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

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

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

Alternating Isotactic Polyhydroxyalkanoates via Site- and Stereoselective Polymerization of Unsymmetrical Diolides

Naturally produced, biodegradable polyhydroxyalkanoates (PHAs) promise more sustainable alternatives to nonrenewable/degradable plastics, but biological PHA’s stereomicrostructures are strictly confined to isotactic (R)-polymers or copolymers of random sequences. Chemical synthesis via catalyzed ring-opening polymerization (ROP) of cyclic (di)esters offers expedient access to diverse PHA microstructures, including those with defined comonomer sequences and tacticities. However, the synthesis of alternating isotactic PHAs has not been achieved by the existing methodologies. Here, we report the design of unsymmetrically disubstituted eight-membered diolides (rac-8DLR1‑R2) and their site- and stereoselective ROP with discrete chiral catalysts, enabling the synthesis of alternating isotactic PHAs, poly(3-hydroxybutyrate-alt-3-hydroxyvalerate) (alt-P3HBV) and poly(3-hydroxybutyrate-alt-3-hydroxyheptanoate) (alt-P3HBHp), with high to quantitative (>99%) alternation and isotacticity and Mn up to 113 kDa and Đ = 1.01. Physical properties of such PHAs are substantially determined by the degree of backbone sequence alternation and tacticity, ranging from amorphous to semi-crystalline materials. The alt-P3HBV shows significantly improved mechanical performance relative to the constituent homopolymers. Intriguingly, enantiomeric (R)-alt-P3HBV and (S)-alt-P3HBV, synthesized by kinetically resolved ROP of rac-8DLMe‑Et, form a stereocomplex with a significantly enhanced Tm (by 53 °C), while the enantiomeric homopolymers do not form a stereocomplex

Crystallization Behavior and Properties of Propylene/4-Methyl-1-pentene Copolymers from a Metallocene Catalyst

Copolymers of isotactic polypropylene (iPP) with 4-methyl-1-pentene (iPP4MP) were prepared with a highly isoselective homogeneous organometallic catalyst in a range of 4-methyl-1-pentene (4MP) concentrations between about 1.7 and 14 mol %. Crystallization from the melt at different crystallizations temperatures have been performed to study the effect of 4MP comonomeric units excluded from the crystals on the crystallization of α and γ forms. All samples crystallize in mixtures of α and γ forms, and for each sample, the fraction of γ form increases with increasing crystallization temperature to achieve a maximum value fγ(max), which depends on the 4MP concentration. Compared to the homopolymer, the maximum fractional amount of γ form fγ(max) rapidly increases with increasing 4MP content achieving the highest value of 92% at low 4MP concentration of 2.2 mol %, and decreases with a further increase of 4MP concentration. These data are compared with analogous data of the maximum amount of γ form that develops in copolymers of iPP with ethylene and butene. This allows comparing the different effects of rejection of defects from the crystals, which produces interruption of the regular propene sequences and shortening of the length of the crystallizable sequences, the inclusion of defects into crystals of α and γ forms, and the effect of the crystallization kinetics. Since 4MP comonomeric units are excluded from the crystals, the behavior of iPP4MP copolymers provides the sole interruption effect, which is highly efficient and produces the highest amount of γ form of 92% at low 4MP concentration of nearly 2 mol %. The observed decrease of fγ(max) at higher 4MP concentrations is due to the too slow crystallization rate of the γ form at these 4MP contents that induces the crystallization of the kinetically favored α form. In fact, crystals of γ forms that develop in these copolymers are highly defective and show melting temperatures lower than those of the α form and, therefore, experience low undercooling at high crystallization temperatures. These results demonstrate that in metallocene iPP copolymers containing a significant amount of constitutional defects, the crystallization of the γ form is favored because of the short regular propene sequences, whereas the crystallization of the α form is always kinetically favored

Crystallization of Propene–Pentene Isotactic Copolymers as an Indicator of the General View of the Crystallization Behavior of Isotactic Polypropylene

The crystallization from the melt in isothermal conditions of metallocene random propene–pentene isotactic copolymers (iPPC5) has been studied. All samples with pentene concentration between 0.5 and 10 mol % crystallize at any crystallization temperature in mixtures of α and γ forms of isotactic polypropylene (iPP) and the amount of γ form increases with increasing crystallization temperature up to a maximum (fγ(max)), which depends on pentene concentration. Pentene defects produce a shortening of the regular propene sequences that in turn induces crystallization of the γ form. At concentrations higher than 6–7 mol %, pentene units are incorporated to a high extent in the crystals of α and trigonal forms, which are stabilized over the γ form, and fγ(max) decreases. The maximum fraction of γ form is, therefore, related to the average length of regular propene sequences and the degree of incorporation of defects in the crystals of α and γ forms. The values of fγ(max) that develop in iPPC5 copolymers have been compared with those that develop in copolymers of iPP with ethylene (iPPC2), butene (iPPC4), and hexene (iPPC6) and in stereoirregular iPPs reported in the literature. Stereoirregular iPPs and iPPC2 copolymers give the same relationship between fγ(max) and the average length of regular propene sequences (LiPP), whereas iPPC4, iPPC5, and iPPC6 copolymers show different behaviors. In particular, iPPC5 copolymers exhibit a behavior intermediate between those of iPPC4 and iPPC6 copolymers. The relationship between fγ(max) and LiPP of iPPC5 copolymers fits perfectly between the relationships found for iPPC4 and iPPC6 copolymers, in agreement with the different types and sizes of comonomers and the different efficiencies of their interruption and inclusion effects. These data give evidence of the general view of the crystallization behavior of iPP, based on the definition of a double role exerted by defects, the interruption effect that shortens the regular propene sequences and favors crystallization of γ form, and the effect of incorporation of defects into the crystalline unit cells of α and γ forms, which favors crystallization of the form that better accommodates the defect into crystals. The relative efficiency of these two effects depends on the type and size of the defect. The different relationships between fγ(max) and LiPP are a result of the equilibrium between interruption and inclusion effects achieved by each defect and confirm that the crystallization of γ form is a perfect indicator of the length of the regular propene sequences and may provide very detailed information on the molecular structure of iPP

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

Synthesis, Structure, and Properties of Poly(isoprene)s of Different Constitutions and Configurations from Catalysts Based on Complexes of Nd, Co, and Fe

Poly­(isoprene)­s of different molecular structures have been synthesized with various catalysts based on complexes of Nd-, Co-, and Fe-bearing pyridylimine, phosphine, and bipyridine ligands. Poly­(isoprene)­s with essentially cis-1,4 structure, 3,4 syndiotactic structure, and regular alternating cis-1,4-alt-3,4 structure have been obtained. Moreover, polymers with an unusual prevalent alternating cis-1,4-alt-3,4 structure but containing cis-1,4 units sequences of different lengths (3 or 5 units) within the chain have also been obtained. The structure and mechanical properties of all of the synthesized poly­(isoprene)­s, some of them completely new, have been investigated. Poly­(isoprene)­s with a cis-1,4 structure and with a perfectly alternating cis-1,4-alt-3,4 structure or prevalent alternating cis-1,4-alt-3,4 structure are amorphous, exhibiting mechanical behavior with a viscous flow at relatively high deformation. The sample of poly­(isoprene) with almost a regular 3,4 syndiotactic structure (about 79% of 3,4 units) crystallizes in the stable orthorhombic form of 3,4-syndiotactic poly­(isoprene). This crystalline sample shows better mechanical properties of deformability and flexibility than the other amorphous poly­(isoprene)­s without a viscous flow with plastic deformation and breaking at relatively high strain around 300–400% and remarkable elastic properties. The plastic deformation and the elastic behavior are associated with a reversible transformation during tensile deformation of the crystalline orthorhombic form into a disordered mesomorphic form, which recrystallizes into the orthorhombic form upon relaxation during elastic recovery. The glass transition temperature of the different poly­(isoprene)­s decreases linearly with the increasing cis-1,4 content, from 16 °C of the poly­(isoprene) with the prevalent 3,4 structure and 21% of cis-1,4 units to −64 °C of the poly­(isoprene) with 98% of cis-1,4 units

Synthesis of stereoregular polymyrcenes using neodymium-, iron- and copper-based catalysts

β-Myrcene polymerization using catalysts derived from pyridyl-imino complexes of neodymium, iron, and copper. The resulting polymers exhibited high stereoregularity. Specifically, the neodymium-based catalyst yielded polymers with a high cis-1,4 configuration (≥97%), whereas the iron and copper catalysts produced a unique structure in stereospecific polymerization: predominantly alternating cis-1,4-alt-3,4 polymers with sequences of cis-1,4 units (five units) along the polymer chain. The polymers were subsequently subjected to structural, thermal, and mechanical characterization.</p

Mechanical Properties and Morphology of Propene–Pentene Isotactic Copolymers

The mechanical behavior of propylene–pentene isotactic copolymers synthesized with a single-center organometallic catalyst in a broad interval of pentene (Pe) concentration, from 3 to 50 mol %, is analyzed. All copolymers show flexible behavior with very high deformation at breaking. The presence and entrance of Pe counits in α and δ forms of isotactic polypropylene (iPP) transform a strong and fragile iPP homopolymer into ductile and flexible materials. The stress at yield decreases as the Pe content increases, but a different behavior has been observed for samples in the α form or in the δ form. In samples in the α form, from 3 to 8.8 mol % of Pe, the stress at yield increases as the thickness of crystalline lamellae increases, while in samples in the δ form containing higher Pe counits, it decreases as the thickness of lamellae increases and Pe content increases. Phase transition of the α form in the mesophase and crystallization of the δ form, in samples with 8–11 mol % of Pe, occur upon deformation and are correlated to the strain-hardening observed at high strain