High performance benzoimidazolyl-based aminophenolate zinc complexes for isoselective polymerization of: Rac -lactide

Article abstract of DOI:10.1039/c9cc04834k

Zinc complexes supported by achiral benzoimidazolyl-based aminophenolate ligands exhibit high catalytic activities and excellent isoselectivities toward the ring-opening polymerization of rac-lactide under mild conditions.

Full text of DOI:10.1039/c9cc04834k

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High Performance Benzoimidazolyl-Based Aminophenolate Zinc
Complexes for Isoselective Polymerization of rac-Lactide
Yanmei Gong and Haiyan Ma*
Received 00th June 2019,
Accepted 00th June 2019
DOI: 10.1039/x0xx00000x
Zinc complexes supported by achiral benzoimidazolyl-based toward the ROP of rac-LA are still very rare,5d,7d,11 and it
aminophenolate ligands feature high catalytic activities and becomes an abstracting challenge for researches to overcome.
excellent isoselectivities toward the ring-opening polymerization of
rac-lactide under mild conditions.
Previously, we reported pyrrolidinyl-based aminophenolate
zinc complex possessing multiple stereogenic centers (Chart
1), which was the first zinc complex achieving relatively high
I

1
Poly(lactic acid) (PLA) is emerging as one of the most promising activity and isoselectivity for the ROP of rac-LA (TOF = 132 h ,
bioderived polymers as well as sustainable and eco-efficient Pm = 0.80, 298 K).7a Further mechanism study on isoselectivity
alternatives to traditional petrochemical-based polymers. The however indicated dominant chain-end control. Inspired by the
environmental advantages of PLA, such as renewability, results, we introduced chiral oxazolinyl pendant and
biodegradability and biocompatibility, have contributed to this constructed a ligand framework with less stereogenic centres.
material a wide range of applications in both commodity and The obtained zinc complex II facilitated the isotactic
medical fields.1 The established synthesis of PLA is the ring- enchainment with high rates and isoselectivities toward the

1
opening polymerization (ROP) of lactides (LAs) normally ROP of rac-LA (TOF = 792-2014 h , Pm = 0.87-0.89, 298 K; TOF =
mediated by well-defined metal complexes, which is efficient in 117 h , Pm = 0.92, 253 K),7d proving to be the most active and

1
obtaining polylactides with high molecular weights, narrow meanwhile highly isoselective catalyst to date together with the

1
molecular weight distributions and controlled stereo- yttrium bis(phenolate) ether complex (TOF = 2280 h , Pm
=

1
microstructures.2 The physicochemical properties and 0.84, 298 K; TOF = 120 h , Pm = 0.90, 258 K)5d and the

1
therefore practical applications of PLA strongly depends on the phosphasalen indium complex (TOF = 480 h , Pm = 0.87, 298 K;

1
stereochemistry of its microstructure, hence the TOF = 29 h , Pm = 0.92, 258 K)11 reported by Lu’s and Williams’
stereoselective ring-opening polymerization of rac-lactide (rac- groups respectively. When achiral benzoxazolyl pendant was
LA) is currently of great interest, particularly those processes introduced, the resultant aminophenolate zinc complex III also
that can produce isotactic PLAs from rac-lactide.3 In this exhibited high isoselectivities for the ROP of rac-LA but with

1
subfield, since pioneering studies in 1996 that aluminum Schiff- reduced activities (TOF = 190-450 h , Pm = 0.88-0.89, 298 K; TOF

1
base complexes exhibited high isoselectivity toward the ROP of = 34 h , Pm = 0.92, 253 K).7e For both systems II and III, chain-
rac-LA by Spassky’s group,4 the isoselective ROP of rac-LA end control was thoroughly involved, and the chirality of the
although limited but has been gradually explored by various pendant group does not contribute to the high isoselectivity. In
metal precursors based on rare-earth metals,5 alkaline-earth comparison to ligands with chiral fragment(s), achiral ligands
metals,6 zinc,7 alkaline metals8 and transition metals9 besides can be synthesized from much cheaper chemicals, which is of
aluminum.10 Nevertheless, up to date, initiators which integrate great benefit to industrial applications; we thus are highly
excellent isoselectivity with high activity as well as good control motivated to expand the ligand framework by varying the
pendant coordination group with the aim of fur-
Shanghai Key Laboratory of Functional Materials Chemistry and Laboratory of
Organometallic Chemistry, School of Chemistry and Molecular Engineering, East
China University of Science and Technology, 130 Meilong Road, Shanghai,
200237, P. R. China.
E-mail: haiyanma@ecust.edu.cn; Fax: +86 21 64253519; Tel: +86 21 64253519
† Electronic Supplementary Information (ESI) available: Full experimental details,
representative NMR spectra and DSC traces of polymers. CCDC 1919496 (
and 1919497 ( ). For ESI and crystallographic data in CIF or other
electronic format See DOI: 10.1039/x0xx00000x.
2),
6
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Chart 1. Previously reported aminophenolate zinc complexes
Fig. 1 The solid state structures of complexes 2 (left) and 6 (right). Hydrogen atoms are
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omitted for clarity.
DOI: 10.1039/C9CC04834K
and the aminophenolate ligand, the corresponding angles of
O1−Zn1−N4 (122.63(7)°), N4−Zn1−N3 (122.66(7)°), and
N4−Zn1−N1 (123.85(7)°) in complex
significantly from ideal 109.5°. The corresponding structural
features of complex are similar, except that shorter
Zn−Nimidazole, Zn−O distances and longer Zn−Nskeleton, Zn−Nsilylamido
distances are observed for complex , likely arising from the
2 are all deviated
6
6
larger size of the o-trityl group of the ligand. It should be further
noted that, in these two structures, the Zn−Nimidazole bond
lengths (2.0669(17) Å for
shorter than the corresponding Zn−Npyrrolidinyl or Zn−Noxazolinyl
bond lengths in our previously reported zinc complexes ( and
analogues: Zn−Npyrrolidinyl, 2.108-2.137 Å; II-III and analogues:
Zn−Noxazolinyl
2.074-2.1124 Å),7a,7e indicating that the
2; 2.051(3) Å for 6) are obviously
I
,
benzoimidazolyl groups exert stronger coordination interaction
with the zinc centre than the other two types of heterocyclic
structures.
Scheme 1. Synthesis of aminophenol proligands and zinc complexes 1-8
-ther improving the catalytic performance of zinc initiators
toward the ROP of rac-LA. Herein, we use benzoimidazolyl with
adjustable steric and electronic facility to replace benzoxazolyl
as the pendant group, and report another series of
aminophenolate zinc complexes (Scheme 1) with excellent
isoselectivities and high activities toward rac-LA polymerization,
meanwhile integrating the advantages of cheap raw materials
and easy preparation.
Complexes 1-8 are active catalysts toward the ROP of rac-LA
to afford isotactic PLAs (Pm
= 0.68−0.89) at ambient
temperature either alone or in the presence of 2-propanol
(Table 1). The polymerization runs normally can reach
completion within a few minutes or hours depending on the
structures of the complexes, and the substituents on the ligand
framework exert remarkable influences on the catalytic
performance of the corresponding zinc complexes.
Benzoimidazolyl-aminophenol proligands L1-8
H with various
substituents were prepared from substituted 2-
chloromethylbenzoimidazoles, primary amines and three
As shown in Table 1, complex
1 with an o-tert-butyl group on
the phenolate ring showed the highest activity among
substituted 2-bromomethylphenols according to modifications complexes
1
−
3
bearing different ortho-substituents. Whereas,
toward the ROP of rac-LA is
of the published procedures (Scheme 1).7e,12 The corresponding
the stereoselectivity of complex
1
zinc complexes 1-8 H
were synthesized via the reactions of L1-8
the lowest (Pm = 0.69). The introduction of an o-trityl group in
complex resulted in high isoselectivity toward the
polymerization of rac-LA (Pm = 0.89), which is in a sharp contrast
to complexes (Cumyl, Pm = 0.79). The
(tBu, Pm = 0.69) and
high level of isoselectivity of complex is corroborated by the
dominant proportion of the mmm tetrad in the homonuclear
decoupled 1H NMR spectrum and the 13C NMR spectrum of the
resulting polymer sample (Figures S19-S20, ESI†). Obviously, the
with Zn[N(SiMe3)2]2 in a 1:1 molar ratio at ambient temperature
and were fully characterized by 1H and 13C {1H} NMR
spectroscopy as well as elemental analysis methods (see ESI†,
Figures S1-S16). Single crystals suitable for X-ray diffraction
3
1
2
3
determination were successfully obtained for complexes
2 and
6
. As depicted in Fig. 1, both complexes possess a monomeric
structure in the solid state, where the zinc atom is four-
coordinated by three donors of the aminophenolate ligand and
isoselectivity of complexes 1-3 is determined essentially by the
one silylamido group, adopting
geometry around the metal center. For complex
a
distorted tetrahedral
, the bond
2
steric bulkiness of the ortho-substituent on the phenolate ring;
the bulkier the ortho-substituent, the higher the isoselectivity
of the corresponding zinc complex is.
Keeping the superior o-trityl group unchanged, the
replacement of the cyclohexyl on the skeleton N atom with an
n-butyl group led to a considerable increase of the catalytic
distances of Zn−Nimidazole and Zn−Nskeleton range from 2.0669(17)
to 2.2106(16) Å, showing that both the imine N atom of the
benzoimidazolyl ring and the skeleton N atom have strong
coordination interactions with the metal centre, which is
1
consistent with the structure in solution inferred from the H
NMR spectroscopic studies. Due to the steric repulsion between
the bis(trimethylsilyl)amido group
activity of complex
4. We tentatively attribute this to the
smaller steric hindrance and a more flexible nature of the n-
butyl group when compared with the rigid cyclohexyl group.
However, when a benzyl group was introduced to the skeleton
N atom, the activity of the corresponding complex
5 was even
higher than that of complex , and complex proved to be the
4
5
most active catalyst in this series of zinc complexes. Herein the
electronic withdrawing effect of the benzyl group seems to
exert a more dominant influence on activity. By comparing the
isoselectvities of complexes 3-5, it is further noticed that the
introduction of the benzyl group brought an unfavorable effect
2 | J. Name., 2012, 00, 1-3
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on the isoselectivity of complex
5
(Pm = 0.85). Meanwhile, it is not clear why the variation of the benzoimidazolyl
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DOI: 10.1039/C9CC04834K
complexes and 4 with a distinct alkyl substituent on the skel- substituent from benzyl to methyl does not improve the
3
isoselectivity as one might suspect from the influence of benzyl
on the skeleton N atom. We tent to attribute to its location
being far away from the coordination centre.
Table 1 ROPs of rac-LA initiated by zinc complexes 1–8 and I-IIIa
d
d
e
Feed
ratio
Time Conv.b
(min)
TOFc
Mn
Mw/Mn
Pm
Cat.
(h
)
(104)
1
(%)
85
80
95
96
90
80
94
94
93
91
78
90
91
91
86
90
76
77
87
89
99
95
94
94
93
From the perspective of structure-property relationships,
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
500:1:0
500:1:1
200:1:1
200:1:1
200:1:0
200:1:1
200:1:1
500:1:0
500:1:1
500:1:0
500:1:1
40
255
800
228
768
193
640
403
868
587
993
92
13.33
3.59
16.97
5.49
4.84
3.24
11.59
3.25
5.42
2.72
29.13
6.35
16.88
5.58
17.69
6.69
1.42
1.12
1.54
1.27
1.32
1.19
1.63
1.34
1.56
1.29
1.38
1.19
1.22
1.18
1.56
1.25
1.19
1.07
1.28
1.13
1.12
1.39
1.32
1.48
1.17
0.69
0.68
0.79
0.78
0.89
0.88
0.88
0.87
0.85
0.85
0.89
0.88
0.89
0.88
0.87
0.87
0.91
0.93
0.85
0.84
0.80
0.87
0.86
0.89
0.88
1
complexes
3, 4, 6 and 7 with a trityl group on the ortho-
12
phenoxide unit and a cyclohexyl or an n-butyl group on the
central skeleton N atom showed the highest isoselectivities (Pm
= 0.88-0.89, see ESI†, Figures S19, S21-S23) among these
complexes, which parallel those observed for complexes II and
III (Chart 1, Table 1). Although the activity of the best performed
50
2
3
4
5
6
7
15
56
15
28

1
complex
7 (TOF = 573 h ) is somewhat lower than that of
13

1
complex II with a chiral oxazolinyl pendant (TOF = 792 h ), it is
significantly improved when compared to that of complex III
19
11

1
with an achiral benzoxazolyl pendant (TOF = 190 h ). We
believe the introduction of benzoimidazolyl in the ligand
framework afford a good opportunity to improve the catalytic
activities of this series of zinc complexes meanwhile
maintaining the high isoselectivities by simply varying the N-
substituent on the benzoimidazolyl unit.
102
25
432
437
840
573
900
19
25
13
45
30
As reported in the literatures, the addition of 2-propanol has
a beneficial effect on the catalytic activities of all these zinc
complexes, meanwhile providing PLAs in a more controlled
manner as indicated by the matched molecular weights with the
calculated ones as well as narrower molecular weight
distributions (PDI = 1.11-1.34).15 The same level of isoselectivity
was maintained in all cases as witnessed by comparing the runs
carried with or without alcohol. Moreover, when THF was used
as the polymerization solvent, the above mentioned structure-
property relationships of these zinc complexes were also found
8 hf
18 hg
25
3.06
2.91
9.40
4.63
3.00
9.31
7.18
15.6
6.46
8.5
418
890
132
792
2014
190
450
8
12
Ih
IIi
90
36
14
IIIj
148
62
i
a
b
[rac-LA]0 = 1.0 M, Feed ratio = [rac-LA]0:[Zn]0:[iPrOH]0, toluene, 25 C
.
and so did the effect of PrOH on activity and stereoselectivity
Determined by 1H NMR spectroscopy. c Turnover frequency (TOF) = mol of
product (polylactides)/mol of catalyst per hour. d Determined by GPC. e Pm is
the probability of forming a new m-dyad, determined by homonuclear
decoupled 1H NMR spectroscopy. fat 20 ℃. gat 40 ℃. h See ref. 7a. I See ref.
7d. j See ref. 7e.
(see ESI†, Table S2).
By carrying the polymerization run at −20 °C in toluene, the
Pm value of the representative complex 7 was improved to 0.91;
further decreasing the reaction temperature to −40 °C, an even
higher isoselectivity of Pm = 0.93 could be achieved (see ESI†,
Figures S24, S25). DSC measurement of this sample indicates a
significantly higher Tm of 186 °C than that of the homochiral
PLLA (Tm = 170 °C), which agrees well with the highly isotactic
chain microstructure and indicates the formation of
stereocomplexed structure (see ESI†, Figure S28).7e
-eton N atom of the ligand displayed more or less the same high
level of isoselectivities (Pm = 0.89, 0.88). In general, the steric
hindrance of substituents in the ligand framework was reported
to play a dominant role in determining the stereoselectivity of
organometallic catalysts adopted for ROP of rac-LA;7d,13
whereas some recent studies did show that the electronic effect
of substituents would also exert a certain influence on the
stereoselectivity.7a,7e,14 Herein, we suggest that substituents
with distinguished electronic effects might influence the
stereoselectivity by varying the coordination parameters
(bonds and angles) around the metal center, which should be
crucial in selective coordination/insertion of monomer.
Complex
4 with high activity and high isoselectivity among
these complexes further promoted us to study its catalytic
performance under melt conditions. As illustrated in Table 2, at
110
tolerate catalyst loadings as low as 0.0002 mol (vs. monomer).
With [rac-LA]0/[
]0/[iPrOH]0 molar ratios of 1000:1:5,
C, in the presence of exogenous alcohol, complex 4 could
4
2000:1:10, 5000:1:50, high monomer conversions were
obtained within 20, 20 and 23 min. respectively, and good
isoselectivities were still maintained (Pm = 0.76-0.78).
To acquire some information about the isoselective
polymerization mechanism of rac-LA enabled by these zinc
complexes, we monitored at first the NMR tube reactions of the
Being different from above, changing the out-sphere
substituent on the benzoimidazolyl moiety from benzyl to
methyl had hardly influence on isoselectivities of the
corresponding zinc complexes 6-8, but led to slight to moderate
decreases of activities. Therefore, the electronic withdrawing
nature of substituent on the benzoimidazolyl moiety is also
benefit for the activity, which increases the Lewis acidity of the
zinc centre to facilitate the monomer coordination. At present,
representative complex
sequentially with 5 equiv. of rac-LA (see ESI†, Figure S29).
3 with ca. 1 equiv. of 2-propanol and
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Table 2 ROPs of rac-LA initiated by zinc complex 4a
Notes and references
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DOI: 10.1039/C9CC04834K
TOFc
Mn
d
d
e
Feed ratio
Time
(min)
Conv.b
(%)
Mw/Mn
Pm
1
2
3
(a) E. Chiellini and R. Solaro, Adv. Mater., 1996, 8, 305; (b) C.-
(h
)
(104)
1
S. Ha and J. A. Gardella, Chem. Rev., 2005, 105, 4205; (c) M. J.
Stanford and A. P. Dove, Chem. Soc. Rev., 2010, 39, 486.
(a) J. M. Becker, R. J. Pounder and A. P. Dove, Macromol. Rapid
Commun., 2010, 31, 1923; (b) S. Slomkowski, S. Penczek and
A. Duda, Polym. Adv. Technol., 2014, 25, 436.
1000:1:5
2000:1:10
5000:1:50
20
20
23
97
98
90
2910
5880
11739
7.90
8.29
4.08
1.70
1.61
1.30
0.78
0.76
0.78
a[rac-LA]0 = 1.0 M, Feed ratio = [rac-LA]0:[Zn]0:[iPrOH]0, 110 C
.
b Determined
(a) J. Slager and A. J. Domb, Adv. Drug Delivery. Rev., 2003, 55
549; (b) K. Fukushima and Y. Kimura, Polym. Int., 2006, 55, 626;
(c) C. M. Thomas, Chem. Soc. Rev., 2010, 39, 165.
,
c
by 1H NMR spectroscopy. Turnover frequency (TOF) = mol of product
d
(polylactides)/mol of catalyst per hour. Determined by GPC. e Pm is the
probability of forming
a new m-dyad, determined by homonuclear
4
5
N. Spassky, M. Wisniewski, C. Pluta and A. Le Borgne,
Macromol. Chem. Phys., 1996, 197, 2627.
decoupled 1H NMR spectroscopy.
(a) P. L. Arnold, J. C. Buffet, R. P. Blaudeck, S. Sujecki, A. J.
Blake and C. Wilson, Angew. Chem., Int. Ed., 2008, 47, 6033;
(b) C. Bakewell, T. P. Cao, N. Long, X. F. Le Goff, A. Auffrant
and C. K. Williams, J. Am. Chem. Soc., 2012, 134, 20577; (c) C.
Bakewell, A. J. White, N. J. Long and C. K. Williams, Angew.
Chem. Int. Ed., 2014, 53, 9226; (d) T. Xu, G. Yang, C. Liu and X.
Lu, Macromolecules, 2017, 50, 515.
The treatment of 2-propanol resulted in the substitution of the
N(SiMe3)2 group boned to the zinc atom, yielding the
isopropoxide derivative “L3ZnOiPr”. The 5-fold sequential
addition of rac-LA to this system led to a fast oligomerization
and the signals assignable to the active oligomer
L3Zn[(OCH(CH3)CO)nOiPr]} could be identified roughly. The end-
6
7
(a) J. Hu, C. Kan, H. Wang and H. Ma, Macromolecules, 2018,
51, 5304; (b) J. Bhattacharjee, A. Harinath, H. P. Nayek, A.
Sarkar and T. K. Panda, Chem. Eur. J., 2017, 23, 9319; (c) A.
Harinath, J. Bhattacharjee, A. Sarkar, H. P. Nayek and T. K.
Panda, Inorg. Chem., 2018, 57, 2503.
{
1
capping groups of a typical oligomer were recognized by H
NMR and MALDI-TOF mass spectroscopy to be hydroxy and
isopropoxyl ester as expected (see ESI†, Figures S30-S31). All
these features are in line with a coordination-insertion
polymerization process.
(a) H. Wang and H. Ma, Chem. Commun., 2013, 49, 8686; (b)
H. Wang, Y. Yang and H. Ma, Macromolecules, 2014, 47, 7750; (c)
S. Abbina and G. Du, ACS Macro Lett., 2014, 3, 689; (d) C. Kan,
Subsequently, preliminary kinetic studies for the ROP of D-LA,
J. Hu, Y. Huang, H. Wang and H. Ma, Macromolecules, 2017,
L-LA and rac-LA were conducted by using complex
initiator. It is found that complex exerts no difference on the
polymerization rates of D-LA and L-LA (D-LA, kapp= (1.54 ±
8 as the
50, 7911; (e) J. Hu, C. Kan and H. Ma, Inorg. Chem., 2018, 57
,
8
11240.
8
9
(a) J. Zhang, J. Xiong, Y. Sun, N. Tang and J. Wu,


1
0.09)×10 min ; L-LA, kapp = (1.55 ± 0.02)×10 min , in
1
1
1
Macromolecules, 2014, 47, 7789; (b) Z. Dai, Y. Sun, J. Xiong, X.
Pan and J. Wu, ACS Macro Lett., 2015, 4, 556; (c) C. Chen, Y.
toluene at 25 ), which however are obviously larger than that
℃
Cui, X. Mao, X. Pan and J. Wu, Macromolecules, 2016, 50, 83;
(d) Y. Sun, J. Xiong, Z. Dai, X. Pan, N. Tang and J. Wu, Inorg.
Chem., 2016, 55, 136; (e) C. Chen, J. Jiang, X. Mao, Y. Cong, Y.
Cui, X. Pan and J. Wu, Inorg. Chem., 2018, 57, 3158.
of rac-LA (kapp= (0.95 ± 0.04)×101 min ) (see ESI†, Figure S32).
In lacking of a polymer exchange process as observed for
analogue zinc systems,7e these kinetic results indicate the
operation of a chain-end control mechanism.

1
(a) E. L. Whitelaw, M. D. Jones and M. F. Mahon, Inorg. Chem.,
2010, 49, 7176; (b) E. L. Whitelaw, M. G. Davidson and M. D.
Jones, Chem. Commun., 2011, 47, 10004; (c) M. D. Jones, S. L.
Hancock, P. McKeown, P. M. Schafer, A. Buchard, L. H.
Thomas, M. F. Mahon and J. P. Lowe, Chem. Commun., 2014,
50, 15967; (d) M. D. Jones, L. Brady, P. McKeown, A. Buchard,
P. M. Schafer, L. H. Thomas, M. F. Mahon, T. J. Woodman and
The stereoerrors in the microstructure of typical polymer
samples were analysed via homonuclear-decoupled 1H NMR
spectroscopy (see ESI†, Figures S19, S21-S26). It shows that the
intensity ratio of rmm : mmr : mrm tetrad signals is about 1:1:1,
suggesting the formation of isotactic stereoblocks along the
polymer chain. The relatively small rmr stereoerror sequence
may be ascribed to consecutive chain end control errors.5b,16
These features further imply that
mechanism is involved in producing isotactic stereoblock PLAs
by these zinc complexes.
In summary, we have prepared a series of benzoimidazolyl-
based aminophenolate zinc complexes exhibiting excellent
isoselectivities (Pm = ca. 0.89) and high activities toward the ROP
of rac-LA at ambient temperature. The replacement of previous
benzoxazolyl pendant with benzoimidazolyl in the ligand
framework provides a good opportunity of improving the
activities of the corresponding zinc complexes meanwhile
maintaining the high isoselectivities. The advantages of cheap
raw materials and easy preparation further afford potential
industrial applications.
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ChemComm
High Performance Benzoimidazolyl-Based Aminophenolate Zinc Complexes for Isoselective Polymerization of rac-Lactide
Yanmei Gonga and Haiyan Ma*,a
Shanghai Key Laboratory of Functional Materials Chemistry and Laboratory of Organometallic Chemistry, School ofVCiewheAmrticisletrOynlianend
Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 2002D3O7,I:P10e.o10p3l9e/’Cs9RCeCp0u4b8l3i4cKof
China
A series of zinc complexes supported by achiral benzoimidazolyl-based aminophenolate ligands were employed as highly active initiators for the
ring-opening polymerization of rac-lactide to afford highly isotactic polymers under mild conditions.
rac-[Zn]
D-LA
Isoselective ROP
L-LA
rac-Lactide
rac-[Zn]
Me
Stereoblock PLA
Pm = 0.84-0.93
R2
R1
N





High isoselectivity!
High activity!
Low catalyst loading!
Immortal polymerization!
Semicrystalline polymer!
N
Zn
N
O
CPh3
N
Si
Si
SZnSN/RZnRN