Organopolymerization of naturally occurring Tulipalin B: a hydroxyl-functionalized methylene butyrolactone

Abstract

The first successful polymerization of the naturally occurring, OH-containing, tri-functional monomer Tulipalin B (_βHMBL) is established. N-Heterocyclic carbene and phosphazene superbase catalysts effectively polymerize _βHMBL into polymers with M_n up to 13.2 kg mol⁻¹. The possible polymer structure is thought to be a branched copolymer of poly(vinyl-ether lactone)s, derived from proposed crossovers between conjugate Michael and oxa-Michael additions, enabled by proton transfer.

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Introduction

The emergence of organopolymerization¹ has profited from the development of powerful organic catalysts and unique mechanistic pathways in the rapidly growing field of organocatalysis.² One such important class of organic catalysts is N-heterocyclic carbenes (NHCs); thanks to their inherently high Brønsted-basicity and nucleophilicity, NHCs have attracted increasing interest for their unique reactivity and selectivity often observed in many different types of organic reactions.³ The scope of the NHC-mediated polymerization⁴ has expanded significantly from the first ring-opening polymerization (ROP) of cyclic esters (lactides)⁵ to group transfer polymerization (GTP) of α,β-unsaturated esters (or acrylates);⁶ step-growth polyaddition of diols and diisocyanates to polyurethanes;⁷ step-growth polycondensation of diols and diesters to polyesters;⁸ step-growth polyaddition of dimethacrylates to unsaturated polyesters,⁹ which is based on fundamental steps of tail-to-tail dimerization (intermolecular umpolung) of methacrylates;¹⁰ and chain-growth conjugate polyaddition of acrylates.¹¹ Less utilized is the basicity of the NHC for polymerization via basic activation of the protic monomer or initiator, but two recent examples invoked such activation mode to achieve oxa-Michael addition polymerization of hydroxyl functionalized acrylates to poly(ester–ether)s with M_n up to 2400 g mol⁻¹ (ref. 12) and common (meth)acrylates in the presence of alcohol to α-alkoxy polyacrylates with M_n up to 8000 g mol⁻¹.¹³ On the other hand, the phosphazene superbase, 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino) phosphoranylid-enamino]-2λ⁵,4λ⁵-catenadi(phosphazene) (^tBu-P₄),¹⁴ is one of the strongest known neutral bases but with low nucleophilicity. Thus, the combination of its strong basicity and low nucleophilicity renders ^tBu-P₄ a potent organic initiator or catalyst to promote various types of polymerization reactions, but primarily as a base to deprotonate a protic initiator¹⁵ or monomer¹⁶ directly to generate anionic active species for polymerization reactions. ^tBu-P₄ has also been reported as a nucleophilic catalyst for GTP initiated by a silyl ketene acetal.¹⁷

Renewable or sustainable polymers have recently gained increasing attention, the research of which has been directed at examining the possibility of replacing petroleum-based raw materials by naturally occurring or biomass-derived renewable feedstocks for the production of polymeric materials in large commodity and specialty chemicals markets.¹⁸ In this context, renewable methylene butyrolactones based on the Tulipalin family, Tulipalin A (Fig. 1), or α-methylene-γ-butyrolactone (MBL), found in tulips,¹⁹ as well as its methyl-substituted derivatives, γ-methyl-α-methylene-γ-butyrolactone (_γMMBL), derived from the biomass sourced levulinic acid,²⁰ and β-methyl-α-methylene-γ-butyrolactone (_βMMBL), derived from the biomass sourced itaconic acid,²¹ have been explored for the prospects of substituting the currently petroleum-based acrylic monomers, such as methyl methacrylate (MMA), in the production of acrylic bioplastics and specialty chemicals.²² Various types of polymerization processes^22a have been employed to polymerize MBL, _γMMBL, and _βMMBL, including radical,²³ anionic,²⁴ group-transfer,²⁵ Lewis pair zwitterionic,²⁶ and metal-mediated coordination²⁷ polymerization methods. NHC-mediated organopolymerization of such monomers has also been investigated.¹¹ Regardless of the polymerization mechanism or process, all homo-polymerizations of the above three Tulipalin A-based monomers proceed exclusively through vinyl addition without ring-opening of the five-membered lactone ring.

Fig. 1 Structures of Tulipalin A, its methyl derivatives, and Tulipalin B.

Tulipalin B, or β-hydroxy-α-methylene-γ-butyrolactone (_βHMBL) (Fig. 1), is a biologically active (antimicrobial), naturally occurring [present as the (S)-enantiomer in tulips],¹⁹ tri-functional monomer containing an exocyclic conjugated double bond, a five-membered lactone, and a hydroxyl group. Recently, its preparation from tulip biomass by a one-step enzyme reaction has been reported.²⁸ Perhaps owing to its structural complexity and presence of the reactive hydroxyl group, polymerization of _βHMBL has not been reported in the open literature. Accordingly, the objective of this study is to investigate organopolymerization of _βHMBL (1) by organic NHC and superbase catalysts.

Results and discussion

Considering the multi-functionality of the _βHMBL monomer, we hypothesized that five possible scenarios could be involved in the polymerization of _βHMBL by nucleophilic/basic organic NHC catalysts as outlined in Scheme 1. First, nucleophilic activation leads to a zwitterionic intermediate that could undergo repeated conjugate Michael addition¹¹ to form vinyl addition polymer structure type I, poly(vinyl lactone) or P_βHMBL, via pathway A. Second, the zwitterionic intermediate could also undergo proton transfer, with another zwitterion or monomer molecule, to generate an alkoxide species that proceeds through oxa-Michael addition²⁹ followed by proton transfer,¹² thus leading to poly(ether lactone) structure type II, via pathway B. Third, basic activation¹³ leads an alkoxide ion pair, which initiates the polymerization through oxa-Michael/proton transfer alternating sequences to form poly(ether lactone) structure type IIIvia pathway C. Fourth, these different pathways could interchange via pathway D, for example, after Michael addition, proton transfer could occur, which crossover to oxa-Michael addition, or vice versa, thereby forming possibly poly(vinyl-ether lactone) copolymers containing both I and II or III structure units, rather than a mixture of homopolymers. Fifth, the above possible pathways focused on the exocyclic conjugated double bond and the hydroxyl functions, but the presence of the third function, the five-membered lactone ring, points to a possible ring-opening polymerization (ROP) pathway (E), especially with a system involving alkoxide initiating/propagating species. However, the ROP of five-membered lactones, including the MBL core, is not possible (for repeated ROP enchainment) under normal conditions, due to thermodynamic reasons,³⁰ but alkoxide species IV resulted possibly from a ring-opening event could proceed with oxa-Michael addition for further chain growth, only if this ring-opening pathway is competitive with other pathways described above.

Scheme 1 Hypothesized possible scenarios involved in the polymerization of _βHMBL by nucleophilic/basic organic NHC catalysts.

As outlined above, the polymerization of the tri-functional _βHMBL by the nucleophilic/basic organic NHC catalysts can be very complicated. Accordingly, we chose four organic initiators/catalysts including two NHCs, I^tBu (strongest nucleophile) and TPT (weaker nucleophile), one superbase, ^tBu-P₄ (strongest base, weak nucleophile), and one radical initiator, AIBN, as a control, for examining their polymerization behavior (Table 1). For solubility reasons, DMF was chosen as the solvent. At ambient temperature (∼25 °C) in a low monomer [M] to initiator [I] ratio of 20, I^tBu promoted _βHMBL polymerization to completion in 24 h, producing a polymer product with M_n = 1.32 × 10⁴ g mol⁻¹ and Đ = 1.40 (run 1). Increasing the [M]/[I] ratio to 100, the polymerization became sluggish, achieving only 27.9% monomer conversion after 72 h (run 2). The M_n value (9.28 × 10³) of the resulting polymer was also lower, due to the low monomer conversion and chain transfer (vide infra). Raising the polymerization temperature to 80 and 100 °C enabled quantitative monomer conversion in 24 h for [M]/[I] = 50, 100, 200 ratio runs or 48 h for the [M]/[I] = 500 run, but the M_n value of the resulting polymers remained in a narrow range from M_n = 7.33 × 10³ g mol⁻¹ to 8.42 × 10³ g mol⁻¹ (runs 3–6), indicating further chain-growth is limited with more monomer presumably due to chain transfer via the OH group. As compared to I^tBu, the polymerization by the less nucleophilic TPT was less effective, achieving lower monomer conversions and producing polymers with lower M_n values for runs with [M]/[I] = 20 (25 °C, run 7) and 50 (80 °C, run 8), but the polymerization with [M]/[I] = 100 (100 °C) was nearly the same (run 9 vs. 4). The results of the polymerization by the superbase ^tBu-P₄ (runs 10–14) were rather similar to those by I^tBu, hinting a common polymerization mechanism for both initiators. Control runs by AIBN showed that this free radical polymerization produced P_βHMBL with lower M_n and Đ values (run 15), especially when carried out in CHCl₃ (M_n = 3.0 × 10³ g mol⁻¹, run 16), due to the poor solubility of the polymer in this solvent and chain transfer by this solvent, and the M_n value was not obviously affected by varying the [M]/[I] ratio from 50 to 1200. Overall, these results showed that _βHMBL can be effectively polymerized into unimodal polymers with M_n up to 1.32 × 10⁴ g mol⁻¹, but the polymerization by the current initiating systems lack control over polymer M_n and Đ values.

Table 1 Selected results of _βHMBL (M) polymerization by NHC and ^tBu-P₄ initiators^a

Run no.	Initiator (I)	[M]/[I]	Temp. (°C)	Time (h)	Conv.^b (%)	M n ^c (kg mol^−1)	Đ (Mw/Mn)
a Monomer concentration [M] = 2.2 mol L^−1 in DMF. b Monomer conversion measured by ^1H NMR. c Number-average molecular weight (Mn) and molecular weight distribution (Đ = Mw/Mn) determined by gel-permeation chromatography (GPC) relative to poly(methyl methacrylate) standards. d Carried out in CHCl3.
1	I^tBu	20	25	24	100	13.2	1.40
2	I^tBu	100	25	72	27.9	9.28	1.15
3	I^tBu	50	80	24	100	7.43	1.50
4	I^tBu	100	100	24	100	8.27	1.32
5	I^tBu	200	100	24	100	7.33	1.24
6	I^tBu	500	100	48	>99	8.42	1.30
7	TPT	20	25	24	84.3	5.28	1.23
8	TPT	50	80	24	88.8	5.16	1.40
9	TPT	100	100	24	>99	8.77	1.24
10	^tBu-P4	20	25	24	88.4	8.60	1.23
11	^tBu-P4	50	80	24	100	6.37	1.50
12	^tBu-P4	100	100	24	100	10.2	1.35
13	^tBu-P4	200	100	24	86.4	11.0	1.29
14	^tBu-P4	500	100	72	>99	8.26	1.30
15	AIBN	500	80	24	62.8	7.45	1.16
16^d	AIBN	500	80	24	96.0	3.00	1.10

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Thermal properties of the resulting polymer materials were analyzed by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The TGA curves (Fig. 2) showed that all the polymer materials, irrespective of the initiator used, exhibited a similar 3-step decomposition profile as well as similar initial and end onset decomposition temperatures of about 200 °C and 475 °C, respectively. However, the amount of the stable residue left at >600 °C varied significantly, from 15% by AIBN, to 22% by TPT, 29% by I^tBu, and 31% by ^tBu-P₄. DSC curves (Fig. 3) showed a similar glass-transition temperature (T_g) of −15 °C by I^tBu, −17 °C by TPT, and −18 °C by ^tBu-P₄, but no apparent T_g was observed before decomposition for the P_βHMBL produced by AIBN.

Fig. 2 TGA curves of the polymers produced by ^tBu-P₄ (red, run 12), I^tBu (blue, run 6), TPT (green, run 9), and AIBN (black, run 15).

Fig. 3 DSC traces of the polymers produced by ^tBu-P₄ (red), TPT (green), I^tBu (blue), and AIBN (black).

Intriguingly, ¹H NMR spectra (Fig. 4) of the polymers produced by I^tBu, ^tBu-P₄ and TPT are essentially superimposable, so are their ¹³C NMR spectra (Fig. S5–S7†), indicating that they all produced more or less the same polymer structure. This observation suggests that either all three organic catalysts follow the same initiation and propagation pathway or they proceed with different initiation pathways but the resulting propagating species are interchangeable thus leading to the similar polymer structure as outlined in Scheme 1. On the other hand, the P_βHMBL produced by AIBN is that of a typical vinyl addition (atactic) polymer (Fig. 4), and its absence of the apparent T_g before its decomposition is consistent with this structure as the analogous polymer of _βMMBL exhibited a high T_g of ∼290 °C.²⁷ However, the polymer produced by I^tBu, ^tBu-P₄ and TPT is different than that by AIBN. The most visible spectroscopic difference is the appearance of the broad signal centered at δ 7.5 ppm in ¹H NMR, which correlates to signals at δ 156–152 ppm in ¹³C NMR established by ¹H–¹³C gHMQC and TOCSY (Fig. S14 and S15†), assignable to trisubstituted olefinic groups (vide infra). The presence of the OH groups in the polymer was confirmed by the H–D exchange experiment that showed the disappearance of the signal at δ 5.5 ppm upon addition of D₂O (Fig. S9 and S10†) and by FT-IR that revealed a broad absorption band centered at ∼3400 cm⁻¹ (Fig. S11†).

Fig. 4 Overlay of ¹H NMR (DMSO-d₆, 400 MHz, 25 °C) spectra of the polymers produced by TPT (green, run 9), ^tBu-P₄ (red, run 12), I^tBu (blue, run 6), and AIBN (black, run 15).

Focusing on the polymer produced by I^tBu, we carried out a series of 1D and 2D NMR (500 MHz) studies in DMSO-d₆ at 80 °C. The ¹³C NMR spectrum (Fig. S5†) showed a complex pattern of resonances, but the 135-DEPT experiment (Fig. S8†) enabled initial assignments of carbon resonances as follows: δ 178–172 ppm for C=O, 156–152 for >C=CH–, 128–126 for >C=CH–, 86.8 (m) for atactic main-chain quaternary C's, 79.3 for –OCH, 75.0 and 73.3 for –OCH₂, 72.1 for –OCH, 71.0 for –OCH₂, 68.8 for –OCH, 44.4 for CH, and 32.1 (m) for CH₂. Next, ¹H–¹³C gHMQC and TOCSY spectra (Fig. S14 and S15†) showed correlation between the peak at δ 7.5 ppm in ¹H NMR and the peaks at δ 156–152 ppm in ¹³C NMR, confirming the assignment of the peak at 7.5 ppm for >C=CH–. These correlation spectra also enabled assignments of the resonances from δ 2.7–2.0 ppm in ¹H NMR for those of CH (lower field) and CH₂ (higher field) protons, as well as the peak at δ 4.8 ppm for –OCH₂ protons and δ 4.4–3.9 ppm for those of –OCH and additional –OCH₂ protons (Fig. 5). In addition, from the ¹H–¹H gCOSY spectrum (Fig. S12†), the proton in >C=CH– at δ 7.5 ppm correlates to the protons in –OCH₂ at δ 4.8 ppm, thus revealing a connectivity of >C=CH–CH₂O–. Furthermore, the ¹H–¹H zTOCSY spectra (Fig. S13†) revealed that both protons in >C=CH–CH₂O– correlate to CH₂ protons centered at about δ 2.6 ppm, and the OH proton correlates to both types of the methylene protons in –OCH₂ and CH₂, revealing the intact hydroxyl lactone motif in the vinyl addition polymer units. Putting all pieces together, the possible structures present in the resulting polymer contain not only poly(vinyl lactone) (type I) and poly(ether lactone) (type II or III) but also a structure containing the trisubstituted olefinic moiety >C=CH–CH₂O–, the origin of which is probably due to dehydration (vide infra).

Fig. 5 MALDI-TOF MS spectrum of the low molecular weight oligomers produced by I^tBu.

The question of whether these structures represent a mixture of different polymers or possibly a copolymer containing all such units was interrogated by solvent fractionation experiments. We reasoned that polyether homopolymers would be soluble in solvents such as chloroform;³¹ accordingly, the crude polymer was subjected to Soxhlet extraction in refluxing chloroform for 30 h. There was no soluble fraction extracted, suggesting that the polymer product was not a mixture of different polymers, which is consistent with the unimodal molecular weight distribution on their GPC traces. Another key question concerns the >C=CH–CH₂O– unit derived from the above spectroscopic analysis and how it is generated in the current polymerization system. In principle, dehydration of an oxa-Michael addition product (see Scheme 1) could lead to the α,β-unsaturated-γ-butyrolactone unit, but its mechanistic pathway is currently unclear. Furthermore, the OH group on the vinyl polymer units could participate in proton transfer and subsequently generate a branching structure via the oxa-Michael addition pathway. In addition, the appearance of several types of >C=CH– signals at δ 156–152 ppm in ¹³C NMR may indicate that dehydration could occur within the vinyl polymer repeat unit to generate more such unsaturated structure. The observed low T_g of −15 °C for the polymer produced by I^tBu is also consistent with a copolymer containing the soft (branched) polyether units.

MALDI-TOF mass spectrum (Fig. 5) of a low molecular weight polymer sample produced by I^tBu showed a series of peaks associated with different sets of isomerides, with the peaks in the same shade-highlighted set representing the same total degrees of copolymerization but with different degrees of dehydration (loss of the mass unit of 18). Therefore, for the polymer produced by I^tBu the masses m/z can be expressed as: m/z = M_end + (114.1–18.0) × m + 114.1 × n, where m and n are the degrees of polymerization for _βHMBL with dehydration and the degrees of polymerization for _βHMBL without dehydration, respectively, while M_end is the MW of chain ends. Take the last set as an example, the peaks in this set can be attributed to the 11th-mer poly(vinyl-ether lactone) copolymer, where the peaks at 1417.216, 1399.187, 1381.157, and 1363.135 represent the copolymers containing 11 units of _βHMBL with 2 water loss, 11 units of _βHMBL with 3 water loss, 11 units of _βHMBL with 4 water loss, and 11 units of _βHMBL with 5 water loss, respectively. According to this analysis, the MW of the chain end is calculated to be 179.66 g mol⁻¹ (Fig. 6), corresponding to I^tBu⁺.

Fig. 6 Plot of the m/z value of the molecular ions (y) vs._βHMBL repeat unit (x).

Conclusions

In summary, we have revealed for the first time the successful polymerization of the naturally occurring, tri-functional monomer Tulipalin B or _βHMBL using nucleophilic/basic organic catalysts and a radical initiator, producing polymer products with molecular weight up to M_n = 1.32 × 10⁴ g mol⁻¹. The polymer produced by AIBN is that of a typical vinyl addition (atactic) polymer, but the polymers produced by NHCs I^tBu and TPT as well as superbase ^tBu-P₄ have rather complex structures, although the structures by I^tBu, TPT, ^tBu-P₄ are essentially the same. Extensive spectroscopic and analytical analyses of the resulting polymer products led to a possible conclusion that the polymer is likely a branched copolymer of poly(vinyl-ether lactone)s, derived from the proposed mechanistic crossovers between conjugate Michael and oxa-Michael additions, enabled by proton transfer due to the presence of OH groups in the monomer and within the polymer chain, followed by dehydration and branching.

Acknowledgements

This work was supported by the US National Science Foundation (NSF-1300267).

Notes and references

1. Selected recent reviews: (a) C. Thomas and B. Bibal, Green Chem., 2014, 16, 1687–1699; (b) H. A. Brown and R. M. Waymouth, Acc. Chem. Res., 2013, 46, 2585–2596; (c) K. Fuchise, Y. Chen, T. Satoh and T. Kakuchi, Polym. Chem., 2013, 4, 4278–4291; (d) H. A. Brown and R. M. Waymouth, Acc. Chem. Res., 2013, 46, 2585–2596; (e) M. K. Kiesewetter, E. J. Shin, J. L. Hedrick and R. M. Waymouth, Macromolecules, 2010, 43, 2093–2107; (f) N. E. Kamber, W. Jeong, R. M. Waymouth, R. C. Pratt, B. G. G. Lohmeijer and J. L. Hedrick, Chem. Rev., 2007, 107, 5813–5840.
2. Selected recent reviews: (a) C. M. R. Volla, I. Atodiresei and M. Rueping, Chem. Rev., 2014, 114, 2390–2431; (b) D. Liu and E. Y.-X. Chen, Green Chem., 2014, 16, 964–981; (c) R. C. Wende and P. Schreiner, Green Chem., 2012, 14, 1821–1849; (d) C. Grondal, M. Jeanty and D. Enders, Nat. Chem., 2010, 2, 167–178; (e) T. Marcelli and H. Hiemstra, Synthesis, 2010, 1229–1279; (f) D. W. C. MacMillan, Nature, 2008, 455, 304–308.
3. Selected recent reviews: (a) D. M. Flanigan, F. Romanov-Michailidis, N. A. White and T. Rovis, Chem. Rev., 2015, 115, 9307–9387; (b) M. N. Hopkinson, C. Richter, M. Schedler and F. Glorius, Nature, 2014, 510, 485–496; (c) D. J. Nelson and S. P. Nolan, Chem. Soc. Rev., 2013, 42, 6723–6753; (d) S. J. Ryan, L. Candish and D. W. Lupton, Chem. Soc. Rev., 2013, 42, 4906–4917; (e) A. Grossmann and D. Enders, Angew. Chem., Int. Ed., 2012, 51, 314–325; (f) X. Bugaut and F. Glorius, Chem. Soc. Rev., 2012, 41, 3511–3522; (g) T. Dröge and F. Glorius, Angew. Chem., Int. Ed., 2010, 50, 6940–6952; (h) S. Díez-González, N. Marion and S. P. Nolan, Chem. Rev., 2009, 109, 3612–3676; (i) F. E. Hahn and M. C. Jahnke, Angew. Chem., Int. Ed., 2008, 47, 3122–3172; (j) D. Enders, O. Niemeier and A. Henseler, Chem. Rev., 2007, 107, 5606–5655; (k) N. Marion, S. Díez-González and S. P. Nolan, Angew. Chem., Int. Ed., 2007, 46, 2988–3000; (l) D. Bourissou, O. Guerret, F. P. Gabbaï and G. Bertrand, Chem. Rev., 2000, 100, 39–91.
4. Selected recent reviews: (a) S. Naumann and A. P. Dove, Polym. Chem., 2015, 6, 3185–3200; (b) M. Fèvre, J. Pinaud, Y. Gnanou, J. Vignolle and D. Taton, Chem. Soc. Rev., 2013, 42, 2142–2172.
5. E. F. Connor, G. W. Nyce, M. Myers, A. Möck and J. L. Hedrick, J. Am. Chem. Soc., 2002, 124, 914–915.
6. (a) K. Fuchise, Y. Chen, T. Satoh and T. Kakuchi, Polym. Chem., 2013, 4, 4278–4291; (b) J. Raynaud, A. Ciolino, A. Baceiredo, M. Destarac, F. Bonnette, T. Kato, Y. Gnanou and D. Taton, Angew. Chem., Int. Ed., 2008, 47, 5390–5393; (c) M. D. Scholten, J. L. Hedrick and R. M. Waymouth, Macromolecules, 2008, 41, 7399–7404.
7. O. Coutelier, M. El Ezzi, M. Destarac, F. Bonnette, T. Kato, A. Baceiredo, G. Sivasankarapillai, Y. Gnanou and D. Taton, Polym. Chem., 2012, 3, 605–608.
8. G. W. Nyce, J. A. Lamboy, E. F. Connor, R. M. Waymouth and J. L. Hedrick, Org. Lett., 2002, 4, 3587–3590.
9. M. Hong and E. Y.-X. Chen, Angew. Chem., Int. Ed., 2014, 53, 11900–11906.
10. (a) T. Kato, Y. Ota, S.-I. Matsuoka, K. Takagi and M. Suzuki, J. Org. Chem., 2013, 78, 8739–8747; (b) S.-I. Matsuoka, Y. Ota, A. Washio, A. Katada, K. Ichioka, K. Takagi and M. Suzuki, Org. Lett., 2011, 13, 3722–3725; (c) A. T. Biju, M. Padmanaban, N. E. Wurz and F. Glorius, Angew. Chem., Int. Ed., 2011, 50, 8412–8415.
11. (a) Y. Zhang, M. Schmitt, L. Falivene, L. Caporaso, L. Cavallo and E. Y.-X. Chen, J. Am. Chem. Soc., 2013, 135, 17925–17942; (b) Y. Zhang and E. Y.-X. Chen, Angew. Chem., Int. Ed., 2012, 51, 2465–2469.
12. S.-I. Matsuoka, S. Namera and M. Suzuki, Polym. Chem., 2015, 6, 294–301.
13. W. N. Ottou, D. Bourichon, J. Vignolle, A.-L. Wirotius, F. Robert, Y. Landais, J.-M. Sotiropoulos, K. Miqueu and D. Taton, Chem. – Eur. J., 2015, 21, 9447–9453.
14. (a) I. Leito, T. Rodima, I. A. Koppel, R. Schwesinger and V. M. Vlasov, J. Org. Chem., 1997, 62, 8479–8483; (b) R. Schwesinger, C. Hasenfratz, H. Schlemper, L. Walz, E. M. Peters, K. Peters and H. G. von Schnering, Angew. Chem., Int. Ed. Engl., 1993, 32, 1361–1363; (c) R. Schwesinger and H. Schlemper, Angew. Chem., Int. Ed. Engl., 1987, 26, 1167–1169; (d) R. Schwesinger, Chimia, 1985, 39, 269–272.
15. (a) J. Zhao, N. Hadjichristidis and Y. Gnanou, Polimery, 2014, 59, 49–59; (b) B. Esswein and M. Möller, Angew. Chem., Int. Ed. Engl., 1996, 35, 623–625; (c) B. Eßwein, N. M. Steidl and M. Möller, Macromol. Rapid Commun., 1996, 17, 143–148; (d) A. Molenberg and M. Möller, Macromol. Rapid Commun., 1995, 16, 449–453; (e) T. Pietzonka and D. Seebach, Angew. Chem., Int. Ed. Engl., 1993, 32, 716–717.
16. M. Schmitt, L. Falivene, L. Caporaso, L. Cavallo and E. Y.-X. Chen, Polym. Chem., 2014, 5, 3261–3270.
17. (a) Y. Chen, K. Fuchise, A. Narumi, S. Kawaguchi, T. Satoh and T. Kakuchi, Macromolecules, 2011, 44, 9091–9098; (b) J.-C. Hsu, Y. Chen, T. Kakuchi and W.-C. Chen, Macromolecules, 2011, 44, 5168–5177; (c) T. Kakuchi, Y. Chen, J. Kitakado, K. Mori, K. Fuchise and T. Satoh, Macromolecules, 2011, 44, 4641–4647.
18. Selected recent reviews: (a) M. J.-L. Tschan, E. Brulé, P. Haquette and C. M. Thomas, Polym. Chem., 2012, 3, 836–851; (b) G. W. Coates and M. A. Hillmyer, A virtual issue on “Polymers from Renewable Resources”, Macromolecules, 2009, 42, 7987–7989; (c) A. Gandini, Macromolecules, 2008, 41, 9491–9504; (d) C. K. Wiliams and M. A. Hillmyer, Polym. Rev., 2008, 48, 1–10; (e) M. A. R. Meier, J. O. Metzger and U. S. Schubert, Chem. Soc. Rev., 2007, 36, 1788–1802.
19. (a) R. R. A. Kitson, A. Millemaggi and R. J. K. Taylor, Angew. Chem., Int. Ed., 2009, 48, 9426–9451; (b) H. M. R. Hoffman and J. Rabe, Angew. Chem., Int. Ed. Engl., 1985, 24, 94–110.
20. (a) L. E. Manzer, ACS Symp. Ser., 2006, 921, 40–51; (b) L. E. Manzer, Appl. Catal., A, 2004, 272, 249–256.
21. R. R. Gowda and E. Y.-X. Chen, Org. Chem. Front., 2014, 1, 230–234.
22. Recent reviews: (a) R. R. Gowda and E. Y.-X. Chen, in Encyclopedia of Polymer Science and Technology, ed. H. F. Mark, Wiley, Hoboken, 4th edn, 2013, vol. 8, pp. 235–271, 2014 (print edition),  DOI:10.1002/0471440264.pst606 (online edition); (b) S. Agarwal, Q. Jin and S. Maji, ACS Symp. Ser., 2012, 1105, 197–212; (c) R. Mullin, Chem. Eng. News, 2004, 82(45), 29–37.
23. Selected recent examples: (a) Y. Higaki, R. Okazaki and A. Takahara, ACS Macro Lett., 2012, 1, 1124–1127; (b) R. A. Cockburn, R. Siegmann, K. A. Payne, S. Beuermann, T. F. L. McKenna and R. A. Hutchinson, Biomacromolecules, 2011, 12, 2319–2326; (c) R. A. Cockburn, T. F. L. McKenna and R. A. Hutchinson, Macromol. Chem. Phys., 2010, 211, 501–509; (d) J. Mosnáček, J. A. Yoon, A. Juhari, K. Koynov and K. Matyjaszewski, Polymer, 2009, 50, 2087–2094; (e) J. Mosnáček and K. Matyjaszewski, Macromolecules, 2008, 41, 5509–5511; (f) G. Qi, M. Nolan, F. J. Schork and C. W. Jones, J. Polym. Sci., Polym. Chem., 2008, 46, 5929–5944; (g) C. U. Pittman Jr. and H. Lee, J. Polym. Sci., Polym. Chem., 2003, 41, 1759–1777; (h) J. W. Stansbury and J. M. Antonucci, Dent. Mater., 1992, 8, 270–273; (i) M. Ueda, M. Takahashi, Y. Imai and C. U. Pittman Jr., J. Polym. Sci., Polym. Chem. Ed. Engl., 1982, 20, 2819–2828; (j) M. K. Akkapeddi, Polymer, 1979, 20, 1215–1216.
24. (a) Y. Hu, L. O. Gustafson, H. Zhu and E. Y.-X. Chen, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 2008–2017; (b) J. Suenaga, D. M. Sutherlin and J. K. Stille, Macromolecules, 1984, 17, 2913–2916; (c) M. K. Akkapeddi, Macromolecules, 1979, 12, 546–551.
25. (a) Y. Zhang, L. O. Gustafson and E. Y.-X. Chen, J. Am. Chem. Soc., 2011, 133, 13674–13684; (b) G. M. Miyake, Y. Zhang and E. Y.-X. Chen, Macromolecules, 2010, 43, 4902–4908; (c) D. Y. Sogah, W. R. Hertler, O. W. Webster and G. M. Cohen, Macromolecules, 1987, 20, 1473–1488.
26. (a) T. Xu and E. Y.-X. Chen, J. Am. Chem. Soc., 2014, 136, 1774–1777; (b) Y. Zhang, G. M. Miyake, M. G. John, L. Falivene, L. Caporaso, L. Cavallo and E. Y.-X. Chen, Dalton Trans., 2012, 41, 9119–9134; (c) Y. Zhang, G. M. Miyake and E. Y.-X. Chen, Angew. Chem., Int. Ed., 2010, 49, 10158–10162.
27. (a) R. R. Gowda and E. Y.-X. Chen, Dalton Trans., 2013, 42, 9263–9273; (b) Y. Hu, X. Wang, Y. Chen, L. Caporaso, L. Cavallo and E. Y.-X. Chen, Organometallics, 2013, 32, 1459–1465; (c) X. Chen, L. Caporaso, L. Cavallo and E. Y.-X. Chen, J. Am. Chem. Soc., 2012, 134, 7278–7281; (d) Y. Hu, G. M. Miyake, B. Wang, D. Cui and E. Y.-X. Chen, Chem. – Eur. J., 2012, 18, 3345–3354; (e) Y. Hu, X. Xu, Y. Zhang, Y. Chen and E. Y.-X. Chen, Macromolecules, 2010, 43, 9328–9336; (f) G. M. Miyake, S. E. Newton, W. R. Mariott and E. Y.-X. Chen, Dalton Trans., 2010, 39, 6710–6718.
28. T. Nomura, E. Hayashi, S. Kawakami, S. Ogita and Y. Kato, Biosci., Biotechnol., Biochem., 2015, 79, 25–35.
29. E. M. Phillips, M. Riedrich and K. A. Scheidt, J. Am. Chem. Soc., 2010, 132, 13179–13181.
30. (a) M. Hong and E. Y.-X. Chen, Macromolecules, 2014, 47, 3614–3624; (b) A. Duda and A. Kowalski, Thermodynamics and kinetics of ring-opening polymerization, in Handbook of Ring-Opening Polymerization, ed. P. Dubois, O. Coulembier and J.-M. Raquez, Wiley-VCH, Weinheim, 2009, ch. 3.
31. (a) T. Wang, M. Yan, X. Sun and D. Quan, Polymer, 2015, 57, 21–28; (b) D. Wang, G. Zhang, Y. Zhang, Y. Gao, Y. Zhao, C. Zhou, Q. Zhang and X. Wang, J. Appl. Polym. Sci., 2007, 103, 417–424.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and characterisations. See DOI: 10.1039/c5qo00262a

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