Preparation, Characterization of Propargyl Terminal Polybutadiene and Its Crosslinked Elastomers Properties

Propargyl terminal Polybutadiene (PTPB) was successfully prepared through hydroxyl terminal polybutadiene (HTPB) end-capping modification. The FTIR and 13C NMR results indicated that the HTPB terminal hydroxyl was thoroughly replaced and yielded the target product, PTPB, with a theoretical propargyl content of 0.66 mmol g−1. In comparison with HTPB, PTPB has a lower viscosity. Using 1,6-diazide hexane as a curing agent, polytriazole crosslinked polybutadiene (PTriPB) elastomers with various functional molar ratios (R) were prepared by CuAAC reaction, and the glass transition temperatures of the resultant PTriPB elastomers were approximately −75 °C, measured by differential scanning calorimetry (DSC), nearly independent of elastomer R values. Mechanical tests indicated, that with the increase in R, the mechanical properties of PTriPB elastomers exhibit a parabolic dependence on R. In addition, the thermal stability of PTriPB elastomers were also studied. The findings revealed some fundamental features of polytriazole crosslinking elastomer prepared by CuAAC reaction.


Introduction
Polybutadiene is a very important liquid rubber because of its hydrolytic resistance and higher flexibility at low temperatures, raised from its segment hydrophobic character and low glass transition temperature; it has been used in a wide array of applications, such as for sealants, binders, adhesives, waterproof and anticorrosion coatings, foams, electrical insulation, elastomers, etc. [1]. However, up until now, most crosslinking reactions of polybutadienes still focus on the polyurethane reaction between hydroxyl terminal polybutadiene (HTPB) and isocyanate compounds [2,3]. However, the reaction typically comes with some disadvantages, such as the sensitivity to water and stringent reaction conditions, which easily raise defective network structures and adversely affect their mechanical properties [4].
The cuprous-catalyzed azide-alkyne Huisgen [3+2] dipolar cycloaddition (CuAAC) reaction is the most appealing click chemistry reaction for generating 1,2,3-triazole compounds with the advantages of high efficiency, regioselectivity, mild reaction conditions, no side reaction, insensitive to water, etc., and it has become a versatile synthetic tool [5][6][7][8]. However, because common cuprous compounds are hardly soluble in the organic solvents, and easily lose catalytic efficiency due to their oxidation or disproportion reaction [9], CuAAC reaction is usually carried out under a nitrogen atmosphere, in a solution in order to make cuprous catalyst stable and uniformly dispersed. For instance, Liu dissolved azide-functional polystyrene and 3,5-Bis(propargyloxy)benzyl alcohol in toluene solvent and used the

Materials
The prepolymer used was hydroxyl terminal polybutadiene (HTPB, hydroxyl value 0.68 mmol g −1 ; molecular weight~6500 g mol −1 by gel permeation chromatography) and provided by Liming Research Institute of Chemical Industry. All other agents were purchased and directly utilized without further explication.

Synthesis of 1,6-diazide hexane
Referring to [16], NaN 3 (1.0 g, 15.0 mmol) was added into a solution of the dichlorohexane (10.0 mmol) in DMF (15.0 mL). The mixture reacted at 60 • C for 10 h, followed by adding distilled water (100.0 mL). The resultant mixture was extracted with ether (3 × 10 mL), and the organic layer was combined and washed three times with distilled water (3 × 10 mL). The solvent ether was removed, and the compound was achieved, whose azide content is 11.9 mmol g −1 .

Preparation of PTPB
Into a three-neck flask fitted with a thermometer, a condenser, a mechanical stirring and nitrogen inlet, 12.5 mmol potassium t-butoxide and 100 mL tetrahydrofuran (THF) were added. After stirring for 30 min at 0 • C, HTPB (14.7 g, 10 mmol hydroxyl) dissolved in 140 mL n-heptane was added and reacted for 3 h. Then, 20 mmol propargyl bromide was added dropwise. After adding propargyl bromide, the resultant mixture was warmed to 30 • C and reacted for another 2 days. The mixture obtained was successively washed twice with 2 × 150 mL brine. The organic layer was separated and evaporated under reduced pressure, to obtain the target product, propargyl terminal polybutadiene (PTPB), at a yield of approximately 14 g. The synthesis route for PTPB is shown in Scheme 1.

Preparation of PTriPB Elastomers
The compositions of polytriazole crosslinked polybutadiene (PTriPB) elastomer are listed in Table 1. All components were mixed uniformly, degassed under vacuum and cured at 50 °C till the characteristic peaks of azide or alkynyl were thoroughly disappeared in virtue of FTIR detection, and PTriPB elastomers crosslinked by bulk CuAAC reaction were obtained. R is the mole ratio of the curing agent 1,6-diazide hexane functional group to the PTPB functional group, and the catalyst used was a copper(I) hexafluoroacetylacetonate cyclooctadiene complex.

Instrumentation
Infrared spectra were recorded on a Nicolet 6700 infrared spectrometer (Waltham, MA, USA). Then, 13 C-NMR spectra were recorded on a Bruker 600 spectrometer (Switzerland), using gated decoupling, a spectrum width of 20 KHz, a recycle delay of 25 s, a 90° pulse and >500 scans. Chemical shifts were reported in ppm, relative to tetramethylsilane (TMS). The molecular weights of HTPB and PTPB were determined by using a WATERS1515 gel permeation chromatograph (Milford, MA, USA), solution concentration 0.1 mg mL −1 , column temperature 40 °C, flux 1 mL min −1 . Viscosity measurements were performed by using a rheostress 300 rheometer (Karlsruhe, Germany) with a 20 mm parallel plate geometry, a gap size 1 mm, a shear rate range 1~100 s −1 , and collected once every 2 s for 60 s. The glass transition temperatures of PTriPB elastomers were tested by differential scanning calorimetry (DSC, 204 F1, Selb, Germany). All the experiments were carried out in a dry nitrogen atmosphere. The temperature ranged from −110 to 60 °C, with a heating rate of 10 K min −1 . At room temperature, the mechanical properties of PTriPB elastomers (specification 100 mm × 20 mm × 5 mm) were measured on an Instron 6022 mechanical tester (Boston, MA, USA), at a crosshead speed of 100 mm min −1 . The thermal gravimetric analysis (TGA) was performed on a Netzsch 209 F1 thermal analyzer (Selb, Germany), under nitrogen atmosphere, at a heating rate of 10 °C min −1 , from 40 to 800 °C. The samples of 10 mg were used, and the gas flow rate was 60 mL min −1 .

PTPB Structure
The FTIR spectra for HTPB and PTPB are shown in Figure 1. HTPB shows major characteristic vibrations, such as C-H stretching vibration at 3007, 2913, and 2848 cm −1 ; C=C stretching vibration at 1641 cm −1 ; and C-H bending vibration at 1431, 961, and 911 cm −1 . As for O-H stretching vibration peak, it is not easily distinguished because of its lower content within HTPB. These are in good agreement with the report [17]. However, in comparison with HTPB, PTPB presents characteristic peaks of propargyl group at 3306 and 627 cm −1 because of the terminal hydroxyl of HTPB having been etherified, in addition to other characteristic peaks of polybutadiene.

Preparation of PT ri PB Elastomers
The compositions of polytriazole crosslinked polybutadiene (PT ri PB) elastomer are listed in Table 1. All components were mixed uniformly, degassed under vacuum and cured at 50 • C till the characteristic peaks of azide or alkynyl were thoroughly disappeared in virtue of FTIR detection, and PT ri PB elastomers crosslinked by bulk CuAAC reaction were obtained. R is the mole ratio of the curing agent 1,6-diazide hexane functional group to the PTPB functional group, and the catalyst used was a copper(I) hexafluoroacetylacetonate cyclooctadiene complex.

Instrumentation
Infrared spectra were recorded on a Nicolet 6700 infrared spectrometer (Waltham, MA, USA). Then, 13 C-NMR spectra were recorded on a Bruker 600 spectrometer (Switzerland), using gated decoupling, a spectrum width of 20 KHz, a recycle delay of 25 s, a 90 • pulse and >500 scans. Chemical shifts were reported in ppm, relative to tetramethylsilane (TMS). The molecular weights of HTPB and PTPB were determined by using a WATERS1515 gel permeation chromatograph (Milford, MA, USA), solution concentration 0.1 mg mL −1 , column temperature 40 • C, flux 1 mL min −1 . Viscosity measurements were performed by using a rheostress 300 rheometer (Karlsruhe, Germany) with a 20 mm parallel plate geometry, a gap size 1 mm, a shear rate range 1~100 s −1 , and collected once every 2 s for 60 s. The glass transition temperatures of PT ri PB elastomers were tested by differential scanning calorimetry (DSC, 204 F1, Selb, Germany). All the experiments were carried out in a dry nitrogen atmosphere. The temperature ranged from −110 to 60 • C, with a heating rate of 10 K min −1 . At room temperature, the mechanical properties of PT ri PB elastomers (specification 100 mm × 20 mm × 5 mm) were measured on an Instron 6022 mechanical tester (Boston, MA, USA), at a crosshead speed of 100 mm min −1 . The thermal gravimetric analysis (TGA) was performed on a Netzsch 209 F1 thermal analyzer (Selb, Germany), under nitrogen atmosphere, at a heating rate of 10 • C min −1 , from 40 to 800 • C. The samples of 10 mg were used, and the gas flow rate was 60 mL min −1 .

PTPB Structure
The FTIR spectra for HTPB and PTPB are shown in Figure 1. HTPB shows major characteristic vibrations, such as C-H stretching vibration at 3007, 2913, and 2848 cm −1 ; C=C stretching vibration at 1641 cm −1 ; and C-H bending vibration at 1431, 961, and 911 cm −1 . As for O-H stretching vibration peak, it is not easily distinguished because of its lower content within HTPB. These are in good agreement with the report [17]. However, in comparison with HTPB, PTPB presents characteristic peaks of propargyl group at 3306 and 627 cm −1 because of the terminal hydroxyl of HTPB having been etherified, in addition to other characteristic peaks of polybutadiene. The 13 C NMR spectra for HTPB and PTPB are shown in Figure 2. Polybutadiene is comprised of the cis/trans-1,4 enchainment, vinyl-1,2 enchainment structures, so it is clear that the peaks at 27.5 and 129.5 ppm are attributed to cis-1,4 enchainment structure, and the peaks at 32.9 and 130.1 ppm are attributed to trans-1,4 enchainment structure. Meanwhile the peaks at 38.3, 43.6, 142.8, and 114.5 ppm are attributed to vinyl-1,2 enchainment structures [18,19]. It should be emphasized that the small peaks at 65.2, 63.8, and 58.6 ppm originate from carbon atoms connected to terminal hydroxyl shown in Figure 2. In the 13 C NMR spectra of PTPB, the typical peaks attributed to polybutadiene backbone still exist at same locations. However, the peaks at 65.2, 63.8, and 58.6 ppm presented in 13 C NMR spectra of HTPB thoroughly disappear in that of PTPB. Instead, the peaks attributed to the propargyl carbon atom 12, 13, and 14 emerge at 58.3, 74.3, and 79.9 ppm, respectively. These suggest that the terminal hydroxyl of HTPB has been thoroughly etherified, having yielded the target product PTPB. The 13 C NMR spectra for HTPB and PTPB are shown in Figure 2. Polybutadiene is comprised of the cis/trans-1,4 enchainment, vinyl-1,2 enchainment structures, so it is clear that the peaks at 27.5 and 129.5 ppm are attributed to cis-1,4 enchainment structure, and the peaks at 32.9 and 130.1 ppm are attributed to trans-1,4 enchainment structure. Meanwhile the peaks at 38.3, 43.6, 142.8, and 114.5 ppm are attributed to vinyl-1,2 enchainment structures [18,19]. It should be emphasized that the small peaks at 65.2, 63.8, and 58.6 ppm originate from carbon atoms connected to terminal hydroxyl shown in Figure 2 . The 13 C NMR spectra for HTPB and PTPB are shown in Figure 2. Polybutadiene is comprised of the cis/trans-1,4 enchainment, vinyl-1,2 enchainment structures, so it is clear that the peaks at 27.5 and 129.5 ppm are attributed to cis-1,4 enchainment structure, and the peaks at 32.9 and 130.1 ppm are attributed to trans-1,4 enchainment structure. Meanwhile the peaks at 38.3, 43.6, 142.8, and 114.5 ppm are attributed to vinyl-1,2 enchainment structures [18,19]. It should be emphasized that the small peaks at 65.2, 63.8, and 58.6 ppm originate from carbon atoms connected to terminal hydroxyl shown in Figure 2. In the 13 C NMR spectra of PTPB, the typical peaks attributed to polybutadiene backbone still exist at same locations. However, the peaks at 65.2, 63.8, and 58.6 ppm presented in 13 C NMR spectra of HTPB thoroughly disappear in that of PTPB. Instead, the peaks attributed to the propargyl carbon atom 12, 13, and 14 emerge at 58.3, 74.3, and 79.9 ppm, respectively. These suggest that the terminal hydroxyl of HTPB has been thoroughly etherified, having yielded the target product PTPB. In the 13 C NMR spectra of PTPB, the typical peaks attributed to polybutadiene backbone still exist at same locations. However, the peaks at 65.2, 63.8, and 58.6 ppm presented in 13 C NMR spectra of HTPB thoroughly disappear in that of PTPB. Instead, the peaks attributed to the propargyl carbon atom 12, 13, and 14 emerge at 58.3, 74.3, and 79.9 ppm, respectively. These suggest that the terminal hydroxyl of HTPB has been thoroughly etherified, having yielded the target product PTPB.

Propargyl Value
Up until now, there has not yet been a directly quantitative analysis method reported for terminal propargyl content. In order to determine the propargyl value of prepolymer PTPB, gel permeation chromatography (GPC) tests were carried out, and the traces of HTPB and PTPB are shown in Figure 3. The two traces are very similar to each other, and the parameters of HTPB and PTPB evaluated by GPC are listed in Table 2. The number average molecular weight of HTPB is 6588 g mol −1 , and that of PTPB is 6678 g mol −1 . Moreover, the polydispersity index of HTPB (2.12) is also very close to that of PTPB (2.11). Prepolymer HTPB and PTPB have virtually identical molecular weight and distribution. These suggest that, in the course of PTPB preparation, few chain extension and break reactions occurred, and HTPB was transformed into PTPB only through end-capping modification. In combination with 13 C-NMR results, it can be inferred that the terminal group content of polybutadiene should stay conservative.

Propargyl Value
Up until now, there has not yet been a directly quantitative analysis method reported for terminal propargyl content. In order to determine the propargyl value of prepolymer PTPB, gel permeation chromatography (GPC) tests were carried out, and the traces of HTPB and PTPB are shown in Figure 3. The two traces are very similar to each other, and the parameters of HTPB and PTPB evaluated by GPC are listed in Table 2. The number average molecular weight of HTPB is 6588 g mol −1 , and that of PTPB is 6678 g mol −1 . Moreover, the polydispersity index of HTPB (2.12) is also very close to that of PTPB (2.11). Prepolymer HTPB and PTPB have virtually identical molecular weight and distribution. These suggest that, in the course of PTPB preparation, few chain extension and break reactions occurred, and HTPB was transformed into PTPB only through end-capping modification. In combination with 13 C-NMR results, it can be inferred that the terminal group content of polybutadiene should stay conservative.  According to the given HTPB hydroxyl value (0.68 mmol g −1 ), the corresponding propargyl content of PTPB is evaluated to be 0.66 mmol g −1 (see Equation (1)), which was used to determine the R values of preparing PTriPB elastomer (see Section 2.4).
where, Cpropargyl is the content of propargyl groups in PTPB, mmol g −1 , Chydroxyl is the content of hydroxyl groups in HTPB, mmol g −1 , and Mpropargyl is the molecular weight of propargyl groups (39 g mol −1 ).

Viscosity
The dependences of viscosities on the shear rates for HTPB and PTPB are shown in Figure 4. The viscosities of both HTPB and PTPB initially rapidly decrease with the increase in shear rates, followed by slow decrease and giving shear-thinning characteristics. Clearly, HTPB has remarkably higher viscosities than PTPB.  According to the given HTPB hydroxyl value (0.68 mmol g −1 ), the corresponding propargyl content of PTPB is evaluated to be 0.66 mmol g −1 (see Equation (1)), which was used to determine the R values of preparing PT ri PB elastomer (see Section 2.4).
where, C propargyl is the content of propargyl groups in PTPB, mmol g −1 , C hydroxyl is the content of hydroxyl groups in HTPB, mmol g −1 , and M propargyl is the molecular weight of propargyl groups (39 g mol −1 ).

Viscosity
The dependences of viscosities on the shear rates for HTPB and PTPB are shown in Figure 4. The viscosities of both HTPB and PTPB initially rapidly decrease with the increase in shear rates, followed by slow decrease and giving shear-thinning characteristics. Clearly, HTPB has remarkably higher viscosities than PTPB. In combination with FTIR, NMR and GPC results, the structure distinguish between HTPB and PTPB only lie in their terminal groups (hydroxyl for HTPB and propargyl for PTPB), so the discrepancy in viscosity of HTPB and PTPB should also come from them. It is well-known that hydroxyls are capable of forming strong hydroxyl bonding interactions with each other, but propargyls are not. This suggests that there still exists hydroxyl bonding within HTPB matrix, having strengthened the interaction among the HTPB chains and exhibiting higher viscosity. In contrast, the terminal propargyl of PTPB has no perceptible interaction with other atoms, and PTPB exhibits a lower viscosity. This is notably beneficial to the process ability of its elastomer preparation.

DSC
In virtue of the hydroxyl value (0.68 mmol g −1 ) and the number average molecular weight (6588 g mol −1 ) of HTPB, it can also be inferred that the hydroxyl average functionality of HTPB is about 4; it is greater than 2 as a result of its transfer reactions occurring during preparation process [20]. This implies that the propargyl functionality of PTPB is also about 4, and the multi-functional PTPB can be cured by adopting a di-functional azido compound as a curing agent through CuAAC reaction. Figure 5 is the non-isothermal differential scanning calorimetric (DSC) curves of polytriazole polybutadiene elastomers (PTriPB) prepared via CuAAC crosslinking reaction between PTPB and 1,6diazide hexane. It is clear that there only exists a step peak at approximate −75 °C for all PTriPB elastomer S1-S6, which are corresponding to the glass transition temperature of polybutadiene chains [21]. The glass transition temperatures are almost independent of elastomer R values. In combination with FTIR, NMR and GPC results, the structure distinguish between HTPB and PTPB only lie in their terminal groups (hydroxyl for HTPB and propargyl for PTPB), so the discrepancy in viscosity of HTPB and PTPB should also come from them. It is well-known that hydroxyls are capable of forming strong hydroxyl bonding interactions with each other, but propargyls are not. This suggests that there still exists hydroxyl bonding within HTPB matrix, having strengthened the interaction among the HTPB chains and exhibiting higher viscosity. In contrast, the terminal propargyl of PTPB has no perceptible interaction with other atoms, and PTPB exhibits a lower viscosity. This is notably beneficial to the process ability of its elastomer preparation.

DSC
In virtue of the hydroxyl value (0.68 mmol g −1 ) and the number average molecular weight (6588 g mol −1 ) of HTPB, it can also be inferred that the hydroxyl average functionality of HTPB is about 4; it is greater than 2 as a result of its transfer reactions occurring during preparation process [20]. This implies that the propargyl functionality of PTPB is also about 4, and the multi-functional PTPB can be cured by adopting a di-functional azido compound as a curing agent through CuAAC reaction. Figure 5 is the non-isothermal differential scanning calorimetric (DSC) curves of polytriazole polybutadiene elastomers (PT ri PB) prepared via CuAAC crosslinking reaction between PTPB and 1,6-diazide hexane. It is clear that there only exists a step peak at approximate −75 • C for all PT ri PB elastomer S1-S6, which are corresponding to the glass transition temperature of polybutadiene chains [21]. The glass transition temperatures are almost independent of elastomer R values.

Mechanical Properties
At room temperature, well above their glass transition temperature, the mechanical properties of PTriPB elastomer S1-S6 were measured and are listed in Table 3. It can be observed that the stress at break (σb) first increases from 0.39 ± 0.02 MPa to 0.70 ± 0.05 MPa and then decreases to 0.24 ± 0.03 MPa, with increasing R, respectively. In contrast, the strain at break (εb) first decreases from 396 ± 18 to 329 ± 24% and then increases to 460 ± 18%. The mechanical property maximum and minimum values simultaneously appear at R = 1.0 (the stoichiometric ratio). Figure 6 shows the typical stressstrain curves of elastomers. It can be observed that the moduli of elastomers first increase and then decrease as a function of R values. Moreover, elastomer S3 with R = 1.0 has maximum moduli. The dependence of the mechanical properties of PTPB elastomers on R presents parabolic.

Mechanical Properties
At room temperature, well above their glass transition temperature, the mechanical properties of PT ri PB elastomer S1-S6 were measured and are listed in Table 3. It can be observed that the stress at break (σ b ) first increases from 0.39 ± 0.02 MPa to 0.70 ± 0.05 MPa and then decreases to 0.24 ± 0.03 MPa, with increasing R, respectively. In contrast, the strain at break (ε b ) first decreases from 396 ± 18 to 329 ± 24% and then increases to 460 ± 18%. The mechanical property maximum and minimum values simultaneously appear at R = 1.0 (the stoichiometric ratio). Figure 6 shows the typical stress-strain curves of elastomers. It can be observed that the moduli of elastomers first increase and then decrease as a function of R values. Moreover, elastomer S3 with R = 1.0 has maximum moduli. The dependence of the mechanical properties of PTPB elastomers on R presents parabolic.  Considering that CuAAC reaction involves no side-reactions, PTriPB elastomer network structures can be conceived to be those schematically shown in Figure 7, to further elucidate the mechanical properties [7]. When R < 1.0, the terminal propargyl content of PTPB is in excess and propargyl groups cannot be completely reacted with azide groups of curing agent 1,6-diazide hexane, causing the formation of many dangling strands in the elastomer network. Consequently, the elastomers give higher apparent effective strand average molecular weight (Ms) and lower apparent effective strand densities (N0). At the stoichiometric ratio, the terminal propargyl content is equivalent to that of the azide groups, and terminal propargyl groups are nearly reacted to completion with azide groups of the curing agent. The resultant network structure approaches that of an integrated lattice, with few propargyl and azido groups in the elastomer (see Figure 7; R = 1.0); additionally, Ms is minimized and N0 is maximized. When R > 1.0, the excess azido groups in the curing agent molecules cannot also be reacted with and are still appended as branches, forming larger meshes. Resultantly, Ms again increases and N0 again decreases. According to Flory elasticity theory, the stress and modulus of crosslinked elastomer are proportional to the density of network chains N0. The greater the density of network chains, the higher the stress and modulus of crosslinked elastomer, and vice versa [22,23]. Consequently, PTriPB exhibits maximum stress and modulus at the stoichiometric ratio because of its maximum network chain density, whereas its minimum strand molecular weight causes the lowest strain at break. Therefore, the dependence of the mechanical properties of PTPB elastomers on R presents as parabolic (see Table 3). Considering that CuAAC reaction involves no side-reactions, PT ri PB elastomer network structures can be conceived to be those schematically shown in Figure 7, to further elucidate the mechanical properties [7]. When R < 1.0, the terminal propargyl content of PTPB is in excess and propargyl groups cannot be completely reacted with azide groups of curing agent 1,6-diazide hexane, causing the formation of many dangling strands in the elastomer network. Consequently, the elastomers give higher apparent effective strand average molecular weight (M s ) and lower apparent effective strand densities (N 0 ). At the stoichiometric ratio, the terminal propargyl content is equivalent to that of the azide groups, and terminal propargyl groups are nearly reacted to completion with azide groups of the curing agent. The resultant network structure approaches that of an integrated lattice, with few propargyl and azido groups in the elastomer (see Figure 7; R = 1.0); additionally, M s is minimized and N 0 is maximized. When R > 1.0, the excess azido groups in the curing agent molecules cannot also be reacted with and are still appended as branches, forming larger meshes. Resultantly, M s again increases and N 0 again decreases. Considering that CuAAC reaction involves no side-reactions, PTriPB elastomer network structures can be conceived to be those schematically shown in Figure 7, to further elucidate the mechanical properties [7]. When R < 1.0, the terminal propargyl content of PTPB is in excess and propargyl groups cannot be completely reacted with azide groups of curing agent 1,6-diazide hexane, causing the formation of many dangling strands in the elastomer network. Consequently, the elastomers give higher apparent effective strand average molecular weight (Ms) and lower apparent effective strand densities (N0). At the stoichiometric ratio, the terminal propargyl content is equivalent to that of the azide groups, and terminal propargyl groups are nearly reacted to completion with azide groups of the curing agent. The resultant network structure approaches that of an integrated lattice, with few propargyl and azido groups in the elastomer (see Figure 7; R = 1.0); additionally, Ms is minimized and N0 is maximized. When R > 1.0, the excess azido groups in the curing agent molecules cannot also be reacted with and are still appended as branches, forming larger meshes. Resultantly, Ms again increases and N0 again decreases. According to Flory elasticity theory, the stress and modulus of crosslinked elastomer are proportional to the density of network chains N0. The greater the density of network chains, the higher the stress and modulus of crosslinked elastomer, and vice versa [22,23]. Consequently, PTriPB exhibits maximum stress and modulus at the stoichiometric ratio because of its maximum network chain density, whereas its minimum strand molecular weight causes the lowest strain at break. Therefore, the dependence of the mechanical properties of PTPB elastomers on R presents as parabolic (see Table 3). According to Flory elasticity theory, the stress and modulus of crosslinked elastomer are proportional to the density of network chains N 0 . The greater the density of network chains, the higher the stress and modulus of crosslinked elastomer, and vice versa [22,23]. Consequently, PT ri PB exhibits maximum stress and modulus at the stoichiometric ratio because of its maximum network chain density, whereas its minimum strand molecular weight causes the lowest strain at break. Therefore, the dependence of the mechanical properties of PTPB elastomers on R presents as parabolic (see Table 3).

Thermal Stability
A thermal gravimetric analysis was performed in order to assess the thermal stability of PT ri PB elastomers, and their thermal gravity analysis (TG) and derivative thermogravimetric analysis (DTG) curves are shown in Figure 8. It is clear that all TG and DTG curves are nearly overlapped, respectively. The thermal weight loss initiates at 369 • C (T 5% ) and finishes at 485 • C, and the accompanying weight loss is approximately 97%. All DTG curves present the only peaks, indicating that the elastomers have a one-step decomposition characteristic. The weight loss rates reach a maximum at about 463 • C, which is characteristic of polybutadiene thermal decomposition [24].

Thermal Stability
A thermal gravimetric analysis was performed in order to assess the thermal stability of PTriPB elastomers, and their thermal gravity analysis (TG) and derivative thermogravimetric analysis (DTG) curves are shown in Figure 8. It is clear that all TG and DTG curves are nearly overlapped, respectively. The thermal weight loss initiates at 369 °C (T5%) and finishes at 485 °C, and the accompanying weight loss is approximately 97%. All DTG curves present the only peaks, indicating that the elastomers have a one-step decomposition characteristic. The weight loss rates reach a maximum at about 463 °C, which is characteristic of polybutadiene thermal decomposition [24]. The thermal cleavage temperature of the triazole groups that formed from azide and propargyl cycloaddition reaction is high up to nearly 570 °C [25,26], so the thermal weight loss process of PTriPB elastomer should only be attributed to the thermal degradation of polybutadiene strands rather than the polytriazole crosslinks. The thermal stability of the PTriPB elastomers depends on the polybutadiene strands, but not the polytriazole crosslinks. Additionally, because the fission temperature of urethane bonds usually lies in the range of 300-400 °C [27], the thermal stability of polytriazole crosslinked polybutadiene elastomers should also be advantageous over that of traditional polyurethane ones.

Conclusions
Propargyl terminal polybutadiene (PTPB) was successfully synthesized through terminal hydroxyl polybutadiene (HTPB) end-capping modification. PTPB has low viscosity because of no hydrogen bonding interaction among the polymeric chains, which is beneficial to the processing performance of the elastomer preparation.
Using 1,6-diazide hexane as a curing agent, we prepared the PTriPB elastomers with various functional molar ratios (R) via CuAAC reaction. The glass transition temperatures of PTriPB elastomers are approximately −75 °C, and the R values hardly affect these values. The mechanical properties of the elastomers parabolically depend on the R values. At the stoichiometric ratio, the stress and modulus give maximum, while the strain gives minimum, simultaneously. The thermal cleavage temperature of the triazole groups that formed from azide and propargyl cycloaddition reaction is high up to nearly 570 • C [25,26], so the thermal weight loss process of PT ri PB elastomer should only be attributed to the thermal degradation of polybutadiene strands rather than the polytriazole crosslinks. The thermal stability of the PT ri PB elastomers depends on the polybutadiene strands, but not the polytriazole crosslinks. Additionally, because the fission temperature of urethane bonds usually lies in the range of 300-400 • C [27], the thermal stability of polytriazole crosslinked polybutadiene elastomers should also be advantageous over that of traditional polyurethane ones.

Conclusions
Propargyl terminal polybutadiene (PTPB) was successfully synthesized through terminal hydroxyl polybutadiene (HTPB) end-capping modification. PTPB has low viscosity because of no hydrogen bonding interaction among the polymeric chains, which is beneficial to the processing performance of the elastomer preparation.
Using 1,6-diazide hexane as a curing agent, we prepared the PT ri PB elastomers with various functional molar ratios (R) via CuAAC reaction. The glass transition temperatures of PT ri PB elastomers are approximately −75 • C, and the R values hardly affect these values. The mechanical properties of the elastomers parabolically depend on the R values. At the stoichiometric ratio, the stress and modulus give maximum, while the strain gives minimum, simultaneously.
The PT ri PB elastomer is characteristic of a one-step thermal decomposition process, and its thermal stability depends on the polybutadiene strands, instead of the crosslinking triazole groups. These findings will help researchers discover the more extensive application of CuAAC in the field of elastomer.
Author Contributions: J.Z. prepared and edited the manuscript, X.G. analyzed the data, N.L. performed the experiments. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the National Natural Science Foundation of China: 51473022.