Crystallization and mechanical properties of metallocene made 1-butene-pentene and 1-butene-hexene isotactic copolymers

Accepted Manuscript Crystallization and mechanical properties of metallocene made 1-butene-Pentene and 1-butene-Hexene isotactic copolymers Oreste Tarallo, Odda Ruiz de Ballesteros, Annalisa Bellissimo, Miriam Scoti, Anna Malafronte, Finizia Auriemma, Claudio De Rosa PII:

S0032-3861(18)30972-8

DOI:

https://doi.org/10.1016/j.polymer.2018.10.045

Reference:

JPOL 20994

To appear in:

Polymer

Received Date: 2 August 2018 Revised Date:

14 October 2018

Accepted Date: 20 October 2018

Please cite this article as: Tarallo O, Ruiz de Ballesteros O, Bellissimo A, Scoti M, Malafronte A, Auriemma F, De Rosa C, Crystallization and mechanical properties of metallocene made 1-butenePentene and 1-butene-Hexene isotactic copolymers, Polymer (2018), doi: https://doi.org/10.1016/ j.polymer.2018.10.045. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Crystallization and Mechanical Properties of Metallocene Made 1-Butene-Pentene and 1-Butene-

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Hexene Isotactic Copolymers

Oreste Tarallo,* Odda Ruiz de Ballesteros, Annalisa Bellissimo, Miriam Scoti, Anna Malafronte, Finizia Auriemma, Claudio De Rosa

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Dipartimento di Scienze Chimiche, Università di Napoli "Federico II", Complesso Monte S. Angelo,

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Via Cintia, I-80126 Napoli, Italy

Keywords: Isotactic polybutene, isotactic copolymers, mechanical properties, structure-

Abstract

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properties relations, phase transition, polymer crystallization, X-rays.

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The structure and the mechanical properties of metallocene-made butene-pentene (iPBC5) and

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butene-hexene (iPBC6) isotactic copolymers are studied. The effect of the presence of pentene and hexene units on the crystallization behavior and the mechanical properties of isotactic polybutene (iPB) are analyzed. All iPBC5 copolymers crystallize from the melt in form II of iPB, which transforms into form I by aging at room temperature. On the contrary, in iPBC6 copolymers the presence of hexene units stabilizes the form II and, for hexene concentrations higher than 11 mol%, prevents the transformation of form II into form I at room temperature. Both iPBC5 and iPBC6 copolymers show mechanical behavior of highly flexible and ductile materials with enhanced ductility compared to iPB, with values of stress at yielding and of Young’s modulus that decrease with increasing comonomer content. In all the iPBC5 copolymers, form II crystallized from the melt 1

ACCEPTED MANUSCRIPT transforms into form I by stretching, whereas in iPBC6 copolymers form II is stabilized and the transformation of form II into form I by stretching is completely inhibited at high hexene

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concentrations.

Introduction

Isotactic poly(1-butene) (iPB) is a thermoplastic polymer of great industrial interest due to

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its excellent mechanical properties as good toughness, flexibility, tear strength, creep and impact resistance. These properties and the retaining of its mechanical properties at elevated temperatures

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could make it an ideal candidate for applications like pressurized tanks, tubes, and hot water pipes [1-3]. However, iPB crystallizes from the melt in form II [2, 5-11], which spontaneously transforms at room temperature into the thermodynamically stable form I. This transition is accompanied by deep structural changes involving a change in the conformation of chains, from the 11/3 helix of

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form II into the 3/1 helix of form I, and a change in their packing from the tetragonal unit cell of form II into the trigonal structure of form I [4, 6-11]. The unique polymorphic behavior of iPB is further complicated by the fact that form I exists in two variants: the crystal modification generated

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via a solid-state transformation of form II by aging at room temperature, usually addressed as form I, and the same trigonal crystal structure with chains in 3/1 helical conformation obtained by direct

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crystallization from melt or solution, usually addressed as form I’ [12]. Melting temperatures of form I is about 130 °C, while that of form I’ is 90-95 °C [12]. Since these modifications have the same crystal structures, the difference of the melting temperatures has been interpreted assuming that form I’ is an imperfect form I and that it is characterized by thinner crystals than those of form I [12, 13-15]. Finally, a third form, form III, is obtained by crystallization form solutions. Form III consists of chains in 4/1 helical conformation packed in an orthorhombic lattice [2,12,13-17]. Form II → Form I transformation is slow at room temperature and atmospheric pressure and it is completed in several days. This process is unfavorable for industrial applications and represents 2

ACCEPTED MANUSCRIPT the main concern that has delayed the commercial diffusion of iPB. Indeed, although the Form II → Form I transformation causes beneficial effects on the properties including increase of rigidity, strength, and melting temperature [2,11,18-20], on the other hand it causes also severe deformations of molded objects due to an increase of density and shrinkage that reflect changes at microscopic

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length scale involving decrease in the cross section by ≈10% and an extension by ≈14% of the chain conformation in crystalline strands.

These facts have inspired many researchers to find suitable strategies to accelerate or inhibit

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the Form II → Form I transition.

The rate of transformation depends on several conditions such as the aging temperature, the

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application of hydrostatic pressures, shear or tensile stresses, as well as the molecular structure, in particular, the molecular mass, the degree of isotacticity, and presence of constitutional defects such as comonomeric units [1,7,8,13,20-26].

As far as the effect of the presence of comonomeric units is concerned, it is well known that

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in the case of copolymers of iPB with various α-olefins synthesized with heterogeneous ZieglerNatta (ZN) catalysts [1,7,8,23], the introduction in the polymer chain of comonomeric units with less than 5 carbon atoms (e.g. ethylene, propylene and pentene) increases the transformation rate,

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while longer linear α-olefins (with number of carbon atoms higher than 5) or branched comonomers (such as 3-methylbutene, 4-methylpentene and 4,4-dimethylpentene), delay the transition and, at a

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threshold concentration, form II is completely stabilized [7, 8]. It is important to remark that these studies have been performed on iPB samples prepared with heterogeneous Ziegler-Natta catalysts that produce mixtures of macromolecules characterized by different microstructure in terms of stereoregularity, molecular masses, concentration of comonomers, and a non-random distribution of stereodefects and comonomer units along the polymer chains. For these reasons, due to the heterogeneity of the composition of those samples, it is not immediate to draw firm conclusions on the real effect of the presence of comonomers in the polymer chains on the crystallization behavior of iPB.. 3

ACCEPTED MANUSCRIPT Recently, we have started investigating the polymorphic behavior and the mechanical properties of a series of 1-butene based isotactic copolymers prepared by metallocene catalysts [2731]. These samples are characterized by a controlled molecular structure in terms of stereoregularity, random distribution of stereodefects and of comonomer units along the polymer

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chains and uniform comonomer concentration, allowing us understanding the effect of each microstructural feature on the polymorphic behavior and on the mechanical properties of iPB [2731]. In particular, the studies performed on metallocene made stereodefective iPB samples

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containing concentration of rr stereodefects higher than 2-3 mol% have allowed to observe, for the first time, the crystallization of the stable Form I directly from the melt at atmospheric pressure [27,

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28]. Moreover, in the case of the copolymers of iPB prepared with metallocene catalysts, it has been found that ethylene units accelerate the Form II → Form I transition at room temperature and at concentration of nearly 6 mol% favors the direct crystallization from the melt of the Form I’ [30]. A completely different behavior has been described for copolymers with longer linear comonomeric

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units. In the case of isotactic butene-octene copolymers [31], it has been observed, indeed, that the presence of octene comonomeric units up to nearly 6 mol% stabilizes the tetragonal Form II, thus decelerating or even preventing the Form II → Form I transformation. For octene concentrations

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higher that 7 mol%, the samples do not crystallize from the melt and the obtained amorphous samples crystallize at room temperature by aging in mixtures of Form II and Form I’. Moreover,

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contrary to expectation [7, 8], the sample with further higher octene concentration (11.5 mol%) crystallizes from the amorphous phase directly in the pure Form I’ [31]. Finally, the presence of stereodefects or comonomeric units also induces a significant modification of the mechanical properties of iPB. In fact stereodefective iPB samples show interesting mechanical properties of high ductility and flexibility that increase with increasing concentration of rr stereodefects [29] while, in the case of copolymers, the development of an elastic behavior has been observed for ethylene concentration higher than 8 mol% [29] and for octene concentration higher than 6-7 mol% [31]. 4

ACCEPTED MANUSCRIPT Aiming at investigating the effect of the presence of comonomeric units on the crystallization and mechanical properties of iPB, in this paper we report a study of the crystallization behavior in unoriented specimens and oriented fibers and of the mechanical properties of isotactic butene-1-pentene (iPBC5) and butene-1-hexene (iPBC6) copolymers,

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synthesized with highly isospecific metallocene catalysts and characterized by a negligible amount of stereodefects and a random distribution of comonomer units. It is worth noting that for the corresponding copolymers obtained by Ziegler-Natta catalyst [7], 1-pentene and 1-hexene

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comonomers had an opposite effect on the Form II → Form I transformation rate: while copolymers with 1-pentene showed an accelerated polymorphic transformation, 1-hexene copolymers showed a

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clear stabilization of Form II.

Experimental section.

Samples of isotactic poly(1-butene) (iPB), isotactic butene-1-pentene (iPBC5) and isotactic

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butene-1-hexene copolymers (iPBC6) were prepared with the zirconocene catalysts A and B of Chart 1, respectively, activated with methylalumoxane. Both catalysts are highly regio- and isospecific in 1-butene polymerization and produce highly regioregular and high molecular mass iPB

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homopolymer and copolymer samples. The catalyst also allows for incorporation of large amount of 1-pentene (C5) and 1-hexene (C6) while keeping high molecular mass. A list of the copolymer

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samples analyzed in this paper is reported in Table 1.

S Me2Si

S

Me2Si ZrCl2

S

ZrCl2

S

A

B

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ACCEPTED MANUSCRIPT Chart 1. Structure of the catalysts used for the preparation of butene-pentene (A) and butenehexene (B) isotactic copolymers. The mass average molecular masses and polydispersity were evaluated by gel permeation chromatography (GPC). Calorimetric measurements were performed with a differential scanning

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calorimeter (DSC-30) by Mettler Toledo in a flowing N2 atmosphere at a scanning rate of 10°C/min. Compression molded films were prepared by melting powders of the as-polymerized samples at temperatures 30-40°C higher than the melting temperatures under a press at very low

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pressure and cooling to room temperature by circulation of cold water in the press plates. Oriented

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fibers have been obtained by stretching at room temperature compression molded films.

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Table 1. Comonomer concentration (mol%), viscosity average molecular mass (Mv), polydispersity (Mw/Mn), concentration of rr stereoerrors and regiodefects (4,1 units), melting temperatures of melt-crystallized not-aged samples (Tm(II)), melting temperature of melt-crystallized samples aged

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at room temperature for long time (years) (Tm), crystallization temperature from the melt (Tc), glass transition temperature (Tg) and crystal form of melt-crystallized and aged samples of the iPBC5 and iPBC6 copolymers and of the corresponding iPB homopolymers, prepared with the catalysts of

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Chart 1. DSC scans are shown in the supporting information file.

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Comonomer [4,1] Mw Mw/Mnb Tm(II)d (°C) Tme (°C) Tcf (°C) Tgg (°C) Crystal formh [rr] (mol%) (mol%)a (mol%) (kg/mol)b iPB1 A 0 0.8 ~0 104 128 60 -29 form I iPBC5-4 A 4.1 0.8 ~0 3.96×102 2.00 97 114 53 -28 form I iPBC5-9 A 9.1 0.8 ~0 4.14×102 2.10 87 110 40 -28 form I iPBC5-15 A 14.7 0.8 ~0 4.35×102 2.10 80 108 55 -31 form I iPBC5-19 A 18.9 0.8 ~0 4.94×102 2.10 71 100 27 -29 form I iPB2 B 0 ~0 0.3 108 125 59 -30 form I iPBC6-3 B 3.2 ~0 0.2 2.62×102 c 2.05 94 111 53 -31 form I iPBC6-6 B 5.9 ~0 0.3 2.60×102 c 2.37 80 100 31 -33 form I + II iPBC6-11 B 11.2 ~0 2.73×102 c 2.18 60 83 31 -32 form II iPBC6-16 B 15.8 ~0 0.3 2.69×102 c 2.13 73 -32 form II a ) From 13C NMR analysis. b) From gel permeation chromatography (GPC). c) For these samples the values of molecular mass are viscosity average molecular masses evaluated from the intrinsic viscosities. d) Measured from DSC scans at heating rate of 10 °C/min of melt-crystallized samples of Figure 1A,A’ (form II). e) Measured from DSC scans at heating rate of 10 °C/min of aged samples of Figure 1C,C’ (crystal forms indicated in the last column). f) Measured from DSC cooling scans from the melt at cooling rate of 10 °C/min. g) Evaluated from the DSC heating curves recorded at heating rate of 10 °C/min of samples previously cooled from the melt to -70 °C at 10 °C/min. h) Evaluated from the X-ray powder diffraction profiles of the samples crystallized from the melt and aged at room temperature for years of Figure 1C and C’.

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Catalyst

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Sample

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ACCEPTED MANUSCRIPT X-ray diffraction patterns were obtained with Ni filtered Cu Kα radiation. The powder profiles were obtained by an automatic Philips diffractometer, whereas the fiber diffraction patterns were recorded on a BAS-MS imaging plate (FUJIFILM) using a cylindrical camera and digitized with a digital imaging reader (Perkin Elmer Cyclone Plus).

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The indices of crystallinity (xc) were evaluated from the X-ray powder diffraction profiles by the ratio between the crystalline diffraction area and the total area of the diffraction profile. The crystalline diffraction area was obtained from the total area of the diffraction profile by subtracting

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the diffraction halo of the amorphous phase after subtraction of a straight baseline and scaling. The diffraction halos of the amorphous phase of the iPB homopolymer samples and the copolymers

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were determined from the diffraction profile of a sample of atactic poly(1-butene). The fraction of crystals of Form I with respect to the crystals of Form II (fI), present in the melt crystallized compression molded films and in samples aged at room temperature, was evaluated from the intensities of the (110)I reflection of Form I at 2θ = 9.9° and the (200)II

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reflection of Form II at 2θ = 11.9°, in the X-ray powder diffraction profiles [23-25]. The mechanical tests were performed at room temperature on unoriented compression molded films and oriented stress-relaxed fibers of the iPB homopolymers, iPBC5 and iPBC6

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copolymers with a universal mechanical tester Zwicky by Zwick Roell, following the standard test method for tensile properties of thin plastic sheeting ASTM D882-83. Rectangular specimens 20

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mm long, 2-4 mm width and 0.3-0.5 mm thick cut from unoriented compression molded films were stretched up to the break or up to a given strain ε = 100×(Lf-L0)/L0, with Lf and L0 the final and initial lengths of the specimen, respectively. Two benchmarks were placed on the test specimens and used to measure elongation. In the mechanical tests the ratio between the drawing rate and the initial length was fixed equal to 0.1 mm/(mm×min) for the measurement of Young’s modulus and 10 mm/(mm×min) for the measurement of stress-strain curves and the determination of the other mechanical properties

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ACCEPTED MANUSCRIPT (stress and strain at yield and at break and tension set). The reported stress-strain curves and the values of the mechanical properties are averaged over at least five independent experiments.

Results and Discussion.

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Crystallization behavior. The X-ray powder diffraction profiles of samples of iPBC5 and iPBC6 copolymers and of corresponding iPB homopolymer samples, prepared with the same catalysts, crystallized from the melt by compression molding and cooling to room temperature (~20°C),

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recorded soon after the preparation of the films, are reported in Figure 1A and 1A’, respectively. The diffraction profiles of the same melt-crystallized samples after aging at room temperature for 8

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months and for longer time (years) are reported in Figure 1B-B’ and 1C-C’, respectively. Both samples of iPB homopolymer (diffraction profiles a of Figure 1A and 1A’) crystallize from the melt in the metastable Form II, as indicated by the presence of the (200)II, (220)II and (213)II+(311)II reflections at 2θ = 11.9, 16.9 and 18.3°, respectively. Upon aging at room

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temperature, Form II transforms into Form I, as indicated by the fact that the reflections of Form II are replaced by the (110)I, (300)I and (220)I + (211)I reflections of Form I at 2θ = 9.9, 17.3 and 20.5°, respectively (Figure 1B-B’, profiles a). After eight months aging, only a minor fraction of

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form II crystal is still present in the samples, that tends to disappear only after a very long aging

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time (Figure 1C-C’, profiles a).

Samples of iPBC5 copolymers with pentene concentrations up to 14.7 mol% (iPBC5-4, iPBC5-9 and iPBC5-15, containing 4.1, 9.1 and 14.7 mol% of pentene, respectively) also crystallize from the melt in form II immediately after the compression molding and cooling the melt to room temperature (profiles b-d of Figure 1A), as the homopolymer sample iPB1 prepared with the same catalyst (profile a of Figure 1A). The sample iPPC5-19 with 18.9 mol% of pentene still crystallizes from the melt in form II (Figure 1A, profile e) and a small amount of crystals of form I develops by cold-crystallization at nearly 26 °C, as evidenced by DSC scans (Figure S1).

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ACCEPTED MANUSCRIPT The degree of crystallinity of the melt-crystallized samples in Form II is reported in Figure 2A. It is apparent that the crystallinity is practically constant with the pentene content, and only for the sample iPPC5-19 with the highest pentene content a lower crystallinity has been observed (Figure 2A).

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For all the iPBC5 melt-crystallized samples, form II transforms into form I by aging at room temperature for eight months (profiles b-e of Figure 1B). A small amount of crystals of form II remains in the samples aged for eight months and even after aging for longer time of the order of

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years (profiles b-e of Figure 1C). Only in the case of sample iPBC5-19 with the highest pentene concentration, the transformation is almost complete after 8 months (Figure 1B, profile e). The

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degree of crystallinity of the aged films in form I is ≈60%, regardless of the pentene content (Figure 2A). The fraction of crystals of form I in the melt-crystallized samples and in the samples aged for years are reported in Figure 2B. It is apparent from these data that the cold-crystallization of form I from the amorphous is favored by high concentrations of pentene comonomeric units and, as in the

temperature.

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case of iPB homopolymer, form II transforms almost completely into form I by aging at room

Figure 1A’ shows that samples of iPBC6 copolymers with hexene concentration up to 11.2

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mol% crystallize from the melt in form II, as the homopolymer sample iPB2 synthetized with the

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same catalyst (profiles b-d of Figure 1A’). The degree of crystallinity of the melt-crystallized samples in form II decreases with increasing hexene content (Figure 2A). The sample iPBC6-16 with hexene content of 15.9 mol%, instead, does not crystallize by cooling from the melt and an amorphous sample is obtained (Figure 1A’, profile e). Only in the case of samples with low hexene concentrations (samples iPBC6-3 and iPBC66, with 3.2 and 5.9 mol% of hexene units, respectively), crystals of form II obtained from the melt transforms, at least in part, into form I (profiles b, c of Figure 1B’), whereas for samples with higher hexene concentrations form II is stable and does not transform into form I by aging (profile d of Figure 1B’). Finally, the amorphous sample with 15.9 mol% of hexene crystallizes upon aging in 10

ACCEPTED MANUSCRIPT form II (profile e of Figure 1B’). For the sample iPBC6-3 with the lowest hexene content only after years of aging at room temperature the transformation of form II in form I is almost complete (Figure 1C’, profile b), while for the sample iPBC6-6 with 5.9 mol% of hexene the transformation is so slow that no further transformation is observed even after years of aging (Figure 1C’, profile

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c). For higher hexene content the Form II → Form I transition is completely inhibited and even for very long aging time, form II crystals obtained by cooling the melt (profile d of Figure 1B’) or by crystallization from the amorphous (profile e of Figure 1B’) are stable and do not transform into

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form I (profiles d-e of Figure 1C’). This indicates that in iPBC6 copolymers the Form II → Form I

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transition is strongly retarded with respect to the homopolymer iPB.

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A - no aging

e

e

iPBC5-19 18.9 mol% C5 form I

d

iPBC5-15 14.7 mol% C5 form I

c

iPBC5-9 9.1 mol% C5 form I

(200)II

(220)II

a

5

(213)II (311)II

10

15

20 25 2θ (deg)

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b

iPB1 form II

30

a

35

5

e

iPBC5-19 18.9 mol% C5 form I

d

iPBC5-15 14.7 mol% C5 form I

c

iPBC5-9 9.1 mol% C5 form I

b

iPBC5-4 4.1 mol% C5 form I

a

iPB form I

(300)I

iPBC5-4 4.1 mol% C5 form I

(213)II (220)I (110)I (311)II (211)I (300)I (200)II

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b

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iPBC5-4 4.1 mol% C5 form II

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c

iPBC5-9 9.1 mol% C5 form II

Intensity (a.u.)

d

(220)I (211)I

10

15

20 25 2θ (deg)

iPB1 form I

30

35

5

10

15

20 25 2θ (deg)

30

35

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Intensity (a.u.)

(200)II

iPBC5-15 14.7 mol% C5 form II

(110)I

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(220)II

iPBC5-19 18.9 mol% C5 form II + form I

Intensity (a.u.)

(110)I

C - aging for years

B - 8 months aging (213)II (220)I (311)II (211)I

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A' - no aging

C' - aging for years

B' - 8 months aging iPBC6-16 15.8 mol% C6

d

(220)I (211)I

(110)I

iPBC6-11 11.2 mol% C6 form II

b (200)II

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20 25 2θ (deg)

iPB2 form II

30

a

35

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b

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(110)I (300) I (200)II

10

15

e

iPBC6-16 15.8 mol% C6 form II

d

iPBC6-11 11.2 mol% C6 form II

c

iPBC6-6 5.9 mol% C6 form II + form I

b

iPBC6-3 3.2 mol% C6 form I + form II

(213)II (220)I (311)II (211)I

(220)II

(110)I (300)I

iPB2 form I + form II

20 25 2θ (deg)

30

35

(220)I (211)I

a

5

10

15

20 25 2θ (deg)

iPB2 form I

30

35

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10

iPBC6-3 3.2 mol% C6 form II + form I

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a

5

c

(213)II (311)II (220)II

iPBC6-6 5.9 mol% C6 form II + form I

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iPBC6-3 3.2 mol% C6 form II

d

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c

Intensity (a.u.)

Intensity (a.u.)

(300)I

iPBC6-6 5.9 mol% C6 form II

(213)II (311)II

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e

iPBC6-11 11.2 mol% C6 form II

(200)II

iPBC6-16 15.8 mol% C6 form II

Intensity (a.u.)

e

Figure 1. X-ray powder diffraction profiles of melt crystallized samples of the iPBC5 (A-C) and iPBC6 (A’-C’) copolymers obtained by compression molding and cooling to room temperature recorded soon after the preparation of the film (A, A’), after aging at room temperature for 8 months (B, B’), and for long aging times (years) (C,C’). The X-ray powder diffraction patterns of the iPB homopolymers obtained with the same catalyst are reported (profiles a). The (110)I, (300)I and (220)I+(211)I reflections of Form I at 2θ = 9.9, 17.3 and 20.5°, respectively, and the (200)II, (220)II and (213)II+(311)II reflections of Form II at 2θ = 11.9, 16.9 and 18.3°, respectively, are indicated. 13

ACCEPTED MANUSCRIPT The fraction of crystals of form I in the melt-crystallized samples before and after aging for years are reported in Figure 2B. In the freshly prepared melt crystallized samples, the amount of form I increases with the pentene concentration for iPPC5 copolymers up to reach values of 4-5%, and is always zero in iPBC6 copolymers. In the aged samples it is almost 90-100% in iPBC5

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copolymers, whereas decreases in iPBC6 copolymers with increasing hexene concentration (Figure 2B).

The melting temperatures of melt-crystallized and aged samples (Table 1) decrease with

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increasing comonomer concentration in both iPBC5 and iPBC6 copolymers. Since the meltcrystallized samples are all in form II, with exception of the sample iPBC5-19 with the highest

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pentene concentration that is crystallized in mixture of form II and form I (Figure 1A), the decrease of the melting temperature of melt-crystallized samples reflects the decrease of melting temperature of crystals of form II of copolymers with the increase of pentene or hexene concentration. The melting temperature of form II in iPBC6 copolymers is lower than that in iPBC5 copolymers (Table

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1) and decreases more rapidly than in iPBC5 copolymers with increasing comonomer content. The melting temperature of the aged samples also decreases with increasing pentene or hexene comonomeric units. However, whereas in iPBC5 copolymers the aged samples are all in form I

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obtained from transformation of form II (Figure 1C), in iPBC6 copolymers the aged samples are in form II and only the sample iPBC6-3 with the lowest hexene concentration is in form I (Figure

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1C’). This explains the higher melting temperature of the aged samples of the iPBC5 copolymers than that of the iPBC6 copolymers (Table 1). Contrary to the melting temperature, the glass transition temperatures, evaluated from the DSC heating curves of the samples previously cooled from the melt, remains practically constant with the comonomer concentration (Table 1). The data of Figures 1 and 2 and Table 1 allow for a comparison with the behaviors of analogous copolymers of iPB with long α-olefins prepared with Ziegler-Natta (ZN) catalysts reported in the old literature [7]. Our data for metallocene copolymers indicates that 1-pentene

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ACCEPTED MANUSCRIPT comonomeric units favor crystallization of form I from the melt already for pentene concentrations of 16-19 mol% (profile e of Figure 1A). A similar effect has been observed in copolymers prepared with ZN catalysts only for concentration of pentene comonomeric units higher than 50 mol% (see Table 5 of reference 7). This difference is clearly due to the random distribution of comonomers in

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the chains of single-center metallocene made copolymers that makes more efficient the effect of comonomers, compared to the non-random and non-uniform comonomer distribution in copolymers prepared with heterogeneous ZN catalysts. In samples prepared with ZN catalysts, because of the

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non-random distribution of pentene units, the chains are characterized by regular butene sequences longer that those in metallocene copolymers at the same total comonomer concentration [30,31].

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This results in the experimental fact that longer butene sequences in ZN copolymers are still able to crystallize in form II even at high pentene concentrations [7].

In the case of iPBC6 copolymers, the introduction of the 1-hexene units stabilizes form II and the form II → form I transition is strongly retarded or even completely inhibited with respect to

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the iPB homopolymer (Figure 1A’-C’). This effect is similar to that showed by analogous samples obtained by Ziegler-Natta catalyst (Table 6 of reference 7). However, in the metallocene made copolymers the crystallization from the melt is prevented at concentrations of hexene comonomeric

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units of 15-16 mol% (profile e of Figure 1A’), lower than that in Ziegler–Natta samples that do not crystallize from the melt for hexene content higher than 48 mol% [7]. This can be still explained by

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the non-random distribution of hexene units in ZN copolymers that makes the regular butene sequences long enough to crystallize even at high hexene concentration. A detailed study of the kinetics of Form II → Form I transformation in metallocene made copolymers of iPB with these linear α-olefins is going to be reported in a forthcoming paper.

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50 40

A

30

iPBC5 no-aging iPBC6 no-aging iPBC5 aged iPBC6 aged

20

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10 0

5 10 15 comonomer concentration (mol%)

20

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0

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crystallinity (%)

60

100 80 60

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f1 (%)

B

40

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20

0

5 10 15 comonomer concentration (mol%)

20

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0

iPBC5 no-aging iPBC5 aged iPBC6 no-aging iPBC6 aged iPB1, iPB2 no-aging iPB1, iPB2 aged

Figure 2. Degree of crystallinity (A) and fraction of crystals of form I (B) of the melt-crystallized samples of iPBC5 and iPBC6 copolymers and corresponding iPB homopolymers, immediately after the compression molding and cooling the melt to room temperature (full symbols), and of the samples aged at room temperature for years (empty symbols). Degrees of crystallinity have been evaluated from the X-ray diffraction profiles of Figure 1A,A’ and C,C’.

16

ACCEPTED MANUSCRIPT The effect of pentene and hexene comonomeric units on the crystallization of iPB and on the form II - form I transformation can be explained in terms of thermodynamic and kinetics effects related to the random distribution of constitutional defects along the iPB chains and partitioning of comonomeric units between crystalline and amorphous phases [30,31]. The degree of partitioning

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depends on the branch length and for long branches, as in the case of butene-octene copolymers reported in our previous paper [31], comonomeric units are mainly excluded from the crystals of both form I and form II of iPB. In the case of iPBC5 and iPBC6 copolymers partial inclusion of

SC

branches in the crystals cannot be excluded. However, the fact that in iPBC6 copolymers form II is stabilized and the form II → form I transition is retarded or even completely inhibited with respect

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to the iPB homopolymer can be easily explained by the effect of stabilization of the complex 11/3 helical conformation of form II of iPB due to the presence of bulky side groups (for hexene units) that makes the 11/3 helical conformation thermodynamically more stable than the 3/1 helix of form I [9,31,32]. In fact, the steric interactions between bulky side groups are alleviated by isodistortion

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of the backbone torsion angles from the exact trans (T) and gauche (G) values of the (TG)n 3/1 helix toward bigger values (T+δ, G+δ)n, typical of complex M/N helical conformation, as the 11/3 helix

crystals.

EP

[32]. This is a “molecular effect” regardless of the presence of hexene units inside or outside the

AC C

In the case of iPBC5 copolymers, the crystallization behavior is similar to that of the iPB homopolymer because of the much more similar lengths of the branches in butene and pentene units. Only at very high concentrations of pentene units (higher than 18 mol%) a small amount of crystals of form I develops from the amorphous by cold-crystallization. This indicates that the effect of stabilization of form II and of the 11/3 helical conformation by the presence of longer branches is balanced by the interruption effect [30,31], that is, the shortening of the average length of crystallizable butene units due to the presence of high amount of randomly distributed constitutional defects (or any type of defects) that have a greater influence on the stability of the complex 11/3 helix of form II rather than on the 3/1 helix of forms I and I’ [30,31]. For higher pentene

17

ACCEPTED MANUSCRIPT concentrations (higher than 18 mol%), the random distribution of these constitutional defects strongly reduces the average length of the crystallizable butene sequences that becomes lower than 11, that is the number of monomeric units per helical period in a 11/3 helix, thus reducing the stability of form II and favoring the crystallization of form I [30,31].

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As for the form II-form I transformation and its kinetics, as stated in the introduction and described in ref. 7, in the case of copolymers produced by Ziegler-Natta catalysts, while copolymers with 1-pentene showed an accelerated polymorphic transformation, 1-hexene copolymers showed a

SC

clear stabilization of form II. The interpretation of our results of stabilization of form I in the case of iPBC5 copolymers at high pentene concentrations due to the shortening effect of 1-butene

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crystallizable sequences, and the stabilization of the 11/3 helical conformation with the consequent stabilization of form II in the case of iPBC6 copolymers, are in good agreement with the findings of ref. 7, and provide a molecular explanation of the role of comonomeric units on the form II→ form

Mechanical Properties.

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I transition.

The mechanical properties of iPBC5 and iPBC6 copolymers have been analyzed for samples

EP

crystallized from the melt in form II without aging (sample of Figure 1A,A’) and for samples aged at room temperature for eight months crystallized in form II or form I or in mixtures of both forms

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(samples of Figure 1B,B’). The corresponding stress-strain curves of compression-molded films of iPBC5 and iPBC6 and of the corresponding iPB homopolymer samples for the non-aged and aged samples are shown in Figure 3 and 4. The values of the mechanical parameters are reported in Table S1 and plotted in Figure 5 as a function of comonomer concentration. For both iPBC5 (Figure 3) and iPBC6 (Figure 4) copolymers, the presence of pentene or hexene comonomeric units produces a remarkable increase of ductility in both melt-crystallized (Figures 3A and 4A) and aged (Figures 3B and 4B) samples, with increase of deformation at break from nearly 350% or 450% of the homopolymer in form II (Figures 3A and 4A) or in form I

18

ACCEPTED MANUSCRIPT (Figures 3B and 4B), respectively, to about 600-900% for the melt-crystallized (Figures 3A and 4A) and aged (Figure 3B and 4B) samples of iPBC5 and iPBC6 copolymers (Figure 5D). In the case of iPBC5 copolymers (Figure 3), since the melt-crystallized samples are almost all in form II (Figure 1A), the yielding is ill-defined and the values of the stress at yielding are low

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and very similar, regardless of the pentene concentration, and similar to that observed in the stressstrain curve of the iPB homopolymer (Figure 3A). As observed in iPB homopolymer [29], the absence of a defined yielding with a nearly homogeneous deformation of samples in form II

SC

indicates that form II is easily deformable and shows a low mechanical resistance to the plastic deformation. The incorporation of pentene comonomeric units does not produce significant

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variation of this behavior and the values of stress at yielding of melt-crystallized and non-aged samples of iPBC5 copolymers are nearly constant with the pentene concentration (Figure 5C). Analogously, the values of Young’s modulus of iPBC5 copolymers in form II are only slightly lower than that of the homopolymer and are almost constant with the pentene concentration (Figure

TE D

5A), according to the invariance of crystallinity with composition (Figure 2A). Finally, the values of stress at break are also nearly constant with the pentene concentration and achieve very high values of 30-40 MPa even though the values of stress at yielding are low (Figure 3A). This is due to

EP

the occurrence of remarkable strain-hardening after yielding (Figure 3A). The stress-strain curves of the aged samples of the iPBC5 copolymers (Figure 3B) show

AC C

well-defined yielding point and values of Young’s modulus and of stress at yield much higher than those of the melt-crystallized samples. Furthermore, while the as-crystallized samples in form II show a regular increase of stress with nearly homogeneous deformation, the corresponding aged samples exhibit after the well-defined yield point a region of constant stress followed by strain hardening (Figure 3B). This different behavior of the aged samples is clearly due to the transformation of form II into form I during aging and, therefore, to the fact that all the aged samples of the iPBC5 copolymers are in form I (Figure 1B). In particular, the increase of the Young's modulus and of the stress at yield upon aging of the melt-crystallized samples, as well as

19

ACCEPTED MANUSCRIPT the presence of an evident yielding phenomenon (Figure 3B), indicate that the crystals of form I obtained by the transformation of form II upon aging at room temperature, are much more rigid and resistant to the plastic deformation than the initial crystals of form II. This increased stiffness, however, does not produce a decrease in ductility compared to the corresponding melt-crystallized

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samples since similar values of deformation at break are observed (Figures 3 and 5D). This time, for the aged samples of iPBC5 copolymers in form I, the values of stress at yielding and of Young’s modulus slightly decrease with increasing pentene concentration (Figure 5A,C). This indicates that

SC

the presence of pentene units has a significant effect on the stability of crystals of form I compared to that on the stability of form II. Finally, also for the aged samples the values of stress at break are

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nearly constant with the pentene concentration and achieve very high values of about 40 MPa, as the iPB homopolymer due to the occurrence of strain hardening (Figure 3B). The data of Figure 3 indicate that incorporation of pentene units allows a modulation of the properties of iPB, resulting in an increase of flexibility and decrease of mechanical strength of iPBC5 copolymers compared to

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EP

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the homopolymer.

20

ACCEPTED MANUSCRIPT A - iPBC5 melt-crystallized

9.1 mol%C5 4.1 mol%C5

30 14.7 mol%C5

20 18.9 mol%C5

10

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stress (MPa)

40

iPB

0 0

100

200

300

400

500

40

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B - iPBC5 aged samples

30 stress (MPa)

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strain (%)

600

14.7 mol%C5

20

iPB

9.1 mol%C5

18.9 mol%C5

10 0

0

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4.1 mol%C5

100 200 300 400 500 600 700 800 900 strain (%)

EP

Figure 3. Stress-strain curves of compression molded films of the iPBC5 copolymers with the indicated pentene (C5) concentration and of the iPB homopolymer. The curves were recorded soon

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after the preparation of the films (sample of Figure 1A) (A) and after aging at room temperature for 8 months (samples of Figure 1B) (B).

21

ACCEPTED MANUSCRIPT 40

A - iPBC6 melt crystallized

35

3.2 mol% iPB 5.9 mol%

25 20 15 11.2 mol%

10 5

15.8 mol%

0

35

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100 200 300 400 500 600 700 800 900 1000 strain (%)

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0

B - iPBC6 aged samples

30

iPB

3.2 mol%

25 20

5.9 mol%

15

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stress(MPa)

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stress (MPa)

30

10

0 0

100

200

EP

5 300

400

500

600

700

11.2 mol%

15.8 mol%

800

900 1000

strain(%)

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Figure 4. Stress-strain curves of compression molded films of the iPBC6 copolymers with the indicated hexene concentration and of the iPB homopolymer. The curves were recorded soon after the preparation of the films (samples of Figure 1A’) (A) and after aging at room temperature for 8 months (samples of Figure 1B’) (B). In the case of the sample iPBC6-16 with 15.8 mol% of hexene that is amorphous immediately after compression molding and cooling to room temperature (see Figure 1A’, profile e), the stress strain curve in A has been recorded after aging at room temperature for one day to allow partial crystallization of form II.

22

ACCEPTED MANUSCRIPT In the case of iPBC6 copolymers (Figure 4), since the hexene units stabilizes the form II both melt-crystallized and aged samples are mostly in form II (Figure 1A’ and B’) and only for the sample iPBC6-3 with the lowest hexene content of 3.2 mol% form II transforms in part into form I by aging and the aged sample is crystallized in a mixture of form I and form II (profile b Figure

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1B’). For this reason, the stress-strain curves of the melt-crystallized and aged samples (Figure 4) are very similar, with nearly homogeneous deformation, improved ductility compared to the homopolymer and strain-hardening at high deformation. The increase of modulus and stress at yield

SC

observed after aging for the iPB homopolymer and for the iPBC5 copolymers is, therefore, not observed for the iPBC6 copolymers because of the absence of transformation of form II into form I

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(Figure 4B). Only for the sample iPBC6-3 with 3.2 mol% of hexene a slight increase of modulus and stress at yield occurs because of the partial transformation of form II into form I (Figures 4 and 5A,C).

For both melt-crystallized and aged samples of iPBC6 copolymers the values of Young

TE D

modulus and stress at yield slightly decrease with increasing hexene concentration (Figure 5A and C). The decrease of the Young's modulus is consistent with the decrease in the degree of crystallinity (Table S1). The values of stress at break of both melt-crystallized and aged samples are

EP

high at nearly 30-40 MPa and are almost constant with the hexene content and decrease only for the samples with the highest hexene content (Figures 4 and 5E).

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For both melt-crystallized and aged samples of iPBC5 and iPBC6 copolymers the values of tension set are always very high indicating that all samples do not show elastic behavior. It is worth recalling that in copolymers of iPB with octene prepared with a similar metallocene catalyst different mechanical properties have been observed [31]. The presence of octene comonomer units also produces increase of flexibility and ductility and samples with octene concentration higher than 6 - 7 mol% show unexpected remarkable elastomeric properties [31].

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ACCEPTED MANUSCRIPT B iPBC5 not-aged iPBC5 aged iPBC6 not aged iPBC6 aged

160 140

εy (%)

100 80 60 40 20 0 0

2 4 6 8 10 12 14 comonomer concentration (mol%)

16

0

18

1000

C

20

iPBC5 not-aged iPBC5 aged iPBC6 not aged iPBC6 aged

18 16

800 700 600

10 8

500

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εb (%)

12

200

4

100

2

18

iPBC5 not-aged iPBC5 aged iPBC6 not aged iPBC6 aged

300

6

16

400

0

0 2

4 6 8 10 12 14 16 comonomer concentration (mol%)

45

E

40 35 30 25

0

EP

20

18

TE D

0

tb (MPa)

σy (MPa)

4 6 8 10 12 14 comonomer concentration (mol%)

D

900

14

σb (MPa)

2

SC

E(MPa)

120

iPBC5 not-aged iPBC5 aged iPBC6 not aged iPBC6 aged

36 33 30 27 24 21 18 15 12 9 6 3 0

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A

180

15

0 0

AC C

5

2

4 6 8 10 12 14 16 comonomer concentration (mol%)

4 6 8 10 12 14 comonomer concentration (mol%)

16

18

F

800 700 600 500 400 300

iPBC5 not-aged iPBC5 aged iPBC6 not aged iPBC6 aged

10

2

iPBC5 not-aged iPBC5 aged iPBC6 not aged iPBC6 aged

200 100

18

0

0

2

4

6 8 10 12 14 16 comonomer concentration (mol%)

18

Figure 5. Average values of Young’s modulus E (A), strain εy (B) and stress σy (C) at the yield point, strain εb (D) and stress σb (E) at break, tension set at break tb (F) of melt-crystallized compression-molded and not-aged samples (full symbols) and of samples aged for eight months at room temperature (open symbols) of the iPBC5 (circles) and iPBC6 (squares) copolymers, and corresponding iPB homopolymers, as a function of comonomer concentration. In the case of the iPBC6-16 sample with 15.8 mol% of hexene that is amorphous immediately after compression

24

ACCEPTED MANUSCRIPT molding and cooling to room temperature, the data of non-aged sample refer to the melt-crystallized sample aged only for few days at room temperature to allow partial crystallization of form II.

Structural analysis during deformation.

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The described mechanical behavior of iPBC5 and iPBC6 copolymers can be rationalized in terms of possible structural transformations occurring during deformation. In the case of iPB homopolymer, form II crystallized from the melt transforms into form I by stretching [25]. This is shown for the

SC

sample iPB1 by the X-ray diffraction patterns of Figure 6A-C. The undeformed melt-crystallized sample is in form II as indicated by the presence of the (200)II, (220)II and (301)II reflections of

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form II at 2θ = 11.9, 16.9 and 18.4° in Figure 6A (as also shown by profile a of Figure 1A). The (110)I, (300)I and (220)I+(211)I reflections of form I at 2θ = 9.9, 17.3 and 20.5° start appearing, already at low deformation of 50%, (Figure 6B) and the reflections of form II are completely replaced by those of form I in the diffraction pattern of the fiber stretched at higher deformation of

TE D

Figure 6C, indicating complete transformation of form II into form I. A low degree of orientation of crystals of form I is achieved [25]. On the other hand, as shown in Figure 7A-C, for the sample iPB1 aged at room temperature crystallized in form I (Figure 7A), only orientation of crystals of

EP

form I occurs upon stretching and no phase transformations take place (Figure 7C) [25]. Therefore,

AC C

oriented films initially crystallized in form II stretched at high deformations, and oriented films of aged samples stretched at high deformations, are both in form I. This accounts for the observation that the mechanical behaviors of iPB at high values of deformation of as-crystallized and aged samples are similar with similar values of stress at break (Figures 3 and 4) [25]. A similar behavior has been observed for the iPPC5 copolymers. The X-ray fiber diffraction patterns, and the corresponding intensity profiles read along the equator, of melt-crystallized nonaged samples of copolymers iPBC5-4, iPBC5-15 and iPBC5-19 with 4.1, 14.7 and 18.9 mol% of pentene, stretched at different degrees of deformation are reported in Figure 6. As also shown in Figure 1A, the undeformed melt-crystallized compression-moulded samples of iPBC5-4 and

25

ACCEPTED MANUSCRIPT iPBC5-15 copolymers with 4.1 and 14.7 mol% of pentene are in form II (Figures 6D,G), whereas the sample iPBC5-19 with 18.9 mol% of pentene is crystallized in a mixture of form I and form II (Figure 6J). In all three samples crystals of form II start transforming into form I by stretching already at low deformation of 50% (Figure 6E,H,K) and the transformation is complete already at

RI PT

200-300% deformation, as indicated by the complete replacement of the (200)II, (220)II and (301)II reflections of form II at 2θ = 11.9, 16.9 and 18.4° present in the patterns of Figure 6D,EG,H,J,K, with the (110)I, (300)I, (220)I and (211)I reflections of form I at 2θ = 9.9, 17.3 and 20.5° in the

SC

patterns of Figure 6F,I,L. In the case of the sample iPBC5-19 with the highest content of pentene of 18.9% mol, the transformation of crystals of form II (present in mixture with crystals of form I) is

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faster and the transition is already complete for deformation of ε = 50% (Figure 6K). It is worth mentioning that similar results have been obtained by recording for very short time the X-ray diffraction patterns of fibers stretched directly at the maximum deformation. This indicate that the transformation of form II into form I in these samples is induced by deformation

TE D

and not by aging at room temperature during recording the diffraction patterns and progressive stretching at different degrees of deformation.

The X-ray diffraction patterns of fibers of the same samples of copolymers iPBC5-4,

EP

iPBC5-15 and iPBC5-19 stretched from the melt-crystallized and aged samples are shown in Figure

AC C

7. As also shown in Figure 1C, the undeformed aged samples are essentially in form I, with a small amount of residual form II (Figure 7D,G,J). Stretching produces poor orientation of crystals of form I and rapid transformation of residual crystals of form II into form I already at low deformation of 50-100%, as evident from the disappearance of the (200)II reflection and by the only presence of (110)I, (300)I, (220)I and (211)I reflections in the patterns of Figure 7E,F,H,I,K,L.

26

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ACCEPTED MANUSCRIPT

27

ACCEPTED MANUSCRIPT

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Figure 6. X-ray fiber diffraction patterns, and corresponding intensity profiles read along the equatorial layer line (A-L) of oriented fibers of the iPB homopolymer (A-C) and of the samples iPBC5-4 (D-F), iPBC5-15 (G-I), and iPBC5-19 (J-L) with 4.1, 14.7 and 18.9 mol% of pentene, respectively, obtained by stretching melt-crystallized compression-molded non aged films of copolymers at the indicated values of strain ε. The (110)I and (200)II reflections of form I and form II of iPB are indicated.

28

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ACCEPTED MANUSCRIPT

29

ACCEPTED MANUSCRIPT

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Figure 7. X-ray fiber diffraction patterns, and corresponding intensity profiles read along the equatorial layer line (A-L) of oriented fibers of the iPB homopolymer (A-C) and of the samples iPBC5-4 (D-F), iPBC5-15 (G-I), iPBC5-19 (J-L) with 4.1, 14.7 and 18.9 mol% of pentene, respectively, obtained by stretching at the indicated values of strain ε compression-molded films aged for 8 months at room temperature. The (110)I and (200)II reflections of form I and form II of iPB are indicated.

30

ACCEPTED MANUSCRIPT For the iPBC6 copolymers, the X-ray fiber diffraction patterns, and the corresponding intensity profiles read along the equator, of melt-crystallized non-aged samples of copolymers iPBC6-3, iPBC6-6, iPBC6-11 and iPBC6-16 with 3.2, 5.9, 11.2 and 15.8 mol% of hexene, respectively, stretched at different degrees of deformation are reported in Figure 8. As shown in

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Figure 1A’, the undeformed melt-crystallized compression-moulded samples of all iPBC6 copolymers are crystallized in form II. Moreover, Figure 1C’ has demonstrated that in the iPBC6 copolymers with hexene concentration higher than 3-4 mol% form II is stable at room temperature

SC

and does not transform in form I during aging. Only in the case of the sample iPBC6-3 with the lowest hexene concentration of 3.2 mol%, crystals of form II transforms into form I (Figure 1C’).

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Figure 8 shows that in iPBC6 copolymers form II is stable also in stretched fibers and the transformation into form I by stretching is retarded or completely inhibited at high hexene concentrations. In fact, only for the sample iPBC6-3 with 3.2 mol% of hexene, stretching produces transformation of initial crystals of form II into form I already at low degrees of deformation of

TE D

100% (Figure 8A), as it is apparent by the appearance of the (110)I reflection of form I in the pattern of Figure 10A and the increase of its intensity at higher deformation (Figure 10B). The transformation is complete at 300% deformation (Figure 8C). In the case of the sample iPBC6-6

EP

with 5.9 mol% of hexene the initial form II of the melt-crystallized sample transforms by stretching only in part in form I (Figure 8D-F). Stretching, indeed, produces orientation of crystals of form II

AC C

and a simultaneous, but only partial, transformation of part of form II into form I. At 300% deformation fibers with mixture of well oriented crystals of form II and form I are obtained (Figure 8F). At the same degree of deformation of 300%, while in the sample iPBC6-3 the transformation of form II into form I is complete (Figure 8C), in the sample iPBC6-6 the transformation is not complete and needs higher degrees of deformation to obtain fibers in the pure form I. On the other hand, the data of Figure 1C’ indicate that for the sample iPBC6-6 only a small amount of form I develops at room temperature even after aging for long time. Also in these samples similar results have been obtained by recording for very short time the X-ray diffraction patterns of fibers

31

ACCEPTED MANUSCRIPT stretched directly at the maximum deformation, indicating that the transformation of form II into form I is induced by deformation and not by aging at room temperature during recording the diffraction patterns and progressive stretching at different degrees of deformation. In the case of the sample iPBC6-11 with 11.2 mol% of hexene the diffraction patterns of

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Figure 8G-I indicate that initial crystals of form II transform in part into form I at low deformations and fibers with mixture of well oriented crystals of form II and form I are obtained at 400% deformation (Figure 8H). At higher deformation of 600%, corresponding to the deformability limit

SC

of the sample, the transformation is complete and a well oriented fiber in the almost pure form I is obtained (Figure 8I). Since the data of Figure 1C’ have demonstrated that in the sample iPBC6-11

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with 11.2 mol% form II is stable at room temperature in the undeformed sample and does not transform into form I during aging (profile d of Figure 1C’), the formation of form I in the fibers of Figure 8G,I is only due to transformation of form II induced by stretching. This indicates that at this high hexene concentration (10-11 mol%), form II is already the most stable form in the powder

TE D

undeformed samples, but form I is still the most stable form in the stretched fibers. Finally, at higher hexene concentration, for the sample iPBC6-16 with 15.8 mol% of hexene, for which form II crystallized from the amorphous sample by aging at room temperature

EP

(profile e of Figure 1B’) does not transform into form I by further aging (profile e of Figure 1C’), the diffraction patterns of Figure 10J-L show that form II is also stable upon stretching and does not

AC C

transform into form I by tensile deformation (Figure 8J-L). Stretching induces orientation of crystals of form II and even at high degree of deformation of 600% only a very small amount of form I develops and well oriented fibers in almost pure form II are obtained (Figure 8L). This indicates that at hexene concentrations higher than 15 mol%, form II is the most stable form both in the undeformed samples and in stretched fibers. It is worth noting from the data of Figures 6-8 that when form II transforms rapidly into form I by stretching at very low degrees of deformation, as in the case of iPB homopolymer [25] and in iPBC5 copolymers (Figure 6), stretching even at high degree of deformation always produce

32

ACCEPTED MANUSCRIPT fibers with poor orientation of crystals of form I (Figure 6C,F,I,L). When the transformation by stretching is retarded and stretching produces orientation of crystals of form II already at low deformations and then at higher deformation transformation of form II into form I, well oriented fibers of form I are obtained, as in the case of samples iPBC6-6 and iPBC6-11 of Figure 8F,I.

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The data of Figure 6-8 also show that in the diffraction patterns of fibers of some samples of iPBC5 and iPBC6 copolymers and of the iPB homopolymer, stretched at low degrees of deformation the (200)II and (220)II reflections of form II at 2θ = 11.9 and 16.9° respectively, tend to

SC

be polarized on layer lines out from the equator, as in Figures 6B for iPB, Figures 6E and H for the samples iPBC5-4 and iPBC5-15 with 4.1 and 14.7 mol% of pentene, respectively, and Figures

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8A,B, 8D and 8J for the samples iPBC6-3, iPBC6-6 and iPBC6-16 with 3.2, 5.9 and 15.8 mol% of hexene, respectively. In the same diffraction patterns also the (301)II reflection at 2θ= 18.4° on the first layer line is more polarized on the meridian. These data indicate that in these samples stretching at low degrees of deformation produces an orientation of the crystals of form II with the

TE D

c-axis oriented in directions approximately perpendicular to the stretching direction. At higher degrees of deformation, before transformation into form I, a standard fiber orientation of the crystals of form II is achieved with the c axis parallel to the stretching direction and normal

EP

polarization of the (200)II and (220)II reflections on the equator and of the (301)II reflection on the first layer line out the meridian. It is worth reminding that this perpendicular orientation has also

AC C

been observed in the case of samples of isotactic polypropylene crystallized in α and γ forms stretched at low degrees of deformation [32-36].

33

AC C

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ACCEPTED MANUSCRIPT

34

ACCEPTED MANUSCRIPT

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Figure 8. X-ray fiber diffraction patterns and corresponding intensity profiles read along the equatorial layer line (A-L) of oriented fibers of the samples iPBC6-3 (A-C), iPBC6-6 (D-F), iPBC6-11 (G-I) and iPBC6-16 (J-L) with 3.2, 5.9, 11.2 and 15.8 mol% of hexene, respectively, obtained by stretching at room temperature and at the indicated values of strain ε melt-crystallized non-aged compression-molded films. The (110)I and (200)II reflections of form I and form II of iPB are indicated.

35

ACCEPTED MANUSCRIPT Conclusions A study of the influence of the presence of pentene and hexene comonomeric units on the crystallization behavior and the mechanical properties of iPB in butene-pentene (iPBC5) and butene-hexene (iPBC6) isotactic copolymers, is reported. Samples of random iPBC5 and iPBC6

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copolymers have been prepared with isospecific metallocene catalysts in a wide range of comonomer concentration, up to 18.9 mol% of pentene and 15.8 mol% of hexene.

The structural characterization has shown that iPBC5 copolymers with pentene

SC

concentration up to 14.7 mol% crystallize from the melt in the metastable form II, as in the iPB homopolymer. Crystals of form II transform at room temperature into form I, as usually observed

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for the iPB homopolymer. Sample with higher pentene concentration of 18.9 mol%, crystallizes from the melt still in form II, but small amount of crystals of form I develops by cold-crystallization at nearly 25 °C. Also in this sample form II transforms into form I by aging at room temperature. Samples of iPBC6 copolymers crystallize from the melt in form II and only for the lowest hexene

TE D

concentration of 3.2 mol%, form II transforms into form I by aging at room temperature. Higher concentrations of hexene units stabilize the tetragonal form II that no longer transforms into form I at room temperature.

EP

The study of the mechanical properties has shown that the presence of pentene or hexene comonomeric units enhances the ductility of iPB, and iPBC5 and iPBC6 copolymers show

AC C

mechanical behavior of highly flexible and ductile materials in both melt-crystallized samples crystallized in form II and in samples aged at room temperature. Samples in form II show easy homogeneous deformation with low yielding stress and strain hardening, whereas the aged samples of iPBC5 copolymers and iPBC6 copolymers with low hexene content, show increase of values of Young modulus and stress at yielding, because of the transformation of form II into form I during aging. This indicates that the crystals of form I, obtained by the transformation of form II upon aging at room temperature, are much more rigid and resistant to plastic deformation than the initial crystals of form II. For iPBC6 copolymers with high hexene concentrations the mechanical

36

ACCEPTED MANUSCRIPT properties do not change upon aging because of the stabilization of form II and absence of transformation of form II into form I. Both iPBC5 and iPBC6 copolymers show similar ductility at all comonomer concentration, but the values of stress at yielding and of Young’s modulus slightly decrease with increasing

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comonomer content.

In all the iPBC5 copolymers, form II crystallized from the melt transforms into form I by stretching. This accounts for the similar mechanical properties at high deformation (similar values

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of stress at breaking) shown by melt-crystallized samples in form II and of aged samples in form I. In the case of iPBC6 copolymers, the transformation of form II into form I by stretching is retarded

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or completely inhibited at high hexene concentrations. This indicates that the presence of high concentrations of hexene units stabilizes the tetragonal form II not only in the unoriented bulk samples but also in stretched fibers.

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gratefully acknowledged.

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Acknowledgements: Financial support from Università di Napoli, Project "Ricerca di Ateneo" is

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ACCEPTED MANUSCRIPT References [1] A. J. Foglia Polybutylene, its chemistry, properties and applications J. Appl. Polym. Sci., Appl. Polym. Symp. 11, (1969), 1-18.

Prog. Polym. Sci. 13, (1988), 37-62.

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[2] L. Luciani, J. Seppala, B. Lofgren Poly-1-butene: its preparation, properties and challenges

[3] U. W. Gedde, J. Viebke, H. Leijstroem, M. Ifwarson Long-term properties of hot-water polyolefin pipes - a review Polym. Eng. Sci. 34, (1994), 1773-87.

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[4] G. Natta, P. Corradini, I.W. Bassi Crystal structure of isotactic poly(1-butene) Nuovo Cimento Suppl. 15, (1960), 52-67.

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[5] G. Natta, P. Pino, P. Corradini, F. Danusso, E. Mantica, G. Mazzanti, G. Moraglio Crystalline high polymers of α-olefins J. Am. Chem. Soc. 77, (1955), 1708-10.

[6] A. Turner-Jones Poly-l-butylene Type II crystalline form J.Polym.Sci., Part B 1, (1963), 455-6. [7] A. Turner-Jones Cocrystallization in copolymers of α-olefins II—Butene-1 copolymers and

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polybutene type II/I crystal phase transition Polymer 7, (1966), 23-59. [8] A. Turner-Jones Crystalline phases in copolymers of butene and 3-methylbutene. J.Polym.Sci., Part C 3, (1965), 591-600.

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[9] V. Petraccone, B. Pirozzi, A. Frasci, P. Corradini Polymorphism of isotactic poly-α-butene.

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Conformational analysis of the chain and crystalline structure of form 2. Eur.Polym.J. 12, (1976),

[10] P. Corradini, R. Napolitano, V. Petraccone, B. Pirozzi Conformational and packing energy for the three crystalline forms of isotactic poly-α-butene Eur.Polym.J. 20, (1984) 931-5. [11] J. Boor Jr, J. C. Mitchell Kinetics of crystallization and a crystal-crystal transition in poly-1butene J.Polym.Sci. Part A 1, (1963), 59-84. [12] V. F. Holland, R. L. Miller Isotactic poly-1-butene single crystals: morphology J.Appl.Phys. 35, (1964), 3241-8.

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ACCEPTED MANUSCRIPT [13] K. W. Chau, P.H. Geil Solution history effects in polybutene-1 J. Macromol. Sci. Phys. B23, (1984),115-42. [14] J. Boor Jr, E. A. Youngman Polymorphism in poly-1-butene-apparent direct formation of modification I J.Polym.Sci. Part B 2, (1964), 903-7.

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[15] T. Miyoshi, A. Mamun D. Reichert Fast Dynamics and Conformations of Polymer in a Conformational Disordered Crystal Characterized by 1H−13C WISE NMR Macromolecules 43, (2010), 3986-89.

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[16] T. Miyoshi, A. Mamun Critical roles of molecular dynamics in the superior mechanical properties of isotactic-poly(1-butene) elucidated by solid-state NMR Polym. J. 44, (2012), 65-71.

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[17] G. Cojazzi, V. Malta, G. Celotti, R. Zannetti Crystal structure of form III of isotactic poly-1butene Macromol.Chem.Phys. 177, (1976), 915-26.

[18] F. Danusso, G. Gianotti The three polymorphs of isotactic poly-1-butene: dilatometric and thermodynamic fusion properties Makromolekulare Chemie. 61, (1963), 139-56.

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[19] F. Danusso, G. Gianotti Isotactic poly-1-pentene. Polymorphism and some physical properties Makromolekulare Chemie 61, (1963), 164-77.

[20] C. Nakafuku, T. Miyaki Effect of pressure on the melting and crystallization behavior of

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isotactic polybutene-1 Polymer 24, (1983), 141-8.

[21] F. Azzurri, A. Flores, G. C. Alfonso, I. Sics, B. S. Hsiao, F. J. Balta Calleja Polymorphism of

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isotactic polybutene-1 as revealed by microindentation hardness. Part II: Correlations to microstructure Polymer 44 (2003), 1641-45. [22] R. J. Schaffhauser Nature of the form II to form I transformation in isotactic polyl-butene J.Polym.Sci., Part B 5, (1967), 839-41. [23] G. Gianotti, A. Capizzi Butene‐1/propylene copolymers. Influence of the comonomeric units on polymorphism Macromol.Chem.Phys. 124, (1969), 152-9.

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ACCEPTED MANUSCRIPT [24] F. Azzurri, A. Flores, G. C. Alfonso, F. J. Balta Calleja Polymorphism of isotactic poly(1butene) as revealed by microindentation hardness. 1. Kinetics of the transformation Macromolecules 35, (2002), 9069-73. [25] F. Azzurri, G. C. Alfonso, M. A. Gómez, M. C. Martì, G. Ellis, C. Marco Polymorphic

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Transformation in Isotactic 1-Butene/Ethylene Copolymers Macromolecules 37, (2004), 3755-62. [26] F. Azzurri, M. A. Gómez, G. C. Alfonso, G. Ellis, C. Marco Time-Resolved SAXS/WAXS Studies

of

the

Polymorphic

Transformation

of

1-Butene/Ethylene

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J.Macromol.Sci.Phys. B43, (2004), 177-89.

Copolymers

[27] C. De Rosa, F. Auriemma, O. Ruiz de Ballesteros, F. Esposito, D. Laguzza, R. Di Girolamo, L.

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Resconi Crystallization Properties and Polymorphic Behavior of Isotactic Poly(1-Butene) from Metallocene Catalysts: The Crystallization of Form I from the Melt. Macromolecules 42, (2009), 8286-97.

[28] C. De Rosa, F. Auriemma, L. Resconi Metallorganic Polymerization Catalysis as a Tool To

Int. Ed. 48, (2009), 9871-74.

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Probe Crystallization Properties of Polymers: The Case of Isotactic Poly(1‐butene). Angew. Chem.,

[29] C. De Rosa, F. Auriemma, M. Villani, O. Ruiz de Ballesteros, R. Di Girolamo, O. Tarallo, A.

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Malafronte Mechanical Properties and Stress-Induced Phase Transformations of Metallocene Isotactic Poly(1-butene): The Influence of Stereodefects. Macromolecules 47, (2014), 1053-64.

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[30] C. De Rosa, O. Ruiz de Ballesteros, F. Auriemma, R. Di Girolamo, C. Scarica, G. Giusto, S. Esposito, S. Guidotti, I. Camurati Polymorphic Behavior and Mechanical Properties of Isotactic 1Butene–Ethylene Copolymers from Metallocene Catalysts. Macromolecules 47, (2014), 4317-29. [31] C. De Rosa, O. Tarallo, F. Auriemma, O. Ruiz de Ballesteros, R. Di Girolamo, A. Malafronte Crystallization behavior and mechanical properties of copolymers of isotactic poly(1-butene) with 1-octene from metallocene catalysts. Polymer 73, (2015), 156-69. [32] C. De Rosa, F. Auriemma. Crystals and Crystallinity in Polymers. Diffraction Analysis of Ordered and Disordered Crystals. John Wiley & Sons, Inc., Hoboken, 2014.

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ACCEPTED MANUSCRIPT [33] C. De Rosa, F. Auriemma, C. Perretta. Structure and Properties of Elastomeric Polypropylene from C2 and C2v-Symmetric Zirconocenes. The Origin of Crystallinity and Elastic Properties in Poorly Isotactic Polypropylene Macromolecules 37, (2004), 6843-55. [34] C. De Rosa, F. Auriemma, G. De Lucia, L. Resconi. From stiff plastic to elastic polypropylene: transformations

during

plastic

polypropylene Polymer 46, (2005), 9461-75.

deformation

of

metallocene-made

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isotactic

[35] F. Auriemma, C. De Rosa. Stretching Isotactic Polypropylene:  From “cross-β” to

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Crosshatches, from γ Form to α Form Macromolecules 39, (2006), 7635-47.

[36] C. De Rosa; F. Auriemma. In "Progress in Understanding of Polymer Crystallization", G.

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Reiter and G. R. Strobl Eds., Lect. Notes Phys. 2007, 714, 345-371. (Chapter 17) Springer-Verlag,

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Berlin-Heidelberg 2007.

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Graphical Abstracts

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Crystallization and Mechanical Properties of

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Metallocene Made 1-Butene-Pentene and 1-ButeneHexene Isotactic Copolymers

Oreste Tarallo,* Odda Ruiz de Ballesteros, Annalisa Bellissimo, Miriam Scoti, Anna Malafronte,

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Finizia Auriemma, Claudio De Rosa

Dipartimento di Scienze Chimiche, Università di Napoli "Federico II", Complesso Monte S. Angelo,

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Via Cintia, I-80126 Napoli, Italy

Highlights

High pentene concentration in butene-pentene copolymers favors cold-crystallization of form I.

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Hexene co-units stabilize form II in isotactic copolymers with butene.

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In pentene/butene copolymer stretching induces form II to form I transition.

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Hexene co-units inhibit form II to form I transition by effect of stretching.

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Pentene or hexene comonomeric units enhance ductility of isotactic polybutene.

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