Stereoselective Lactide Polymerization: the Challenge of Chiral Catalyst Recognition

A model for stereoselective ring opening polymerization (ROP) of rac-lactide promoted by chiral aluminum systems is reported based on DFT calculations. The mechanism of enantiomorphic site control dictated by the chiral catalyst shows unusual features, including active site reorganization on the reaction path, which add complexity and need to be taken into account when addressing the challenge of chiral catalyst recognition

DataSheet1_Mechanistic insights on 1-butene polymerization catalyzed by homogeneous single-site catalysts: a DFT computational study.PDF

Isotactic poly (1-butene) (iPB) is an interesting semi-crystalline thermoplastic material characterized by notable physical and mechanical attributes encompassing superior creep and stress resistance, elevated toughness, stiffness, and thermal endurance. These distinctive features position iPB as a viable candidate for specific applications; however, its widespread utilization is hindered by certain inherent limitations. Indeed, iPB manifests an intricate polymorphic behavior, and the gradual and spontaneous transition of the kinetically favored form II to the thermodynamically favored form I during aging introduces alterations to the material’s properties. Despite its potential, the attainment of iPB with an exceedingly high molecular mass remains elusive, particularly when employing homogeneous catalysts renowned for their efficacy in propene polymerization. In this study we analyze the mechanistic aspects governing 1-butene polymerization by using DFT calculations modelling the regioselectivity of 1-butene insertions and the termination reactions occurring after primary (1,2) and secondary (2,1) insertions. Finally, the isomerization pathways leading to the formation of 4,1 units in iPB samples synthesized by homogenous catalysts is also discussed. All these aspects, furnish a mechanistic picture of the main drawbacks of an “old” but still interesting material.</p

Self-Nucleation in Stereodefective Isotactic Polypropylene: The Impact of Stereodefects on the Melt Memory

The memory of crystals in the melt of stereodefective samples of isotactic polypropylene (iPP), characterized by different concentrations of rr stereodefects from 0.49 to 10.5 mol %, was analyzed. Experiments of self-nucleation and annealing have demonstrated that high contents of rr stereodefects, largely incorporated in the crystals of iPP, produce a significant memory of crystals in the melt that persists up to high temperatures well above the melting temperature. For low stereodefect concentrations (lower than 2–3 mol %), the memory of the crystals is erased at temperatures (Ts,DI‑DII) only few degrees above the end of the melting endotherm (Tm,end), whereas for contents of rr defects higher than 3–4 mol %, the memory of crystals persists even upon heating at temperatures much above the end of the endothermic signal. The width of the heterogeneous melt Domain II, in terms of range of temperatures in the melt in which the memory exists and self-nucleation takes place, and the difference between the temperature at which the isotropic melt begins Ts,DI‑DII and the end of the melting endotherm Tm,end increase with the increase of defects concentration. The higher the amount of stereodefects and the lower the melting temperature of iPP, the higher the temperature at which the self-nuclei must be heated to cancel the memory of crystals. These results indicate that a significant memory of iPP crystals exists in the melt not only in copolymers of iPP with noncrystallizable comonomeric units but also for iPPs containing small defects largely incorporated in the crystals. During crystallization of these stereodefective iPPs, the selection of the crystallizable segments of suitable length, which has been considered responsible for the formation of the heterogeneous melt and self-nuclei, should be less demanding thanks to the incorporation of stereodefects in the crystallizable sequences. However, upon successive heating to melt at low temperatures these highly irregular produced crystals, the diffusion and homogenization of all long and short sequences is in any case not easy, also considering the low temperature, and portions of partitioned sequences are left in the melt acting as efficient self-nuclei upon cooling and crystallization from the melt. The melt-memory attributed to these self-nuclei and the process of self-nucleation induce crystallization of the γ form, while crystallization from the isotropic melt induces crystallization of the α form, also in the case of samples with high concentrations of stereodefects that should crystallize in the γ form

Keto-Polyethylenes with Controlled Crystallinity and Materials Properties from Catalytic Ethylene–CO–Norbornene Terpolymerization

Recent advances in Ni(II) catalyzed, nonalternating catalytic copolymerization of ethylene with carbon monoxide (CO) enable the synthesis of in-chain keto-functionalized polyethylenes (keto-PEs) with high-density polyethylene-like materials properties. Addition of norbornene as a bulky, noncrystallizable comonomer during catalytic polymerization allows tuning of the crystallinity in these keto-PE materials by randomly incorporated norbornene units in the polymer chain, while molecular weights are not adversely affected. Such crystallinity-reduced keto-PEs are characterized as softer materials with better ductility and may therefore be more suited for, e.g., potential film applications

Synthesis and Characterization of 4‑Methyl-1-Pentene/1,5-Hexadiene Isotactic Copolymers with Enhanced Low-Temperature Mechanical Performance

Novel 4-methyl-1-pentene/1,5-hexadiene isotactic copolymers (iP4MPHD) incorporating methylene-1,3-cyclopentane (MCP) cyclic co-units with concentrations in the range 4.4–17.6 mol % have been synthesized by using the dimethylpyridylamidohafnium/organoboron catalyst. The influence of the MCP cyclic co-unit on the crystallization behavior and the mechanical properties of the isotactic poly­(4-methyl-1-pentene) (iP4MP) homopolymer has been investigated in detail. iP4MPHD copolymers with comonomer content up to 11 mol % crystallize in form II of iP4MP from the polymerization solution and in the stable form I of iP4MP from the melt, whereas the sample with the highest concentration (17.6 mol %) of 1,5-hexadiene (1,5-HD) is amorphous and does not crystallize from either solution and melt. All crystalline samples exhibit high melting temperatures, always above 120 °C, and a controlled glass transition temperature close to the room temperature (28–30 °C). Incorporation of MCP units into iP4MP chains produces an improvement in flexibility and allows tailoring of deformability while retaining high mechanical resistance and transparency of the homopolymer. Interestingly, the high deformability is maintained at low temperature (50 °C below the glass transition temperature), suggesting a cooperative role of both amorphous and crystalline phases in the deformation mechanism that enhances ductility. All stress–strain curves of the different copolymers present an unusual second maximum at strains higher than the yielding point. Diffraction patterns recorded during deformation have revealed that this second maximum is associated with the crystallization under stretching of a highly disordered crystalline mesophase never described in the literature

Crystal Structure of Atactic and Isotactic Poly(3-hydroxy-2,2-dimethylbutyrate): A Chemically Recyclable Poly(hydroxyalkanoate) with Tacticity-Independent Crystallinity

The crystal structures of isotactic and atactic poly(3-hydroxy-2,2-dimethylbutyrate) (P3H(Me)2B) are presented. Samples of atactic P3H(Me)2B (at-(R/S)-P3H(Me)2B) and of the R and S enantiomers of isotactic P3H(Me)2B (it-(R)-P3H(Me)2B and it-(S)-P3H(Me)2B, respectively) have been synthesized by ring opening polymerization of racemic and chiral dimethyl-butyrolactones employing a superbase catalyst. A 1:1 racemic mixture of the two R and S enantiomers (it-(R,S)-P3H(Me)2B) has been also prepared. Both the atactic and the pure enantiomer isotactic polymers crystallize showing identical diffraction patterns, indicating crystallization in the same crystalline form and identical crystal structure. The racemic mixture it-(R,S)-P3H(Me)2B also crystallizes giving identical diffraction pattern. This is one of the few examples of crystallization of an atactic polymer despite the configurational disorder and probably it is the first structurally confirmed example of crystallization of atactic and isotactic polymers in the identical crystal structure. This fascinating tacticity-independent crystallinity explains the remarkable thermal and mechanical behaviors of P3H(Me)2B, which is thermally stable and melt-processable and chemically recyclable to the monomer. The crystal structure has been resolved by analysis of X-ray powder diffraction and X-ray fiber diffraction of oriented fibers, combined with conformational analysis based on methods of density functional theory. The ordered crystal structure of the isotactic pure enantiomer it-(R)-P3H(Me)2B (or it-(S)-P3H(Me)2B) is described by chains in a nearly trans-planar conformation with chain axis of 4.7 Å packed in an orthorhombic unit cell with axes a = 13.30 Å, b = 9.99 Å, and c = 4.75 Å according to the chiral space groups P21212 or P212121. The atactic polymer at-(R/S)-P3H(Me)2B crystallizes in the same orthorhombic unit cell having only slightly larger a axis, a = 13.94 Å, b = 10.03 Å, and c = 4.75 Å, with chains characterized by a disordered succession of R and S monomers and a distorted trans-planar conformation that keeps a straight chain axis and the same periodicity of 4.7 Å of the ordered pure enantiomer. This proposed model of the conformation of at-(R/S)-P3H(Me)2B explains the crystallization of the atactic polymer. The crystal structure of at-(R/S)-P3H(Me)2B is, therefore, characterized by the packing of disordered chains in nearly trans-planar conformation according to both the chiral space groups P21212 or P212121 and the achiral space groups Pna21 or Pnn2

Metallocenes and Beyond for Propene Polymerization: Energy Decomposition of Density Functional Computations Unravels the Different Interplay of Stereoelectronic Effects

Stereoselective propene polymerization mechanisms promoted by C1-symmetric transition metal (TM) catalysts with nonmetallocene and ansa-metallocene ligands have been revisited by density functional theory (DFT) calculations combined with a molecular descriptor for steric analysis (%VBur) and a state-of-the-art interpretative tool based on the Activation Strain Model (ASM) and a Natural Energy Decomposition Analysis (NEDA). While DFT results suggested a close similarity for mechanisms and stereoselectivities for these catalyst classes, the ASM-NEDA analysis unraveled that different stereoelectronic effects play the dominant role depending on the ligand framework. The insights achieved by such analysis on the “naked” cationic active species were also confirmed by adding the counterion in the calculations, thus allowing a better understanding of olefin polymerization mechanism(s) governed by TM catalysts

Metallocenes and Beyond for Propene Polymerization: Energy Decomposition of Density Functional Computations Unravels the Different Interplay of Stereoelectronic Effects

Stereoselective propene polymerization mechanisms promoted by C1-symmetric transition metal (TM) catalysts with nonmetallocene and ansa-metallocene ligands have been revisited by density functional theory (DFT) calculations combined with a molecular descriptor for steric analysis (%VBur) and a state-of-the-art interpretative tool based on the Activation Strain Model (ASM) and a Natural Energy Decomposition Analysis (NEDA). While DFT results suggested a close similarity for mechanisms and stereoselectivities for these catalyst classes, the ASM-NEDA analysis unraveled that different stereoelectronic effects play the dominant role depending on the ligand framework. The insights achieved by such analysis on the “naked” cationic active species were also confirmed by adding the counterion in the calculations, thus allowing a better understanding of olefin polymerization mechanism(s) governed by TM catalysts

Combining Cyclic Units and Unsaturated Pendant Groups by Propene/1,5-Hexadiene Copolymerization Toward Functional Isotactic Polypropylene

The precise use of a widely available and inexpensive metallocene catalyst enabled the synthesis of isotactic polypropylene copolymers characterized by the copresence of randomly distributed cyclic units in the backbone and unsaturated pendant units employing 1,5-hexadiene as comonomer. Optimization of the polymerization conditions avoided the cross-linking phenomena that negatively affects the material processing and final properties, resulting in good yields of samples featuring high molecular masses and a precisely controlled microstructure. Such polypropylene-based copolymers exhibit a broad spectrum of properties ranging from thermoplastic to surprising elastomeric behavior, with the additional value of being functionalizable by post-polymerization reactions

Breaking Symmetry Rules Enhance the Options for Stereoselective Propene Polymerization Catalysis

An example of breaking “Ewen’s symmetry rule” for olefin catalysis polymerization is proposed by DFT calculations. Catalyst precursors with Cs symmetry are suggested to promote the isotactic propene polymerization by a modification of the active site geometry obtained via coordination with AlH–alkyl species in solution. The origin of stereocontrol in olefin polymerization is due to a dual mechanism dictated by the chiral catalyst. These findings may expand the toolbox for promoting stereoselective olefin polymerization by transition metal catalysts