Application of successive self-nucleation and annealing (SSA) to poly(1-butene) prepared by Ziegler–Natta catalysts with different external donors

Abstract

Alkoxysilane compounds R₁R₂Si(OMe)₂ were used as external donors in 1-butene polymerization with MgCl₂-supported Ziegler–Natta catalysts. The structure of the prepared iPB was characterized by ¹³C NMR and GPC. The thermal properties of poly(1-butene) (iPB) were studied by DSC. The crystallization behavior and sequence length distribution of poly(1-butene) were investigated using successive self-nucleation and annealing (SSA) thermal fractionation technology. The SSA results indicated that steric hindrance of an external donor has more influence on the properties of iPB, as each melting point peak and the enthalpy of fusion of iPB gradually increased with an increase in steric hindrance of the external donor. Considering all properties, cyclopentyl-isopropyl-dimethoxysilane had an advantage compared to other external donors in the polymerization of 1-butene.

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Introduction

Isotactic poly(1-butene) (iPB) exhibits interesting physical and mechanical properties, such as heat resistance, creep degeneration resistance, good environmental stress cracking resistance, and high tenacity. Therefore iPB could be used as a pipe, film and sheet material, and especially as a material for hot water pipes.^1,2

Isotactic poly(1-butene) was first synthesized using Ziegler–Natta catalysts.^3,4 Although the catalyst used for preparing PE and PP was similar to that used for iPB, the technology of 1-butene polymerization was difficult to implement, which led to higher cost and limited its commercial development. An external donor (D_e) was one of the important parts of the catalyst system during 1-butene polymerization.¹ The external donor was the key point in obtaining an isotactic polymer with a higher degree of isotacticity⁵ and the desired physicochemical properties of the product. In recent years, researchers had much interest in alkoxysilanes as external donors for Ziegler–Natta catalysts. External donors had more influence on the microstructure of polypropylene.^6,7 Busico et al.⁸ proposed a three-site model to explain the effects of external donors on catalyst efficiency and polypropylene stereoregularity. In this model, successive adsorption of D_e on the catalyst changed the stereochemical environment of the active center, which could turn atactic centers into isotactic centers. This model could also apply to other polyolefins.

Successive self-nucleation and annealing (SSA)^9–11 was regarded as an effective technique for characterizing the microstructure of polypropylene. Our research group had studied isotactic sequence length and its distribution in PP prepared by a Ziegler–Natta catalyst with different alkoxysilanes as external donors using SSA,¹² which could be connected with many macro-properties such as mechanical, thermal and processing properties.

Although there are many reports on the influence of external donors on propylene polymerization and polypropylene, investigations of the effect of external donors on 1-butene polymerization and poly(1-butene) are few. Kudinova et al.¹³ reported that use of polydentate phosphine oxides as electron donors for TiCl₄–MgCl₂ catalysts could obtain highly isotactic poly(1-butene) and the isotactic index could reach 92.4% when the external donor was iso-AmP(O) (CH₂OMe)₂. Ren et al.¹⁴ studied diphenyl-dimethoxysilane and cyclohexyl-methyl-dimethoxysilane as external donors for TiCl₄–MgCl₂ catalysts. These external donors could improve the catalytic activity slightly and sharply increased the isotactic index, and cyclohexyl-methyl-dimethoxysilane has been found to be a more effective electron donor than diphenyl-dimethoxysilane.

In this study, poly(1-butene)s were prepared by a Ziegler–Natta catalyst with different external donors. The effects of the external donor on the molecular weight, molecular weight distribution and isotactic index of poly(1-butene) were investigated. Especially, the SSA technique was first used for studying the microstructure and properties of poly(1-butene).

Experimental

Materials

MgCl₂-supported Ziegler–Natta catalyst (SAL, Ti content of 2.5%), triethylaluminum (TEA) and 1-butene (polymerization grade) were provided by YanShan Petrochemical Co. Ltd. Cyclohexyl-methyl-dimethoxysilane (Donor-C), diisopropyl-dimethoxysilane (Donor-P), dicyclopentyl-dimethoxysilane (Donor-D), dicyclohexyl-dimethoxysilane (Donor-H) and cyclopentyl-trimethoxysilane (CPTMS) were provided by LuJing Chemical. Isobutyl-methyl-dimethoxysilane (Donor-MB), isopropyl-methyl-dimethoxysilane (Donor-MP), cyclopentyl-methyl-dimethoxysilane (Donor-MD), isobutyl-isopropyl-dimethoxysilane (Donor-PB) and cyclopentyl-isopropyl-dimethoxysilane (Donor-PD) (Fig. 1) were synthesized by our group.

Fig. 1 Structure of external donors with different substituent groups.

Polymerization of 1-butene

In a typical experiment, a 5 L stainless reactor equipped with a mechanical stirrer was degassed at 70 °C, and then the Ziegler–Natta catalyst (15 mg), TEA (7.92 mmol) in n-hexane solution and the alkoxysilane compound (Si/Ti = 20) were added. Then hydrogen (0.04 MPa) and 1-butene were charged into the system. The mixture was stirred at a stirring rate of 300 rpm. The autoclave reactor was heated to 40 °C and stirred for 60 min. Unreacted 1-butene was removed and the mixture subsequently dried under vacuum at 60 °C until the weight of the polymer was constant. Catalyst activity was determined in terms of the amount of produced PB (kg) per the amount of catalyst used (g).

Measurement

¹³C NMR spectra of polymers were recorded with a DMX 300M (Bruker). Polymer solutions were prepared with 80 mg of polymer in 0.5 ml deuterated o-dichlorobenzene at 373 K.

The molecular weights (M_n and M_w) and molecular weight distribution (MWD) of samples were determined by PL-GPC 220 high-temperature gel permeation chromatography at 413 K, using 1,2,4-trichlorobenzene as a solvent, and the flow rate was 1.0 ml min⁻¹. Calibration was achieved using polystyrene as the standard sample.

The isotacticity of the polymer obtained was determined by extracting the polymer with boiling ether in a Soxhlet extractor. The boiling-ether-insoluble fraction was a crystalline polymer and recognized as iPB. The weight percentage of ether-insoluble polymer in a whole sample was defined as the isotactic index (I.I.).

Differential scanning calorimetry (DSC) measurements were performed on a Q2000 instrument under a nitrogen atmosphere. The heating rate was 10 °C min⁻¹ in the range from 40 °C to 180 °C. The degree of crystallinity was calculated according to the following formula:

X_c = ΔH_m/ΔH⁰_m

where ΔH_m is the heat of fusion obtained from the DSC curve, ΔH⁰_m (62 J g⁻¹) is the heat of fusion of perfectly crystalline iPB where the crystal is II.¹⁵ Then, for the purpose of evaluating the microstructure of the iPB, self-nucleation experiments (SN)^9,16,17 and successive self-nucleation and annealing experiments (SSA)^17,18 were employed as follows:

Self-nucleation experiments (SN). The self-nucleation and annealing experiment using DSC was originally reported by Fillon et al.¹⁹ for isotactic polypropylene (PP).

The successive self-nucleation and annealing (SSA) protocol employed was very similar to that reported previously. The detailed procedure was as follows: (a) the sample was heated to 180 °C and maintained at such a temperature for 5 min to erase its previous thermal history; (b) the sample was cooled to 30 °C at 20 °C min⁻¹ to its initial “standard state”; (c) the sample was heated at 20 °C min⁻¹ to a selected self-seeding temperature (T_s) located in the final melting temperature range of the sample; (d) the sample was held at this T_s for 5 min. This isothermal treatment at T_s results in partial melting and, depending on the T_s, in the annealing of unmelted crystals, whereas some of the melted species may isothermally crystallize (after being self-nucleated by the unmelted crystals); (e) the sample was cooled from T_s to 30 °C at a rate of 10 °C min⁻¹, where the effects of SN would be revealed by the crystallization behavior of the sample; (f) steps (c), (d), and (e) were repeated at progressively lower T_s. The number of repetitions (cycles) can be chosen to cover the entire melting range of the sample with a “standard” thermal history or a shorter range; (f) finally, the sample was heated to 180 °C at 10 °C min⁻¹, where the effect of the entire SN and annealing treatment would also be revealed by the melting behavior of the sample.

Successive self-nucleation/annealing experiments. (a) The sample was heated to 180 °C and maintained at such a temperature for 5 min to erase its previous thermal history; (b) the sample was cooled to 30 °C at 20 °C min⁻¹ to its initial “standard state”; (c) the sample was heated at 20 °C min⁻¹ to T_s and maintained at that temperature for 5 min. The first applied T_s temperature was chosen so that the polymer would only self-nucleate (i.e. T_s would be high enough to melt all the crystalline regions except for small crystal fragments that can later self-seed the polymer during cooling); (d) the sample was cooled from T_s to 30 °C at a rate of 20 °C min⁻¹; (e) the sample was heated to a new T_s temperature which was 5 °C lower than the previous T_s and maintained at that temperature for 5 min; (f) steps (d) and (e) were repeated until the entire melting range of the original sample was covered; (g) finally, the sample was heated to 180 °C at a rate of 10 °C min⁻¹.

Results and discussion

The external donor exerted a strong effect on the catalyst activity, isotactic index (I.I.) and molecular weight, as well as the molecular weight distribution of iPB. The catalyst activity, molecular weight, molecular weight distribution and isotactic index (I.I.) of iPB were as shown in Table 1.

Table 1 Results of molecular weight and molecular weight distribution of iPB

Sample (or donor)^a	Activity (kg PB gcat^−1)	Mn^b (10^4 g mol^−1)	Mw^b (10^4 g mol^−1)	Mw/Mn^b	I.I.^c (%)
a Reaction temperature was 40 °C.b Gel permeation chromatography (GPC) measurements were performed using a PL-GPC 220; the solvent was 1,2,4-trichlorobenzene.c The isotactic index.
Donor-MP	20.4	13.9	46.5	3.36	97.8
Donor-MB	13.3	16.7	48.5	2.90	96.4
Donor-MD	17.7	16.7	53.8	3.22	97.0
Donor-C	9.8	16.1	63.6	3.96	97.0
Donor-P	20.0	32.4	77.2	2.38	97.6
Donor-PB	18.0	25.6	70.2	2.75	98.0
Donor-PD	17.1	27.0	79.1	2.93	98.0
Donor-D	7.3	29.7	92.1	3.09	97.0
Donor-H	3.3	17.0	60.5	3.55	96.2
Donor-B	11.7	25.4	73.5	2.90	97.2
CPTMS	4.5	27.0	81.1	3.01	95.4

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The catalyst activity was influenced by the structure of the external donors. The results were related to the size and number of alkoxy groups and the size of hydrocarbon groups of the alkoxysilane. For the polymerization results, the lowest catalyst activity was obtained with Donor-H and the highest catalyst activity was obtained with Donor-MP and Donor-P. As reported by Seppala et al.,^20,21 the most important factor for the catalyst activity and the properties of the obtained polymer was the number of alkoxy groups attached to the silicon atom, where more alkoxy groups led to effective deactivation of the active centers of the catalyst. Moreover, the size of the hydrocarbon group bonded to the silicon atom was a significant factor too. If the hydrocarbon group was the right size, the active centers were deactivated selectively and the isotactic index increased. When R₁ was methyl or isopropyl in R₁R₂Si(OMe)₂, with an increase in the R₂ size of the external donors, the isotactic degree of iPB showed an increasing trend. A generally accepted mechanism was that the alkoxysilane could complex with both the active site and the cocatalyst (TEA in this work). Bulky substituents on the alkoxysilane were required to prevent the external donor from leaving the catalyst surface through complexation with the cocatalyst;²² therefore, the isotactic degree of the obtained iPB was higher. However, when the steric hindrance of the substituent groups of the external donors was too large, complexation became weak; thus, the isotactic degree of iPB decreased.

The weight-average molecular weight (M_w) was influenced by the structure of the external donor. From Table 1, the M_w increased with increasing size of the hydrocarbon groups in the external donor, except for CPTMS. The M_w of iPB ranged from 46.5 × 10⁴ to 92.1 × 10⁴ g mol⁻¹ by changing the structure of the external donor. The change in molecular weight distribution was not obvious, which suggested the molecular weight distribution was correlated with the Ziegler–Natta catalyst, but not directly with the external donor.

Polymer characterization

Analysis of ¹³C NMR spectrum. The high-temperature ¹³C NMR spectrum of the iPB is shown in Fig. 2. There are four peaks in the aliphatic region. The peaks observed at δ = 40.3 and δ = 34.8 ppm were assigned to the main-chain carbons of iPB. The peak observed at δ = 27.8 ppm was assigned to the side-chain methylene carbon which was directly bonded to the main chain of the iPB; the remaining peaks belonged to the main chain of atactic PB. The peak observed at δ = 11.0 ppm was assigned to the methyl carbon in the ethylene side chain. Compared with iPB using Donor-PD, iPB using Donor-MB had a large amount of atactic PB (the side-chain methylene carbon which was directly bonded to the main chain of the atactic iPB was appeared at 26–27 ppm), which suggested the stereoregularity of poly(1-butene) using external donors with large substituent groups was greater than that of poly(1-butene) using external donors with small substituent groups. This phenomenon was the same as with the results of the isotactic index of iPB. According to the reports of Proto and Zhang,^23,24 the stereospecificity of the catalyst system was affected by the structure of external donors and it was found that an increase in the bulk of alkyl groups increased the isotacticity. However, because the stereoregularity of the obtained iPB was higher, the content of atactic PB was very low; thus, the intensity of the peak of the atactic structure was very weak and the instrumental resolution was lower. Therefore, the stereoregularity of iPB was difficult to accurately calculate from ¹³C NMR.

Fig. 2 ¹³C NMR spectra of iPB using different external donors.

Thermal and crystallization behaviors of iPB. The thermal and crystallization behaviors of iPB were studied by differential scanning calorimetry. The DSC results of poly(1-butene) are shown in Fig. 3 and Table 2. As can be seen from Fig. 3 and Table 2, the melting temperature and crystallization temperature of the prepared iPB showed a trend of increasing with an increase in steric hindrance of the substituent groups of the external donors. However, when the steric hindrance of the substituent groups of the external donors was too large, the melting point of the obtained iPB decreased to some extent with an increase in steric hindrance of the substituent groups of the external donors. For example, the melting temperature of iPB using Donor-H was 116.3 °C, which was lower than that of iPB using Donor-D and higher than that of iPB using Donor-C. When the external donor was Donor-D, the melting temperature reached 118.1 °C and the crystallization temperature reached 80.7 °C.

Fig. 3 DSC melting curves and crystallization curves of iPB using different external donors.

Table 2 Influences of external donors on the thermal performance of iPB

Sample (or donor)	Tm^a/°C	ΔHm^b/(J g^−1)	Xc^c%	Tc^d/°C	ΔHc^e/(J g^−1)	I.I.^f(%)
a Melting temperature as determined from the peak maximum value in the endothermic curve of DSC.b Value of the endothermic enthalpy of melting as determined by DSC.c Degree of crystallization calculated from the value of the endothermic enthalpy.d Crystallization temperature as determined from the exothermic curve of DSC.e Value of the endothermic enthalpy of crystallization as determined by DSC.f Isotactic index of poly(1-butene).
Donor-MP	112.6	32.3	52.1	68.4	32.3	97.8
Donor-MB	111.2	29.3	47.3	62.9	31.8	96.4
Donor-MD	113.6	27.6	44.5	65.6	32.0	97.0
Donor-C	115.7	32.4	52.3	73.5	35.0	97.0
Donor-P	117.1	36.0	58.1	76.8	38.6	97.6
Donor-PB	116.9	35.2	56.7	75.6	37.1	98.0
Donor-PD	118.0	32.6	52.7	78.5	34.9	98.0
Donor-D	118.1	32.8	52.9	80.7	35.8	97.8
Donor-H	116.3	32.0	51.6	73.3	34.8	96.2
Donor-B	116.1	35.0	56.4	73.4	36.9	97.2
CPTMS	110.9	26.8	43.3	65.6	29.9	95.4

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Fillon et al.¹⁹ studied the self-nucleation behavior of iPP during self-annealing procedures by DSC and the self-annealing temperature range was divided into three domains. In Domain I, or the “complete melting domain”, the crystallization temperature (T_c) upon cooling from T_s remained constant and no self-nucleation could be detected. Domain II or the “self-nucleation domain” occurred when heat treatment at T_s caused a shift in the crystallization temperature to higher temperatures with a decreasing self-nucleation temperature. Finally, in Domain III or the “self-nucleation and annealing domain”, annealing and self-nucleation took place simultaneously.¹⁰

Crystallization and melting curves during SN at different annealing temperatures using Donor-P as an example were studied, as shown in Fig. 4. Compared to the standard crystallization temperature obtained at a T_s of 132 °C, the crystallization temperature obtained at T_s values from 128 °C to 132 °C was unchanged, indicating that the nucleation density of the samples remained constant in Domain I and crystallization, nucleation, or aggregation of poly(1-butene) macromolecules did not occur.^11,25 When T_s was 126 °C and remained constant at 126 °C for 5 min, self-nucleation of poly(1-butene) occurred and its crystallization temperature shifted to a higher temperature. This was because the unmelted component of poly(1-butene) could induce self-nucleation in the process, which could reduce the energy barrier to crystallization. Fig. 4 shows that the melting peak increased markedly when the T_s was 118 °C, which, because it showed poly(1-butene) began self-nucleation and annealing, suggested that the sample remained in Domain III. When the annealing temperature was lower than 119 °C, the crystallization peaks became wider and the lamellae started to grow thick. The nucleation behavior with Donor-P illustrated that the minimum annealing temperature was 119 °C. The optimal annealing temperature T_s of each sample was tested as in the above self-nucleation experiments and the results are listed in Table 3.

Fig. 4 DSC crystallization and melting curves at different annealing temperatures.

Table 3 Domains and optimal range of T_s for each sample

Sample	Domain-I	Domain-II	Domain-III	Ts/°C
Donor-MP	Ts > 126	116 < Ts < 126	Ts ≤ 116	116
Donor-MB	Ts > 126	115 < Ts < 126	Ts ≤ 115	115
Donor-MD	Ts > 127	117 < Ts < 127	Ts ≤ 117	117
Donor-C	Ts > 127	119 < Ts < 127	Ts ≤ 119	119
Donor-P	Ts > 127	119 < Ts < 127	Ts ≤ 119	119
Donor-PB	Ts > 128	120 < Ts < 128	Ts ≤ 120	120
Donor-PD	Ts > 130	119 < Ts < 130	Ts ≤ 119	119
Donor-D	Ts > 128	120 < Ts < 128	Ts ≤ 120	120
Donor-H	Ts > 128	119 < Ts < 128	Ts ≤ 119	119
Donor-B	Ts > 128	119 < Ts < 128	Ts ≤ 119	119
CPTMS	Ts > 125	115 < Ts < 125	Ts ≤ 115	115

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Study of SSA experimental parameters

Effect of annealing time on self-nucleation behavior of poly(1-butene). DSC thermal history is the memory of the previous crystal structure of a molten polymer. It was reported that the thermal history memory could be reduced when the annealing time was increased.^19,26,27 Therefore, we only studied the effect of different annealing times upon the SN behavior of the sample with Donor-P when the annealing temperature was 119 °C. The results are shown in Fig. 5 and Table 4. When the annealing time t_s = 0, the crystallization temperature T_c (∼88.9 °C) was higher than the crystallization temperature for standard cooling. With an increase in annealing time, the crystallization temperature gradually decreased, but it was always higher than the crystallization temperature for standard cooling. The result revealed that an increasingly short annealing time could not effectively eliminate the relevant thermal history memory when the annealing temperature was 119 °C.

Fig. 5 Crystallization behavior in SN for different annealing times of sample with Donor-P.

Table 4 Crystallization behavior in SN for different annealing times of sample with Donor-P

ts/min	Tc/°C	ΔHc/J g^−1
Std-cooling	76.77	41.32
0	89.44	38.73
2	80.78	36.60
5	80.51	37.04
10	80.34	36.56
15	80.33	36.94

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Fig. 6 shows the effect of annealing time on the SSA melting curve of the sample with Donor-P. With an increase in annealing time, the separation of the melting peak became clear and the melting peak increased. When the annealing time was 10 min, it could not only effectively eliminate the effect of thermal history, but also the separation of the melting peak was better.

Fig. 6 SSA melting curves for different annealing times t_s of sample with Donor-P. DSC heating and cooling rates were 20 °C min⁻¹. The annealing temperatures of the five steps were 120 °C, 115 °C, 110 °C, 105 °C and 100 °C.

Effect of heating and cooling rates on SSA behavior of poly(1-butene). Pijpers et al.²⁸ have introduced high-speed calorimetry concepts that could be advantageously applied to thermal fractionation experiments. These concepts showed that an increment in heating rate is compensated by a reduction in sample mass. Based on the study of Lorenzo et al.,¹⁸ the weight of the DSC sample was selected as 3 mg. Fig. 7 shows SSA melting curves for two different heating and cooling rates of the sample with Donor-P at an annealing temperature T_s = 119 °C and an annealing time t_s = 10 min. As can be seen from Fig. 7, the thermal fractionation of the different heating and cooling rates (10 °C min⁻¹ and 20 °C min⁻¹) was very similar. For the low-temperature melting peak, the effect of thermal fractionation at 20 °C min⁻¹ was better than at 10 °C min⁻¹. A higher melting peak appeared for the two different heating and cooling rates, and the melting temperature (134.5 °C) at 20 °C min⁻¹ was slightly above the melting temperature (133.7 °C) at 10 °C min⁻¹. For poly(1-butene), a heating and cooling rate of 20 °C min⁻¹ could either complete SSA thermal fractionation or save testing time.

Fig. 7 SSA melting curves for different heating and cooling rates of sample with Donor-P.

Influence of annealing temperature interval on SSA experiments with poly(1-butene). In the SSA thermal fractionation process, the annealing temperature interval has a major impact on the thermal analysis of polymers. Müller et al.²⁵ reported that an appropriate annealing temperature interval was favorable for obtaining better thermal fractionation. Fig. 8 shows SSA melting curves for the width of the fractionation window of the sample with Donor-P when the annealing temperature was 119 °C, the annealing time was 10 min and the heating and cooling rates were 20 °C min⁻¹. When the annealing temperature interval was 3 °C, the short isotactic sequence of the chain segments during the annealing period was not crystallized, leading to incomplete separation. When the annealing temperature interval was 5 °C, the resolution significantly increased and the separation of the low-temperature component was clear. However, when the annealing temperature interval continued to increase, the resolution decreased. It was because the rate of crystallization was fast that the crystallization was incomplete.

Fig. 8 SSA melting curves for the width of the fractionation window of sample with Donor-P.

The above results show that the different parameters of SSA have an important influence on the SSA thermal fractionation process. We first determined the annealing temperature of different samples. In order to better analyze the SSA, we selected an annealing time of 10 min, an annealing temperature interval of 5 °C and heating and cooling rates of 20 °C min⁻¹ based on the results of the SN experiment.

Application of SSA to research the sequence length distribution of poly(1-butene) using different external donors. Accurate characterization of the sequence length distribution of poly(1-butene) could contribute to better understanding of the structure and properties of poly(1-butene). The crystallization behavior of poly(1-butene) was studied by SSA thermal fractionation technology. The melting curves of iPB after SSA treatment are shown in Fig. 9. Each melting point peak and the enthalpy of fusion ΔH_m are listed in Table 5. When R₁ was methyl in R₁R₂Si(OMe)₂, the enthalpy of fusion ΔH_m of poly(1-butene) rose from 36.7 J g⁻¹ to 44.8 J g⁻¹, which increased with the increase in the volume of the R₂ substituent on the alkoxysilane. For other poly(1-butene) samples, this phenomenon was in accordance with the above rules, except for poly(1-butene) with Donor-H. This might be because the right volume of hydrocarbon substituents on alkoxysilanes was conducive to coordination between alkoxysilane donors and the active center, but too bulky hydrocarbon substituents on alkoxysilanes could prevent coordination, so the catalyst activity was reduced and the content of ash in the poly(1-butene) increased, which led to the high enthalpy of fusion ΔH_m.

Fig. 9 SSA melting curves of poly(1-butene) and fitted curves of samples using Peakfit 4.12 software.

Table 5 SSA results of poly(1-butene) samples

Sample	I.I./%	Ts/°C	ΔHm/J g^−1	Tm1/°C	n1/%	Tm2/°C	n2/%	Tm3/°C	n3/%
Donor-C	97.0	119	44.8	134.0	2.0	120.5	78.6	115.4	19.5
Donor-MD	97.0	117	44.5	133.1	1.3	120.2	81.1	114.9	17.6
Donor-MP	97.8	116	40.6	132.1	2.1	119.1	86.2	113.7	11.7
Donor-MB	96.4	115	36.7			118.9	85.3	114.7	14.7
Donor-PD	98.0	119	47.0	133.8	3.4	121.9	81.0	117.9	15.7
Donor-P	97.6	119	43.7	134.1	2.4	122.0	67.0	117.3	30.5
Donor-PB	98.0	120	43.9	134.3	2.8	122.4	78.1	118.1	19.1
Donor-MP	97.8	116	40.6	132.1	2.1	119.1	86.2	113.7	11.7
Donor-D	97.8	120	46.2	134.0	2.7	122.1	81.0	117.6	16.4
Donor-PD	98.0	119	47.0	133.8	3.4	121.9	81.0	117.9	15.7
Donor-MD	97.0	117	44.5	133.1	1.3	120.2	81.1	114.9	17.6
CPTMS	95.4	115	37.5			118.1	84.9	112.6	15.1
Donor-P	97.6	119	43.7	134.1	2.4	122.0	67.0	117.3	30.5
Donor-D	97.8	120	46.2	133.9	2.7	122.1	81.0	117.6	16.4
Donor-H	96.2	119	48.0	133.5	1.3	122.1	74.3	117.6	24.4
Donor-B	97.2	119	46.1	134.0	1.8	122.4	74.8	117.6	23.4

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In order to quantitatively evaluate the variations in each peak, the isotactic sequence length and distribution of poly(1-butene) were calculated by Peakfit 4.12 software. Fig. 9 shows the SSA melting curves of poly(1-butene) and the fitted curves of samples using Peakfit 4.12 software. The SSA results and the relative contents of all peaks on the SSA curve of poly(1-butene) are presented in Table 5. It has been proved that the higher melting temperature on the SSA melting curves corresponded to the higher isotacticity and isotactic sequence length in the molecular chains.²⁹ As can be seen from Table 5, there were no peak 1 in the melting curves of poly(1-butene) prepared with Donor-MB or CPTMS, indicating that the highest isotactic sequence length of poly(1-butene) prepared with Donor-MB and CPTMS was lower than for that prepared with other donors. This might be because the smaller volumes of the substituents on Donor-MB and CPTMS caused the absence of the most highly isotactic active centers in the catalytic system with two donors. In the SSA melting curves of poly(1-butene) the main melting point peak of poly(1-butene) was peak 2, and when the substituent on the external donor was smaller the melting temperature was lower than 120 °C, such as with Donor-MP, Donor-MB and CPTMS. The rest of the peak 2 values of poly(1-butene) were higher than 120 °C and the melting temperature was closer, indicating that larger substituents were conducive to obtaining high-performance poly(1-butene). The SSA results show that with increasing steric hindrance of the external donor, the melting temperature of iPB exhibited an increasing trend. For example, when R₁ was methyl in R₁R₂Si(OMe)₂, with an increase in the hindrance of R₂ the melting temperature of peak 1 increased gradually from 132.1 °C to 134 °C and the melting temperatures of peak 2 and peak 3 increased, which showed that the sequence length of iPB gradually increased. When R₁ and R₂ were the same substituent, the melting temperature was similar, but the relative contents of all peaks on the SSA curve of poly(1-butene) were different. The SSA results for iPB using Donor-PD and Donor-D were similar, and better than other poly(1-butene) samples, but the isotactic index and catalyst activity of iPB using Donor-PD were superior to those of iPB using Donor-D. Therefore, Donor-PD had an advantage relative to other external donors in the polymerization of 1-butene.

The lamellar thickness can be estimated from the SSA results by the Thomson–Gibbs equation:^12,30

where T⁰_m = 409.25 K (equilibrium melting temperature), ΔH₀ = 1.35 × 10⁸ J m⁻³, σ = 17.1 × 10⁻³ J m⁻² (surface energy) and L_i is the lamellar thickness.³¹ The lamellar thickness of iPB after SSA thermal fractionation was calculated and is listed in Table 6. When R₁ was methyl in R₁R₂Si(OMe)₂, with increasing steric hindrance of R₂, the lamellar thickness of iPB gradually increased. However, when R₁ was isopropyl, the lamellar thickness of iPB using Donor-P, Donor-PD and Donor-PB was higher than the lamellar thickness of iPB using Donor-MP. The lamellar thickness of iPB prepared using Donor-PB was the highest (the lamellar thickness of peak 1 was 28.80 nm, the lamellar thickness of peak 2 was 3.78 nm, and the lamellar thickness of peak 3 was 2.88 nm). When R₁ and R₂ were the same substituent, the lamellar thicknesses of peak 2 and peak 3 were the same, but the lamellar thickness of peak 1 was different, and the highest lamellar thickness of peak 1 of iPB using Donor-P was 25.92 nm.

Table 6 Lamellar thicknesses of poly(1-butene) samples after SSA thermal fractionation

Sample	L1/nm	L2/nm	L3/nm
Donor-C	24.68	3.32	2.50
Donor-MD	17.28	3.26	2.45
Donor-MP	12.96	3.05	2.31
Donor-MB		3.01	2.42
Donor-PD	22.54	3.65	2.85
Donor-P	25.92	3.68	2.76
Donor-PB	28.80	3.78	2.88
Donor-MP	12.96	3.05	2.31
Donor-D	24.68	3.70	2.80
Donor-PD	22.54	3.65	2.85
Donor-MD	17.28	3.26	2.45
CPTMS		2.88	2.21
Donor-P	25.92	3.68	2.76
Donor-D	23.56	3.70	2.80
Donor-H	19.94	3.70	2.80
Donor-B	24.68	3.78	2.80

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Moreover, in order to extend the analysis, we introduced the terms “arithmetic average L_n”, “weighted average L_w”, and broadness index L_w/L_n:³²

where n_i is the normalized peak area and L_i is the lamellar thickness for each fraction. The results for all samples are listed in Table 7. As can be seen from Table 7, when R₁ was methyl in R₁R₂Si(OMe)₂, with increasing steric hindrance of R₂, L_n, n_w and I increased gradually. But when R₁ was isopropyl, the variation in L_n, n_w and I showed no regularity. It was found that iPB prepared using Donor-PB has the highest isotactic component and a shorter medium component relative to other iPB, but the amount of the medium component was higher, therefore the sequence length distribution was broader.

Table 7 Lamellar thickness statistical parameters of iPB samples prepared using different external donors

Sample	Lw/nm	Ln/nm	I = Lw/Ln
Donor-C	6.15	3.59	1.71
Donor-MD	4.11	3.30	1.25
Donor-MP	3.84	3.17	1.21
Donor-MB	2.94	2.92	1.01
Donor-PD	7.03	4.17	1.69
Donor-P	7.00	3.93	1.78
Donor-PB	8.35	4.31	1.94
Donor-MP	3.84	3.17	1.21
Donor-D	7.00	4.12	1.70
Donor-PD	7.03	4.17	1.69
Donor-MD	4.11	3.30	1.25
CPTMS	2.80	2.78	1.01
Donor-P	7.00	3.93	1.78
Donor-D	6.69	4.09	1.64
Donor-H	4.67	3.69	1.27
Donor-B	5.98	3.93	1.52

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Conclusions

In this study, poly(1-butene) was prepared by Ziegler–Natta catalysts using silane external donors. The influence of steric hindrance of the external donor on the structure of poly(1-butene) was studied by DSC and ¹³C NMR. The crystallization behavior and sequence length distribution of poly(1-butene) samples were studied by the successive self-nucleation and annealing calorimetric technique. It was found that the optimum annealing time t_s, annealing temperature interval and heating and cooling rates could enhance the separation of the different crystalline components. The results showed that iPB has higher isotacticity and stereoregularity by bulk polymerization. The SSA results showed the melting temperature of iPB increased with an increase in steric hindrance of the external donor; subtle differences in the temperature have a large impact on the lamellar thickness of iPB. When R₁ was methyl in R₁R₂Si(OMe)₂, L_n, n_w and I increased gradually with increasing steric hindrance of R₂. But when R₁ was isopropyl, the tendency of L_n, n_w and I showed no regularity.

Acknowledgements

The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (no. 51073170, 50703044, 51403216).

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