs in Promoting Hybrid Filler Networking and Synergism with Carbon Black in the Mechanical Behavior of Filled Polyisoprene
No full textThe filler networking process promoted by multiwalled CNTs is studied in neat and CB-filled poly(1,4-cis-isoprene) matrices. TEM analysis, tensile, dynamic-mechanical, and electrical measurements reveal that the CNTs form a filler network at low concentration in neat PI and a continuous hybrid filler network at a lower
CNT concentration in the presence of CB, with a remarkable increase of the nonlinear dynamic- mechanical behavior of the nanocomposites at low deformation. A synergistic effect between CB and CNTs is demonstrated. The addition of CNTs to the CB-filled PI matrix leads to initial modulus values much larger than those calcu- lated by simple addition of the two initial moduli of the composites containing only CB and only CNTs, respectively
Cycloolefin copolymers with new transition metal catalysts
No full textIl complesso [Ti(k2-N,O-{2,6-F2C6H3N=C(Me)C(H)=C(CF3)O})2Cl2] (1) è stato valutato come catalizzatore per la polimerizzazione vivente di etilene (E) con norbornene (N) dopo attivazione con metilalluminossano dry (d-MAO) in un range di temperature da 25 a 90 °C. La reazione di polimerizzazione eseguita a diversi rapporti [N]/[E] ha dato copolimeri P(E-co-N) stereoirregolari, alternanti e ad alto peso molecolare. La natura vivente del sistema 1/d-MAO è stata dimostrata tramite cinetica a 50 °C. Questo sistema catalitico è stato impiegato per la sintesi di copolimeri a blocchi così come PE-block-P(E-co-N) con un blocco di PE cristalino e un blocco amorfo di P(E-co-N) così come P(E-co-N)1-block-P(E-co-N)2, avente diversi contenuti di norbornene nei segmenti e quindi aventi diversi valori di Tg. Inoltre, uno studio NMR multinucleare è stato effettuato per stabilire la natura delle specie responsabili per la copolimerizzazione vivente E-N impiegando il sistema catalitico 1/d-MAO. L'analisi ha riguardato (i) il sistema catalitico (1 con MAO); (ii) le specie che si formano dopo l'aggiunta simultanea di etilene 13C arricchito e norbornene; e (iii) le specie che si formano durante l'omopolimerizzazione del norbornene. I risultati supportano l'esistenza di interazioni non covalenti dei sostituenti orto-fluoro col centro di titanio, che contribuiscono al carattere vivente della reazione di copolimerizzazione.Complex [Ti(k2-N,O-{2,6-F2C6H3N=C(Me)C(H)=C(CF3)O})2Cl2] (1) was evaluated as catalyst for living copolymerization of ethylene (E) with norbornene (N) upon activation with dried methylaluminoxane (d-MAO) at temperatures between 25 and 90 °C. Copolymerization performed at different [N]/[E] feed ratios afforded stereoirregular alternating high molar mass P(E-co-N) with narrow molar mass distribution. The living nature of E-co-N copolymerization by 1/d-MAO was demonstrated by kinetics at 50 °C. This catalyst system was used for the synthesis of block copolymers such as PE-block-P(E-co-N) with a crystalline PE block and an amorphous P(E-co-N) block as well as P(E-co-N)1-block-P(E-co-N)2, having different norbornene contents in the segments and thus having different Tg values.
Moreover, a multinuclear NMR spectroscopic study was performed to assess the nature of species responsible for living E-N copolymerization reaction using the catalytic system 1/d-MAO. The analysis involved (i) the catalytic system (1 with MAO); (ii) the species formed after simultaneous addition of 13C-enriched ethylene and norbornene and after further addition of 13C-enriched ethylene; and (iii) species formed during norbornene homopolymerization. The results supported the existence of noncovalent interactions of ortho-fluoroaryl substituents with the titanium center, which contribute to the living character of the copolymerization reaction.DIPARTIMENTO DI CHIMICA, MATERIALI E INGEGNERIA CHIMICA GIULIO NATTA27RESNATI, GIUSEPPEFARAVELLI, TIZIAN
Copolymerization Catalyzed by salen-Type Cr(III) Complexes
No full textOrganic quaternary phosphonium salts are used in conjunction with chromium salen complexes in the alternating copolymerization of CO2 and cyclohexene oxide (CHO) or propylene oxide (PO). A highly efficient formation of atactic polycarbonate with narrow distribution and molecular weights comparable or superior to those obtained with bis(triphenylphosphine)iminium (PPN+) salts can be achieved with phosphonium salts such as triphenyl(4-pyridinylmethyl)phosphonium chloride hydrochloride (UHFFA) and tetraphenylphosphonium chloride (TPPCl). For CHO-based copolymers, a structure-property correlation can be observed: the Tg value seems to be related both to the amount of ether linkages and to the molecular weight. In addition, in the case of CHO, FTIR/ATR spectroscopy on the pretreated catalytic system can reliably predict the success of the copolymerization before the introduction of the epoxide monomer. Equilibria between the cation and anion in the phosphonium salts under study and the competition between epoxide and X- have been shown to play a role in the activity, molecular weight, selectivity, and polymer properties. A general view of these equilibria and of the polymerization mechanism has been given as well as a rationale of differences in CHO and PO copolymerization behavior and polymer properties
Propene-cycloolefin polymerization
No full textHighly active metallocenes and other single site catalysts have opened up the possibility of polymerizing cycloolefins such as norbornene (N) or of copolymerizing them with ethene (E) or propene (P). The polymers obtained show exciting structures and properties. E-N copolymers are industrially produced materials, with variable and high glass transitions depending on the wide range of their microstructures. By realizing the possibility in great variety of stereoregularity of propene and norbornene units and the difference in comonomer distribution, P-N copolymers were expected to have fine tuned microstructures and properties. Moreover, P-N copolymers should be characterized by higher Tg-values than E-N copolymers with the same norbornene content and molar mass. A review of the state of the art of P-N copolymerization by ansa-metallocenes of C2 symmetry, namely rac-Et(Ind)2ZrCl2 (I-I) and rac-Me2Si(Ind)2ZrCl2 (I-II), and rac-Me2Si(2-Me-Ind)2ZrCl2 (I-III), and of catalysts of Cssymmetry, namely (tBuNSiMe2Flu)TiMe2 (IV-I) and derivatives, is given here. Special emphasis is given to microstructural studies of P-N copolymers, including stereo- and regioregularity of propene units as well as of comonomer distribution, stereoregularity of norbornene units, and the structure of chain end-groups. This information allows us to find a rationale for the catalytic activities and the copolymer properties
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