crystallization properties of elastomeric polypropylene from alumina-supported tetraalkyl zirconium...

Crystallization properties of elastomeric polypropylene from

alumina-supported tetraalkyl zirconium catalysts

Claudio De Rosaa,*, Finizia Auriemmaa, Clementina Speraa, Giovanni Talaricoa,Markus Gahleitnerb

aDipartimento di Chimica, Universita degli Studi di Napoli ‘Federico II’, Complesso Monte S. Angelo, Via Cintia, 80126 Napoli, ItalybBorealis GmbH, St Peterstr. 25, A-4021 Linz, Austria

Received 29 January 2004; received in revised form 1 June 2004; accepted 16 June 2004

Abstract

An analysis of the crystallization properties of fractions of elastomeric polypropylene (ELPP) prepared with Al2O3-supported tetraalkyl

zirconium catalyst is presented. A comparison with the polymorphic behavior of isotactic polypropylene (iPP) samples prepared with a single

center homogeneous metallocene catalyst is also shown. The ELPP sample has been fractionated by extraction with boiling solvents. The

irregular fraction insoluble in pentane and soluble in hexane crystallizes from the melt almost totally in the g form, whereas the more

stereoregular fraction, insoluble in n-heptane, crystallizes mainly in the a form. The relative amount of g form crystallized from the melt is

much lower than the one formed in samples of metallocene-made iPP samples with a similar average content of isotactic stereosequences.

Since the g form crystallizes in chains having short regular isotactic sequences, these data indicate that in the fractions of the ELPP sample

the regular isotactic sequences are longer than those present in chains of metallocene-made iPP. In particular, in the more irregular crystalline

fractions of the ELPP sample the chains are characterized by a stereo-block microstructure, consisting in the presence of quite regular

isotactic sequences alternating with irregular sequences, the latter containing most of the defects. The presence of stereoblocks allows

crystallization of these highly irregular fractions, even in the presence of a very high content of defects, and accounts for the elastomeric

properties shown by this sample. The whole ELPP sample is constituted mostly by amorphous ether soluble (40%) and pentane soluble (26%)

fractions and shows elastic properties due to the high molecular weight of chains in all the fractions and the crystallization of isotactic

sequences present in the stereoblocks.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Elastomeric polypropylene; Alumina-supported tetraalkyl zirconium catalysts; Polymorphic behavior

1. Introduction

The first example of thermoplastic elastomeric poly-

propylene was described by Natta [1–3], and obtained by

fractionating polypropylenes made with conventional tita-

nium- and vanadium-based catalysts. The elastic properties

of this material were interpreted in terms of a stereoblock

microstructure of the chains, consisting of alternating blocks

of atactic, noncrystallizable sequences and more regular

isotactic sequences, which are able to crystallize producing

a physical cross-linked network.

0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymer.2004.06.037

* Corresponding author. Tel.:C39-081-674346; fax:C39-081-674-090.

E-mail address: [email protected] (C. De Rosa).

In recent years, several authors have described new

strategies for synthesizing elastomeric stereoblock poly-

propylene, using different heterogeneous and homogeneous

metallocene catalysts. High molecular weight stereoblock

polypropylene has been prepared with highly active

heterogeneous catalysts consisting of transition metal alkyls

R4M (where MZTi, Zr or Hf and RZbenzyl, neopentyl,

neophyl) supported on Al2O3 [4–6]. This material is not

homogeneous, as fractions of different stereoregularity can

be separated by conventional extraction with boiling

solvents, but shows elastomeric properties as-polymerized.

The elasticity is attributed to a high molecular weight

ether-soluble fraction containing chains with stereoblock

microstructure, with short isotactic crystallizable blocks

alternating to longer stereoirregular sequences. The isotactic

Polymer 45 (2004) 5875–5888

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C. De Rosa et al. / Polymer 45 (2004) 5875–58885876

blocks can cocrystallize with the larger isotactic sequences

of the more stereoregular components, forming a cross-

linked elastomeric network [4–6].

Thermoplastic elastomeric polypropylene has recently

been obtained with different classes of metallocene

catalysts. Unbridged zirconocene catalysts, described by

Coates and Waymouth [7,8], produce a reactor blend of

stereoblock polypropylene which can be separated in

fractions of different tacticities and melting points. Elasto-

meric polypropylene has also been produced by C1-

symmetric metallocene catalysts, as described by Chien

[9], Collins [10–13] and Rieger [14]. In these cases, low

melting or amorphous polypropylenes having a more

homogeneous distribution of tacticity are obtained.

More recently, Resconi has developed a C2v-symmetric

ansa-zirconocene catalyst, that efficiently produces high

molecular weight atactic polypropylene [15,16], and C2-

symmetric catalysts, which produce poorly isotactic poly-

propylene [17], showing elastic properties. Also in these

cases the polymers present a homogeneous distribution of

tacticity, being fully soluble in diethyl ether. The elasticity

is due to the high degree of entanglement, in the case of the

high molecular weight atactic polypropylene [15,16], and to

the crystallization of the very short isotactic sequences in a

predominantly amorphous material, in the case of the poorly

isotactic polypropylene [17,18], producing the physical

crosslinks of the elastomeric network.

In most of these examples of elastomeric polypropylene

the presence of stereoblocks, consisting of more regular

isotactic sequences alternating with atactic, or more

stereoirregular, sequences, have been invoked to explain

the elastic properties. The regular isotactic blocks may be

very short or longer depending on the catalytic system. In

any case, the crystallization of the regular isotactic

sequences induces the formation of the elastomeric network.

Recent studies of the crystallization properties of

isotactic polypropylene (iPP) samples produced with

metallocene catalysts have clearly shown that the length

of the isotactic sequences strongly influence the poly-

morphic behavior of iPP [18–26]. It has been demonstrated

that when the fully isotactic sequences are very short, iPP

crystallizes in the g form, whereas very long regular

isotactic sequences generally crystallize only in the a form

[19,24–26]. The formation of the g form is therefore favored

by the presence of stereodefects (mainly rr isolated triads)

[19,24–26], and/or regiodefects (mainly 2,1 and 3,1

insertions) [19,22], and also by the presence of consti-

tutional defects, like comonomer units [21,23], which,

indeed, shorten the regular isotactic sequences.

In chains of iPP samples prepared with metallocene

catalysts the distribution of defects is random and the length

of fully isotactic sequences is roughly inversely related to

the content of insertion errors [24,25]. As a consequence,

even a small amount of defects reduces the length of the

regular isotactic sequences inducing the crystallization of

the g form [19,24,25]. The content of g form increases with

increasing the concentration of defects, due to the reduced

length of the regular isotactic sequences [25], and increases

by crystallization from the melt with increasing the

crystallization temperature [25].

We have recently shown for the first time that also the

kind of distribution of defects along the polymer chains

influences the polymorphic behavior of iPP and the

crystallization of the g form [24]. We have, indeed,

demonstrated that in the case of stereoblock polypropylene,

prepared with oscillating metallocene catalysts [7,8], the

amount of g form which develops in the melt-crystallization

procedures, is much lower than that obtained for iPP

samples having the same overall concentration of defects,

but prepared with stereorigid metallocene catalysts, which

produce a random distribution of defects [24]. This has been

explained considering that in the stereoblock polypropylene

most of the defects are segregated in stereoirregular

(atactic), non-crystallizable, blocks, which alternate to

more regular isotactic sequences, long enough to crystallize

in the a form [24]. This result confirms that the g form

crystallizes when the fully isotactic sequences are very short

[24,25]. Our studies have, therefore, demonstrated that it is

possible to obtain information about the microstructure of

iPP chains (distribution of defects and inclusion of defects

in the crystals) through the indirect method of the structural

analysis [24]. We have been able, indeed, to deduce the

blocky microstructure of the elastomeric polypropylene

from its crystallization properties and have proposed a

method to evaluate the average length of the regular

isotactic sequences [24].

With this simple idea in mind, the crystallization of iPP

samples obtained with commercial heterogeneous Ziegler–

Natta catalytic systems in the a form can be easily explained

considering that in these samples the distribution of defects

in the polymer chains is not random. The majority of defects

are segregated in a small fraction of poorly crystallizable

macromolecules or in small portions (more irregular blocks)

of long regular chains, producing in any case long regular

isotactic sequences. For this reason, even a not negligible

amount of defects does not greatly influence the crystal-

lization properties of iPP. In fact, the regular isotactic

sequences are long enough to crystallize in the a form, even

for a relatively high overall concentration of defects, with

melting temperatures generally higher than those observed

in metallocene iPP [27].

In this paper, the crystallization properties and the

polymorphic behavior of elastomeric polypropylene pre-

pared with the heterogeneous alumina supported tetraalkyl

zirconium catalysts [4–6] are analyzed and compared with

the behavior of metallocene-made iPP samples. From the

amount of g form, which develops by crystallizations from

the melt, information about the microstructures of the

various fractions of elastomeric polypropylene are obtained.

Moreover the most important parameter that influences the

crystallization and the related mechanical properties

(strength and resilience), that is the average length of the

C. De Rosa et al. / Polymer 45 (2004) 5875–5888 5877

regular isotactic sequences, has been evaluated with the

method of the structural analysis proposed in Ref. [24].

2. Experimental section

2.1. As-prepared samples and fractionation

The sample of elastomeric polypropylene ELPP was

supplied by Borealis GmbH (Linz, Austria). The sample

was prepared with Al2O3-supported tetraneophyl zirconium

catalyst (Chart 1) in an advanced version of the standard

procedure [4] developed at Borealis. Liquid propylene was

used as polymerization medium here. The whole as-

polymerized unfractionated sample is here identified with

the term ‘as-prepared sample’.

The ELPP sample was fractionated by exhaustive

Kumagawa extraction using sequentially boiling diethyl

ether (Et), pentane (P), hexane (Hs) and heptane (H). The

fractions soluble in the boiling diethyl ether and pentane

were recovered by evaporation of the solvent at 30 8C under

vacuum, whereas the fractions soluble in hexane and

heptane were precipitated from solutions by adding acetone.

Fractions soluble in diethyl ether (ELPP(EtS)), insoluble in

diethyl ether and soluble in pentane (ELPP(EtI-PS)),

insoluble in pentane and soluble in hexane (ELPP(PI-

HsS)), insoluble in hexane and soluble in heptane

(ELPP(HsI-HS)) and insoluble in heptane (ELPP(HI)),

were separated (Table 1). All fractions were characterized

by solution viscosimetry, 13C NMR spectroscopy, differen-

tial scanning calorimetry (DSC) and X-ray diffraction

(Table 1).

Three samples of metallocene-made iPP (samples R1, R2

and R3) were provided by Dr Luigi Resconi of Basell

Polyolefins (Ferrara, Italy). They were synthesized at

different temperatures using the single-center catalyst rac-

isopropylidene[bis(3-trimethylsilyl-indenyl)] zirconium

Chart 1. Tetraneophyl zirconium, the main component of the heterogeneous

catalyst used for the synthesis of the elastomeric polypropylene ELPP

sample.

dichloride, activated with methylaluminoxane (MAO), as

described in Ref. [28]. As shown by Resconi et al. [28], this

catalyst produces iPP samples characterized by chains

containing only defects of stereoregularity (mainly isolated

rr triads), whose amount depends on the polymerization

temperature. The distribution of defects along the chains is

random and no defects of regioregularity are present. The

molecular weights, the melting temperatures and the main

microstructural characteristics of the three samples are

reported in Table 2.

2.2. Isothermal melt-crystallizations

The various fractions of the sample ELPP were

isothermally crystallized from the melt at different

temperatures. Powder specimens were placed in a sealed

vial in a N2 atmosphere, melted at 180 8C in a

thermostatic oil bath and kept at this temperature for

5 min; they were then rapidly cooled to the crystal-

lization temperature, Tc, moving the sample into a second

thermostatic oil bath already regulated at the crystal-

lization temperature Tc. The samples are kept at this

temperature, still in a N2 atmosphere, for a time tc long

enough to allow complete crystallization at Tc. The

samples were then taken out from the bath and rapidly

cooled to room temperature, dipping the vial in cold

water. In the various isothermal crystallizations, the

crystallization time tc is different depending on the

crystallization temperature. The shortest time is 24 h for

the lowest crystallization temperature and increases with

increasing crystallization temperature, up to two weeks

for the highest crystallization temperature. The melt-

crystallized samples were then analyzed by X-ray

diffraction.

2.3. Non-isothermal melt-crystallizations

The various fractions of the ELPP sample were also

crystallized from the melt in non-isothermal conditions by

cooling the melt to room temperature. Powder specimens of

the crystalline fractions ELPP(PI-HsS), ELPP(HsI-HS) and

ELPP(HI) were melted at 180 8C in DSC, kept for 10 min at

this temperature in a N2 atmosphere, then cooled to room

temperature at cooling rates of 40, 20, 10, 2.5 and 1 8C/min.

The crystallized samples were then analyzed by X-ray

diffraction.

2.4. Characterization

The intrinsic viscosities were measured in 1,2,3,4-

tetrahydronaphtalene solutions at 135 8C, using standard

Ubbelohde viscosimeter, with a single point technique

[29a]. The molecular weight was evaluated from the

intrinsic viscosities according to ½h�ZKð �MvÞa;using the

parameters reported for isotactic polypropylene aZ0.74,

kZ1.93!10K4 dl/g [29b].

Table 1

Results of the fractionation of the sample ELPP

Fractions Weight (%) [h] (dl/g) Mva Tm (8C)b DHm (J/g) xc

c

ELPP(EtS) 40 1.88 2.43!105 – – –

ELPP(EtI-PS) 26 2.10 2.82!105 – – 0.03d

ELPP(PI-HsS) 7 2.14 2.89!105 105 25 0.12

ELPP(HsI-HS) 13 2.16 2.92!105 134 28 0.13

ELPP(HI) 14 2.02 2.68!105 151 92 0.44

Whole sample

ELPP

100 2.09 2.79!105 150 17 0.08

Weight fractions (%), intrinsic viscosities ([h]), molecular masses (Mv), melting temperatures (Tm), melting enthalpies (DHm) and crystallinity indices (xc) of

the sample ELPP and of the corresponding four fractions.a From the intrinsic viscosities using the parameters of the Mark–Houwink equation aZ0.74, kZ1.93!10K4dl/g [29b].b Peak temperatures from the DSC curves recorded at heating rate of 10 8C/min (Fig. 3).c From the melting enthalpies DHm, xcZDHm/DHm

o assuming as thermodynamic melting enthalpy DHmoZ209.5 J/g [30].

d From the X-ray powder diffraction profile of Fig. 2c.


The 13CNMR spectra of the various fractions of the sample

ELPP were recorded on a Varian XL-200 spectrometer

operating at 200 MHz in the Fourier transform mode of 10%

w/v polymer solutions in deuterated tetrachloroethane (also

used as internal standard) at 70 8C (fractions ELPP(EtS),

ELPP(EtI-PS), ELPP(PI-HsS)) and at 125 8C (fractions

ELPP(HsI-HS) and ELPP(HI)). In these conditions for each

fraction homogeneous solutions are obtained.

X-ray powder diffraction patterns were obtained at room

temperature with an automatic Philips diffractometer using

Ni-filtered CuKa radiation.

The relative amount of crystals in the g form present

in our samples was measured from the X-ray diffraction

profiles, as suggested by Turner-Jones et al. [31], by

measuring the ratio between the intensities of the (117)greflection at 2qZ20.18, typical of the g form, and the

(130)a reflection at 2qZ18.68, typical of the a form:

fgZ Ið117Þg=½Ið130ÞaC Ið117Þg�: The intensities of

(117)g and (130)a reflections were measured from the

area of the corresponding diffraction peaks above the

diffuse halo in the X-ray powder diffraction profiles.

The amorphous halo has been obtained from the X-ray

diffraction profile of an atactic polypropylene, then it

was scaled and subtracted to the X-ray diffraction

profiles of the melt-crystallized samples.

The calorimetric measurements were performed with a

differential scanning calorimeter (DSC) Perkin–Elmer

DSC-7 in a flowing N2 atmosphere. The melting tempera-

tures of the samples were taken as the peak temperature of

the DSC curves.

Table 2

Polymerization temperatures (Tpol), concentrations of the isotactic pentad ([mmm

melting temperatures of the as-prepared powders (Tm) of the three iPP sa

trimethylsilyl)(indenyl)]zirconium/dichloride/MAO catalytic system [28]

Samples Tpol (8C) [mmmm] (%) [m] (%) [mm] (%)

R1 20 89.0 95.5 93.3

R2 50 87.4 94.8 92.1

R3 60 83.4 93.1 89.7

a Melting temperature measured from DSC scans of the ‘as prepared’ samples

3. Results and discussion

3.1. As-prepared sample and fractions

The sample ELPP has been fractionated with boiling

diethyl ether, pentane, hexane and heptane as described in

Section 2. The weight fractions and the physical properties

(melting temperature, intrinsic viscosity, molecular weight)

of the five fractions ELPP(EtS), ELPP(EtI-PS), ELPP(PI-

HsS), ELPP(HsI-HS) and ELPP(HI) are reported in Table 1.

The 13C NMR spectra in the region of the methyl carbon

atom resonance of the five fractions are reported in Fig. 1.

The concentrations of the pentad stereosequences in the five

fractions, evaluated from the 13C NMR spectra of Fig. 1, are

shown in Table 3. It is apparent that the ELPP sample is

mainly constituted by the more stereoirregular fractions

soluble in pentane and diethyl ether, and that all the

fractions have a high molecular weight. This feature

represents one of the main difference of this sample with

the iPP samples prepared with the traditional MgCl2-

supported Ziegler–Natta catalysts, whose stereoirregular

fractions have generally low molecular weights. Other

important differences concern the microstructure of the

single fractions, including the more stereoregular ones, as

will be discussed in the following.

The X-ray powder diffraction profiles of the as-prepared

unfractionated sample ELPP and of the five corresponding

fractions are reported in Fig. 2. The fractions soluble in

diethyl ether and pentane (ELPP(EtS) and ELPP(EtI-PS))

are basically amorphous (only a negligible crystallinity is

m]) and of the meso diad ([m]), triads distribution, molecular masses (Mw),

mples prepared with the highly regiospecific rac-isopropyliden[bis(3-

[mr] (%) [rr] (%) Mw Tm (8C)a

4.4 2.3 110000 144

5.2 2.6 75000 141

6.9 3.4 66000 137

recorded at heating rate of 10 8C/min.

Fig. 1. 13C NMR spectra in the region of the methyl carbon atoms resonance of the fractions ELPP(EtS) (A), ELPP(EtI-PS) (B), ELPP(PI-HsS) (C), ELPP(HsI-

HS) (D) and ELPP(HI) (E) of the ELPP sample.


observed in the diffraction profile of the fraction ELPP(EtI-

PS) of Fig. 2(c)), due to the very low stereoregularity (Table

3).

The other fractions, as well as the unfractionated sample,

show a significant crystallinity, which increases for

fractions insoluble in high boiling temperature solvents

(Fig. 2(d)–(f)). We recall that a and g forms of iPP present

similar X-ray diffraction profiles, the main difference being

the position of the third strong diffraction peak, which

occurs at 2qZ18.68 ((130)a reflection) in the a form [33],

and at 2qZ20.18 ((117)g reflection) in the g form [34]. It is

apparent from Fig. 2 that the crystalline fractions are mainly

crystallized in the a form of iPP, as indicated by the

presence of the (130)a reflection at 2qZ18.68 of the a form,

and the absence of the (117)g reflection at 2qZ20.18 of the g

Table 3

Concentration of pentad stereosequences, from the 13C NMR spectra of Fig. 1, in

Fractions sample

ELPP

[mmmm]

(%)

[mmmr]

(%)

[rmmr]

(%)

[mmrr]

(%)

ELPP(EtS) 15.4 11.2 5.9 13.2

ELPP(EtI-PS) 25.9 11.8 5.1 12.8

ELPP(PI-HsS) 53.5 13.5 2.7 10.6

ELPP(HsI-HS) 62.0 10.0 1.2 8.6

ELPP(HI) 93.8 2.7 – 2.5

a The resonance of low intensity at 20.90 ppm in the spectrum of Fig. 1A corresp

20.78 ppm corresponds mostly to the sequences rrmrrm and rrmrrr [32].b The resonance of low intensity at 20.92 ppm in the spectrum of Fig. 1B corresp

20.81 ppm corresponds mostly to the sequences rrmrrm and rrmrrr [32].c The resonance at 20.95 ppm in the spectrum of Fig. 1C corresponds to the m

mostly to the sequences rrmrrm and rrmrrr[32].d The broad resonance at 20.85 ppm in the spectrum of Fig. 1D is due to the c

form in the X-ray diffraction profiles of Fig. 2(d)–(f). The

low intensity of the (130)a reflection of the a form, with

respect to that expected for crystals in the pure a form [33],

observed in the diffraction profiles of some fractions (Fig.

2(d) and (e)), as well as of the as-prepared sample (Fig.

2(a)), indicates that structural disorder (of the kind

described in the Refs.[18] and [25]) in the stacking of

bilayers of three-fold helical chains along the b axis is

present in the crystals. According to the model of disorder

proposed in Refs. [18] and [25], consecutive bilayers of

chains may face each other with the chain axes either

parallel (like in the a form) or nearly perpendicular (like in

the g form), producing disordered modifications intermedi-

ate between a and g forms (a/g disorder) [25].

The DSC heating curves of the unfractionated ELPP

the various fractions of the sample ELPP

[mmrm]C

[rmrr] (%)

[rmrm]

(%)

[rrrr]

(%)

[rrrm]

(%)

[mrrm]

(%)

17.4a 7.8 10.0 12.8 6.3

14.5b 5.2 9.6 10.5 4.6

5.3c 2.4 4.4 3.6 4.0

5.4d 2.0 3.8 3.7 3.3

0.2 – – – 0.8

onds to the mmmrmr sequence, whereas the resonance of higher intensity at

onds to the mmmrmr sequence, whereas the resonance of higher intensity at

mmrmr sequence, whereas the broad resonance at 20.82 ppm corresponds

ontributions of mmmrmr, rrmrrm and rrmrrr sequences.

Fig. 2. X-ray powder diffraction profiles of the as-prepared ELPP sample

(a) and of the corresponding fractions ELPP(EtS) (b), ELLP(EtI-PS) (c),

ELPP(PI-HsS) (d), ELPP(HsI-HS) (e) and ELPP(HI) (f). The (130)areflection at 2qZ18.68 of the a form is indicated.

Fig. 3. DSC curves, recorded at heating rate of 10 8C/min, of the as-

prepared ELPP sample (a) and of the corresponding fractions ELPP(PI-

HsS) (b), ELPP(HsI-HS) (c) and ELPP(HI) (d).


sample and of the corresponding crystalline fractions are

reported in Fig. 3. The melting temperatures and the

crystallinities evaluated from the DSC curves are reported in

Table 1. The broad DSC curve of the unfractionated sample

(Fig. 3(a)) is a result of the melting of the various fractions

and shows peak melting temperature at 150 8C, due

basically to the melting of the more crystalline fraction

ELPP(HI) (Fig. 3(d)), even though this fraction is present in

the sample ELPP with low concentration (14%, see Table

1).

Fig. 4. X-ray powder diffraction profiles of samples of the as-prepared

unfractionated ELPP sample isothermally crystallized from the melt at the

indicated temperatures. The (130)a reflection of the a form at 2qZ18.68

and the (117)g reflection of the g form at 2qZ20.18 are indicated.

3.2. Isothermal melt-crystallizations

The unfractionated ELPP sample and the corresponding

three crystalline fractions have been isothermally crystal-

lized from the melt at different temperatures and analyzed

by X-ray diffraction.

The X-ray powder diffraction profiles of samples

obtained by isothermal crystallization of the as-prepared

unfractionated ELPP sample are reported in Fig. 4. The

diffraction profile of the as-prepared sample (already shown

in Fig. 2(a)) is also reported in Fig. 4(a) for comparison. The

crystallization behavior of the whole sample is a result of

the crystallization properties of the more stereoirregular

fractions, which are present with high concentrations (Table

1). A low level of crystallinity is achieved at any crystal-

lization temperature.

It is apparent that in the diffraction profiles of all the

melt-crystallized samples, as well as of the as-prepared

sample, the (117)g reflection at 2qZ20.18 of the g form is

practically absent or shows very low intensity. This does not

necessarily indicate that in these samples crystals of the gform are absent. In fact, the relative intensities of the


reflections observed in the profiles of Fig. 4 do not

correspond to those of crystals of the pure a form. In

particular the presence of the sharp reflection at 2qZ16.78,

corresponding to (040)a or (008)g reflections of a and gforms, respectively, and the low intensity of the (130)areflection at 2qZ18.68 in the diffraction profiles of Fig. 4,

suggest that these samples are crystallized in disordered a/gmodifications intermediate between a and g forms [18,25].

As demonstrated in Refs. [18] and [25], the presence of this

kind of structural disorder (as in the model of Fig. 8 of Ref.

[25]) reduces the intensities of both (130)a and (117)g, and

does not affect the intensity of the diffraction peak at 2qZ16.78 ((040)a or (008)g reflection), which remains sharp and

with high intensity, as actually observed in the diffraction

profiles of Fig. 4.

The X-ray powder diffraction profiles of samples of the

three fractions ELPP(PI-HsS), ELPP(HsI-HS) and ELP-

P(HI), isothermally crystallized from the melt are reported

in Fig. 5. The diffraction profiles of the three fractions

(already shown in Fig. 2), as obtained after fractionation

without any thermal treatments (here defined ‘as-fractio-

nated samples’) are also reported in Fig. 5 (profiles a) for

comparison. It is apparent that, whereas the as-fractionated

samples are basically in the a form (profiles a of Fig. 5), the

diffraction profiles of samples crystallized from the melt of

Fig. 5 always present the (117)g reflection at 2qZ20.18 of

the g form, indicating that for all the fractions, crystals

of the g form develop when the samples are crystallized

from the melt (profiles b–e of Fig. 5(A), (C) and profiles b–g

of Fig. 5(B)).

In particular, the fraction ELPP(PI-HsS), initially in the aform with low crystallinity (profile a of Fig. 5(A)),

crystallize from the melt at high crystallization temperatures

almost totally in the g form, although with low crystallinity,

as indicated by the high intensity of the (117)g reflection of

the g form at 2qZ20.18, and the very low intensity of the

(130)a reflection of the a form at 2qZ18.68 in the

diffraction profiles d–e of Fig. 5(A). At lower crystallization

temperatures mixtures of crystals of a and g forms are

obtained, as indicated by the presence of both (130)areflection of the a form at 2qZ18.68 and (117)g reflection of

the g form at 2qZ20.18 in the diffraction profiles b,c of Fig.

5(A). However, as discussed above, also for these samples

the presence of the sharp reflection at 2qZ16.78, corre-

sponding to the (040)a or (008)g reflections of the a and gforms, respectively, and the low intensities of both (130)aand (117)g reflections at 2qZ18.68 and 20.18, respectively,

of the a and g forms, respectively, in the diffraction profiles

b,c of Fig. 5(A), indicate that these samples are crystallized

in disordered a/g modifications intermediate between the aand g forms [18,25].

The more stereoregular fractions ELPP(HsI-HS) and

ELPP(HI) crystallize from the melt in mixtures of crystals

of a and g forms, as indicated by the presence of both (130)aand (117)g reflections at 2qZ18.6 and 20.18, respectively, in

the diffraction profiles of Fig. 5(B) and (C), and levels of

crystallinity higher that those obtained for the fraction

ELPP(EtI-PS) are achieved (Fig. 5(B) and (C)). The content

of g form is higher in the samples of the more irregular

fraction ELPP(HsI-HS) and increases with increasing the

crystallization temperature. The intensity of the (117)greflection at 2qZ20.18 of the g form is, indeed, similar to

that of the (130)a reflection of the a form in the samples of

the fraction ELPP(HsI-HS) crystallized at high temperatures

(profile e–g of Fig. 5(B)), whereas it is much lower in the

case of the samples of the more stereoregular ELPP(HI)

fraction (Fig. 5(C)).

For all the fractions of sample ELPP, the relative

intensity of the (117)g reflection of the g form at 2qZ20.18, with respect to that of the (130)a reflection of the aform at 2qZ18.68, increases with increasing the crystal-

lization temperature, reaches a maximum value and then

decreases for a further increase of the crystallization

temperature. The relative amount of the g form with respect

to the a form, fg, for the various samples is reported in Fig. 6

as a function of the crystallization temperature. The content

of g form increases with increasing the crystallization

temperature and a maximum amount of g form is obtained

at temperatures in the range 120–130 8C, for the more

crystalline fractions ELPP(HsI-HS) and ELPP(HI), and at

90 8C for the less crystalline fraction ELPP(PI-HsS).

The analysis of the microstructures of the fractions of the

ELPP sample, performed by 13C NMR (Fig. 1 and Table 3),

has shown that the iPP chains of the more stereoregular

fraction ELPP(HI) present small concentration of defects of

stereoregularity mainly represented by isolated rr triads. In

the 13C NMR spectrum of Fig. 1(E), besides the resonance

of the isotactic pentad mmmm, the three signals relative to

the pentad stereosequences mmmr, mmrr and mrrm with

intensity ratios close to 2:2:1 (Table 3) are present,

according to a site control of the propagation mechanism.

The chains of the less stereoregular fractions ELLPP(EtI-

PS), ELPP(PI-HsS) and ELPP(HsI-HS) present rr defects,

the concentrations of the mrrm pentad being 4.6, 4.0 and

3.3%, respectively (Fig. 1(B)–(D) and Table 3), but also

isolated r diads and rather long syndiotactic sequences, as

indicated by the presence in the spectra of Fig. 1(B)–(D) of

resonances at 20.8–20.9 ppm, corresponding to mmrm and

rmrr pentads, and at 20.3 ppm corresponding to rrrr

sequences. The concentration of the fully syndiotactic rrrr

pentad is quite high in the more irregular fractions (10.0 and

9.6% for the chains of the fractions ELPP(EtS) and

ELPP(EtI-PS), respectively) and decreases in the more

regular fractions (4.4% and 3.8% in the fractions ELPP(PI-

HsS) and ELPP(HsI-HS), respectively, and negligible in the

fraction ELPP(HI), Fig. 1 and Table 3). The presence of the

very weak diffraction peak at 2qZ128 in the X-ray powder

diffraction profiles of some samples of the fraction

ELPP(PI-HsS) crystallized from the melt of Fig. 5(A) (for

instance profiles d,e), indicates that a very small, almost

negligible, amount of crystals of syndiotactic polypropylene

are obtained. The reflection at 2qZ128 corresponds, indeed,

Fig. 5. X-ray powder diffraction profiles of samples of the fractions ELPP(PI-HsS) (A), ELPP(HsI-HS) (B) and ELPP(HI) (C) isothermally crystallized from

the melt at the indicated temperatures. The (130)a reflection of the a form at 2qZ18.68 and the (117)g reflection of the g form at 2qZ20.18 are indicated.


to the 200 reflection arising from the crystals of the stable

polymorphic form of syndiotactic polypropylene with

chains in two-fold helical conformation [35].

It is worth noting that in the case of iPP samples prepared

with MgCl2-supported Ziegler–Natta catalysts, depending

on the Lewis bases added to the catalyst/cocatalyst system

as internal or external donors, longer syndiotactic

sequences, able to form significant amount of crystals of

sPP, are generally present in chains of some fractions [27].

The results of the structural analysis of Fig. 6 indicate

Fig. 6. Relative content of g form of iPP, fg; evaluated from the X-ray

diffraction profiles, in the samples isothermally crystallized from the melt

of fractions of the sample ELPP, as a function of the crystallization

temperature Tc. (&) fraction ELPP(PI-HsS); (:) fraction ELPP(HsI-HS);

(C) fraction ELPP(HI). The stereoregularity of the samples, as

concentration of the isotactic pentad mmmm, is also shown.

that for these complex microstructures of the elastomeric

polypropylene produced with the Al2O3-supported catalyst,

the amount of g form, which may develop by melt-

crystallizations, increases with increasing the concentration

of defects. The maximum amounts of g form, fgðmaxÞ;

evaluated from the maxima of the curves of Fig. 6, are

nearly 26% for the fraction ELPP(HI), 60% for the fraction

ELPP(HsI-HS) and 90% for the fraction ELPP(PI-HsS).

This result is in agreement with the hypothesis reported in

the literature that the g form crystallizes in samples or

fractions of iPP characterized by chains having short regular

isotactic sequences [19,24–27].

3.3. Comparison with metallocene iPP

The crystallization properties of the elastomeric polypro-

pylene sample have been compared with the polymorphic

behavior of samples of iPP prepared with metallocene

catalysts [25]. The amounts of g form obtained by melt-

crystallization of iPP samples prepared with the single-center

metallocene catalyst rac-isopropyliden[bis(3-trimethylsilyl)

(indenyl)]ZrCl2 (samples R1, R2 and R3) [28], taken from

Refs. [24,25], are compared in Fig. 7 with the data obtained

for the ELPP sample. As described by Resconi et al. [28],

this metallocene catalyst produces iPP samples character-

ized by chains containing only defects of stereoregularity

(mainly isolated rr triads), whose amount depends on the

polymerization temperature. The distribution of defects


along the chains is random and no defects of regioregularity

are present. This catalyst may be, therefore, taken as a

reference catalyst for the single-center metallocene catalysts

because it produces very simple chain microstructure,

providing the first example of metallocene-made iPP not

including regioerrors [28].

As discussed in Refs. [24,25], for the metallocene iPP

samples the content of g form increases with increasing the

crystallization temperatures, with a maximum at tempera-

tures of 120–130 8C, and increases with increasing the

concentration of defects of stereoregularity (Fig. 7). The

study of the crystallization properties of metallocene-made

iPP samples has allowed explaining the trend of curves of

the kind of Fig. 7 [24,25], in particular the increase of the

content of g form with increasing crystallization tempera-

ture up to a maximum value, as a result of two competing

kinetic and thermodynamic effects. For samples of iPP

containing an appreciable amount of stereodefects, the

formation of the g form is favored, since the rr stereo-

defects, and the a/g structural disorder are highly tolerated

in crystals of the g form [18,25]. As a consequence, a high

amount of g form develops in the slow crystallizations at

high temperatures. At lower crystallization temperatures,

the fast crystallization of the a form is instead kinetically

favored, giving a low amount of g form. With increasing the

crystallization temperature the amount of g form increases,

but at very high crystallization temperatures the crystal-

lization of the defective g form is too slow because of its

lower melting temperature [25] and the more perfect a form

becomes again kinetically favored, so that the amount of gform decreases [24,25]. Moreover these data have indicated

that the crystallization of the g form is favored in iPP

samples characterized by short regular isotactic sequences

[24,25]. The lower the degree of isotacticity, the higher the

maximum amount of the crystallized g form ðfgðmaxÞÞ;

Fig. 7. Comparison between the relative contents of g form of iPP, fg; as a

function of the crystallization temperature Tc, for metallocene-made iPP

samples Ri (taken from Refs. [24,25]), and the fractions of the ELPP

sample, isothermally crystallized from the melt. (&) fraction ELPP(PI-

HsS), [mmmm]Z53.5%; (:) fraction ELPP(HsI-HS), [mmmm]Z62.0%;

(C) fraction ELPP(HI), [mmmm]Z93.8%; (B) sample R1, [mmmm]Z89.0%; (6) sample R2, [mmmm]Z87.4%; (,) sample R3, [mmmm]Z83.4%.

evaluated from the maxima of the curves of Fig. 7. In

particular, fgðmaxÞz78% for the sample R1 ([mmmm]Z89.0%) and 90% for the less stereoregular samples R2 and

R3 ([mmmm]Z87.4 and 83.4%, respectively) [24,25].

The relative stability of a and g forms of iPP is related to

the average length of the fully isotactic sequences

comprised between two successive interruptions, and the

g form crystallizes more easily when the regular isotactic

sequences are short [19,24,25]. The number of interruptions

depends, in turn, on the content of defects and the degree of

segregation of the defects along the polymer chain. Since

the samples Ri have a random distribution of errors, the

average length of the fully isotactic sequences, hLisoi, is

inversely proportional to the content of errors. Moreover,

since these samples contain only isolated rr triad defects, the

average length of the fully isotactic sequences may be

roughly assumed as hLisoiz1/[rr]. As a consequence, the

amount of g form at a given crystallization temperature,

turns out to be higher for the less stereoregular sample R3

(with hLisoiZ29 monomeric units) than for the samples R2

(hLisoiZ38 monomeric units) and R1 (hLisoiZ43 monomeric

units) [24,25].

As discussed in Section 3.2, the fractions of the ELPP

sample show a similar behavior, with a notable difference in

the maximum amount of the crystallized g form. It is

apparent from the comparison of Fig. 7 that the more

stereoregular fraction ELPP(HI) present a content of g form

much lower than that obtained in the sample R1 at any

crystallization temperature. The sample R1, having

[mmmm]Z89% (Table 2), develops a maximum amount

of g form fgðmaxÞZ78% (Fig. 7), against the values of only

26% observed for the fraction ELPP(HI), having [mmmm]Z94% (Table 3).

In the hypothesis that the g form preferably crystallizes

when the regular isotactic sequences are relatively short [19,

24,25], these data indicate that the chains in the sample

Fig. 8. Maximum amount of g form of iPP obtained upon melt-

crystallization procedures for various samples of iPP, fgðmaxÞ; as a

function of the average length of fully isotactic sequences, hLisoi, comprised

between two consecutive interruptions (defects). (,) samples Ri from

Refs. [25,26]; (6) data from Ref. [22]; (C) data from Ref. [19]; (:) data

from Ref. [23]. The values of fgðmaxÞ evaluated from the Fig. 7 for the

fractions of the ELPP sample, ELPP(PI-HsS), ELPP(HsI-HS), ELPP(HI)

are also shown as horizontal dashed lines. The intercepts of these dashed

lines on the straight line give the average lengths of fully isotactic

sequences hLisoi for the fractions of the ELPP sample.


ELPP(HI) present regular isotactic sequences much longer

than those of the chains of the sample R1, according with

the lower concentration of defect. Both samples R1 and

ELPP(HI) contain indeed only isolated rr triad defects with

contents [rr]Z2.3% for the sample R1 and 0.8% for the

fraction ELPP(HI).

The more irregular fractions of the ELPP sample,

ELPP(HsI-HS) and ELPP(PI-HsS) develop a much higher

amount of g form (Fig. 7). It is apparent, however, that even

though the fraction ELPP(HsI-HS) is less stereoregular

([mmmm]Z62%, Table 3) than the metallocene samples Ri,

it shows a maximum amount of g form of 60%, much lower

than those observed for the samples Ri. As discussed above

the fraction ELPP(HsI-HS) is characterized by a high

concentration of rr defects (3.3%, Table 3), a appreciable

content of r defects and a high concentration of syndiotactic

sequences ([rrrr]Z3.8%, Table 3). Despite this high

concentration of defects, the sample ELPP(HsI-HS) crystal-

lizes preferably in the a form at low crystallization

temperatures and a maximum amount of g form fgðmaxÞ

of only 60% is achieved at high crystallization temperatures

(Fig. 7). This indicates that in this fraction the regular

isotactic sequences are still rather long, even in the presence

of a so high overall concentration of defects, and in any case

longer than those of the chains of the more stereoregular

samples R1, R2 and R3, which contain only 2.3, 2.6 and

3.4% of rr defects (Table 2).

This result can be explained considering that the chains

of the elastomeric polypropylene, produced with the

heterogeneous catalysts, and of metallocene-made iPP

present a different distribution of defects. In the metallo-

cene-made iPP samples Ri the distribution of defects of

stereoregularity along the macromolecular chains is random

so that even a small concentration of defects produces

frequent interruptions of the isotactic propagation and the

regular isotactic sequences turn out to be relatively short

even for samples with quite high stereoregularity [25]. This

explains the high concentration of g form (78–90%)

obtained in the relatively high isotactic samples R1, R2

and R3 (Fig. 7).

In the sample ELPP(HsI-HS) the higher amount of

stereodefects is not randomly distributed along the chains.

Most of the defects (rr and r stereodefects and longer

syndiotactic sequences rrrr) are segregated in more

irregular noncrystallizable (or hardly crystallizable) por-

tions of the chains. The more regular isotactic sequences

would be, therefore, rather long and still able to crystallize

in the a form, and the amount of the g form turns out to be

not very high, even though the overall degrees of

stereoregularity, as evaluated by the 13C NMR spectra, is

very low.

A rough evaluation of the average length of the isotactic

sequences in polypropylene chains may be obtained with the

empirical method proposed by us in the Ref. [24] for

stereoblock polypropylene produced with unbridged metal-

locene catalysts. The method is based on the use of a

calibration plot of the maximum amount of the g form,

fgðmaxÞ; as a function of the average length of the fully

isotactic sequences hLisoi, obtained for iPP samples prepared

with metallocene catalysts having a random distribution of

defects. The average length of the fully isotactic sequences

may be evaluated as hLisoiZ1/3, with 3 the total concen-

tration of errors determined by the 13C NMR analysis. For

these samples the maximum amount of g form, which can

be obtained for the various samples by melt-crystallization

procedures, is roughly linear with the logarithm of hLisoi (see

Fig. 7 of Ref. [24]). If this linear relationship is kept general

whichever the distribution of defects, it is possible to find

the length of the fully isotactic sequences also for

polypropylene samples characterized by nonrandom distri-

bution of defects from the values of fgðmaxÞ: The calibration

plot of fgðmaxÞ as a function of hLisoi is reported in Fig. 8 and

the indirect method is applied to the various fractions of the

ELPP sample. From the values of fgðmaxÞZ26% and 60%,

evaluated from Fig. 7 for the fractions ELPP(HI) and

EL(HsI-HS), values of the average length of the isotactic

sequences hLisoi of 160 and 55 monomeric units for the

chains of the fractions ELPP(HI) and EL(HsI-HS), respect-

ively, have been found (Fig. 8).

It is worth noting that the highly stereoregular fraction

ELPP(HI) contains basically only isolated rr triads defects

([rr]Z0.8%), the content of other kinds of stereoirregula-

rities being below 0.2% (Fig. 1(E), Table 3). If these defects

were randomly distributed along the chains, as in the case of

metallocene-made iPPs, the average length of fully isotactic

sequences hLisoi would correspond to hLisoiz1/[rr]Z125

monomeric units. This value is only slightly lower than the

values of 160 monomeric units obtained by the structural

method of Fig. 8. This possibly indicates that for the fraction

ELPP(HI) the distribution of stereo-defects is more uniform

and nearly approaches that of iPP samples prepared with

single site metallocene catalysts rather than that of iPP

samples prepared with MgCl2-supported Ziegler–Natta

catalysts. A further evidence comes from the analysis of

the melting properties of the fraction ELPP(HI) compared

with that of metallocene-made iPP and Ziegler–Natta iPP

samples. The melting temperature of the fraction ELPP(HI)

is, indeed, nearly 150 8C (Table 1), similar to that of

metallocene-made iPP samples having similar stereoregu-

larity [19–25], but much lower than the melting temperature

generally observed for heptane insoluble fractions of

Ziegler–Natta iPP samples (160 8C) [27].

In the case of the more irregular fraction ELPP(HsI-HS),

the value of 55 monomeric units of the average length of the

isotactic sequences, rather high compared to the high

concentration of defects, suggest that in this fraction the

chains are characterized by a stereo-block microstructure

consisting in long, rather regular, isotactic sequences linked

to more irregular sequences containing most of the

stereodefects.

Figs. 6 and 7 show that the most irregular crystalline

fraction ELPP(PI-HsS) crystallizes almost totally in the g


form, the maximum amount of g form, fgðmaxÞ being nearly

90%, similar to that obtained for the samples R2 and R3. It

is worth noting, however, that the metallocene iPP samples

R2 and R3, having [mmmm]Z87.4 and 83.4%, respectively

(Table 2), are much more stereoregular than the fraction

ELPP(PI-HsS), which is characterized by [mmmm]Z53.5%

(Table 3). This clearly indicates that fractions of the

elastomeric ELPP sample may crystallize from the melt

almost totally in the g form only when the concentration of

defects is much higher than that needed for the metallocene-

made iPP samples. In fact the samples R2 and R3 contain

only 2.6 and 3.4% of randomly distributed rr defects (Table

2), sufficient to make the regular isotactic sequences very

short (hLisoiZ30–40 monomeric units), inducing crystal-

lization almost totally in the g form (Fig. 7). The fraction

ELPP(PI-HsS) is characterized by chains containing a high

concentration of defects, consisting in isolated rr triads

(4.0%), isolated r diads and long syndiotactic sequences

([rrrr]Z4.4%, Table 3). Only with a so high concentration

of defects, sufficiently short isotactic sequences are obtained

so that an almost total crystallization in the g form is

induced. From the value of fgðmaxÞZ90% and from Fig. 8,

an average length of the isotactic sequences hLisoi of 25

monomeric units has been evaluated for the chains of the

fraction ELPP(PI-HsS), similar to that of the sample R3

(Table 3).

This result indicates that very different amounts of

defects produce similar average lengths of the isotactic

sequences in the samples R3 and ELPP(PI-HsS), and

suggests that also for the fraction ELPP(PI-HsS) most of

the defects are segregated in more irregular noncrystalliz-

able portions of the chains, producing slightly more regular

isotactic sequences able to crystallize. These sequences,

however, contain probably appreciable amount of defects

and tend to crystallize in the g form (Fig. 5(A)). This

microstructure is in agreement with the experimental

observation that the fraction ELPP(PI-HsS) crystallizes

either from solution (Fig. 2(d)) or from the melt (Fig. 5(A))

with a relatively high crystallinity and melting temperature

(105 8C, Table 1), despite the high content of defects.

The presence of stereoblocks, consisting in regular

isotactic blocks alternating to atactic, or more stereoirre-

gular, sequences, in some fractions of the ELPP sample,

in particular in the fraction soluble in hexane-insoluble in

heptane (ELPP(HsI-HS)) and in the fraction insoluble in

pentane-soluble in hexane (ELPP(PI-HsS)), accounts for the

elastic properties shown by this materials [4–6]. In these two

fractions the length and the microstructure of the more

regular crystallizable isotactic sequences are different. In

the more irregular fraction ELPP(PI-HsS) the more regular

crystallizable blocks, linked to the atactic sequences, are

actually poorly isotactic containing an appreciable amount

of stereodefects. These short isotactic blocks (hLisoiZ25

monomeric units) crystallize preferably in the g form. In the

more regular fraction ELPP(HsI-HS) the isotactic blocks are

more regular and still rather long, even in the presence of a

high concentration of defects (hLisoiZ55 monomeric units),

and tend to crystallize in mixtures of the a and g forms. The

elastic properties shown by the unfractionated whole ELPP

sample [4–6], which is constituted mostly by amorphous

fractions (40% fraction ELPP(EtS) and 26% fraction

ELPP(EtI-PS)), are due to the two effects of the high

molecular weight shown by all the fractions (even the more

irregular fractions, see Table 1) and the crystallization of

isotactic sequences present in the stereoblocks. The poorly

isotactic blocks of the irregular fractions can cocrystallize

with the longer isotactic sequences of the more stereoregular

component (fraction ELPP(HI)), producing cross-linking in

the amorphous matrix and forming the elastomeric network

[4–6].

It is worth noting that, as discussed above, the main

difference between the elastomeric polypropylene ELPP

sample and the iPP samples prepared with the traditional

MgCl2-supported Ziegler–Natta catalysts is in the micro-

structure of the chains of the various fractions. The ELPP

sample is mainly constituted by the more stereoirregular

fractions soluble in pentane and diethyl ether, and all

the fractions have high molecular weights. The stereo-

irregular fractions of the Ziegler–Natta iPP are generally

minor components of the whole sample and have, instead,

generally, lower molecular weights. Moreover, in the ELPP

samples, even the more stereoregular fractions are charac-

terized by chains having relatively short regular isotactic

sequences, compared to the very long regular sequences in

the chains of Ziegler–Natta iPP samples [27,32,36]. The

relatively short length of the isotactic sequences in the ELPP

sample is clearly demonstrated by the appreciable amount

of g form which crystallizes in all of the three crystalline

fractions of the ELPP sample (Figs. 6 and 7), even in the

more stereoregular ELPP(HI) fraction. In particular, the

latter fraction presents an average length of isotactic

sequences and a melting temperature similar to those of

metallocene-made iPP samples having similar stereoregu-

larity [19–25], but much lower that those observed in

heptane insoluble fractions of Ziegler–Natta iPP samples

[27]. As discussed above, this indicates that in the fraction

ELPP(HI) the distribution of defects in the polymer chains

is more uniform and approaches that of chains of

metallocene-made iPP. These microstructural features of

the various fractions are the basis of the elastomeric

properties shown by the ELPP sample.

3.4. Non-isothermal melt-crystallizations

The crystallization from the melt of the sample ELPP has

also been studied in non-isothermal conditions. The X-ray

diffraction profiles of the fractions ELPP(PI-HsS),

ELPP(HsI-HS) and ELPP(HI) crystallized by cooling the

melt to room temperature at cooling rates of 1, 2.5, 10, 20

and 40 8C/min are reported in Fig. 9. Also in this case the

samples crystallize in mixtures of a and g forms. It is

apparent that for all samples the crystallization of the a form

Fig. 9. X-ray powder diffraction profiles of samples of the fractions ELPP(PI-HsS) (A), ELPP(HsI-HS) (B) and ELPP(HI) (C) of the ELPP sample crystallized

by cooling the melt to room temperature at the indicated cooling rates. The (130)a reflection of the a form at 2qZ18.68 and the (117)g reflection of the g form at

2qZ20.18 are indicated.

Fig. 10. Relative content of g form of iPP, fg; evaluated from the X-ray

diffraction profiles of Fig. 9, in fractions of the ELPP sample crystallized by

cooling the melt to room temperature as a function of the cooling rate. (&)

fraction ELPP(PI-HsS); (:) fraction ELPP(HsI-HS); (C) fraction

ELPP(HI). In the case of the fraction ELPP(PI-HsS) only the point at

cooling rate of 1 8C/min is reported because at higher cooling rates only a/g

disordered modifications are obtained, with diffraction profiles character-

ized by the absence of both (130)a and (117)g reflections (Fig. 9A),

preventing the evaluation of the amount of g form.


is favored by fast cooling rates, whereas slow cooling rates

induce the crystallization of the g form, as already shown

for other iPP samples [25,37]. The intensity of the (117)greflection at 2qZ20.18, indeed, increases with decreasing

the cooling rate. The content of g form is very high for the

irregular fraction ELPP(PI-HsS) crystallized at very low

cooling rate (profile a in Fig. 9(A)). At higher cooling rates

this fraction crystallizes instead in a/g disordered modifi-

cations with low crystallinity, as indicated by the very low

intensities of both (130)a and (117)g reflections at 2qZ18.6

and 20.18 in the X-ray diffraction profiles b–e of Fig. 9(A).

The more stereoregular fraction ELPP(HI) crystallizes

basically in the a form with an appreciable amount of gform only at very low cooling rate (1 8C/min, profile a of

Fig. 9(C)).

The content of g form, evaluated from the X-ray

diffraction profiles of Fig. 9, is reported in Fig. 10 as a

function of the cooling rate. From these data the values of fgat zero cooling rate can be approximately estimated,

obtaining the maximum amounts of g form, fgðmaxÞ;

which can be obtained for each sample in these conditions

(90 and 35% for the fractions ELPP(HsI-HS) and ELPP(HI),

respectively). These values are only slightly higher than the

maximum amount of g form obtained by isothermal

crystallization from the melt, deduced from the maxima of

the curves of Fig. 6. These data clearly indicate that, as

already shown for metallocene-made iPP [25], the crystal-

lization of a and g forms of iPP depends on thermodynamic

and kinetic factors. The a form is always kinetically favored

over the g form; even in the case of samples or fractions of

iPP characterized by chains with relatively short regular

isotactic sequences (fraction ELPP(HsI-HS)), which

crystallize almost totally in the g form when slowly

crystallized from the melt by slow cooling the melt to

room temperature (profile a of Fig. 9(B)), the a form is

obtained when the crystallization is very fast by quench-

ing the melt to room temperature (profiles d, e of

Fig. 9(B)) or by isothermal crystallization at low

temperatures (profile b in Fig. 5(B)).


4. Conclusions

The crystallization properties of fractions of elastomeric

polypropylene prepared with Al2O3-supported tetraneophyl

zirconium catalyst has been analyzed and compared with

the behavior of iPP samples prepared with a single center

homogeneous metallocene catalyst.

The ELPP sample has been fractionated by extraction

with boiling solvents. The various fractions have been

crystallized from the melt and the polymorphic behavior has

been analyzed by X-ray diffraction. Appreciable amounts of

g form of iPP are obtained in more stereoirregular fractions.

In particular, the fraction insoluble in pentane and soluble in

hexane (ELPP(PI-HsS)) crystallizes from the melt almost

totally in the g form. The most stereoregular fraction

insoluble in n-heptane (ELPP(HI)) crystallizes mainly in the

a form.

The relative amount of g form crystallized from the melt

is, however, much lower that that observed in samples of

metallocene-made iPP samples, which contain an amount

of defects much lower than that present in some fractions of

the ELPP sample. This indicates that in these fractions the

regular isotactic sequences are longer than those present in

chains of metallocene-made iPP.

The different polymorphic behavior of metallocene iPP

and ELPP samples is related to the different distribution of

defects in the chains generated by the different kinds of

catalytic systems. While in the iPP samples prepared with

the single site metallocene catalyst the defects are randomly

distributed along the chains, in the fractions of the ELPP

sample the majority of the defects are segregated in a small

fraction of poorly crystallizable macromolecules or in more

irregular portions of the chain, so that much longer fully

isotactic sequences can be produced, leading to the crystal-

lization of the a form even for a relatively high overall

concentration of defects.

These data indicate that in the more irregular fractions of

the ELPP sample, ELPP(HsI-HS) and ELPP(PI-HsS)

fractions, the chains are characterized by a stereo-block

microstructure, consisting in the presence of quite regular

isotactic sequences alternating with more irregular

sequences. Most of the defects are segregated in the

irregular blocks.

The presence of stereoblocks allows crystallization of

these highly irregular fractions of the ELPP sample, even in

the presence of a very high content of defects, and accounts

for the elastomeric properties shown by this sample [4–6].

The structural analysis has also pointed out the main

difference between the elastomeric polypropylene ELPP

sample and the iPP samples prepared with the traditional

MgCl2-supported Ziegler–Natta catalysts. The ELPP

sample is mainly constituted by the more stereoirregular

fractions soluble in pentane and diethyl ether, and all

the fractions have high molecular weights. The stereirre-

gular fractions of the Ziegler–Natta iPP are generally minor

components of the whole sample and have, instead,

generally, lower molecular weights. Moreover, in the

ELPP samples, even the more stereoregular fractions are

characterized by chains having relatively short regular

isotactic sequences, compared to the very long regular

sequences in the chains of the Ziegler–Natta iPP samples

[27,32,36]. In particular for the most isotactic fraction

ELPP(HI) the distribution of defects in the polymer chains

is more uniform and approaches that of chains of

metallocene-made iPP.

The results reported in this paper confirm the idea

suggested by us in ref. 24 that the structural analysis of iPP,

in particular the crystallization of the g form, may give

information about the microstructure of the polymer chains,

and may be used as a practical tool to test the degree of

segregation of defects, revealing the presence of

stereoblocks.

Acknowledgements

Financial supports from the ‘Ministero dell’Istruzione,

Universita e Ricerca Scientifica’ of Italy (PRIN 2002 and

Cluster C26 projects) are gratefully acknowledged. We

thank Dr Luigi Resconi of Basell, Ferrara, Italy for

providing the metallocene-made polypropylene samples.

The NMR polymer characterizations were carried out at

Centro di Metodologie Chimico-Fisiche, University of

Naples ‘Federico II’.

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crystallization properties of elastomeric polypropylene from alumina-supported tetraalkyl zirconium...

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