Sequence-Controlled Ring-Opening Polymerization - American

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Chapter 23

Sequence-Controlled Ring-Opening Polymerization: Synthesis of New Polyester Structures Emilie Brulé,1,2 Carine Robert,1 and Christophe M. Thomas1,* 1PSL

Research University, CNRS - Chimie ParisTech, Institut de Recherche de Chimie Paris, 75005 Paris, France 2Sorbonne Universités, UPMC Univ Paris 06, UFR926, F-75005, Paris, France *E-mail: [email protected]; Fax: +33(0)143260061; Tel: +33(0)144276721

In order to obtain alternating/stereocontrolled polymers, coordination polymerization using well-defined metal complexes has played a leading role in the last two decades. We have described selected published efforts to achieve these research goals using discrete, structurally well-characterized metal complexes. Keywords: Catalysis; Polyester; Ring-Opening Polymerization; Alternating

Introduction Of the variety of biodegradable polymers known, linear aliphatic polyesters have a leading position and are commonly produced by ring-opening polymerization (ROP) of cyclic esters (1, 2). In contrast to polyesterification, ROP of cyclic monomers proceeds under mild reaction conditions and avoids the formation of small molecule byproducts. Among the various ROP processes, including anionic, cationic, organocatalytic and coordination-insertion, the latter has gained increasing attention (3). It is now commonly accepted that the most efficient method for the production of well-controlled polyesters © 2014 American Chemical Society In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


in terms of molecular weight, composition and microstructure, is ROP with metal-coordination initiators (4). Therefore, a large number of investigations have been directed towards synthesizing efficient metal-based initiators and studying their reactivities. However, current aliphatic polyesters are far from being optimal and tailor-made structures are certainly needed. In the case of simple aliphatic polyesters, the available options for molecular optimization are relatively limited. Indeed, only a few macromolecular parameters can be varied, e.g. chain length, molecular weight distribution, chain-ends, architecture and microstructure. In particular, the control over chain microstructure (i.e. tacticity and monomer sequences) may lead to highly-optimized macromolecules with tailored properties (5). However, when polymers are made from more than one different type of monomer in a single pot process, it is difficult to control their primary structure to any significant extent. Catalyst design is an efficient option for sequence/stereochemistry control. In this regard, the application of readily available stereopure monomers in association with stereocontrolled ROP has enabled the facile manipulation of the tacticity of the resultant polyesters, considerably affecting their properties. For instance it has been already demonstrated that tacticity may substantially influence the degradation rates of synthetic polyesters (6, 7). In order to obtain alternating polymers, coordination polymerization using well-defined metal complexes has played a leading role in the last two decades. The goal of the proposed article is the description of catalysts that allow higher order sequence control of this latter type, and create alternating macromolecular structures with advanced properties. Herein, three practical examples will be discussed. It will first be shown that ROP techniques are interesting tools for preparing macromolecular architectures containing sequence-defined segments. For instance, heterotactic polylactides and alternating/syndiotactic polyhydroxyalkanoates prepared by ROP will be presented in this chapter. Also, comonomer sequences can be directly controlled in a ROP process. This aspect will be discussed in the second section of this chapter.

Synthesis of Heterotactic PLA from rac-Lactide The earliest examples were reported by Coates with a class of β-diiminate zinc complexes (8). These achiral derivatives featured high activities and selectivities, affording highly heterotactic polylactide (PLA) (Pr up to 0.94) from racemic lactide (rac-LA), by incorporating the (R,R)- and (S,S)-LA in an alternating fashion (Figure 1). Notably it was shown that the aryl groups’ substituents on the β-diiminate ligand exert a significant influence upon the course of the polymerizations, affecting both the degree of stereoselectivity and the rate of polymerization. For instance, complex 1c exhibited the highest activity and stereoselectivity of the zinc complexes studied for the polymerization of rac-lactide to PLA. However, changing the ligand substituents from isopropyl to n-propyl or ethyl groups resulted in a decrease in heterotacticity (Figure 2). 350 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 1. Synthesis of heterotactic PLA. In 2004, Gibson and coworkers reported high levels of heterotacticity with aluminum initiators stabilized by tetradentate phenoxyamine (salan-type) ligands (9) (e.g. complexes 2c and 2d, Figure 2). The microstructures of the poly(lactide)s obtained were also found to be dependent upon the ancillary ligand substituents (Pr up to 0.96). In particular, it was shown that the size of the nitrogen substituents R1 plays a crucial role on the tacticity of the polymer produced using these initiators. On the basis of the single-crystal X-ray analysis of complex 2d, Gibson proposed that the alkylamino backbone substituents can closely approach the site of polymer chain growth and thereby influence monomer selectivity.

Figure 2. Stereoselective systems for the heterotactic ROP of rac-lactide. Chisholm investigated a series of tris-pyrazolyl borate (TpR, R = iPr, tBu for instance) calcium complexes for lactide polymerization (Figure 2) (10). The monomeric amide or aryloxide complexes of the form (TptBu)CaX (3a-b) were shown to be highly active and stereoselective, leading to heterotactic PLA (Pr = 0.90). The use of bulky substituent as seen in the TptBu ligand is necessary to confer single-site living polymerization behavior and stereoselectivity in the ring-opening event. Moving towards a different metal-based system, Thomas and Davidson have recently reported the first examples of single-site germanium initiators for the ROP of LA (Figure 3) (11, 12). Catalytic experiments showed that the C3-symmetric Ge-based isopropoxide system 4 was active for the solvent-free ROP of rac-LA, to provide heterotactic-enriched PLA (up to 0.82). Having identified an active catalyst which displays promising selectivity, Davidson decided to investigate different germanium complexes with the aim of optimizing the selectivity and activity of germanium-based single-site initiators for ROP of LA. Unfortunately, the synthetic route used for 4 proved to be unsuitable for other Ge-OiPr complexes. 351 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 3. Synthesis of germanium-based complex 4. In 2009 Hillmyer and Tolman prepared highly heterotactic PLA (0.86 < Pr < 0.94 at 25°C, Pr = 0.97 at 0°C) from rac-LA using an easily accessible catalyst prepared in situ from indium trichloride, benzyl alcohol and triethylamine (13). The resulting robust system was found to operate under a variety of reaction conditions to yield heterotactic PLA with controlled molecular weight and a narrow molecular weight distribution. As early as 2002, Coates described the catalytic behavior of a new heteroleptic yttrium alkoxide complex (14). Although this derivative revealed higher activity than the corresponding aluminum derivative, no stereoselectivity was observed for the polymerization of rac-LA. From this study, other research groups anticipated that rare-earth metal complexes supported by multidentate bis(phenoxide) ligands would be of interest in order to achieve effective ROP of rac-LA (15). For instance, Okuda reported the synthesis of several lanthanoid complexes such as 5 and 6 supported by 1,ω-dithiaalkanediyl-bridged bis(phenoxide) ligands (Figure 4) (16). Once again, the ancillary ligand proved to be crucial for polymerization selectivity. Among these bis(phenoxide) derivatives, scandium complexes 5a and 6a showed high heterotactic selectivity (Pr up to 0.95) which was attributed to a dynamic monomer-recognition process involving interconversion of the ligand configuration.

Figure 4. Lanthanide complexes supported by dichalcogen bridged bis(phenoxide) ligands. The reactivity of yttrium, neodymium and lanthanum-based metal derivatives supported by amino-bis(phenoxide) ligands was also investigated (17). In 352 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


particular, the yttrium amido complexes 7a-d (Figure 5) have demonstrated an interesting ability for mediating the heterotactic living polymerization of rac-LA (Pr up to 0.96) (18). It was demonstrated that the steric bulk of the ortho-phenoxide substituents plays a crucial role in achieving high selectivity for the chain-end controlled polymerization of rac-lactide. For instance, complex 7b, which bears cumyl (α,α-dimethylbenzyl) groups in the ortho positions of the phenoxides, produced heterotactic-enriched polymer (Pr = 0.90), whereas complex 7a gave lower selectivity (Pr = 0.80) for heterotactic PLA. By using bulkier groups as R1 substituents, PLAs produced by 7c-d were found to be substantially more heterotactic, with Pr values of respectively 0.95 and 0.96.

Figure 5. Stereoselective yttrium-based systems for the heterotactic ROP of rac-lactide.

Synthesis of Syndiotactic PHB from rac-BBL Despite the increasing number of studies dealing with the ROP of rac-β-butyrolactone using metal complexes for the last decade, there is still a limited number of initiators capable of producing highly syndiotactic-enriched PHB (Figure 6) (19, 20). Gross first showed that (nBu)3Sn(OMe) 8a was able to polymerize rac-BBL with a moderate probability of racemic linkage between monomer Pr of 0.70 at 40°C (Figure 7) (21). When the temperature is increased, the selectivity decreased but the reactivity was improved with 70% yield of low molecular weight PHB after several days at 75°C. With less bulky (nBu)2Sn(OMe)2 8b complex, a Pr value of 0.72 could be reached at 0°C (22). Kricheldorf further studied the influence of the number of butyl groups coordinated to the tin metal center and thus compared the reactivity of the least steric (nBu)Sn(OMe)3 8c (23). It was found that (nBu)2Sn(OMe)2 8b exhibited the highest activity with molecular weight lower than 10 000 g.mol-1, and that the stereoselectivity increased with the steric hindrance around the metal. Bis(tributyl) and bis(triaryl) 353 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


tin oxides 9a-b (22) were also tested for the ROP of rac-BBL and induced similar syndiotacticity of 0.6-0.7 depending on temperature, suggesting a chain-end control of stereoselectivity rather than catalyst site. In the case of dialkyl tin oxide 10 as initiators, the nature of alkyl group is important since variation of reactivity and stereoselectivity was observed. Indeed while Me2SnO 10a was inactive, Et2SnO 10b and Bu2SnO 10c displayed highest reactivity with Mn up to 80 000 g.mol-1 and similar Pr of 0.72 at 100°C and 50°C respectively (24). It was the first example of selectivity occurring at high temperature. Surprisingly the bulkiest di(octyl)tin oxide 10d led to poorer selectivity. More recently, the use of distannoxanes 11 as catalysts (25, 26) was investigated but still moderate and similar syndiospecificity was obtained (Pr = 0.58-0.67) for the ROP of rac-BBL. Since the reports using Sn(IV) complexes as catalysts for the preparation of syndiotactic enriched PHBs, almost all recent studies deal with the use of discrete metal complexes of group 3 and lanthanides except one using metals of group 4. Various bi- tri- and tetradentate ligands bearing nitrogen and/or oxygen atoms have been selected and initiators were either metal amides or alkoxides.

Figure 6. Synthesis of iso- or syndiotactic PHBs by the ROP of rac-BBL.

Figure 7. Sn(IV) complexes as initiators for the syndiotactic ROP of rac-BBL. 354 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Bidentate pro-ligand guanidine was chosen by Carpentier and co-workers (27) who reported the synthesis new bis(guanidinate) isopropoxide complexes of yttrium 12a and lutetium 12b which display interesting syndioselectivity for the ROP of rac-BBL (Figure 8). Pr between 0.80 and 0.84 were obtained depending on the solvent of polymerization (THF or toluene) and a chain-end control mechanism was confirmed using 13C NMR spectroscopy. Although the two complexes displayed similar selectivity, they differed regarding their reactivity. The yttrium complex 12a was much more active than its lutetium counterpart 12b with TOF of 50 h-1 and 2 h-1 respectively.

Figure 8. Rare earth metal complexes.

The same group also investigated the synthesis of rare earth metal (Sc, Y, La) complexes bearing sterically demanding o-substituted bis(naphthoxide)-pyridine and -thiophene tridentate ligands (28) (Figure 8). The complexes were characterized and shown to be mononuclear, 5-coordinate around the metal (coordination of amido group and THF) and Cs-symmetrical. Among all of the complexes synthesized, bis(naphthoxide)-pyridine of yttrium 13a and lanthanum 13b were the only catalysts affording syndiotactic-enriched PHB with a high probability for racemic linkage of 0.87 and 0.86 respectively, in toluene. In order to reach a value of 0.87 using complex 13a, 1 equivalent of isopropanol was added to generate in situ the isopropoxide complex and the reaction temperature was conducted at 50°C instead of 20°C to increase the reactivity (TOF of 200 h-1 vs 20 h-1 at 20°C without addition of isopropanol). While yttrium amido complex 13a (without isopropanol addition) afforded a Pr lowered to 0.76, the lanthanum-amido complex 13b displayed itself high syndioselectivity (Pr = 0.86) and was more productive than 13a even at 20°C (TOF 720 h-1). Several examples of yttrium complexes bearing tetradentate ligands have also been reported as efficient initiators for the production of syndiotactic PHB. Thomas et al. (29, 30) investigated the activity and selectivity of amino-alkoxy-bis(phenoxide) complexes 14a-c prepared in situ from the amido N(SiHMe3)2 counterparts (Figure 9). These complexes led to controlled polymerization of rac-BBL with polydispersity around 1.1 and experimental molecular weights close to theoretical ones. The authors showed that complex 10b was able to convert 1740 equivalents and that 14b and 14c are very productive 355 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


with TOF up to 23500 h-1. Moreover these complexes displayed the highest probability of racemic linkage to date (Pr = 0.94) at room temperature for 14c and at -20°C for 14b. Complex 14a which contains tert-butyl groups in ortho positions was less syndioselective (Pr = 0.81). The authors further studied the influence of the nature of the ortho substituent situated on the phenoxide moieties on the resulting tacticity. Complexes substituted with chloro or CMe2(4-CF3C6H4) groups were also synthesized and tested and the chloro complex led to atactic polymer while the CF3-aryl analogue induce syndiotactic stereocontrol of 82% (31). DFT computations on model intermediates showed the importance of the presence of an ortho-aryl substituent for high syndiotactivity such as on 14b-c since this aryl is involved in a stabilized CH···πinteraction with the methylene group of opened BBL. Electronic properties are thus involved in the mechanism of polymerization as well as a chain-end control mechanism by acyl cleavage of BBL, confirmed by NMR studies. Thomas and coworkers also looked at the influence of the syndiotactic degree of the polymers on the thermal properties and it was demonstrated that the melting point (Tm) of the polymers increased linearly with the syndiotacticity. The highest Tm reached 183°C and corresponded to the 94% enriched syndiotactic PHB obtained with 14c.

Figure 9. Amino-alkoxy-bis(phenolate) alkoxide-yttrium complexes for the synthesis of highly syndiotactic PHB. More recently, Yao and co-workers utilized a similar amino-bis(phenol) pro-ligand to that of complex 14a but the methoxide side chain was replaced by an amine group (Figure 10) (32). Moreover the rare earth metal (Y, Yb, Er, and Sm) complexes alkoxides were isolated and not prepared in situ as it was the case for 14a-c. Three yttrium-alkoxide complexes 15a-c were obtained by addition of 2,2,2-trifluoroethanol, benzyl alcohol and 2-propanol. In the case of ytterbium, erbium and samarium, only 2,2,2-trifluoroethoxide complexes 16a-c were synthesized and the X-ray structure of all the complexes were obtained. For the polymerization of rac-BBL, it was shown that the polymerization was controlled with a narrow distribution of the PHB formed (Mw/Mn < 1.26) and that the activity of the complexes followed the trend of Yb>Er>Y>>Sm, which corresponds to an increase of ionic radii of the rare earth metals. The ytterbium complex 16a displayed the highest productivity of these initiators with a TON of 1900 and a TOF of 12000 h-1 while a TOF of 45 h-1 was obtained with the samarium 356 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


analogue 16c. No further characterization of the polymer was given. On the contrary, regarding the selectivity of the PHB formed, the metal did not have any influence as 82% syndiotactic-enriched polymers were obtained in all cases in toluene. There was no influence of the nature of the alkoxide neither since the three different yttrium complexes exhibited the same activity and selectivity. A 1H NMR study of the polymer obtained using the benzyloxy yttrium complex 15b and analysis of the chain-end groups revealed a coordination-insertion mechanism but some crotonate was also observed, typical of elimination side-reaction.

Figure 10. Amine-bridged amino-bis(phenoxide) alkoxy-rare earth metal and salan amido yttrium complexes. Other tetradentate pro-ligands often used in catalysis are Schiff bases of salanand salen-type. Pellechia et al. (33) reported the synthesis of amido-yttrium complexes of salan-type 17a-b and binaphthyl-bridged salen-type for the ROP of rac-BBL (Figure 10). Complex 17a was the most active although much less productive than the amino-bis(phenoxide) rare earth metal complexes described above (TOF of only 122 h-1 at 70°C). The syndioselectivity was also moderate as a probability of racemic linkage Pr of 0.81 was obtained with the best initiator 17a at 20°C and 70°C. The salen-type complexes produced PHBs syndiotactic-enriched at 76% at 20°C. The less selective initiator was the bulky and rigid adamantylsubstituted salan complex 17b (Pr = 0.64 at 20°C). Considering this latter result the author suggested that the selectivity originated from both the chirality of last inserted BBL unit (chain-end control mechanism) and the chirality of the ligand around the metal center (enantiomorphic site control). Only one report deals with group 4 metal complexes for the production of syndiotactic-enriched PHB. Davidson et al. used amino-tris(phenol) pro-ligands L1H3-L3H3 bearing substituents with different steric and electronic properties (Figure 11) (34). Titanium(IV), zirconium(IV) and hafnium(IV) isopropoxide complexes 18a-c were synthesized by addition of the appropriate precursor to the pro-ligands. In the solid state, dimer complexes were formed whereas in solution monomeric species are present, and the authors suggested that the initiator is predominantly monomeric in the presence of an excess of rac-BBL. Narrow distributions were obtained for the polymers in the presence of all complexes but the titanium initiators 18a did not induce any selectivity and displayed only poor activity. The zirconium and hafnium analogs 18b-c were more active although not very productive with similar TON and TOF of 300 and 40-50 h-1 respectively. Moreover syndiotactic-enriched PHB were obtained with both metal complexes 357 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


albeit moderate degrees of stereocontrol 64-76% were observed. The hafnium initiators were the most selective (Pr of 0.74-0.76) in the presence of ligands containing a tert-butyl group in para position (L2) of the third phenoxide group or without substitution (L1) (Figure 12). The chloro-substituted Hf(IV) complex {Hf(L3)(OiPr)}2 led to a poorer selectivity of 68%. The MALDI-TOF analysis of the polymers indicated that the chain-end groups were isopropoxide and a proton on the other side.

Figure 11. Tris(phenol) ligands coordinated to group 4 metals.

Figure 12. Tris(phenoxide) zirconium and hafnium(IV) complexes for the synthesis of PHB. Other PHAs have also been prepared in a sequence-control approach using yttrium complexes. Thomas et al. (35) investigated the copolymerization of rac-BBL and rac-allyl-β-butyrolactone (rac-allylBBL) using the tetradentate amino-alkoxy-bis(phenoxide) yttrium-amido complex 7a (Figure 13). Syndiotactic-enriched random copolymers were produced with a probability of racemic linkage between 0.80 and 0.84. Different ratios of both monomers were studied and in all cases 80% of rac-allylBBL was converted. A plot conversion vs time carried out for a 1:1 ratio of monomers showed that both monomers were consumed at the same time. The copolymers formed were determined to be monomodal and characterized by NMR spectroscopy. Moreover narrow distribution was observed as well as accordance of the experimental and theoretical molecular weights when up to 300 equivalents of monomers polymerized. Thermal analyses revealed that the copolymer is semi-crystalline but an increase of the amount of rac-allylBBL decreased the crystallinity of the copolymer (decrease of Tm and Tg). This result was predictable as the homopolymer poly(rac-allylBBL) is amorphous. The allyl side-chains of the copolymer were further functionalized by hydroxy groups and epoxides 358 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


without any change of distribution in the resulting polymer, and similar thermal properties to pristine copolymer were obtained. The encapsulation and release of L-leuprolide with some of these copolymers was also investigated (36).

Figure 13. Synthesis of syndiotactic-enriched random copolymers.

Instead of polymerizing a mixture of racemic different monomers, Thomas and co-workers also decided to start from a mixture of enantiomerically pure BBL and β-lactone (different from BBL) of opposite absolute configuration to prepare alternating copolymers (Figure 14) (37). Regarding the catalyst, the addition of isopropanol to a salan yttrium-amido complex gave rise to a bimetallic salan yttrium-isopropoxide 19 which was the most efficient initiator compared to the amido complex. The rate of copolymerization was found to be similar for both monomers with different substitution although polymerization of each of them separately proceeded at different rate. Depending on the nature of the side chain of the β-lactone, alternations between 91 and 94% were obtained. Thomas et al. also reported the one-pot synthesis of this yttrium-isopropoxide dimer 19 along with the dimer (salan)2Y2(μ-OiPr)(μ-OH) by direct reaction of the salan pro-ligand and yttrium isopropoxide (38). The polymerization of rac-BBL with the mixture of these dimers was also investigated and highly syndiotactic PHB (Pr = 0.90) was formed. DFT calculations of the ROP of rac-BBL were undertaken with a salanY(OiPr) in order to have a better understanding of the origin of the selectivity. It was first demonstrated that the initiation steps for (R)- or (S)-BBL were similar and that whatever enantiomer is inserted first, a syndiotactic polymer chain is preferred over an isotactic one.

Synthesis of Aliphatic Polyesters by Alternating Copolymerization Although the ring-opening polymerization of cyclic esters takes place under mild reaction conditions without any byproducts, the low availability of structurally diverse monomers restricts the scope of the polymer architecture (Figure 15) (39, 40). Alternatively, the ring-opening copolymerization of epoxides with cyclic anhydrides has the potential to produce a wide range of polymer backbone structures (41). 359 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 14. ROP of a mixture of enantiopure β-lactones catalyzed by {salanY(OiPr)}2 19.

Figure 15. Synthetic approaches to aliphatic polyesters.

The coupling of epoxides with cyclic anhydrides to afford polyesters was first described by using tertiary amines (42) in the 1960s. Then Inoue reported a system of aluminum tetraphenylporphyrin (TPP)AlX 20a-c with a covalently bound axial ligand and a quaternary ammonium or phosphonium salt, as an effective catalyst for the copolymerization of phtalic anhydride (PA) and propylene oxide (PO). These systems provided an alternating polyester with unusually narrow molecular weight distribution and without any side reactions such as chain transfer or termination (Figure 16) (43). 360 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 16. Metalloporphyrin complexes. Taking into account how the diffusion of the residue of metallic compounds used as an initiator influences environment before and after biodegradation of the aliphatic copolymers, Maeda and Nishimura used common and nontoxic magnesium diethoxide to afford poly(ethylene succinate) and itaconic acid-based polymeric network respectively (44). However, catalysts reported for the ring-opening copolymerization of cyclic anhydrides with epoxides exhibit relatively long reaction times (days) and produce polyesters with low molecular weight values (

Sequence-Controlled Ring-Opening Polymerization - American

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