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1H NMR and 13C NMR Investigation ofMicrostructures of Carboxyl-TerminatedButadiene Acrylonitrile RubbersDongming Liu a , Jian Kang a , Pengyu Chen a , Xinpu Liu a & Ya Cao aa State Key Laboratory of Polymer Materials Engineering , PolymerResearch Institute of Sichuan University , Chengdu , People'sRepublic of ChinaAccepted author version posted online: 04 Jun 2012.Publishedonline: 19 Oct 2012.

To cite this article: Dongming Liu , Jian Kang , Pengyu Chen , Xinpu Liu & Ya Cao (2013)1H NMR and 13C NMR Investigation of Microstructures of Carboxyl-Terminated ButadieneAcrylonitrile Rubbers, Journal of Macromolecular Science, Part B: Physics, 52:1, 127-137, DOI:10.1080/00222348.2012.695622

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Journal of Macromolecular Science R©, Part B: Physics, 52:127–137, 2013Copyright © Taylor & Francis Group, LLCISSN: 0022-2348 print / 1525-609X onlineDOI: 10.1080/00222348.2012.695622

1H NMR and 13C NMR Investigationof Microstructures of Carboxyl-Terminated

Butadiene Acrylonitrile Rubbers

DONGMING LIU, JIAN KANG, PENGYU CHEN, XINPU LIU,AND YA CAO

State Key Laboratory of Polymer Materials Engineering, Polymer ResearchInstitute of Sichuan University, Chengdu, People’s Republic of China

Two carboxyl-terminated butadiene acrylonitrile (CTBN) copolymers, named Lan-1and Lan-2, were synthesized by solution radical polymerization. They had similarphysical properties with the commercial sample Hycar1300×13, but had higher vis-cosity. The results of 1H NMR and 13C NMR revealed that Hycar1300×13 had morebranched chains and more acrylonitrile-1,4 butadiene bonded units, but had loweracrylonitrile–acrylonitrile and acrylonitrile-1,2 butadiene bonded units, compared withLan-1 and Lan-2. This improved the flexibility of the molecular chains of Hycar1300×13,and correspondingly decreased its viscosity.

Keywords acrylonitrile, carboxyl-terminated butadiene, microstructure, NMR,polymer sequence, structure-properties relationship, viscosity

Introduction

Carboxyl-terminated butadiene acrylonitrile copolymers (CTBN), a kind of reactivetelechelic polymer, have been developed since the 1960s. They usually had relativelylow molecular weights, were called liquid rubber, and were widely used in tougheningthermosets without significant loss in other properties.[1–3] In most cases of CTBN tough-ening epoxy resins, the CTBN was initially dissolved into the epoxy resin; then, duringthe cure process, the rubber phase separated as a discrete particulate phase.[4–11] Morecarboxyl end-groups on CTBN would promote the formation of chemical bonds and acrosslinking network with the epoxy matrix, which resulted in an increase of toughness ofthe material.[12,13] Acrylonitrile units would help increase the compatibility between CTBNand epoxy. However, higher contents of carboxyl groups and acrylonitrile units lead toserious intrachain and interchain interactions, consequently, a higher viscosity,[14] whichwas unfavorable for CTBN to be added in the epoxy matrix.[15–17] It is thus a challengingproject to prepare CTBN with high contents of acrylonitrile units and carboxyl groups andlow viscosities, simultaneously. Generally, the viscosity of a bulk polymer is influencedby the flexibility of polymer chains, chain branching, and molecular weight.[18–20] Detailed

Received 10 January 2012; accepted 8 May 2012.Address correspondence to Ya Cao, State Key Laboratory of Polymer Materials Engineering,

Polymer Research Institute of Sichuan University, Chengdu 610065, People’s Republic of China.E-mail: caoya@scu.edu.cn

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128 D. Liu et al.

structure characterization of CTBN is crucial to improve the understanding of the relation-ship between microstructure and viscosity.

To characterize the microstructure of liquid rubber, infrared spectra,[21] Raman spec-tra,[22] 1H NMR, and 13C NMR[23–25] are often used. Among these methods, 1H NMR and13C NMR spectrum have been used for quantified analysis of molecular sequences of cis-1,4; trans-1, 4; and vinyl-1, 2 units in polybutadiene[25] and butadiene acrylonitrile copoly-mers.[26,27] Though the microstructure of liquid rubbers has been extensively studied, the re-lationship between the molecular structure and properties, such as viscosity, is not very clear.

In this paper, the microstructure of two laboratory synthesized CTBN samples, Lan-1,Lan-2, and a commercial sample Hycar1300×13, were characterized by 1H NMR and 13CNMR. Their contents of branches and 1,2-butadiene-bonded-1,4- butadiene, 1,2-butadiene-bonded-acrylonitrile, 1,2-butadiene-bonded-1,2-butadiene units were calculated. The influ-ence of these sequence contents, branches and molecular weight on the viscosity of CTBNwas investigated.

Experimental

Materials

Butadiene was supplied by Lanzhou Petroleum Chemical (China). Acrylonitrile and ethanolwere obtained from Kelong Chemical (China). The commercial sample, Hycar1300×13was the product of Emerald Chemical Co. Ltd. (USA).

Solution Polymerization of CTBN

Lan-1 and Lan-2 CTBN were synthesized by solution polymerization with ethanol as a sol-vent, and diacid acyl peroxide as an initiator. The monomer ratios of butadiene/acrylonitrilewere 75/25 for Lan-1 and 70/30 for Lan-2, respectively. The mixture of monomers, solventand initiators were introduced into a high-pressure autoclave reactor and heated to 110◦Cfor 4 h. The synthetic route of CTBN is presented in Scheme 1. The carboxyl groups ofCTBN were derived from the scission of the diacid initiators used in the polymerization,and were presumed to be located primarily at chain ends. After the polymerization, theautoclave was cooled to room temperature, and the mixture was dried in a vacuum oven for24 h at room temperature.

Characterization

The molecular weights and their distributions of the CTBN samples were determinedby Waters (USA) 515 gel permeation chromatography (GPC) using a series of threeultrastyragel columns HT2, HT3, and HT4. Polystyrene standards were used for calibration.

Scheme 1. Synthetic route for preparation of CTBN.

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Microstructures of CTBN Rubbers 129

The measurements were carried out at a tetrahydrofuran (THF) flow rate of 1 mL/min at35◦C, using Waters (USA) 2414 refractive index detection.

The acrylonitrilecontents of CTBN samples were determined by a Perkin–Elmer (USA)2400 element analyzer. The carboxyl group contents were determined by chemical titrationwith alcoholic potassium hydroxide.

The viscosities of the CTBN samples were characterized in a Bohlin Gemini 200rheometer (Malvern, UK) with PP40 parallel plate geometry at a gap size of 1000 µm. Thedata were recorded at 70◦C with shear rates from 0.01 to 100 per s.

The NMR spectra were recorded at room temperature with a Bruker (Switzerland) AVII spectrometer at 400 MHz for 1H NMR and 600 MHz for 13C NMR, respectively, withsolutions of samples in CDCl3. Quantitative 13C NMR analysis was performed with thespectra, which were recorded with inverse gated proton decoupling and long pulse delaysto provide long relaxations of the 13C nuclei. Under these conditions, the integral intensityof the 13C NMR signals is proportional to the number of carbon atoms of a certain type.

Results and Discussion

Basic Properties of CTBN

Figure 1 shows the GPC traces of the CTBN samples; the data of molecular weights are listedin Table 1. Obviously, Lan-1, Lan-2, and Hycar1300×13 have similar molecular weightand molecular weight distribution. However, they have totally different apparent viscosities(shown in Fig. 2). Hycar1300×13 had a much lower viscosity compared with Lan-1 andLan-2. Other properties, including carboxyl group contents determined by chemical titra-tion, and acrylonitrile contents measured by the element analyzer, are also listed in Table 1.Compared with Hycar1300×13, Lan-2 had higher molecular weight fractions (a high MWtail shown in the GPC trace), which would cause more molecular entanglement in the bulkpolymer, correspondingly, a higher viscosity. Compared with Lan-1, Hycar1300×13 hadsimilar carboxyl group content, narrower molecular weight distribution, higher molecularweight and acrylonitrile content, but much lower viscosity. Generally, higher molar weightand higher contents of acrylonitrile units in liquid rubber would lead to higher viscosity,[14]

which is contrary to the results of Hycar1300×13. This unusual phenomenon, we suggest,should be attributed to the different microstructures of Lan-1 and Hycar1300×13.

Figure 1. GPC traces of CTBN samples.

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Table 1Basic properties of CTBN

Samples Lan-1 Lan-2 Hycar1300×13

Appearance viscous darkyellow liquid

viscous darkyellow liquid

viscous lightyellow liquid

Mn 5600 7000 6200Mw 14000 22400 12400Mw

/Mn 2.5 3.2 2.0

Carboxyl content (mmol/g) 0.66 0.54 0.57Acrylonitrile content (%) 20.9 25.6 26.0viscosity (Pa.S, 70◦C) 40 53 13

Microstructure Characterization of CTBN

Figure 3 shows 1H NMR spectra of the CTBN samples. The overall spectra of the CTBNsamples were similar, while the integral intensities of each peak had some differences indetail, indicating that the microstructures of CTBN samples were different.

The proton spectra of the unsaturated and saturated units regions are enlarged in Figs. 4and 5, respectively. There are two isomeric units of butadiene, 1,2-butadiene units and 1,4-butadiene units, in CTBN samples. The C=C bonds distributions in the two isomericbutadiene units and relative mole fraction of 1,2-butadiene, 1,4-butadiene, acrylonitrileunits, and methyl groups in CTBN samples could be calculated; they are listed in Tables 2and 3, respectively. The content of acrylonitrile calculated from 1H NMR spectrum was inreasonable accordance with data determined by the element analyzer. There are signals ofmethyl groups in the 1H NMR spectra, and the methyl groups can only located at the endof molecular chains because there are no methyl groups in the reactants. The end groups ofbackbone chains are carboxylates. Therefore, it can be speculated that the CTBN molecules

Figure 2. Apparent viscosity of CTBN samples at 70 ◦C with shear rates from 0.01 to 100 per s.

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Figure 3. The 1H NMR spectra of Lan-1, Lan-2, and Hycar1300×13.

have some branches if they have methyl groups, and the branch content could be evaluatedby the content of methyl end-groups (listed in Table 3). The branches in CTBN are mainlydue to the chain transfer side reactions between living chain radicals and macromolecules.

Obviously, Hycar1300×13 had less 1,2-butadiene units but more branches than Lan-1or Lan-2 had. Compared with 1,4-butadiene units, the rigid vinyl groups in 1,2 -butadieneunits would decrease the flexibility of the molecular chains, and correspondingly wouldlead to higher viscosity of CTBN for the same molecular weight. The chain branchingwould increase the distance between molecular backbones, decrease the chain length, coildimension, and the interchain interactions; consequently, they would result in a lowerviscosity of a given CTBN. Therefore, Hycar1300×13 has a lower viscosity comparedwith Lan-1 or Lan-2, even though Hycar1300×13 has higher acrylonitrile content.

Figure 4. The 1H NMR enlarged spectra of CTBNs in 4.5–6.0 ppm.

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Figure 5. The 1H NMR enlarged spectra of CTBN in 0.5–3.0 ppm.

In Fig. 5, the area of peak d was less than half the area of peak g, these peaks beingattributed to two possible structures in the CTBN samples:

Scheme 2. Structures of acrylonitrile-bonded-1,2-butadiene and acrylonitrile-bonded-1,4-butadiene.

In formula (I), the chemical shifts of methylene protons (g) in acrylonitrile and methy-lene linked to acrylonitrile in 1,2-butadiene units were found at 1.6 ppm due to the lowinfluence of double bonds conjugation effect. Therefore, in Formula I, the amount of gprotons will be four times the amount of d protons (g = 4 d). However, in formula (II),

Table 2Distribution of C=C Bonds in units of 1,2-butadiene and 1,4-butadiene

C=C Bonds distribution

Samples 1,2-butadiene 1,4-butadiene

Hycar1300×13 15.20% 84.80%Lan-1 17.89% 82.11%Lan-2 16.92% 83.08%

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Table 3Relative mole fraction of some units in CTBN

Relative mole fraction

Units Hycar1300×13 Lan-1 Lan-2

CH CH2 10.14% 13.96% 12.21%CH CH 56.56% 64.01% 59.95%CN 29.19% 19.52% 25.64%CH3 4.11% 2.51% 2.20%

the chemical shifts of methylene protons (e) linked to acrylonitrile in 1.4-butadiene unitsare found at 2.0–2.3 ppm due to the large influence of double bonds conjugation effect.Therefore, in formula II, the g type protons will be twice the d protons (g = 2 d). Theacrylonitrile-bonded-acrylonitrile units could be neglected due to the low reactivity ofacrylonitrile monomer (the reactivity ratio of acrylonitrile and butadiene monomer arer[acrylonitrile] = 0.02 and r[butadiene] = 0.3, respectively) and relative low added contents ofacrylonitrile. Therefore, the relative contents of formula (I) and (II), presented in Table 4,could be used to evaluate the distribution of 1,2-butadiene and 1,4-butadiene units follow-ing acrylonitrile units in the molecular chains; they could be calculated by the followingequations:

X(I) =12A(g) − A(d)

A(d)

X(II) = 1 − X(I)

where X(I) and X(II) are the relative contents of formula (I) and formula (II), A is theintegral area of the corresponding peak.

Obviously, Hycar1300×13 had much less content of acrylonitrile-bonded-1,2- butadi-ene units (formula I) than other two CTBN samples. For the formula I, two rigid units bondtogether in the molecular chains, which would harm the molecular flexibility. Therefore,the small content of formula I, we suggest, should be the other reason for Hycar1300×13to maintain its low viscosity.

Figure 6 shows the 13C NMR spectra of the CTBN samples. The assignments ofthe main units sequences have been reported in the literature.[28] The relative contentsof 1,2-butadiene-bonded-acrylonitrile sequences and 1,2-butadiene-bonded-1,4-butadienesequences (corresponding peaks shown in Fig. 7) are listed in Table 5. The content ofeach sequence reflects the distribution of acrylonitrile or 1,4-budadiene units added after

Table 4Distribution of 1, 2-butadiene and 1, 4-butadiene followed acrylonitrile units

Samples Acrylonitrile-bonded-1,2-butadiene Acrylonitrile-bonded-1,4-butadiene

Hycar1300×13 1.74% 98.26%Lan-1 29.29% 70.71%Lan-2 19.57% 80.43%

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Table 5Distribution of 1, 4-butadiene and acrylonitrile followed 1, 2-butadiede units

Samples 1,2-butadiene-bonded-1,4-butadiene 1,2-butadiene-bonded-acrylonitrile

Hycar1300×13 50.0% 50.0%Lan-1 55.6% 44.4%Lan-2 49.5% 50.5%

Figure 6. The 13C NMR spectra of CTBN between 110 and 150 ppm.

Figure 7. The 13C NMR enlarged spectra of CTBN between 136 and 145 ppm.

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Table 6Distribution of acrylonitrile, 1, 2-butadiene and 1, 4-butadiene followed acrylonitrile units

acrylonitrile-bonded- acrylonitrile-bonded-1, acrylonitrile-bonded-1,Samples acrylonitrile 2-butadiene 4-butadiene

Hycar1300×13 0.8% 6.6% 92.6%Lan-1 1.0% 28.2% 70.8%Lan-2 1.4% 21.0% 77.6%

Figure 8. The 13C NMR enlarged spectra of CTBN between 113 and 124 ppm.

1,2-butadiene units in the molecular chain. The relative contents of 1,2-butadiene-bonded-acrylonitrile sequences for the three CTBN samples are similar. The contentsof acrylonitrile-bonded-1,2-butadiene sequences, acrylonitrile-bonded-1, 4-butadiene se-quences, and acrylonitrile-bonded-acrylonitrile sequences could be calculated from Fig. 8;they are listed in Table 6. The relative content of each sequence could be used to evaluatedthe distribution of 1,2-butadiene, 1,4-butadiene or acrylonitrile units added after acryloni-trile units in the molecular chains. Obviously, Hycar1300×13 had the lowest content ofacrylonitrile-bonded-1,2-butadiene sequences, which is in reasonable accordance with theconclusion obtained from 1H NMR spectrum. Therefore, the viscosity of Hycar1300×13was much lower than that of Lan-1 or Lan-2.

Conclusions

In this study, the relationship between viscosity and microstructure of CTBN was studiedin detail. Compared to Lan-1 or Lan-2, Hycar1300×13 had similar contents of carboxylgroups and acrylonitrile, but much lower viscosity.

The results of 1H NMR and 13C NMR showed that the contents of acrylonitrile-bonded-1,2-butadiene and acrylonitrile-bonded-acrylonitrile sequences in Hycar1300×13are much lower than that of Lan-1 and Lan-2, but the content of branches in Hycar1300×13was higher. All of these made Hycar1300×13 have much lower viscosity than Lan-1 andLan-2.

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Acknowledgment

This work was supported by the Program for New Century Excellent Talents in University(NCET-10-0562).

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