Y. Gu, et al.
Polymer170(2019)24–30
Gronowitz groups [22] reported respectively that hydrozirconation led
to the cleavage of carbon-oxygen bond in acyclic vinyl ethers, such as
vinyl ethyl ether and β-methoxy styrene. And Caulton et al. also re-
ported that Schwartz's reagent induced the cleavage of carbon-fluorine
bond in vinyl fluoride [23]. But, under the hydrozirconation, the
cleavage of carbon-carbon bond in alkene was hardly reported [24,25].
In this work, we found that the [Cp2ZrHCl]n could not only cleave
carbon-chlorine bond in polychloroprene (PCP), but also break down
the C]C bonds in PCP. At the same time, it was found that β-alkyl
elimination reaction causing carbon-carbon bond cleavage was pro-
moted via chlorine self-assisted manners, that is, chlorine in PCP would
be eliminated in the form of Cp2ZrCl2 once C]C bonds in PCP pro-
ceeded hydrozirconation addition. PCP could be cleaved into end-
functionalized oligomers with controlled molecular weights under mild
conditions. More importantly, efficiently catalytic cleavage of C]C
bonds in PCP is achieved in the combination of [Cp2ZrClH]n with LiH
and H2. We believe that this research will shed some light on the C]C
bonds cleavage in organic synthesis and bring novel inspiration to re-
cycle wasted polymers by organometallic catalysts.
Table 1
The molecular weights and PDI of chain-scission products from PCP.
)
Mn (kg mol−1
)
1
2
3
4
5
6
7
0
–
–
109.9
12.8
3.5
1.9
1.2
2.14
1.95
1.88
1.44
1.61
1.54
1.50
0.05
0.1
0.3
0.5
1.0
2.0
5.9
2.3
1.7
0.8
–
0.4
0.2
a
Conditions: THF, 60 °C, 3 h.
Molar ratio of ZrH and the C]C bonds in PCP.
Calculated Mn by NMR spectroscopy.
b
c
d
From GPC versus polystyrene standards (Fig. S1).
2.4. Hydrozirconation procedure of CPP
The reaction procedure was the same as above PCP. However,
purified procedure was as following: Products were extracted by hexane
after removing the solvent THF under water aspirator pressure. Then,
hexane was also removed under water aspirator pressure to obtain final
products that were characterized directly by NMR spectroscopy.
2. Experimental section
2.1. Materials and general measurements
3. Results and discussion
Solvent THF was distilled from sodium/potassium alloy with ben-
zophenone before use. Cp2ZrHCl was prepared according to published
procedure [26] using diisobutyl aluminium hydride (iBu2AlH, 1 M in
hexane). Bis(cyclopentadienyl) zirconium dichloride (Cp2ZrCl2, 99%),
2,3-dichloro-1-propene (98%) and copper(I) iodide (CuI, 99.998%) was
purchased from Alfa Aesar. Phenethylmagnesium chloride (1.0 M in
THF) was purchased from Acros. Polychloroprene (PCP) was a com-
mercial product from Dupont. 2-Chloro-5-phenyl-1-pentene (CPP) was
synthesized by reaction between 2,3-dichloro-1-propene and phe-
nethylmagnesium chloride. Nuclear magnetic resonance (NMR) spectra
were performed on Bruker AVANCE 500 (500 MHz for 1H NMR,
77 MHz for 2H NMR in CHCl3 using CDCl3 as standards
(δD = 7.26 ppm), 125 MHz for 13C NMR) spectrometers and Bruker
AVANCE 400 (400 MHz for 1H NMR, 100 MHz for 13C NMR). Relative
molecular weight was determined by gel permeation chromatography
(GPC) on TOSOH HLC 8220 GPC at 40 °C using THF as an eluent against
linear polystyrene standards.
3.1. Controlled PCP chain scission by [Cp2ZrHCl]n
A designed amount of [Cp2ZrHCl]n (ZrH) powder was added to THF
solution of PCP and vigorously stirred for 3 h at 60 °C, then quenched
with acidified methanol. The isolated products were characterized by
GPC, and the results were summarized in Table 1. It can be seen that the
molecular weight (Mn) of chain-scission products is decreased with the
increase of ZrH loading, suggesting that the molecular weight of chain-
scission products is much controlled by the loading of ZrH. The chain-
scission product with 0.2 kg mol−1 was obtained when the molar ratio
of added ZrH to C]C bonds in PCP was equal to 2.0 (entry 7 in
Table 1), indicating that PCP could be nearly cleaved into oligomers
composed of three 2-chloro-1,3-butadiene monomers. Besides, chain-
scission products are relatively narrower distributed (PDI ≈ 1.6), im-
plying the chain-scission reaction is well controlled.
Next, the microstructures of chain-scission products are investigated
in detail. From 1H NMR measurement (Fig. 1), it can be seen that the
microstructures of chain-scission products are very similar to that of
starting PCP, that is to say, there are no side reactions happening to
disturb the microstructures of PCP during chain scission. In organic
synthesis, hydrozirconation of alkene is generally accompanied with a
few hydrogenation side reactions of C]C bonds [22,28–30]. However,
the characteristic signal of eCHCle [31] is not detected by 1H NMR
spectroscopy (Fig. 1b–d), indicating that the hydrogenation side reac-
tion does not occur in the case of PCP chain scission. As shown in 1H
NMR (Fig. 1b), primary protonic signals of chain-scission products were
still assigned to characteristic unit of PCP, eCH2C(Cl) = CHCH2e, al-
though there were three new peaks at δH = 1.65, 1.26, and 0.89 ppm.
These three peaks were assigned to chain-end groups eCH]CHCH3
(Fig. S2), eCH2CH3, and eCH3 (Fig. 1), respectively. In addition, chain-
end functionalization of chain-scission products was obtained by iodine
(I2) as quenching reagents. As shown in Fig. 1d, three new character-
istic peaks at δH = 3.19, 1.83, and 1.41 ppm were assigned to eCH2I,
eCH2CH2I and eCH2CH2CH2I, respectively. Their coupling relationship
was further confirmed by 1H 1H COSY and HSQC NMR spectra (Figs. S3
and S4). In conclusion, the microstructures of chain-scission products
are also much controlled during the chain scission.
2.2. Preparation of CPP
CPP was synthesized according to the published literature [27].
Firstly, 2,3-dichloro-1-propene (1.0 g) was added to the THF (10 ml)
suspension of CuI (0.096 g) at room temperature and stirred for 10 min.
Then cooled to −40 °C, phenethylmagnesium chloride (11.3 ml, 1.0 M
in THF) was added dropwise for 5 min. The reaction mixture was
warmed slowly to room temperature and stirred for overnight. Finally,
it was quenched with saturated aqueous solution of NH4Cl, and ex-
tracted with Et2O (10 ml × 4). The combined organic extracts were
dried (Na2SO4) and evaporated. The residue was dissolved in hexane,
filtered through silica gel and the filtrated was evaporated under re-
duced pressure. The product (0.8 g) was used without further pur-
ification.
2.3. General PCP cleavage procedure
Under an inert atmosphere of argon using standard Schlenk line
techniques or M-Braun glovebox, [Cp2ZrHCl]n power was added to THF
solution of PCP, and stirred for 3 h at 60 °C, then quenched with acid-
ified methanol. Chain-scission products were precipitated in methanol
and dried in vacuum oven at 30 °C to a constant weight.
25