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Polyhydroxyalkanoates (PHAs) have attracted increasing interest as sustainable plastics because of their biorenewability and biodegradability in the ambient environment. However, current semicrystalline PHAs face three long-standing challenges to broad commercial implementation and application: lack of melt processability, mechanical brittleness, and unrealized recyclability, the last of which is essential for achieving a circular plastics economy. Here we report a synthetic PHA platform that addresses the origin of thermal instability by eliminating α-hydrogens in the PHA repeat units and thus precluding facile cis-elimination during thermal degradation. This simple α,α-disubstitution in PHAs enhances the thermal stability so substantially that the PHAs become melt-processable. Synergistically, this structural modification also endows the PHAs with the mechanical toughness, intrinsic crystallinity, and closed-loop chemical recyclability.

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 γforms. All samples crystallize in mixtures of α and γforms, and for each sample, the fraction of γform 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 γform 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 γform 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 γforms, 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 γform 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 γform at these 4MP contents that induces the crystallization of the kinetically favored α form. In fact, crystals of γforms 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 γform is favored because of the short regular propene sequences, whereas the crystallization of the α form is always kinetically favored.

Technologically important thermosets face a long-standing end-of-life (EoL) problem of non-reprocessability, a more sustainable solution of which has resolved to nascent vitrimers that can merge the robust material properties of thermosets and the reprocessability of thermoplastics. However, the lifecycle of vitrimers is still finite, as they often suffer from significant deterioration of mechanical performance following multiple reprocessing cycles, analogous to mechanical recycling, and they often show undesired creep under working conditions. To address these two key limitations, we have developed a cross-linked semi-crystalline polythioester with both dynamic covalent bonds and intrinsic crystallinity and chemical recyclability, affording a vitrimeric system that exhibits not only reprocessability and crystallinity-restricted creep but also complete chemical recyclability to initial monomer by catalyzed depolymerization in solution or bulk. Therefore, reported herein is an "infinite" vitrimer system that is empowered with a facile closed-loop EoL option once serial reprocessing deteriorates performance and the material can no longer meet the application requirements. Specifically, the polythioester vitrimer was constructed by copolymerization of a bicyclic thioester with a bis-dithiolane, producing dynamically cross-linked polythioesters with excellent property tunability, from amorphous to semi-crystalline states and melting transition temperatures from 91 to 178 °C.

The crystallization behavior of propene–ethylene (iPPC2), propene–butene (iPPC4), propene–pentene (iPPC5), and propene–hexene (iPPC6) isotactic copolymers, prepared with single center metallocene catalysis, is described. Thanks to the perfectly random distribution of comonomers along the macromolecules, these copolymers can be considered as model for the crystallization behavior of isotactic polypropylene (iPP) that is mainly defined by the average length of the regular propene sequences. A model of the crystallization behavior of iPP is described based on the definition of a double effect of defects (stereodefects and comonomers), the interruption effect of defects excluded from the crystals that shortens the length of the crystallizable propene sequences inducing crystallization of the γ form, and the effect of inclusion of defects in the crystals of α form, γ form or δ form that induces the crystallization of the form whose lattice is less disturbed by the defect.

The crystallization behavior of random propene-octene isotactic copolymers (iPPC8) prepared with a homogeneous metallocene catalyst has been studied. Samples of iPPC8 with low octene content up to about 7 mol% were isothermally crystallized from the melt at various crystallization temperatures. The samples crystallize in mixtures of the α and γ forms of isotactic polypropylene (iPP). The relative amount of γ form increases with increasing crystallization temperature, and a maximum amount of γ form (fγ(max)) is achieved for each sample. The crystallization behavior of iPPC8 copolymers is compared with the crystallization from the melt of propene–ethylene, propene–butene, propene–pentene, and propene–hexene copolymers. The results show that the behavior of iPPC8 copolymers is completely different from those described in the literature for the other copolymers of iPP. In fact, the maximum amount of γ form achieved in samples of different copolymers of iPP generally increases with increasing comonomer content, while in iPPC8 copolymers the maximum amount of γ form decreases with increasing octene content. The different behaviors are discussed based on the inclusion of co-monomeric units in the crystals of α and γ forms of iPP or their exclusion from the crystals. In iPPC8 copolymers, octene units are excluded from the crystals giving only the interruption effect that shortens the length of regular propene sequences, inducing crystallization of the γ form at low octene concentrations, lower than 2 mol%. At higher octene concentration, the crystallization of the kinetically favored α form prevails.

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 γforms of isotactic polypropylene (iPP) and the amount of γform 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 γform. 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 γform, and fγ(max) decreases. The maximum fraction of γform is, therefore, related to the average length of regular propene sequences and the degree of incorporation of defects in the crystals of α and γforms. 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 γform, and the effect of incorporation of defects into the crystalline unit cells of α and γforms, 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 γform is a perfect indicator of the length of the regular propene sequences and may provide very detailed information on the molecular structure of iPP.

Crystallization and phase separation in the melt in semicrystalline block copolymers (BCPs) compete in defining the final solid state structure and morphology. In crystalline–crystalline di-block copolymers the sequence of crystallization of the two blocks plays a definitive role. In this work we show that the use of epitaxial crystallization on selected crystalline substrates allows achieving of a control over the crystallization of the blocks by inducing crystal orientations of the different crystalline phases and a final control over the global morphology. A sample of polyethylene-block-syndiotactic polypropylene (PE-b-sPP) block copolymers has been synthesized with a stereoselective living organometallic catalyst and epitaxially crystallized onto crystals of two different crystalline substrates, p-terphenyl (3Ph) and benzoic acid (BA). The epitaxial crystallization on both substrates produces formation of highly ordered morphologies with crystalline lamellae of sPP and PE highly oriented along one direction. However, the epitaxial crystallization onto 3Ph should generate a single orientation of sPP crystalline lamellae highly aligned along one direction and a double orientation of PE lamellae, whereas BA crystals should induce high orientation of only PE crystalline lamellae. Thanks to the use of the two selective substrates, the final morphology reveals the sequence of crystallization events during cooling from the melt and what is the dominant event that drives the final morphology. The observed single orientation of both crystalline PE and sPP phases on both substrates, indeed, indicates that sPP crystallizes first onto 3Ph defining the overall morphology and PE crystallizes after sPP in the confined interlamellar sPP regions. Instead, PE crystallizes first onto BA defining the overall morphology and sPP crystallizes after PE in the confined interlamellar PE regions. This allows for discriminating between the different crystalline phases and defining the final morphology, which depends on which polymer block crystallizes first on the substrate. This work also shows that the use of epitaxial crystallization and the choice of suitable substrate offer a means to produce oriented nanostructures and morphologies of block copolymers depending on the composition and the substrates.

The crystallization behavior of butene-ethylene random isotactic copolymers prepared with a Ziegler-Natta catalyst, in particular the surprising crystallization of form II of isotactic polybutene in samples with a high ethylene concentration, provides novel evidence of the long-debated blocky molecular structure of these "random" copolymers.

The crystallization and thermal properties of copolymers of isotactic poly(1-butene) (iPB) with ethylene (iPBEt) synthesized with a Ziegler-Natta catalyst and ethylene (E) content between 7 and 45 mol % are presented. The crystallization properties provide evidence of a multiblock molecular structure of these copolymers. The samples have been separated in two or three fractions by standard fractionation in boiling solvents. Samples with ethylene concentration lower than 16 mol % surprisingly crystallize in form II of iPB, and crystallization of polyethylene also occurs for the maximum ethylene content of 45 mol %. Even more surprising is the result that the melting temperature of crystals of any polymorphic form of iPB that crystallize in these samples do not depend on the ethylene concentration. This suggests that long crystallizable butene sequences are present even at high ethylene concentration. In all fractions, the chains are composed of multiblocks where the comonomers are segregated, with blocks characterized by long crystallizable butene sequences and blocks with shorter butene sequences. In the different fractions, the average length of butene sequences is nearly the same. The different fractions are featured by different amounts of the constituting blocks.

The mechanical properties and the structural transformations occurring during deformation of some commercial grades of ethylene/1-octene multiblock copolymers (OBCs) obtained from chain shuttling technology are analyzed. The samples are characterized by a statistical multiblock architecture, where soft and amorphous blocks with high octene concentration (approximate to 18.9 mol %) alternate with hard and crystalline blocks with low octene concentration (approximate to 0.5 mol %). The length of blocks (BL) and the number of blocks/chain (NB) change from chain to chain according to a statistical distribution. The selected samples have molecular mass in the range 85-130 kg/mol, percentage of hard segments in the range 15-27%, and a melting temperature of approximate to 120 degrees C. The average molecular masses of the hard blocks M-H and soft blocks M-S are in the ranges 2-3 kg/mol and 6-15 kg/mol, respectively, whereas the number of blocks/chain is in between 2 and 17. Even though the samples are characterized by similar octene concentration, small difference in molecular mass, and fractional content of hard blocks, they show remarkable differences of mechanical properties, depending on the average BL and NB values, encompassing from those of strong elastomers, in the case of samples with low block length and high number of blocks/chain, to those of soft elastomers, in the case of samples with high block length and low number of blocks/chain. The differences in the mechanical properties of OBC samples are amplified by stretching at high temperatures. Not previously stretched films obtained by compression molding show only partial recovery of the initial dimensions in mechanical cycles of stretching and releasing the tension, with values of recovered strain higher than 50% at 25 degrees C. However, the resultant specimens obtained by release of the tension show good elastomeric properties in a wide deformation range at 25 degrees C and, in the case of the samples with high strength, also at 60 degrees C. Fiber diffraction analysis reveals that by stretching at high deformation the orientation of the crystals is accomplished by mechanical melting and formation of an oriented amorphous phase, namely, involving the hard segments extracted from the crystals. Upon releasing the tension recrystallization occurs, and the high degree of orientation achieved by the crystals and amorphous phase is lost.

Syndiotactic propene–hexene (sPPC6) and propene–octene (sPPC8) copolymers have been synthesized in a wide range of compositions with a syndiospecific metallocene catalyst. The samples are crystallized up to a comonomer content of nearly 20 mol% with crystallinity, melting and glass transition temperatures that decrease with increasing concentrations of comonomers. The copolymers show elastic properties that depend on the crystallization behavior. The as-prepared samples with a comonomer content lower than 2 mol% crystallize in mixtures of the helical form I and form II of syndiotactic polypropylene (sPP), the latter containing conformational kink band defects and the former containing a b/4 shift disorder. Both sPPC6 and sPPC8 copolymers crystallize from the melt in disordered modifications of form I characterized by disorder in the alternation of right- and left-handed two-fold helical chains along the axes of the orthorhombic unit cell of form I of sPP. At high crystallization temperatures, more ordered modifications of form I develop for samples with a low comonomer concentration and form II surprisingly crystallizes in samples with a high comonomer content. The crystallization of form II rather than the ordered form I in melt-crystallized samples is a valuable result because it indicates that it is possible to tune the polymorphic transformations and the elastic properties of these copolymers through tuning of the branches concentrations.

The mechanical properties of copolymers of syndiotactic polypropylene (sPP) with 1-hexene and 1-octene have been studied and correlated with the structural transformations occurring during stretching and relaxation. These copolymers crystallize in disordered modifications of form I of sPP, with crystallinity, melting, and glass-transition temperatures that decrease with increasing comonomer content. All copolymers show remarkable mechanical properties and elastic behavior, with great improvement of flexibility and ductility with respect to the sPP homopolymer already at small comonomer concentrations. For low comonomer content up to 4-5 mol %, the elastic recovery is associated to a reversible conformational phase transition between the trans-planar form III or the trans-planar mesophase and the helical form II, which gives an enthalpic contribution to the elasticity. As the comonomer content increases, the trans-planar conformation is progressively destabilized and the observed elastic properties are not associated with any conformational transformation during stretching and relaxation. In these cases, copolymers behave as conventional thermoplastic elastomers where elasticity is purely entropic and crystals only act as topological constraints of the elastomeric network. These copolymers represent an exemplary case of the possibility to combine in the same materials desired and apparently incompatible physical properties of elasticity, crystallinity, and strength by controlling the crystallization behavior, achieved by tailoring the chemical structure in the synthetic process.

The mechanical properties and elastic behavior of Ziegler-Natta isotactic 1-butene-ethylene (BuEt-ZN) copolymers, having a broad composition from 1.7 to 45.5 mol% of ethylene units, are studied. The multi-site nature of the catalytic system determines a non-uniform distribution of the comonomers along the polymer chains; the copolymers present a blocky microstructure and composed of fractions of chains made of long and short butene sequences, which can be separated by solvent fractionation procedures. Incorporation of ethylene induces improving of drawability and decrease of rigidity with respect to homopolymer, associated with the lower crystallinity, and outstanding elastomeric behavior when the ethylene amount exceeds 5.5 mol%. In these copolymers the presence of irregular fractions of chains with short butene sequences, showing crystallization of the defective and low-melting temperature form I′ upon aging of the amorphous at room temperature, is responsible for the good elasticity of the materials.

A study of the crystallization behavior and kinetics of form II-form I transition in isotactic butene-ethylene copolymers (iPBEt) prepared with Ziegler-Natta catalyst with ethylene concentration in the range 9–44 mol% is presented. The samples have been separated with boiling solvents in different fractions containing chains with different ethylene concentration. Crystallization of form II from the melt is observed in each fraction even at high ethylene concentration, up to nearly 16 mol%. For higher ethylene content the samples do not crystallize from the melt but still crystallize during aging at room temperature in form I′. The comparison with the crystallization behavior of iPBEt copolymers prepared with homogeneous metallocene catalysts, characterized by a random distribution of ethylene units, has shown that the form II-form I transformation in Ziegler-Natta copolymers is much slower than in metallocene copolymers having the same ethylene concentrations and for the metallocene copolymers a low concentration of ethylene units of nearly 6 mol% is sufficient to prevent crystallization from the melt of form II. This indicates that in the chains of Ziegler-Natta iPBEt copolymers the regular butene sequences interrupted by the ethylene units are much longer than those of the chains of copolymers prepared with metallocene catalysts of the same ethylene concentration, giving further evidence of the multiblock structure of Ziegler-Natta iPBEt copolymers.

Isotactic propylene-butene copolymers [i(P-co-B)] with precise and controlled molecular structures were synthesized with various organometallic catalysts having different stereoselectivities. Stereoregular and stereodefective samples of i(P-co-B) with 1-butene (B) content variable in the whole range of composition were synthesized. All samples crystallize regardless of composition, indicating cocrystallization of propene and 1-butene units, which are incorporated in the unit cells of polymorphic forms of isotactic poly(propylene) (iPP) and isotactic poly(1-butene) (iPB). The copolymers show a continuum change of crystal morphology with the composition, transforming from big spherulites to bundle-like and needle-like crystals, to granular crystals. The cocrystallization allows maintaining high crystallinity of copolymers for any composition and provides an opportunity to develop outstanding mechanical properties that can be tailored by changing the isotacticity and composition. This allows, ideally, combining in the same material the different properties of stiffness of iPP and flexibility of iPB. These copolymers show, indeed, mechanical properties intermediate between iPP and iPB, ranging from stiffness/brittleness and ductility/flexibility depending on the composition and isotacticity, with high strength and Young's modulus that may be regulated by the stereoregularity of the iPP and iPB sequences, which is, in turn, dictated by the catalyst structure.

In this review, the phenomenon of polymorphism in polymers is discussed on the basis of the thermodynamic stability of different polymorphs that defines the conformations of chains in the crystalline lattice and the packing mode. In particular, we show that the symmetry and energy stability of the chain conformation, the crystal density and entropy of packing, and the presence of defects and disorder in the crystals are the main factors that define the mode of crystallization of polymers and induce polymorphism. How specific polymorphs are selected and the effects of the molecular structure and processing conditions on polymorphism are analyzed. How crystallization of particular polymorphs affects the material's properties and ultimate applications is also discussed. The polymorphic behaviors of isotactic polypropylene and poly(1-butene) are described as examples of how processing and the controlled modification of the molecular structure of polyolefins using methods of controlled synthesis that allow controlled incorporation of configurational and constitutional defects may drive crystallization of a desired polymorph. The possibility to select a particular polymorph using these strategies results in a tool to tailor the physical and mechanical properties of polymers.

The crystalline morphology of ethylene/1-octene (EO-BC) statistical multiblock copolymers is investigated by transmission electron microscopy (TEM). EO-BCs are an important class of thermoplastic elastomers characterized by alternating crystalline, octene-poor, hard blocks and amorphous, octene-rich, soft blocks and statistical distribution of block length (BL) and number of blocks/chain (NB). Adopting similar crystallization conditions, samples with only subtle differences in the chain microstructure, are shown to display different solid-state morphology as probed by TEM and different melt rheology. In particular, the homogeneous or heterogeneous lamellar morphology which develops in the solid state reflects the tendency of the samples to show homogeneous or heterogeneous melt-rheology, respectively. As EO-BCs are a reactor blend of non-uniform chains, it is argued that the solid state and melt phase behavior of EO-BCs is controlled by differences in the relative abundance of chains including a prevalence of hard blocks of short and/or high length.

The phase behavior of ethylene/propylene(/diene) (EP(D)M) co- (and ter-)polymers characterized by ethylene content in the range 64–78 wt%, and comparatively low level of crystallinity, in the range 1–15%, is analyzed during stretching, through collection of wide angle X-ray scattering data. All samples experience crystallization by effect of stretching, but whereas for the samples with ethylene content ≥71% crystallinity starts increasing already after yielding, the nearly amorphous sample, with 64 wt% ethylene content, experiences crystallization only at strain above a threshold (≈170%). In all cases, a high degree of orientation of the amorphous phase is achieved already at low deformations. It is shown that for the samples with high ethylene content, the pre-existing crystals undergo initial fragmentation and/or mechanical melting at low deformations, and successive oriented recrystallization at higher deformations. The role of oriented amorphous phase, along with the role of pre-existing crystals as nuclei able to foster further crystallization is highlighted.

In this paper the morphologies and crystallization properties of crystalline block copolymers are reviewed. The equilibrium morphologies expected in crystalline-amorphous diblock copolymers and the different modes of crystallization, such as, breakout, templated crystallization, confined crystallization and pass through crystallization are described. The effects of confinement on crystallization mode and crystallization kinetics are also analyzed in detail. The paper is focused on the morphologies and nanostructures produced in crystalline block copolymers based on polyolefins. Early studies performed on block copolymers containing highly defective polyethylene block, synthesized by hydrogenation of block copolymers containing 1,4-polybutadiene blocks prepared by classic anionic living polymerization, are described in the first part of the paper. Then, the paper features recent advances in the synthesis and characterization of crystalline block copolymers containing stereoregular polyolefins, such as stereoregular polypropylene and linear polyethylene prepared using novel organometallic catalysts able to promote the living and stereoselective polymerization of α-olefins. Strategies for creating nanoscale patters exploiting the ability to induce desired orientations of the crystalline phase through epitaxial crystallization of the crystallizable blocks on crystalline substrates are described. According to this strategy, the desired orientations of the crystalline phase may be suitably designed and act as templates to create the desired final ordered nanostructure.

The synthesis of a poly(3,5-tricyclo[2.2.1.0(2,6)]heptylene) [poly(norbornadiene) (PNBD)] mediated by the TiCl4/Et2AlCl catalytic system is reported. This polymer is characterized by a rigid nortricyclene repeating structure and unique 3,5-enchainment and is obtained through a transannular cationic polymerization of norbornadiene involving both double bonds. The new PNBD is crystalline with high glass transition and melting temperatures due to the rigid tricyclic repeating structure. C-13 NMR spectra indicate that the chains of PNBD contain only exo/exo (xx) and exo/endo (xn) nortricyclene repeating monomeric units. Models of the chain conformation and crystal structure of PNBD are also reported. Models of low conformational energy of isotactic and syndiotactic chains of PNBD have been obtained from the regular enchainment of xx and xn units. The model of the isotactic (xx)(m) chain with s(2/1) helical symmetry assumes a nearly trans-planar conformation, whereas the models of the syndiotactic (xn-xn)(m) and (nx-xn)(m) chains with tc and ti symmetries, respectively, assume the conformations (TG(-)G(-)TG(+)G(+))(n) and (G(-)TTTG(+)T)(n), respectively. Ideal limit ordered models of the crystal structure have been obtained from the X-ray powder diffraction data and the conformational energy analysis. A good agreement between experimental and calculated powder diffraction profiles has been obtained for a limit ordered model of packing of syndiotactic chains (nx-xn)(m) of PNBD with ti symmetry in an orthorhombic unit cell with axes a = 6.6 angstrom, b = 11.0 angstrom, and c = 6.84 angstrom according to the monoclinic space group P2(1)/c. The real crystal structure of PNBD is disordered because of the possible presence of configurational disorder in the chains due to possible random enchainment of nortricyclene monomeric units in the xx, xn and nx configurations that produces conformational and packing disorder.

The crystallization behavior and the morphology of blends obtained by mixing a highly crystalline and isotactic poly(1-butene) (iPB) and a copolymer of isotactic polypropylene (iPP) with 26 mol% of 1-hexene (C3C6 copolymer) have been studied. The C3C6 copolymer does not crystallize from the melt but the amorphous sample crystallizes into the trigonal form of iPP by aging at room temperature. All the blends crystallize from the melt in form II of iPB and the aging at room temperature induces transformation into form I and crystallization of the C3C6 copolymer. Both the melt crystallization of form II and its transformation into form I of iPB are retarded in the blends by the presence of the C3C6 component, whereas the crystallization from amorphous phase of the C3C6 copolymer is accelerated thanks to the nucleating effect of the preformed iPB crystals. A good inter-mixing of the two components occurs by solution blending. However, the initial state of the blend obtained by precipitation from solution is not at equilibrium and at 200 °C phase separation takes place by effect of spinodal decomposition, and the degree of demixing increases with increasing the residence time at 200 °C. The final morphology of the iPB/C3C6 blends depends on the composition and can be controlled and modulated by changing time and temperature of melt treatment.

The yield behavior during uniaxial drawing of isotactic random copolymers of propene with ethylene (iPPEt), 1-butene (iPPBu), 1-pentene (iPPPe), 1-hexene (iPPHe), and 1-octadecene (iPPOc) is analyzed within the framework of our current understanding of deformation properties of semicrystalline polymers, that is, the intrinsic stability of lamellar crystals and related polymorphism phenomena, along with the ability of entangled amorphous chains to transmit stress. The samples selected for analysis were synthesized using single-site metalorganic catalysts, are highly stereoregular, and contain small amounts of regiodefects caused by secondary 2,1 erythro units. Moreover, the interchain and intrachain distribution of comonomeric units is uniform. In the case of iPPEt copolymers, samples containing ≈3.5 mol% stereodefects were also studied. The yield behavior of these samples depends on the kind and concentration of defects, and is directly related to the level of inclusion in and exclusion from crystals of the comonomeric units. Apart from iPPBu copolymer samples with high butene content, the yield stress of all samples increases with the thickness of lamellar crystals according to a common trend, regardless of comonomer. In the case of iPPBu copolymers containing a high concentration of butene units, the yield stress decreases with increasing lamellar thickness. The increase in yield stress with lamellar thickness is rationalized in terms of the micromechanical model of crystallographic slips, based on thermal activation of screw dislocations. The parameters of the model describing the yield behavior are the critical free energy required to form a screw dislocation and the shear modulus associated with the slip planes of the dislocations. These were set as identical to those deduced for isotactic polypropylene homopolymer samples (iPP) crystallized under different conditions. Study of the yield behavior of these copolymers extends the use of the dislocation model to a set of samples crystallized under similar conditions but characterized by differences in comonomeric unit, degree of crystallinity, lamellar thickness, polymorphism, and intrinsic flexibility of the chain backbone. The results indicate that for a homogeneous class of propene-based copolymers, namely crystallized in the α-form of iPP under similar conditions, lamellar thickness controls the level of plastic resistance provided that the concentration of structural irregularities in the crystals is not too high. iPPBu copolymers with high comonomer concentration do not obey this rule because of the high level of inclusion of comonomers in the crystals, which induces an increase in lamellar thickness but also a decrease in crystal stability.

[This corrects the article DOI: 10.1021/acs.macromol.3c00710.]., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)