Nonpolar Olefins Research Articles

Unveiling the Detailed Mechanism and Origins of Chemo-, Regio-, and Stereoselectivity of Rare-Earth Catalyzed Alternating Copolymerization of Polar and Nonpolar Olefins.

The direct copolymerization of polar and nonpolar olefins is of great interest and significance, as it is the most atom-economical and straightforward strategy for the synthesis of functional polyolefin materials. Despite considerable efforts, the precise control of monomer-sequence and their regio- and stereochemistry is full of challenges, and the related mechanistic origins are still in their infancy to date. Herein, the mechanistic studies on the model reaction of Sc-catalyzed co-syndiospecific alternating copolymerization of anisylpropylene (AP) and styrene were performed by DFT calculations. The results suggest that the subtle balance between electronic and steric factors plays an important role during monomer insertions, and a new amino-dissociated mechanism was proposed for AP insertion at chain initiation. AP insertion follows the 2,1-si-insertion pattern, which is mainly controlled by steric factors caused by the restricted MeO···Sc interaction. As for styrene insertion, it prefers the 2,1-re-insertion manner and its regio- and stereoselectivities are influenced by steric repulsions between the inserting styrene and the polymer chain or the ligand. More interestingly, it is found that the alternating monomer-sequence is mainly determined by the "steric matching" principle, which is quantitatively expressed by the buried volume of the metal center of the preinserted species. The concept of steric pocket has been successfully applied to explain the different performances of several catalysts and other alternating copolymerization reactions. The insightful mechanistic findings and the quantitative steric pocket model present here are expected to promote rational design of new rare-earth catalysts for developing regio-, stereo-, and sequence-controlled copolymerization of specific polar and nonpolar olefins.

A disubstituted-norbornene-based comonomer strategy to address polar monomer problem

The transition-metal-catalyzed copolymerization of olefins with polar comonomers is a direct strategy to access polar-functionalized polyolefins in an economical manner. Due to the intrinsic poisoning effect of polar groups towards Lewis acidic metal centers and the drastic reactivity differences of polar comonomers versus non-polar olefins, it is challenging to develop catalysts that provide the desired polymer molecular weight, comonomer incorporation, and activity. In this contribution, we tackle this issue from a comonomer perspective using 5,6-disubstituted norbornenes, which are highly versatile, easily accessible, inexpensive, and capable of introducing two functional groups in a single insertion. More importantly, they are only mildly poisoning due to the presence of long spacers between double bonds and polar groups, and are not prone to β-hydride elimination due to their cyclic structures. As strong π-donors, they can competitively bind to metal centers versus olefins. Indeed, phosphine-sulfonate palladium catalysts can catalyze the copolymerization of ethylene with 5,6-disubstituted norbornenes and simultaneously achieve a high polymerization activity, copolymer molecular weight, and comonomer incorporation. The practicality of this system was demonstrated by studying the properties of the resulting polymers, copolymerization in hydrocarbon solvents or in bulk, recovery/utilization of unreacted comonomer, molecular weight modulation, and large-scale synthesis.

Early Transition Metal Catalysis for Olefin-Polar Monomer Copolymerization.

Introducing polar functional groups into widely used polyolefins can enhance polymer surface, rheological, mixing, and other properties, potentially upgrading polyolefins for advanced, value-added applications. The metal catalyst-mediated copolymerization of non-polar olefins with polar comonomers represents the seemingly most straightforward, atom- and energy-efficient approach for synthesizing polar functionalized polyolefins. However, electrophilic early transition metal (groups 3 and 4)-catalyzed processes which have achieved remarkable success in conventional olefin polymerizations, encounter severe limitations here, largely associated with the Lewis basicity of the polar co-monomers. In recent years, however, new catalytic systems have been developed and successful strategies have emerged. In this Minireview, we summarize the recent progress in early transition metal polymerization catalyst development, categorized by the catalytic metal complex and polar comonomer identity. Furthermore, we discuss advances in the mechanistic understanding of these polymerizations, focusing on critical challenges and strategies that mitigate them.

Copolymerization of Nonpolar Olefins and Allyl Acetate Using Nickel Catalysts Bearing a Methylene-Bridged Bisphosphine Monoxide Ligand

Bisphosphine monoxide ligand 1 with a methylene linker was applied to the nickel catalyzed olefin polymerization. In situ generated phenylnickel chloride complex 2 and NaBArF4 mediated the homopoly...

Rhodium-Catalyzed Enantioselective Hydroselenation of Heterobicyclic Alkenes.

A highly efficient Rh(I)/(S)-xyl-Binap catalytic system is developed for the asymmetric hydroselenation of various nonpolar olefins with diselenides. Under these mild reaction conditions, a wide range of heterobicyclic alkenes give selenol-incorporated adducts in excellent enantioselectivities (up to 97%) along with high yields (up to 96%) by overcoming self-promoted racemic hydroselenation. The strategy is also applied for kinetic resolution of unsymmetric oxabenzonorbornadiene.

Co-syndiospecific Alternating Copolymerization of Functionalized Propylenes and Styrene by Rare-Earth Catalysts.

The precise control of monomer sequence and stereochemistry in copolymerization is of much interest and importance for the synthesis of functional polymers, but studies toward this goal have met with only limited success to date. Now, the co-syndiospecific alternating copolymerization of methoxyphenyl- and N,N-dimethylaminophenyl-functionalized propylenes with styrene by half-sandwich rare-earth catalysts is reported. This reaction efficiently afforded the corresponding functionalized propylene-alt-styrene copolymers with a perfect alternating sequence and excellent co-syndiotacticity (rrrr >99 %), thus constituting the first example of co-stereospecific alternating copolymerization of polar and non-polar olefins.

Valencene as a naturally occurring sesquiterpene monomer for radical copolymerization with maleimide to induce concurrent 1:1 and 1:2 propagation

Valencene, a naturally occurring sesquiterpene that can be obtained from various citrus fruits, was copolymerized as an unconjugated 1,1-disubstituted bulky vinyl monomer by radical copolymerization with conjugated monomers possessing electron-withdrawing substituents, such as maleimides, acrylates, methacrylates, acrylonitrile, and methacrylonitrile, in toluene and PhC(CF3)2OH. The fluorinated alcohol increased the copolymerizability, as in the radical copolymerization of other unconjugated non-polar olefins with those electron-acceptor comonomers. In particular, the copolymerization of valencene and N-phenylmaleimide in PhC(CF3)2OH proceeded via concurrently occurring 1:1 and 1:2 propagation, in which the monomer reactivity ratio of maleimide and valencene to the maleimide radical with the penultimate valencene unit was close to 1 according to a kinetic analysis of the copolymerization based on the penultimate model. Therefore, the copolymerizability of valencene was higher than that of limonene, which is a similar unconjugated 1,1-disubstituted vinyl monomer with a smaller cyclohexenyl substituent that undergoes selective 1:2 radical copolymerization with maleimide derivatives under the same conditions. Reversible addition-fragmentation chain transfer RAFT copolymerization using a trithiocarbonate resulted in copolymers with controlled molecular weights and chain-end groups. The biobased copolymers of valencene and N-phenylmaleimide showed relatively high glass transition temperatures (Tg) of approximately 220–240 °C and relatively high thermal stability with 5% decomposition temperatures (Td5) of approximately 330–340 °C due to their rigid main-chain structures.

Environmentally benign stereoselective polymerizations of polar as well as nonpolar olefins by a new postmetallocene Ti(IV) salicylate complex at ambient temperature in aqueous emulsion

A new Titanium (IV) complex, dichlorobis(salicylato)titanium(IV) [TiL2Cl2] (1) {LH = Salicylic acid} has been synthesized and characterized by elemental analysis, 1H NMR, 13C NMR, IR and UV-visible spectroscopy. As a post-metallocene complex it’s olefin polymerization activity has been investigated at room temperature in aqueous emulsion. In presence of tetraphenylborate as cocatalyst, it has been found to exhibit moderate to high activity in the range of 103 in homopolymerization and copolymerization of styrene and methyl methacrylate to produce syndiotactic rich, ultrahigh molecular weight polymers with low molecular weight distribution. The polymer latex produced has been analyzed by DLS, HRTEM and FESEM which revealed that the latex is highly stable and contains polymer particles having size around 50 nm. The microstructures, molecular weights and thermal properties of the polymers have also been determined by 1H-NMR, 13CNMR, GPC and DSC analysis. The polymers have also been characterized by 1H-NMR, 13C-NMR, GPC and DSC analysis.

1:2‐sequence‐regulated radical copolymerization of naturally occurring terpenes with maleimide derivatives in fluorinated alcohol

AbstractNaturally occurring bulky terpenes, such as (+)‐ and (–)‐limonene and (–)‐β‐pinene, were quantitatively copolymerized with maleimide (MI) derivatives (i.e., phenyl‐, cyclohexyl‐, ethyl‐, and unsubstituted‐MI) in PhC(CF3)2OH solvent via selective 1:2‐alternating propagation governed by the penultimate effect, which resulted in 1:2‐sequence regulated polymers with relatively high glass transition temperatures and optical activities. Similar petroleum‐derived bulky olefins possessing cyclohexenyl, cyclohexyl, or additional α‐methyl substituents were copolymerized with phenylmaleimide via preferential 1:2‐alternating propagation with a slightly lower selectivity. A further decrease in the bulkiness of nonpolar olefins increased the 1:1‐alternating sequence. The copolymerization of limonene and acrylonitrile also proceeded approximately via 1:1‐alternating propagation, in which the penultimate effect was less observable. Furthermore, when methylene chloride was used instead of fluorinated alcohol for the copolymerization of limonene and phenylmaleimide, the length of the sequence of MI units increased. Thus, in addition to the characteristic MI skeleton, the bulky and rather specific structure of either limonene or β‐pinene induces 1:2‐selective propagation via the penultimate effect, whereas the fluoroalcohol diminishes the homopropagation of MI via a hydrogen bonding interaction with the MI unit. RAFT copolymerization of limonene and various MI derivatives in PhC(CF3)2OH successfully proceeded to give the end‐to‐end 1:2‐sequence‐regulated copolymer with a selective initiating sequence and predominant capping sequence using an appropriate RAFT agent. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Striking a Compromise: Polar Functional Group Tolerance versus Insertion Barrier Height for Olefin Polymerization Catalysts

The coordination–insertion random copolymerization of polar and nonpolar olefins holds great potential for the design of new polymers with targeted properties. However, examples of catalysts capable of such polymerization still remain scarce, the majority of which are based on PdII and some also on NiII. So far, the apparent superiority of PdII has not been rationalized. In this work, the catalytic potential of a broad range of transition metals is assessed by investigating their polar functional group tolerance and insertion barrier heights for realistic and comparable complexes. Multivariate regression models suggest that the π-back-donation ability of the metal plays an important role in both the polar functional group tolerance and the insertion barrier height. Specifically, the polar functional group tolerance and insertion barrier height were found to correlate positively, indicating that a compromise must be struck. PdII seems to strike this balance optimally, thus explaining its prominent position...

Dual Behavior of Cationic Palladium Pentafluorophenyl Complexes as Catalysts for the Homopolymerization of Acrylates and of Nonpolar Olefins

[Pd(Pf)(NCMe)(N−N)]BF4, (Pf = C6F5; N−N = ethylenediamine (en, 1), 2,2′-bipyridine (2), (2,6-iPrC6H3)N═C(Me)C(Me)═N(2,6-iPrC6H3) (diimine, 3)) are very efficient initiators of the radical polymeriz...