Polybutadienes synthesized by various polymerization methods are widely used for the commercial manufacture of tires, rubber goods, sealants, paints, and adhesives [1, 2]. The structure of polybutadienes has been comprehensively studied in the literature and it has been shown that the polymer chain can consist of 1,4-cis-, 1,4-trans-, and 1,2-units [1–6]. Significantly less attention has been paid to studying the structure of the head and end units in polybutadiene. This is due to the difficulty of identifying relatively weak signals of carbon atoms of terminal units in the NMR spectra [5, 6]. However, such information is important because it allows one to understand the details of the butadiene polymerization mechanism and to predict the physicochemical properties of the polymer and possible methods for its modification.

The present work is devoted to the implementation of a new method for identifying the structure of the head and end units of polybutadiene, which is based on NMR experiments using T2-filter.

The selected sample of polybutadiene was synthesized by cationic polymerization of butadiene in methylene chloride using a TiCl4–CCl3COOH catalyst system under the following conditions: [С4Н6] = 2.0 mol/L, [TiCl4]  =  1  ×  10–2  mol/L,  [СCl3СOOH]  =  2  ×  10–2 mol/L, temperature 0°С, and reaction time 2 min. The yield of polybutadiene was 37.5 wt %, Mn = 7200 Da, Mw/Mn = 5.9, the unsaturation was 66 mol % of the theoretical. The 13С NMR spectra of the polymer in CDCl3 were recorded on a Bruker Avance III 400 spectrometer in the Center for Magnetic Resonance” Research Park, St. Petersburg State University, by the procedure described elsewhere [7]. In T2-filtered 13С NMR experiments, the CPMG pulse sequence was used, the echo was repeated 1522 times, the number of acquisitions was 10 200, and the spin-echo delay was 320 µs.

Figure 1 shows the 13С NMR spectrum and the T2-filtered 13С NMR spectrum of polybutadiene.

Fig. 1.
figure 1

Aliphatic region of the (a) 13C NMR spectrum (acquisition time 44.5 h) and (b) T2-filtered 13C NMR spectrum (acquisition time 15 h) of polybutadiene.

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The overview of the 13С NMR spectrum in Fig. 1a is typical of polybutadienes synthesized by cationic polymerization [2, 5]. The polymer chain of the studied polybutadiene sample consists mainly of 1,4-trans- units and some number of 1,2-units, which are distributed, as a rule, as single units located between 1,4-trans-units [5]. It should be noted that 1,4-cis-units are completely absent in the “cationic” polybutadiene [2, 5].

The 13C NMR spectrum of polybutadiene shows five pronounced signals (denoted by numbers 1–5 in Fig. 1a) arising from the carbon atoms of the 1,4-trans- and 1,2-units of the backbone in various combinations. Methylene carbon atoms C-1 and C-4 in the triad of 1,4-trans units

are responsible for dominant signal 4 at δ 32.6 ppm (Fig. 1a) [3–6]. For the 1,2-unit located between two 1,4-trans-units

methylene carbon atom C-1 and methine carbon atom C-2 give rise to signals 3 and 1 at δ 33.8 and 43.3 ppm, respectively (Fig. 1a). Signal 5 at δ 30.0 ppm (Fig. 1a) is due to methylene carbon atom C-4 of the 1,4-trans-unit connected to methylene carbon atom C-5 in the 1,2-unit [3–6]:

Signal 2 at δ 38.0 ppm (Fig. 1a) is assigned to methylene carbon atom C-4 in the 1,4-trans-unit, which is connected to methine carbon atom C-6 in the 1,2-unit [3–6]:

In addition to pronounced signals 1–5, the 13C NMR spectrum of polybutadiene shows a number of other weak signals (Fig. 1a). It should be noted that the identification of the structures responsible for these signals is significantly complicated due to the low intensity of the observed signals.

The use of T2-filter during NMR experiments led to a significant change in the pattern of the 13C NMR spectrum of polybutadiene (Fig. 1b). First, a dramatic increase in the intensity of the signal at δ 17.8 ppm is observed in the T2-filtered 13C NMR spectrum (Fig. 1b) as compared with the signal in the “standard” spectrum (Fig. 1a). Hereinafter, by the increased intensity of a signal, we mean the growth of its relative, rather than absolute, integrated intensity as compared to the signals of the units of the main polymer chain. According to [5], a signal at δ 17.8 ppm belongs to the methyl carbon atom in the head 1,4-trans-unit of the polybutadiene polymer chain (structure HI):

Methylene carbon atom HI/4 (hereinafter, the structures are denoted by Roman numerals and the carbon atoms in the structures, by Arabic numerals) in the head 1,4-trans-unit gives rise to a strong signal at δ 32.6 ppm (Fig. 1b).

Second, in the T2-filtered 13C NMR spectrum, the intensity of signals at δ 19.1 and 40.1 ppm assigned to the HII/1 and HII/2 carbon atoms in the head 1,2-unit increased significantly [5] (structure HII):

Third, there is a noticeable increase in intensities of the signals at δ 29.2, 30.7, and 47.0 ppm assigned to carbon atoms HIII/6, HIII/5, and HIII/1, respectively, in the head unit of the polymer chain of polybutadiene with a tert-butyl group [5] (structure HIII):

The head unit with the HIII structure is formed due to the presence of isobutylene impurities in the initial butadiene [5]. The signal of methylene carbon atom HIII/4 contributes to the intense signal at δ 32.6 ppm (Fig. 1b).

In addition, the T2-filtered spectrum reveals a significant increase in the intensities of signals of the end chlorine-containing units of the polymer chain of polybutadiene with 1,4-trans-(EI structure) and 1,2- structure (EII structure):

For the end chlorine-containing unit EI, the signal at δ 45.0 ppm is due to methylene carbon atom EI/4 [5], and the signal of methylene carbon atom EI/1 is part of the intense signal at δ 32.6 ppm. For the end chlorine-containing 1,2-unit (EII structure), one could unambiguously identify in the T2-filtered spectrum (Fig. 1b) signals at δ 37.5 and 61.9 ppm corresponding to carbon atoms EII/1 and EII/2, respectively [5] (Fig. 1b).

As can be seen from Fig. 1b, regardless of the structure of the terminal units, a significant increase in the signal intensities of all carbon atoms of the head and end polybutadiene units is observed in the aliphatic region of the T2-filtered 13C NMR spectrum. A similar increase in the signal intensities of the carbon atoms of the terminal units is also observed in the olefin region of the T2-filtered NMR spectrum of the polybutadiene sample.

It is known that the use of T2-filter in NMR experiments makes it possible to differentiate the signals of carbon atoms in fragments of a macromolecular chain of different mobility by suppressing the signals of rapidly relaxing carbon atoms in structures with relatively low mobility [8]. The polymer chain of the “cationic” polybutadiene contains a significant amount of branched and cross-linked supramolecular structures formed as a result of the reaction of the growing chain transfer to the double bond of “own” or “alien” macromolecule [2, 5, 7]. It is logical to assume that such polymer chains are characterized by reduced mobility; therefore, in the T2-filtered NMR spectra, the intense signals of rapidly relaxing carbon atoms in the units of the main polymer chain are suppressed. This makes it possible to significantly increase the sensitivity for weak signals of slower relaxing carbon atoms in the more mobile head and end units of the polymer; therefore, the relative intensity of such signals in the T2-filtered spectrum increases significantly.

Thus, NMR experiments using T2-filter during the study of the structure of polybutadiene polymer chains have significantly increased the intensity of the signals of carbon atoms in the terminal units. This greatly simplifies the task of identifying the structure of the head and end units in polybutadiene.