Carbon-carbon (CC) backbone polymers are a class of polymer materials widely used in daily life.
Such polymers are usually obtained by polymerization of olefinic monomers or single-carbon (C1) units
The polymerization of diazocarbonyl compounds is also one of the convenient methods to construct CC backbone polymers.
This strategy can obtain CC backbone polymers with polar substituents distributed on each backbone carbon.
Natta catalysts are more difficult to achieve
At present, the reactive controllable polymerization of diazotate esters is still widely concerned by many chemists, and it is still a challenge to use non-precious metal initiation systems to realize reactive controllable polymerization of diazolate esters
The research group of Prof.
Zongquan Wu from Hefei University of Technology/Jilin University has done a series of research work in the field of preparation of helical polycarbene by reactive controlled polymerization of diazotes ( J.
2018, 140, 17773–17781; Macromolecules 2019, 52, 7260–7266; ACS Macro Lett.
2022, 11, 179–185)
The research group of Professor Wu Zongquan recently reported for the first time a non-precious metal thiophene-based Ni(II) catalytic system, established a method for the controlled polymerization of diazotes with Ni(II) activity, and obtained a series of molecular weight (Mn) controllable, molecular weight Narrow distribution (Mw/Mn) of polycarbene, and Ni(II)-catalyzed one-pot sequential active polymerization of thiophene and diazotate to obtain π-conjugated polythiophene-polycarbene copolymer with adjustable molecular weight and structure and proposed a possible mechanism for the controlled polymerization of diazotes with Ni(II) catalytic activity, as shown in Figure 1
(a) Ni(II)-catalyzed active polymerization of diazotes; (b) Ni(II)-catalyzed sequential polymerization of thiophenes and diazotes.
Optimization of polymerization conditions
The research team first optimized the polymerization conditions and tried a series of catalytic systems composed of different Ni compounds (R'–Ni–X) and different ligands (L1–L6) to induce benzyl diazoacetate (1a) The polymerization reaction was studied, as shown in Figure 2
The results show that: the catalytic system composed of π-allyl Ni compounds and L1-L6 can only obtain oily oligomeric poly-1am, and the phenyl Ni(II) catalyst (Ph-Ni(L)Br) initiates the 1a single Bulk polymerization can obtain polycarbene with Mn of ~3 kDa and Mw/Mn of 1.
The dimerized thiophene Ni(II) catalyst (BT–Ni(dppp)Cl) exhibited better results than other catalysts when initiating the polymerization of 1a monomer, by tuning the ratio of the initial concentration of monomer to Ni(II) [1a ]0/[Ni(II)]0, a series of polycarbene poly-1am with different Mn with narrow Mw/Mn (<1.
20) can be obtained.
II)]0 increases linearly, and can maintain a narrow Mw/Mn, as shown in Figure 3, the polymerization reaction is applicable to diazo monomers 1a-1d
These results suggest that BT-Ni(dppp)Cl is an active catalyst for the polymerization of diazonium esters, possibly following a living polymerization mechanism
Screening of Ni(II) catalytic systems
(a) Size exclusion chromatograms of poly-1ams with different [1a]0/[Ni]0; (b) Mn and Mw/Mn of poly-1am with different [1a]0/[Ni(II) ]0.
(The catalyst is BT–Ni(dppp)Cl; test conditions: THF as solvent, temperature at 25 °C)
Polymerization kinetics studies
Next, the research team studied the polymerization kinetics of BT–Ni(dppp)Cl-initiated polymerization of diazo monomer 1a (internal standard is polystyrene; Mn = 41.
4 kDa, Mw/Mn = 1.
02), and the results are shown in Figure 4.
With the prolongation of reaction time, the molecular weight of the obtained polymer gradually increased (Fig.
4a), and the conversion rate of monomers reached 85% after 5 h
The curve of –Ln([M]/[M]0) as a function of polymerization time indicates that the polymerization follows a first-order reaction mechanism with a reaction rate constant of 8.
3 × 10−5 s−1 (Fig.
Furthermore, the Mn of the resulting polymers is proportional and linearly related to the conversion of 1a, with Mw/Mn <1.
20 for all polymers (Fig.
To further demonstrate the controllable activity of this polymerization reaction, monomer 1a was added to a solution of freshly prepared poly-1a50 (Mn = 5.
52 kDa, Mw/Mn = 1.
19), and the Mn was increased to 8.
47 kDa by size exclusion chromatography
The above results show that the BT-Ni(dppp)Cl catalyst-induced diazotate polymerization follows an activity-controlled polymerization mode
(a) Size exclusion chromatography of 1a polymerization initiated by BT-Ni (dppp)Cl catalyst with polystyrene (PSt) as internal standard; (b) 1a conversion and -Ln ([M]/[M ]0) value as a function of polymerization time; (c) Mn and Mw/Mn of polymer versus 1a conversion
(THF, 25 °C, [1a]0/[Ni]0 = 100)
Synthesis of π-conjugated copolymers
Since BT-Ni(dppp)Cl showed excellent performance during the polymerization of diazotate, the team tried to study the functional polymer with similar Ni(II)-complex ends to initiate the living polymerization of diazoacetic acid, thereby obtaining Functional block copolymers
First, 2-bromo-3-hexyl-5-chloromagnesium thiophene (2) (THF, 25 °C, [1a]0 = 0.
50 M, [1a] was polymerized in situ with Ni(dppp)Cl2 using CTP or Grignard metathesis mechanism.
]0/[Ni(II)]0 = 20), as shown in Fig.
Polythiophene poly-220 (P3HT) with Mn of 6.
71 kDa and Mw/Mn of 1.
19 can be obtained.
Monomer 1a was added to the solution of poly-220 and reacted at room temperature for 24 h.
The molecular weight was significantly increased and the molecular weight distribution was relatively poor.
The narrow π-conjugated copolymer poly(220-b-1a40) (71% yield, Mn = 12.
63 kDa, Mw/Mn = 1.
18) is shown in Figure 5
Further research results show that the reaction of the functional polymer poly-2m block comonomer 1a with a structure similar to BT-Ni (dppp)Cl at the end also proceeds according to the activity-controlled polymerization mechanism
(a) Size exclusion chromatography of polymers poly-220 and poly(220-b-1a40).
(b) Mn and Mw/Mn pairs of polymer poly(220-b-1an)s [1a] Graph of the 0/[Ni]0 ratio.
Using the above-mentioned functional polymer P3HT-initiated diazotate polymerization strategy, a series of π-conjugated copolymers with controllable molecular weight and structure, such as amphiphilic block copolymers, can be prepared
Studying its self-assembly behavior in different solvents and verifying it with atomic force microscopy (AFM), the research team finally proposes a possible mechanism for the controlled polymerization of diazotes with Ni(II) catalytic activity
The significance of this study is to establish a method for the controllable polymerization of diazotes with non-noble metal Ni(II) catalysts, and to easily synthesize CC backbone polymers and π-conjugated interpolymers with substituents on each backbone carbon atom.
The segmented copolymers provide new synthetic methods and research ideas for the preparation of organic light-emitting materials and optical device materials
The above related research results were published in Nature Communications (Nat.
13, 811 (2022))
The co-first authors of the paper are Zhou Li, a teacher from the School of Chemistry and Chemical Engineering, Hefei University of Technology, and Xu Lei, a postdoctoral fellow, and the corresponding author is Professor Wu Zongquan.
Currently, they have joined the State Key Laboratory of Supramolecular Structure and Materials, Jilin University