Crystallization of Metallocene-Made Isotactic Polypropylene: Disordered Modifications Intermediate...

Crystallization of Metallocene-Made Isotactic Polypropylene:Disordered Modifications Intermediate between the R and γ Forms

Finizia Auriemma* and Claudio De RosaDipartimento di Chimica, Universita di Napoli “Federico II”, Complesso Monte S. Angelo, Via Cintia,80126 Napoli, Italy

Received April 25, 2002; Revised Manuscript Received August 12, 2002

ABSTRACT: A detailed analysis of the structure and the thermal behavior of samples of isotacticpolypropylene (iPP) prepared with a highly regiospecific metallocene catalysts is presented. The samplespresent variable amounts of stereodefects (isolated rr triads) and provide the first example of metallocene-made iPP not including regioirregularities. They crystallize from the melt in mixtures of the R and γforms. The content of the γ form increases with increasing the crystallization temperature, reaches amaximum value, and then decreases for a further increase of the temperature. The crystallization of theγ form is favored by the presence of stereodefects. The thermal analysis has indicated that the crystalsof γ form melt at lower temperature than the crystals of R form obtained at the same crystallizationtemperature. A comparison between the experimental X-ray diffraction profiles of the melt-crystallizedsamples and the calculated profiles for models of structure has shown that the γ form obtained in thesesamples always presents structural disorder, which produces local situation of packing typical of the Rform. The amount of structural disorder decreases with increasing the crystallization temperature andthe degree of stereoregularity of the samples. The presence of disorder in the crystals of γ form accountsfor their lower melting temperature. This analysis indicates that metallocene isotactic polypropylenecrystallizes in a continuum of disordered modifications intermediate between the R and γ forms, and theamount of disorder present in the crystals depends on the crystallization conditions and on thestereoregularity of the sample.

Introduction

With the development of metallocene catalytic sys-tems for the isospecific polymerization of olefins, newisotactic polypropylenes (iPP) showing a wide varietyof microstructures have become available. Dependingon the specific metallocene complex used as catalyst,as well as on the polymerization conditions (i.e., mono-mer concentration, temperature, nature of cocatalyst,etc.), iPP samples containing different amounts andcombinations of defects of stereoregularity (primaryinsertions with the wrong enantioface, or stereodefects)and regioregularity (meso and racemo secondary inser-tions, or regiodefects) can be obtained.1

The polymorphic behavior of these iPP samples, and,in particular, the relative stability of the R and γ formsunder common conditions of crystallization, is com-pletely different from that of iPP samples prepared withheterogeneous catalysts and is closely related to theirmicrostructure.2-8

Commercial iPP, prepared with the traditional het-erogeneous Ziegler-Natta catalytic systems, generallycrystallizes in the stable R form.9 The γ form may beobtained only under special conditions, i.e., by crystal-lization from the melt at elevated pressures (about 5000atm)9,10 or by crystallization at atmospheric pressureof low molecular weight samples11 and of copolymerscontaining small amounts (in the range 5-20 mol %) ofother olefins.12

iPP samples prepared with homogeneous metallocenecatalysts crystallize more easily in the γ form, even atatmospheric pressure and for high molecular weightsamples.2-8 The different polymorphic behavior of iPPsamples prepared with heterogeneous and homogeneouscatalysts is related to the different type and distributionalong the chains of insertion mistakes, that is, stereo-defects and regiodefects, generated by the different

kinds of catalytic systems. The distribution of defects,in turn, influences the average length of the crystalli-zable (fully isotactic) sequences.

The studies published so far have contributed toclarify the factors that induce the crystallization of theR and γ forms.2-12 All the data indicate that when thefully isotactic sequences are very short, iPP crystallizesin the γ form, whereas very long regular isotacticsequences generally crystallize only in the R form. Inthe chains of iPP samples prepared with metallocenecatalysts the distribution of defects is random, and thelength of fully isotactic sequences is roughly inverselyrelated to the content of insertion errors. As a conse-quence, even a small amount of defects reduces thelength of the regular isotactic sequences and the γ formcrystallizes.2,5-8 In the case of iPP samples obtainedwith heterogeneous Ziegler-Natta catalytic systems,instead, the majority of the defects may be segregatedin a small fraction of poorly crystallizable macromol-ecules, so that much longer, fully isotactic sequences canbe produced, leading to the crystallization in the R formeven for a relatively high overall concentration ofdefects.9

It has been shown that for iPP samples prepared withmetallocene catalysts mixtures of the R and γ forms aregenerally obtained by isothermal crystallization fromthe melt,2,5,6 and the content of the γ form increases withincreasing content of defects.2,6,8 In the extensive workof Alamo et al.,2-4 the influence of the molecular weightand of the type and content of defects on the crystal-lization of the γ form has been investigated. Theformation of the γ form seems to be favored by thepresence of stereodefects (mainly rr isolated triads),2and/or regiodefects (mainly 2,1 and 3,1 insertions),2,5

and also by the presence of constitutional defects, likecomonomeric units.4,6

9057Macromolecules 2002, 35, 9057-9068

10.1021/ma020648r CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 10/17/2002

The kind of distribution of defects along the polymerchains also influences the crystallization of the γ form.It has been recently shown8 that in the case of stereo-block polypropylene, prepared with oscillating metal-locene catalysts,13-15 the amount of γ form whichdevelops in the melt-crystallization procedures is muchlower than that obtained for iPP samples having thesame overall concentration of defects, but prepared withstereorigid metallocene catalysts, which produce arandom distribution of defects.8 This has been explainedconsidering that in the stereoblock polypropylene mostof the defects are segregated in stereoirregular (atactic),noncrystallizable, blocks, which alternate to more regu-lar isotactic sequences, long enough to crystallize in theR form.8 This result confirms that the γ form crystallizeswhen the fully isotactic sequences are very short.2

Several issues related to the formation of the γ formare, however, still not completely clarified. For instance,Alamo and VanderHart3,4 have recently shown that iniPP samples from metallocene catalysts defects of ste-reoregularity, like isolated rr triads, regio 2,1 mesodefects,3 or even constitutional defects, like ethylenecomonomeric units in copolymers of iPP with lowamount of ethylene,4,6 are included in the crystallineregions. Since these samples crystallize in mixtures ofR and γ forms, it is not clear whether a preferentialsegregation of defects in the crystals of one of the twopolymorphic forms occurs. In other words, it is still notclear which crystalline form, R or γ, is able to toleratesuch defects at the lowest cost of energy and whetherthe defects, in turn, play a role in favoring the crystal-lization of one form over the other, besides the alreadymentioned effect of shortening the regular isotacticsequences.

Another important issue is the observation that theamount of γ form obtained by isothermal crystallizationsfrom the melt first increases with increasing the crys-tallization temperature (Tc), reaches a maximum at acritical value of Tc, and then decreases for higher Tc.2,6,8

This behavior seems to be quite general and does notdepend on the amount and kind of defects or on theirdegree of segregation. In fact, also in the case ofstereoblock polypropylene, the same result has beenfound.8 Alamo et al.2 have suggested that this behavioris related to the fact that the concentration of crystal-lizable sequences, i.e., those of length that equal orexceed the requirement for a nucleus of critical size,changes with the temperature. Moreover, the relativestability of the R and γ forms at different crystallizationtemperature, their stability in the presence of defects,and finally their different melting behavior, may playan important role. In ref 2 it has been also shown thatthe melting temperature of the crystals in the γ form isalways 5-10 °C lower than that of the crystals of the Rform. Since the crystal packing of the two forms is quiteequivalent,16 this may be due to the small size of the

crystals in the γ form and/or to the presence of highdegree of disorder in the γ crystals.

In this paper all these issues have been analyzedperforming a detailed study of the structure, the poly-morphic behavior, and the thermal behavior of differentsamples of iPP having different amounts of defects ofstereoregularity, prepared with a highly regiospecificmetallocene catalyst at different temperatures. Thefeatures in the X-ray diffraction profiles and the thermalbehavior of the various samples have been interpretedon the basis of a disordered model of packing for the γform, which implies the idea of crystallization of disor-dered modifications intermediate between the R and γforms. It is worth noting that metallocene iPP samplesanalyzed so far always include regio 2,1, eventually 3,1errors, besides the normal stereoirregularities (isolatedrr triads).2-6 Therefore, the samples here analyzedprovide the first example of metallocene-made iPP notincluding regioerrors. It is suggested that the easyinclusion of rr triads in the crystals3,7 possibly favorsthe formation of these disordered modifications in theγ domains. This analysis allows to establish a moreunified view of the crystallization of metallocene iPP.

Experimental SectionThree samples of iPP, R1, R2, and R3, were synthesized at

different temperatures using the single-center catalyst rac-isopropylidene[bis(3-trimethylsilylindenyl)]zirconium dichlo-ride, activated with methylaluminoxane (MAO), as describedin ref 17. This catalytic system is highly regiospecific andproduces relatively high molecular weight iPP samples (thepolydispersion index of the molecular masses is around 2) witha moderate stereoregularity and a random distribution ofdefects, mainly represented by isolated rr triads. The molec-ular weights, the melting temperatures, and the main micro-structural characteristics of the three samples are reportedin Table 1. The samples provide the first example of metal-locene-made iPP not including regioerrors.

The samples were isothermally crystallized from the meltat different temperatures. Compression-molded specimenswere melted at 180 °C and kept for 5 min at this temperaturein a N2 atmosphere; they were then rapidly cooled to thecrystallization temperature, Tc, and kept at this temperature,still in a N2 atmosphere, for a time tc long enough to allowcomplete crystallization at Tc. The samples were then rapidlycooled to room temperature and analyzed by wide-angle X-raydiffraction. In the various isothermal crystallizations, thecrystallization time tc is different depending on the crystal-lization temperature. The shortest time is 24 h for the lowestcrystallization temperatures and increases with increasing thecrystallization temperature, up to 2 weeks for the highestcrystallization temperatures.

X-ray powder diffraction patterns were obtained at roomtemperature with an automatic Philips diffractometer usingNi-filtered Cu KR radiation and at higher temperatures usingan attached Anton Paar TTK camera.

The calorimetric measurements were performed with adifferential scanning calorimeter (DSC) Perkin-Elmer DSC-7at different scan rates of 2.5, 10, 20, and 40 °C/min in a flowingN2 atmosphere.

Table 1. Polymerization Temperatures (Tpol), Concentrations of the Pentad [mmmm] and the Dyad [m], TriadsDistribution, Molecular Masses (Mw), Fractions of Stereoerrors (E), Average Lengths of Fully Isotactic Sequences (⟨Liso⟩),

and Melting Temperatures of the As-Prepared Powders (Tm) of the Three IPP Samples Prepared with the HighlyRegiospecific rac-isopropyliden[bis(3-trimethylsilyl)(indenyl)]zirconium/MAO Catalytic System17

samples Tpol (°C) [mmmm] (%) [m] (%) [mm] (%) [mr] (%) [rr] (%) Mw ε (%)⟨Liso⟩ (monomeric

units) Tm (°C)a

R1 20 89.0 95.5 93.3 4.4 2.3 110 000 2.3 43 144R2 50 87.4 94.8 92.1 5.2 2.6 75 000 2.6 38 141R3 60 83.4 93.1 89.7 6.9 3.4 66 000 3.4 29 137

a Melting temperature measured from DSC scans of the “as-prepared” samples (heating rate 10 °C/min).

9058 Auriemma and De Rosa Macromolecules, Vol. 35, No. 24, 2002

The relative amount of crystals in the γ form present in oursamples was measured from the X-ray diffraction profiles, assuggested by Turner-Jones et al.,18 by measuring the ratiobetween the intensities of the (117)γ reflection at 2θ ) 20.1°,typical of the γ form, and the (130)R reflection at 2θ ) 18.6°,typical of the R form: fγ ) I(117)γ/[I(130)R + I(117)γ]. Theintensities of (117)γ and (130)R reflections were measured fromthe area of the corresponding diffraction peaks above thediffuse halo in the X-ray powder diffraction profiles. Theamorphous halo has been obtained from the X-ray diffractionprofile of an atactic polypropylene, and then it was scaled andsubtracted from the X-ray diffraction profiles of the melt-crystallized samples.

Results and Discussion

X-ray Diffraction and Thermal Analysis. As al-ready reported in ref 8, the samples R1, R2, and R3 (Risamples) of iPP crystallize from the melt in a mixtureof crystals of the R and γ forms. The X-ray powderdiffraction profiles of the as-prepared samples Ri andof the samples isothermally crystallized from the meltat different temperatures, already reported in ref 8, areshown again in Figure 1. We recall that the R and γforms of iPP present very similar X-ray diffractionprofiles, the main difference being the position of thethird strong diffraction peak, which occurs at 2θ ) 18.6°((130)R reflection) in the R form19 and at 2θ ) 20.1°((117)γ reflection) in the γ form.20,21 The presence of thediffraction peak at 2θ ) 18.6° and the absence of thereflection at 2θ ) 20.1° in the X-ray diffraction patternsof the as-prepared R1 and R2 samples (profiles a inFigure 1, A and B, respectively) indicates that thesesamples are basically in the R form. The as-preparedpowder of the less stereoregular sample R3 is insteadcrystallized in a mixture of crystals of the two forms ofiPP, since both reflections of the R and γ forms arepresent in the X-ray diffraction profile (profile a inFigure 1C).

The diffraction patterns of the melt-crystallizedsamples present both (130)R and (117)γ reflections,indicating that the γ form develops by crystallizationfrom the melt. The relative intensity of the (117)γreflection of the γ form at 2θ ) 20.1°, with respect tothat of the peak of the R form at 2θ ) 18.6° ((130)Rreflection), increases with increasing the crystallizationtemperature, reaches a maximum value, and thendecreases for a further increase of the crystallizationtemperature. The relative amount of the γ form withrespect to the R form, fγ, for the various samples isreported in Figure 2 as a function of the crystallizationtemperature. The content of the γ form increases withincreasing the crystallization temperature and a maxi-mum amount is obtained, for each sample, at temper-atures in the range 120-130 °C. Similar curves havealready been reported and discussed in refs 2 and 6 fordifferent samples of iPP prepared with different met-allocene catalysts. As already observed in the litera-ture,2,6,8 the content of γ form increases with increasingthe concentration of defects. The crystallization of theγ form is favored in iPP samples characterized by shortregular isotactic sequences. The lower the degree ofisotacticity, the higher is the maximum amount of thecrystallized γ form (fγ(max)). In particular, fγ(max) ≈78% for the sample R1 ([mmmm] ) 89.0%) and 90% forthe less stereoregular samples R2 and R3 ([mmmm] )87.4 and 83.4%, respectively).

The DSC curves, recorded at heating rate of 10 °C/min, of the melt-crystallized samples R1, R2, and R3

are reported in Figures 3A, 4A, and 5A, respectively.All the samples show broad melting endotherms char-acterized by double peaks. The peak at lower temper-ature is present as a shoulder of the peak at highertemperature in the DSC scan of the samples crystallizedat low temperatures (for instance, at Tc ) 100 °C inFigures 3A and 4A and at Tc ) 90 °C in Figure 5A). Athigher crystallization temperatures the two peaks arewell separated and clearly evident. For the threesamples, the area of the peak at low temperatureincreases with increasing the crystallization tempera-ture, up to Tc ) 130 °C for samples R1 and R2 (curvesd in Figures 3A and 4A) and Tc ) 120 °C for sample R3(curve d in Figure 5A), and then it decreases for afurther increase of the crystallization temperature, asshown, for instance, for sample R1 in Figure 3A (curvee). Moreover, whereas the two peaks are separated bynearly 15 °C in the DSC scan of the samples crystallized

Figure 1. X-ray powder diffraction profiles of the as-preparediPP samples and of specimens of the same samples isother-mally crystallized form the melt at the indicated tempera-tures: (A) sample R1; (B) sample R2; (C) sample R3. The (130)Rand (117)γ reflections at 2θ ) 18.6° and 20.1°, respectively,typical of the R and γ forms of iPP, respectively, are alsoindicated.

Macromolecules, Vol. 35, No. 24, 2002 Crystallization of Isotactic Polypropylene 9059

at low temperatures (for instance at Tc ) 120 °C, inFigure 3A), they approach each other at higher crystal-

lization temperatures and are separated by less than10 °C in the DSC scans of the samples crystallized, forinstance at 140 °C (curve e in Figure 3A). This indicatesthat the materials crystallized at high crystallizationtemperatures melts in a more narrow range of temper-atures.

In the case of samples crystallized at highest tem-peratures (for instance at 140 °C for sample R1) anadditional broad endotherm is present at very lowtemperatures (in the range 120-130 °C) (curve e inFigure 3A). This is probably due to the melting of thematerial which does not crystallize at Tc, even for verylong crystallization times, but crystallizes during thequenching to room temperature. The contribution of thisendotherm to the global heat of melting is only 5-10%and cannot be further reduced by further increasing thecrystallization time.

The melting behavior of metallocene iPP samplescrystallized from the melt has been extensively studiedby Alamo et al.2 They have shown that the multiplepeaks present in the DSC scans are due to the melting

Figure 2. Content of the γ form of iPP, evaluated from theX-ray diffraction profiles, fγ, in the samples isothermallycrystallized from the melt, as a function of the crystallizationtemperature, Tc: (9) sample R1, [mmmm] ) 89%; (b) sampleR2, [mmmm] ) 87.4%; (2) sample R3, [mmmm] ) 83.4%.

Figure 3. DSC curves at 10 °C/min of the samples R1isothermally crystallized from the melt at the indicatedtemperatures (A). DSC curves at different heating rates ofsome samples R1 crystallized from the melt at the indicatedtemperatures (B).



of the two polymorphic forms of iPP.2 The γ form melts,generally, at lower temperatures and, in some samples,shows two different melting peaks, which have beenassociated with different crystals of γ form havingdifferent morphologies,2 also according to the atomicforce microscopy observation.5 These hypotheses couldbe valid also for our samples, which are characterizedby mixtures of crystals of R and γ forms (Figure 1), eventhough the double peak shape of the endotherms in theDSC scans of Figures 3-5 could also be due, at least inpart, to the occurrence of recrystallization phenomenaduring the heating. To analyze the possibility of recrys-tallization, DSC scans were performed at differentheating rates (parts B of Figures 3-5). It is apparentfrom Figures 3B, 4B, and 5B that in all the DSC scansthe relative areas of the two melting peaks do notchange significantly by changing the heating rate. Thisindicates that the two melting peaks are not the resultof the occurrence of recrystallization but are mostly dueto the melting of the two crystalline polymorphs of iPP,even though the occurrence of not relevant recrystalli-zation phenomena cannot be excluded.

This hypothesis has been confirmed by the X-raydiffraction patterns of the melt-crystallized samplesrecorded at different temperatures reported in Figure6. For some selected melt-crystallized samples, theX-ray diffraction profiles have been recorded at roomtemperature and at temperatures corresponding to theonset of the melting endotherm and to the two peaksin the corresponding DSC curve. According to the resultsof Alamo et al.,2 in all the analyzed samples, the X-raydiffraction profile remains unchanged from room tem-perature (profiles a in Figure 6) up to the temperaturecorresponding to the onset of the melting peaks (profilesb in Figure 6). In the X-ray diffraction patterns recordedat higher temperatures the intensities of the reflectionsof the γ form (mainly the (117)γ at 2θ ) 20.1°) decreaseprogressively over a broad range of temperatures (aslarge as 15 °C) (profiles c in Figure 6). The reflectionsof the γ form disappear at temperatures higher thanthe first melting peak in the DSC scan, and for tem-peratures corresponding to the maximum of the secondpeak, only reflections of the R form are generally presentin the X-ray diffraction patterns (profiles d in Figure6). These data clearly indicate that, as already shownby Alamo et al.2 for different iPP samples, the peak atlow temperature in the melting endotherm correspondsbasically to the melting of the γ form, while that at hightemperature corresponds to the melting of crystals ofthe R form.

In the case of the samples crystallized at the highesttemperatures, which present in the DSC scan an ad-ditional broad endotherm at low temperature, due tothe melting of a fraction of the material crystallizedduring the quenching to room temperature (for instance,curve e in Figures 3A), the X-ray diffraction profilerecorded at a temperature higher than that of this broadendotherm shows a reduction of the intensity of thereflection of the γ form. For instance, the case of thesample R1 crystallized at 140 °C is reported in Figure6B. The X-ray diffraction profile recorded at 140 °C,between the broad low-temperature endotherm and thehigh-temperature double peak shaped endotherm, showsa reduction of the intensity of the (117)γ at 2θ ) 20.1°(profile b in Figure 6B) with respect to that in the X-raydiffraction pattern at room temperature (profile a inFigure 6B). This indicates that the fraction of material,which does not crystallize isothermally at the crystal-lization temperature but crystallizes during the quench-ing to room temperature, is basically in the γ form.Moreover, as also shown by Alamo et al.,2 this demon-strates that the decrease of the content of γ form at highcrystallization temperatures in the curves of Figure 2is authentic and not an artifact due to the fact that afraction of the material is unable to crystallize at thehighest crystallization temperatures.

Once the two melting peaks in the DSC scans of thesamples crystallized from the melt have been attributedto the melting of the R and γ crystals, it is possible toevaluate the content of γ form in these samples fromthe relative area of the peaks in the DSC scans: fγ(DSC)) ∆Hγ/∆Ht, where ∆Hγ is the area of the peak at lowtemperature and ∆Ht is the total enthalpy of melting.The values of the content of γ form so evaluated arereported in Figure 7 as a function of the crystallizationtemperature. The same behavior as that of the curvesof Figure 2 is apparent. The content of γ form increaseswith increasing the crystallization temperature, reachesa maximum for Tc around 120-130 °C, and then



decreases for further increase of Tc. As in Figure 2, thecontent of γ form increases with decreasing the degreeof stereoregularity of the samples.

The crystallization of the Ri samples has also beenstudied in nonisothermal conditions. The X-ray diffrac-tion profiles of the sample R3 crystallized by cooling themelt at cooling rates of 40, 20, 10, 5, and 2.5 °C/minare reported in Figure 8 as an example. Also in this casethe samples crystallize in mixtures of R and γ forms.Fast cooling rates favor the crystallization of the R form,whereas for slow cooling rates the crystallization of theγ form is favored. This is in agreement with resultsreported in the literature for other iPP samples.22 Thecontent of γ form, evaluated from the X-ray diffractionpatterns of the kind shown in Figure 8, is reported in

Figure 9 as a function of the cooling rate for all threesamples. The extrapolation to zero cooling rate of thevalues of fγ gives the maximum amounts of γ form,fγ(max), which can be obtained for each sample in theseconditions. These values nearly coincide with the maxi-mum amount of γ form obtained by isothermal crystal-lization from the melt, deduced from the maxima of thecurves of Figure 2.

The results of the present analysis indicate thatsamples of iPP prepared with metallocene catalystscrystallize from the melt in mixtures of crystals of R andγ forms. The crystals of the two polymorphic forms meltat different temperatures: the γ form at lower temper-atures, the R form at higher temperatures. The meltingtemperature of both forms increases with increasing the

Figure 6. X-ray powder diffraction profiles of iPP samples isothermally crystallized form the melt recorded at 25 °C (a), at thetemperature corresponding to the onset of the melting endotherm (b), and at temperatures corresponding to the two peaks of themelting endotherm (c, d). The corresponding DSC scan recorded at 10 °C/min is also shown (e). (A) sample R1 crystallized at Tc) 120 °C; (B) sample R1 crystallized at Tc ) 140 °C; (C) sample R2 crystallized at Tc ) 100 °C; (D) sample R2 crystallized at Tc) 130 °C; (E) sample R3 crystallized at Tc ) 90 °C; (F) sample R3 crystallized at Tc ) 110 °C. The (130)R and (117)γ reflectionsat 2θ ) 18.6° and 20.1°, respectively, typical of the R and γ forms of iPP, respectively, are also indicated.


crystallization temperature and the degree of stereo-regularity. The γ form melts in a rather broad range oftemperature (about 10 °C), which decreases with in-creasing the crystallization temperature, whereas theR form melts in a range of only 3 °C. This is probablydue to the fact that the crystals of γ form present a highdegree of structural disorder,7 as we will discuss in thenext section, and/or include a large amount of defects.The crystals of the R form are, instead, probably moreperfect and also of larger dimension and melt at highertemperatures and in a narrower range of temperature.In the next section the crystal structure of these melt-crystallized iPP samples, and the possible presence ofstructural disorder, are analyzed in detail through thecomparison of the X-ray powder diffraction profiles ofFigure 1 with the diffraction profiles calculated formodels of structure which include mixtures of R and γforms and disorder in the regular packing of the chains.

Model of Disordered Structures and Method ofCalculation of the X-ray Diffraction Patterns

A comparison between the modes of packing of thechains in the R and γ forms is reported in Figure 10. Inboth forms chains in 3/1 helical conformations areorganized in double layers. In the R form (Figure 10B),the bilayers are piled along the crystallographic bR

direction with all chain axes parallel to each other.19

In the γ form (Figure 10A), the bilayers are piled alongthe cγ direction, and the chain axes in adjacent bilayersare nearly perpendicular to each other (tilted by 81°).20,21

In a recent paper we have shown that a low stereo-regular iPP sample crystallizes by stretching in disor-dered modifications of iPP.7 Pure R and γ forms werenever obtained, but only disordered modifications in-termediate between the R and γ forms, with structuresclose to the R or γ forms depending on the draw ratio,were obtained.7 The proposed model of disorder is shownin Figure 11. Consecutive bilayers of chains may faceeach other with the chain axes either parallel (like inthe R form) or nearly perpendicular (like in the γ form)(Figure 11). Two adjacent bilayers have chains paralleleach other with a probability pR or nearly perpendicularwith a probability pγ ) 1 - pR. In this model ofdisordered structure, ordered domains in the R or γforms are present inside the same crystal, giving riseto a mixture at molecular level of the R and γ forms(Figure 11). In the ref 7 we have shown that, differentdisordered modifications, containing different amountsof R-like or γ-like arrangements of the chains, dependingon the draw ratio, can be obtained.

Figure 7. Content of the γ form of iPP, evaluated from theDSC curves of Figure 3, fγ(DSC), in the samples isothermallycrystallized from the melt, as a function of the crystallizationtemperature Tc: (9) sample R1, [mmmm] ) 89%; (b) sampleR2, [mmmm] ) 87.4%; (2) sample R3, [mmmm] ) 83.4%.

Figure 8. X-ray powder diffraction profiles of sample R3crystallized form the melt by cooling to room temperature atthe indicated cooling rates. The (130)R and (117)γ reflectionsat 2θ ) 18.6° and 20.1°, respectively, typical of the R and γforms of iPP, respectively, are also indicated.

Figure 9. Content of the γ form of iPP, evaluated from theX-ray diffraction profiles of the kind shown in Figure 8, fγ, inthe Ri samples crystallized from the melt by cooling to roomtemperature at different cooling rates, as a function of thecooling rate. The maximum amounts of the γ form obtainedby extrapolation to zero cooling rate are also indicated. (9)sample R1, [mmmm] ) 89%; (b) sample R2, [mmmm] ) 87.4%;(2) sample R3, [mmmm] ) 83.4%.

Figure 10. Models of packing proposed for the γ (A) and theR (B) forms of iPP. The dashed horizontal lines delimit bilayersof chains. Subscripts R and γ identify unit cell parametersreferred to the monoclinic and orthorhombic unit cells of theR and γ forms, respectively. R and L identify rows of all right-and all left-handed helices, respectively.


We believe that the same kind of structural disordermay also be present in the samples isothermally crys-tallized from the melt reported in this paper, because asimple mixture of crystals of the pure R and γ formsdoes not account for the intensities of the reflections inthe X-ray powder diffraction patterns of Figure 1. Inthese patterns, indeed, the intensities of the (110)R (or(111)γ), (130)R, and (117)γ reflections at 2θ ) 14°, 18.6°,and 20.1°, respectively, are generally lower than thoseexpected by the diffraction of separate crystals of pureR and γ forms. In some patterns the (130)R and (117)γreflections are almost absent (for instance the patterna of Figure 1C), and only a diffuse scattering is insteadobserved in the corresponding 2θ range. The experi-mental observations cannot be explained by a mixtureof R and γ crystals. As shown in ref 7, the presence ofR/γ disorder in the crystalline domains, of the kind ofFigure 11, induces fluctuations in the relative intensitiesof the Bragg peaks and introduces some diffuse scat-tering. The diffuse scattering concentrates in verynarrow regions of the diffraction diagrams, i.e., mainlyaround 2θ ≈14° (reflections (110)R and (111)γ), in the2θ range 18-20° (where the reflections (130)R and (117)γoccur), and around 2θ ≈ 21.9° (reflections (1h31)R and(026)γ). Moreover, the intensities and the positions ofother strong reflections, such as the (040)R or (008)γ (2θ≈ 17°) and the (111)R or (202)γ (2θ ≈ 21.1°), are notaffected by the presence of disorder and/or by therelative amounts of the R and γ phases present in agiven sample. Therefore, the presence of this kind ofdisorder would reduce the intensities of the (110)R,(111)γ, (130)R, and (117)γ reflections, whereas the (040)Ror (008)γ and the (111)R or (202)γ reflections wouldremain strong and sharp, according to the experimentalobservations. We have performed a quantitative analy-sis through a direct comparison of the experimental

X-ray diffraction data with those calculated for suitablemodel structures.

We have calculated the X-ray powder diffractionprofiles for models of structure of the kind of Figure 11,including variable amounts of R/γ disorder, and com-pared the experimental X-ray diffraction pattern of thesamples crystallized from the melt at different temper-atures (Figure 1) to the calculated profiles.

The method of calculation has already been describedin the ref 7, in the case of oriented iPP samples. In thispaper, instead, we calculate the diffraction profiles ofunoriented powder samples; therefore, the calculatedintensity (Ic) is evaluated by integrating the intensityfunction I(s,F,æ) in the reciprocal space over surfaceswith radius equal to the modulus of the reciprocalscattering vector s, |s| ) 2 sin θ/λ, where F and æ arethe spherical polar reciprocal coordinates, F the anglebetween s and the z axis of a reference frame, and æthe angle between the projection of s in the a*-b* planeand the a* axis. The surface integral is given by theequation

The intensity function I(s,F,æ) is evaluated according toeq 2 of ref 7. With reference to the three latticedirections 1, 2, and 3 shown in Figure 11, we havedefined in ref 7 bilayers of kinds 1 or 2 as bilayers withchains having their axes along the directions 1 or 2,respectively (Figure 11). The conditional probabilitiespij (i, j ) 1, 2) were defined in ref 7 as the probabilitiesthat a bilayer of kind i is followed along the direction 3by a bilayer of kind j. At the interface of adjacentbilayers, local arrangements of the chains like in the Ror γ forms are obtained for i ) j or i * j, respectively.The regular stacking of bilayers of kind 1 (or 2) alongthe direction 3 gives rise to ordered domains with thechains packed like in the R form, all the chain axes beingparallel (p11 ) 1, or p22 ) 1). The regular alternation ofbilayers of kind 1 and 2, instead, gives rise to domainsordered like in the γ form of iPP (p12 ) p21 ) 1). Sincein this paper we calculate the powder diffraction pro-files, no preferred orientations of the chains are con-sidered; therefore, the distribution of the X-ray diffrac-tion intensity depends on a single parameter, i.e., therelative amount of R-like or γ-like arrangements ofadjacent bilayers, pR or pγ, respectively, with pR + pγ )1. Hence, with respect to ref 7, the calculation issimplified assuming pii ) pjj ) pR and pij ) pji ) pγ.

The structure factors of the bilayers of the kinds 1and 2, V1 and V2, respectively, are calculated using thefractional coordinates of the atoms in the bilayersdeduced from those reported in the ref 20. In thecalculations, the presence of up/down disorder is as-sumed;19,20 therefore, the structure factors V1 and V2are average values, evaluated assuming that each siteof the lattice is occupied by up and down chains withthe same probability. A Bernoullian distribution of thesize of the crystallites along the directions 1, 2, and 3is assumed,23 L1, L2, and L3 being the apparent averagelengths of the crystallites along the three directions 1,2, and 3, respectively.

According to the suggestions obtained from the ex-perimental X-ray diffraction and melting behavioranalyses, the calculations of the powder diffractionprofiles were performed in the hypothesis that in thesamples two crystalline phases are present, crystals in

Figure 11. Model of the R/γ disorder occurring in thedisordered modifications of iPP intermediate between the Rand γ forms. Consecutive aRcR (aγbγ) bilayers of chains arestacked along bR (cγ) with the chain axes either parallel ornearly perpendicular, making R-like or γ-like arrangementsof the bilayers. R and L identify rows of all right- and all left-handed chains, respectively. The three lattice directions 1, 2,and 3 are indicated.

Ic ) 14π∫0

πsin F dF ∫0

2πI(s,F,æ) dæ (1)


the pure R form and crystals in the γ form, the lattereventually containing different amounts of R/γ disorder.For the sake of simplicity, the apparent size of thecrystallites was assumed equal for the two crystallinephases. In particular, the value of L3 was deduced,applying the Scherrer’s formula, from the half-heightwidth of the (008)γ reflection (for the γ form) and (040)Rreflection (for the R form) at 2θ ) 17° in the experimen-tal X-ray powder diffraction profiles of Figure 1. Thevalues of L1 (assumed equal to L2 in all the calculations)have been deduced from the half-height width of theequatorial reflections, (110)R and (130)R for the R formand (111)γ and (117)γ for the γ form.

The above assumptions are quite reasonable andhighly consistent with the results of some kinetic,morphological, and SAXS studies on Ziegler-Natta iPPsamples melt crystallized at high pressures24,29 and onmetallocene-made iPP samples, melt crystallized atatmospheric pressure.2,5 During the crystallization fromthe melt, R and γ crystals form almost simultaneouslysince the early stages.2,25 The R and γ lamellae showthe helical stems inclined about 50° with respect to thelamella surface. Moreover, R and γ lamellae do not existas isolated, independent entities.24 In these systemsindeed, the γ lamellae have been observed to form stacksof lamellae branched off the R lamellae.30 Branchedlamellae and mother lamellae correspond to a samethickness and give rise to a variety of structures, i.e.,spherulites5,24,25,29 and elongated and bundle-like enti-ties.5 Therefore, as observed in ref 24, the stem lengths

of the R and γ phase obtained in the same conditions ofpressure and temperature are close to one another. Thiscorrespond to assume that the R and γ population ofcrystals have the same average dimensions along direc-tions 1 and 2 of Figure 11, coinciding with the cR andaR lattice directions for R crystals, respectively, to thediagonal lattice directions for the γ crystals.

Results of the Calculations

Some experimental X-ray powder diffraction profilesof Figure 1 of the samples Ri isothermally crystallizedfrom the melt at different temperatures are reportedin Figure 12 after the subtraction of the amorphous haloand the correction for the polarization factor. Thecalculated profiles, which give the best agreement withthe experimental profiles, are also shown in Figure 12.The X-ray diffraction profiles are calculated for mixturesof R and γ forms and for different amounts of R/γdisorder present in the crystals of the γ form. Eachcalculated profile is, therefore, the sum of the contribu-tion of the diffraction of crystals in the pure R form (alsoshown in Figure 12 as dashed profiles) and the contri-bution of the diffraction of disordered crystals of the γform, containing a suitable amount of R/γ disorder (likein the model of Figure 11). The amount of disorder isindicated by the values of the probability pγ. The valuesof the average size of the crystallites used in thecalculations are also indicated in Figure 12. For eachsample Ri, the size of the crystallites increases with

Figure 12. Comparison between some experimental X-ray powder diffraction profiles of Figure 1 of the iPP samples isothermallycrystallized from the melt at the indicated temperatures, after the subtraction of the amorphous halo and the correction for thepolarization factor (upper profiles), and calculated X-ray diffraction profiles, which give the best agreement with the experimentalprofiles (lower profiles). The X-ray diffraction profiles are calculated for mixtures of crystals in the pure R form and in defectiveγ form, containing different amounts of R/γ disorder (like in the model of Figure 11). For each calculated profile the contributionof the diffraction of crystals in the pure R form is also shown as dashed profile. The amount of disorder in the model of disorderedγ form (like the model of Figure 11) is indicated by the values of pγ. The fraction of γ form in the calculated profiles is alsoindicated as (fγ)c. The values of the average size of the crystallites L1, L2, and L3 used in the calculated profiles are also shown.(A) sample R1; (B) sample R2; (C) sample R3.


increasing the crystallization temperature, being in allcases of the order of 100-140 Å.

It is apparent from Figure 12 that, for all the samplescrystallized from the melt, disorder in the crystals ofthe γ form is almost always present. The amount ofstructural disorder in the γ form decreases with in-creasing the crystallization temperature; it correspondsto the presence of 20% of R-like arrangements of thechains in the samples crystallized at low temperatures(i.e., pγ ) 0.8, pR ) 1 - pγ ) 0.2 for the samples R1 andR2 crystallized at Tc ) 100 °C in parts A and B of Figure12, respectively, and for the sample R3 at Tc ) 90 °C inFigure 12C) and decreases to only 5% of R-like arrange-ments in the samples crystallized at higher tempera-tures (i.e., pγ ) 0.95, pR ) 1 - pγ ) 0.05 for the samplesR1 and R2 at Tc ) 120 °C in parts A and B of Figure12, respectively, and for the sample R3 at Tc ) 130 °Cin Figure 12C). It is worth noting that for the moststereoregular samples R1 and R2 the disorder in the γform is basically absent in the samples crystallized atthe highest temperature (pγ ) 1 at Tc ) 140 °C in Figure12A,B), whereas for the less stereoregular sample R3,disorder is always present even at the highest crystal-lization temperature (pγ ) 0.95 at Tc ) 130 °C in Figure12C). Moreover, it is also worth noting that the contri-bution of the crystals in the pure R form (dashed profilesin Figure 12) is very small (about 10%) for the lessstereoregular samples R3 (Figure 12C), whereas it ishigher for the most stereoregular samples R1 (Figure12A).

The presence of disorder in the γ form, shown by thecalculations, is in agreement with the experimentalobservations that the crystals of the γ form, obtainedby crystallization from the melt, melt at lower temper-atures and in a wider range than the crystals of the Rmodification formed at the same crystallization tem-perature (Figures 3-5). Moreover, the reduction of theamount of disorder with increasing the crystallizationtemperature (Figure 12) accounts for the larger increaseof the melting temperature of the γ form than that ofthe R form with increasing the crystallization temper-ature (Figures 3-5).

Since the calculated profiles of Figure 12, which fitthe experimental profiles of the melt-crystallized samples,are characterized by the sum of the contributions of theR and γ crystals, the calculated content of the γ formmay be evaluated from the ratio between the area ofthe calculated profile giving the contribution of the γform (Aγ) and the area of the whole calculated profile(A), Aγ/A. This parameter basically corresponds to thatused for the evaluation of the γ content in the experi-mental profiles, fγ, through the method suggested byTurner-Jones18 and described in the Experimental Part.Since in our model the crystals of the γ form containstructural R/γ disorder, which introduces some variableamount of R-like arrangements of the chains, thecontribution due to the diffraction of these disorderedcrystals (Aγ) does not give the correct amount of γ-likearrangements of the chains in the crystals. The realfraction of γ-like arrangements, (fγ)c, may be evaluatedby multiplying the contribution due to the diffractionof the disordered crystals of the γ form (Aγ) by the orderparameter pγ, (fγ)c ) pγAγ/A. The values of (fγ)c calculatedfor the three samples are reported in Figure 13 as afunction of the crystallization temperature. The calcu-lated curves of Figure 13 are similar to those of Figure2, evaluated from the experimental X-ray diffraction

patterns in the approximation of the method of Turner-Jones.18 The values of (fγ)c are slightly lower than thoseof fγ of Figure 2, indicating that the content of γ formevaluated with the method of Turner-Jones is an upperlimit of the effective γ-like arrangements present in oursamples.

It is worth mentioning that the presence of disorderedmodifications intermediate between the R and the γforms of iPP was already hypothesized by Campbell etal.24 on the basis of morphological observations andsmall-angle X-ray scattering (SAXS) experiments onZiegler-Natta iPP samples crystallized from the meltat elevated pressures. In ref 24 it was observed that thecoexistence and the concurrent growth of R and γlamellae at the same temperature and pressure occursince the early stages of the crystallization; small-angleexperiments, not showing any evidence for a bimodalpopulation of lamellar thickness, indicated that the Rand γ lamellae do not exist in identifiable separatestacks even at pressures where the amount of R crystalsis small. On the other hand, the observed lineardependence of the relative amount of γ form with thepressure induced the authors of ref 24 to suggest thehypothesis that both crystal forms are present in eachlamella, rather than existing as separate lamellae,giving rise to disordered modifications intermediatebetween the R and the γ form.

Conclusions

Samples of iPP prepared with metallocene catalystscrystallize from the melt in mixtures of crystals of R andγ forms. The content of the γ form increases withdecreasing degree of stereoregularity of the samples.The crystals of the γ form melt at temperatures lowerthan those of the R form crystallized at the sametemperature. The melting temperature of both formsincreases with increasing crystallization temperatureand degree of stereoregularity. The γ form melts in arather broad range of temperatures (about 10 °C), whichdecreases with increasing crystallization temperature.

The comparison between calculated and experimentalX-ray diffraction profiles has indicated that a simplemixture of crystals of R and γ forms does not accountfor the X-ray powder diffraction patterns. Crystals ofthe γ form obtained in these samples always presentstructural disorder, characterized by defects in theregular packing of bilayers of chains with axes orientedalternatively along two nearly perpendicular directions.

Figure 13. Calculated content of the γ form of iPP, evaluatedfrom the calculated X-ray diffraction profiles of Figure 12, (fγ)c,for the samples isothermally crystallized from the melt, as afunction of the crystallization temperature Tc. (9) sample R1,[mmmm] ) 89%; (b) sample R2, [mmmm] ) 87.4%; (2) sampleR3, [mmmm] ) 83.4%.


This disorder produces a local situation of packingtypical of the R form, with some adjacent bilayers havingparallel chains. The amount of structural disorderdecreases with increasing crystallization temperatureand degree of stereoregularity of the sample. Thepresence of disorder in the crystals of γ form accountsfor their lower melting temperature. The results of ourcalculations indicate that the length of the crystallitesalong the chain axes (i.e., L1 and L2), assumed equalfor both the populations of R and γ crystals, are in therange 100-140 Å and increase with the decrease of theamount of stereoirregularities and the increase ofcrystallization temperature. Considering that the heli-ces in the R and γ crystals produced in these systemsform an angle of about 50° with respect to the lamellasurface,4,25,26 the lamellar thickness of our samplescorresponds to 80-110 Å, in good agreement with theresults of recent studies on Ziegler-Natta iPP crystal-lized at elevated pressure.25,26,29

This analysis indicates that iPP prepared with met-allocene catalysts crystallizes in a continuum of disor-dered modifications intermediate between the R and γforms, the amount of disorder being dependent on thecrystallization conditions and on the stereoregularity ofthe sample.

The crystallinity of the analyzed samples is alwayshigh (60-65%) regardless of the crystallization temper-ature and the amount of defects. This is probably dueto the fact that some stereodefects can be included inthe crystalline phase. For instance, isolated rr triadsare defects possibly included in the crystals at a low costof conformational and packing energy, as shown in ref7 and demonstrated in ref 3 by accurate NMR experi-ments. Probably these defects are better tolerated in thecrystals of the γ form than in those of the R form, andas shown in ref 7, their inclusion in the crystals mayinduce, itself, the R/γ structural disorder.

On the basis of this complex polymorphic behavior ofiPP prepared with metallocene catalysts, the reason forthe presence of maxima in the curves of the content ofγ form as a function of the crystallization temperaturemay be explained as a result of two competing kineticand thermodynamic effects. For the present samples ofiPP, containing an appreciable amount of stereodefects,the formation of the γ form is thermodynamicallyfavored, since the rr defects (the only one present inthese samples) and the R/γ structural disorder arehighly tolerated in the γ form, less in the crystals ofthe R form. As a consequence, a high amount of γ formdevelops in the slow crystallizations at high tempera-tures. At lower crystallization temperatures the fastcrystallization of the R form is instead kineticallyfavored, giving a low amount of γ form. With increasingcrystallization temperature the amount of γ form in-creases, but at very high crystallization temperatures(higher than 130 °C) the crystallization of the defectiveγ form is too slow because of its lower melting temper-ature, and the more perfect R form becomes againkinetically favored, so that the amount of γ formdecreases.

Mezghani and Phillips reported important worksconcerning the phase diagram of iPP and the equilib-rium melting point of the γ phase.26 The two polymorphshave nearly the same thermodynamic stability, inagreement with the packing energy calculations of Ferroet al.16 In ref 26, the temperature of transition betweenthe R and γ form (in the limit of a system of infinite

chain length and free of defects, forming infinite dimen-sions crystals) was also evaluated, and the polymorphicbehavior of a commercial iPP sample, prepared viatraditional heterogeneous Ziegler-Natta catalytic sys-tems, was discussed in terms of the established phasediagram. In agreement with the thermodynamic predic-tions, the pure γ form can be obtained only at pressureshigher than 100 MPa and low undercooling, whereasfor pressures lower than 25 MPa, only the R form isobtained. In a large region of the phase diagram,however, i.e., at low pressure and high undercooling,for conditions where only the R form would be expectedto form, both forms are found to be present. Thesedeviations from the ideal were attributed to the hetero-geneous nature of Ziegler-Natta systems: in thesesystems, chains or a portion of chains with a highercontent of defects than the average fraction, favor theγ phase under conditions where the average moleculeproduce R crystals. Also, a phase transition between thetwo polymorphs has never been observed so far, prob-ably because of kinetic reasons.26 Metallocene-made iPPsamples represent a more homogeneous system fromthis point of view (not generally fractionable), yetheterogeneous since, in the limit of a random distribu-tion of errors, the distribution of the length of fullyisotactic sequence Liso is broad (⟨Liso

2⟩/⟨Liso⟩ ) 2) andbecomes progressively more flat the higher the averagevalue (⟨Liso⟩). Therefore, these systems generally producemixtures of R and γ crystals upon melt crystallizationprocedures for the same reasons stated for Ziegler-Natta iPP systems in ref 26; i.e., long stems of fullyisotactic sequences crystallize in the R form, and shorterones containing a major number of defects are unableto crystallize in the R modification at that temperatureand hence give the γ form. The R crystals produced bysuch short portions of chains would be, indeed, ther-modynamically unstable because of the too short lamel-lar thickness. Moreover, the thickness cannot increase,for instance, by inclusions of defects in the crystalsbecause the inclusion of (rr) defects in the R crystals isnot easy and not compatible with the maintaining ofthe parallelism of the helical chains.7 In other words,the short isotactic sequences crystallize in the γ formbecause in the γ crystals the inclusion of “punctual”defects, as for instance isolated rr triads,7 and “planar”defects, as for instance R/γ stacking fault disorder of thekind shown in Figure 11, is at a low cost of packing andconformational energy and entropically favored.

Acknowledgment. We thank Dr. L. Resconi ofBasell Poliolefine Italia for providing and selecting thethree iPP samples and for having stimulated this study.Financial support from the “Ministero dell’Universitae della Ricerca Scientifica e Tecnologica” (PRIN 2000and Cluster C26 projects) is gratefully acknowledged.

References and Notes

(1) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev.2000, 100, 1253.

(2) Alamo, R. G.; Kim, M. H.; Galante, M. J.; Isasi, J. R.;Mandelkern, L. Macromolecules 1999, 32, 4050.

(3) VanderHart, D. L.; Alamo, R. G.; Nyden, M. R.; Kim, M. H.;Mandelkern, L. Macromolecules 2000, 33, 6078.

(4) Alamo, R. G.; VanderHart, D. L.; Nyden, M. R.; Mandelkern,L. Macromolecules 2000, 33, 6094.

(5) Thomann, R.; Wang, C.; Kressler, J.; Mulhaupt, R. Macro-molecules 1996, 29, 8425.

(6) Thomann, R.; Semke, H.; Maier, R. D.; Thomann, Y.; Scher-ble, J.; Mulhaupt, R.; Kressler, J. Polymer 2001, 42, 4597.


(7) Auriemma, F.; De Rosa, C.; Boscato, T.; Corradini, P.Macromolecules 2001, 34, 4815.

(8) De Rosa, C.; Auriemma, F.; Circelli, T.; Waymouth, R. M.Macromolecules 2002, 35, 3622.

(9) Bruckner, S.; Meille, S. V.; Petraccone, V.; Pirozzi, B. Prog.Polym. Sci. 1991, 16, 361.

(10) Kardos, J. L.; Christiansen, W.; Baer, E. J. Polym. Sci. 1966,A-2 (4), 777. Pae, K. D.; Morrow, D. R.; Sauer, J. A. Nature(London) 1966, 211, 514. Pae, K. D. J. Polym. Sci. 1968, A-2(6), 657. Sauer, J. A.; Pae, K. D. J. Appl. Phys. 1968, 39, 4959.Morrow, D. R. J. Macromol. Sci., Phys. Ed. 1969, B3, 53.

(11) Lotz, B.; Graff, S.; Wittmann, J. C. J. Polym. Sci., Polym.Phys. 1986, 24, 2017. Kojima, M. J. Polym. Sci. 1967, 5, 245.Kojima, M. J. Polym. Sci. 1968, A-2 (6), 1255. Morrow, D.R.; Newman, B. A. J. Appl. Phys. 1968, 39, 4944.

(12) Turner-Jones, A. Polymer 1971, 12, 487.(13) Coates, G.; Waymouth, R. M. Science 1995, 267, 217.(14) Hu, Y.; Krejchi, M. T.; Shah, C. D.; Myers, C. L.; Waymouth,

R. M. Macromolecules 1998, 31, 6908.(15) Witte, P.; Lal, T. K.; Waymouth, R. M. Organometallics 1999,

18, 4147.(16) Ferro, D. R.; Bruckner, S.; Meille, S. V.; Ragazzi, M. Macro-

molecules 1992, 25, 523.(17) Resconi, L.; Piemontesi, F.; Camurati, I.; Sudmeijer, O.;

Ninfant’ev, I. E.; Ivchenko, K. V.; Kuz’mina, L. J. Am. Chem.

Soc. 1998, 120, 2308.(18) Turner-Jones, A.; Aizlewood, J. M.; Beckett, D. R. Makromol.

Chem. 1964, 75, 134.(19) Natta, G.; Corradini, P. Nuovo Cimento Suppl. 1960, 15, 40.(20) Bruckner, S.; Meille, S. V. Nature (London) 1989, 340, 455.(21) Meille, S. V.; Bruckner, S.; Porzio, W. Macromolecules 1990,

23, 4114.(22) Perez, E.; Zucchi, D.; Sacchi, M. C.; Forlini, F.; Bello, A.

Polymer 1998, 40, 675.(23) Allegra, G.; Bassi, I. W. Gazz. Chim. Ital. 1980, 110, 437.(24) Campbell, R. A.; Phillips, P. J.; Lin, J. S. Polymer 1993, 34,

4809.(25) Mezghani, K.; Phillips, P. J. Polymer 1997, 38, 5725.(26) Mezghani, K.; Phillips, P. J. Polymer 1998, 39, 3735.(27) Meille, S. V.; Phillips, P. J.; Mezghani, K.; Bruckner, S.

Macromolecules 1996, 29, 795.(28) Bruckner, S.; Phillips, P. J.; Mezghani, K.; Meille, S. V.

Macromol. Rapid Commun. 1997, 18, 1.(29) Angelloz, C.; Fulchiron, R.; Damillard, A.; Chabert, B.; Fillit,

R.; Vautrin, A.; David, L. Macromolecules 2000, 33, 4138.(30) Lotz, B.; Graff, S.; Straupe, C.; Wittmann, J. C. Polymer 1991,

32, 2902.

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