Ring-opening polymerization,

This polymerization should be more favorable, giving 

Addition polymerization may be initiated by anions, cations, or radicais, or effected by metal complexes. 

Radical polymerization requires an initiator such as AiBN and proceeds by a radicai chain process to give a largely regioregular, but generally atactic or somewhat syndiotactic polymer. Termination involves recombination or disproportionation. The process is tolerant of impurities, but molecular weight distributions are broad. 

Cationic polymerization is initiated by protons, other cations, or Lewis acids. It proceeds by the most stable carbocation to give a regioregular polymer, with variable tacticity. Termination involves reaction with an anion or proton transfer between chains. 

Anionic polymerization is initiated by BuLi or another anion and proceeds via the most stable carbanion to give a regioregular polymer. The process is terminated by exposure to oxygen, water, or COj. The process requires very pure, dry monomers, and solvents. 

Ring opening polymerization produces a small number of synthetic commercial polymers. Probably the most important ring opening reaction is that of caprolactam for the production of nylon 6  

Although no small molecule gets eliminated, the reaction can be considered a condensation polymerization. Monomers suitable for polymerization by ring opening condensation normally possess two different functional groups within the ring. Examples of suitable monomers are lactams (such as caprolactam), which produce polyamides, and lactons, which produce polyesters. 

Ring opening polymerization may also occur by an addition chain reaction. For example, a ring opening reaction polymerizes trioxane to a polyacetal in the presence of an acid catalyst. Formaldehyde also produces the same polymer  

These water soluble polymers are commercially known as carbowax. 

The ring opening of cycloolefins is also possible with certain coordination catalysts. This simplified example shows cyclopentene undergoing a first-step formation of the dimer cyclodecadiene, and then incorporating additional cyclopentene monomer units to produce the solid, rubbery polypentamer  

Ring opening polymerization (ROP) of cyclic oligoesters is an emerging method of polymerization that has some advantages compared to melt phase polycondensation (71). In this method, the polyester is prepared from low-viscosity precursors by an entropi-cally driven process that evolves without releasing of volatiles and emission of heat. 

ROP has been recently applied for the synthesis of polyesters derived from 1,4-CHDM. The cyclic oligomers have to be previously synthesized in solution or extracted from thermally depolymerized PCT (38,72). Thus, cyclic oligomers of PCT have been prepared by reaction of 1,4-CHDM with terephthaloyl chloride under diluted conditions and then fractionated by size in a silica gel column (39). The main fraction collected was that containing from dimer to pen-tamer with the trimer being the major component. The ROP of these cycles carried out at 310 °C and using different catalysts yielded high molecular weight PCT. 

The properties of polyesters and copolyesters containing cyclo-hexylenedimethylene units are greatly influenced by the type of diacid used and the stereoisomeric composition of the 1,4-CHDM used for their synthesis. Since the cis/trans ratio of commercially available 1,4-CHDM is 30/70, the properties are referred to the polymers synthesized with this isomeric ratio unless a different composition is explicitly mentioned. 

Ring opening polymerization is a chain reaction with the carbene as chain carrier. Polymers are not formed by consecutive macrocycUzation, but instead cyclic oligomers are obtained when the carbene end reacts with a double bond within the polymer chain, by intramolecular metathesis [31]. 

The theory predicts a bimodal molecular weight distribution of polyocte-namers and polydodecenamers prepared by metathesis of cyclooctene. The low molecular weight products were assumed to be linear polymers [32]. Further support for this carbene chain reaction is that the ratio of 

Ci6j C24/C16, C28/C16 and C32/C16 are essentially constant throughout the ring opening polymerization. The constancy of the C12/C16 ratio is especially significant in this respect, since the pairwise mechanism would have required cross metathesis of the cyclooctadiene and cyclobutene, which is unUkely [33]. 

The alcoxycarbene [W(=CMeOEt)(CO)5] is used as a catalyst precursor for polymerization of cyclopentene, in the presence of the cocatalyst EtAlCl2 [34] but other carbene complexes e.g. [W(=CPh2)(CO)5], can be used without a cocatalyst for the same reaction [35]. 

In accordance with this alkylidene—metallacyclobutane mechanism, metathesis of alkynes has been performed using tungsten metallacarbyne complexes [36]. The analogy between the reactions of olefins and acetylenic hydrocarbons has led to the assumption that metallacyclobuta-dienes could be intermediates for this reaction. 

Additionally, ring-opening polymerization can be used to prepare polymers which cannot easily be prepared by other methods, e.g. poly(phosphazene)s. Some important ring-opening polymerizations are listed in Table 2.12. 

TABLE 2.12 Some important ring-opening polymerizations 

In the polymerization of ethylene oxide initiation takes place by addition of to the epoxide oxygen atom to yield a cyclic oxonium ion (I) which is in equilibrium with the corresponding open-chain carbocation (II) 

Both species can propagate (I) via ring-opening of the cyclic oxonium ion upon nucleophilic attack at a ring carbon atom by the epoxide oxygen atom in another molecule of monomer, and (II) via its addition to monomer in a reaction similar to the initiation step. In each case the initial product of propagation has a terminal cyclic oxonium ion formed from the newly added molecule of monomer 

when less stable counter-ions such as AlC are employed, both V and ir can rearrange to give 

HgureS. General scheme for a ring-opening poljrmerization. 

Much of the interest in ring-opening polymerizations stems from the fact that the polymers formed may have lower densities than the monomers from which they are derived (i.e. volume expansion may accompany polymerization).168-171 This is in marked contrast with conventional polymerizations which typically involve a nett volume contraction. Such polymerizations are therefore of particular interest in adhesive, mold filling, and other applications where volume 

Reviews on radical ring-opening polymerization include those by Sanda and Endo,172 Klemm and Schultz,173 Cho,1 4 Moszner et at.,175 Endo and Yokozawa176 

The ring-opening reaction usually results in the formation of a new unsaturated linkage. When this is a carbon-carbon double bond, the further reaction of this group during polymerization leads to a crosslinked (and insoluble) structure and can be a serious problem when networks are undesirable. In many of the applications mentioned above, crosslinking is desirable. 

2-Phenyl-1-vinylcyclobutane (37) is also reported to give partial ring-opening174 while the vinylpropiolactones 38 and 39 give 100% ring-opening with loss of carbon dioxide.174 

However, it may also reflect the greater ease with which the larger ring systems can accommodate lhe slereoelectronic requirements for fi-scission (Section 2.3.4). n Substituents (e.g. CH j, Ph) which lend stabilization to the new radical center, or increase strain in the breaking bond, also favor ring-opening (Tabic 4.5), 

For ring-opening to compete effectively with propagation, the former must be extremely facile. For example with Ap-lO -lO M the rate constant for ringopening ( (j) must be at least -10 -10 s- to give 99% ring-opening in bulk 

Many heterocyclic compounds can be polymerized by ring opening under certain conditions with ionic initiators, to produce linear macromolecules. Amongst these are cyclic ethers, cyclic sulfides, cyclic acetals, cyclic esters (lactones), cyclic 

When a ring is opened to form a linear polymer, the propagation step may resemble either chain or stepwise polymerization. It was pointed out in Section 4.8 that an equilibrium may be set up between open-chain and ring structures when the monomer (i.e., the ring) is as reactive as growing polymer. In making nylon 6 from caprolactam, no condensation is involved because the product has the same 

FIGURE 4.18 Ring opening by olefin metathesis. (Data from Calderon, N., and R. L. Hinrichs, Chem. Tech., 4, 627,1974.) (a) Polymerization of cyclopentane (b) olefin metathesis (c) cross metathesis with acrylic olefin. 

Recently researchers have examined the polymerization of polycarbonates and polyesters from macrocycles, ring structures made up of dozens of repeat units. The advantage of this approach is that a low-melting-point, low-viscosity macrocycle can be molded in shape and then converted to a polymer [49]. This process has not yet been commercialized. 

Another common feature of most ring-opening schemes is the low heat of polymerization. Polybutadiene has a very similar structure to polypentenamer, but the isothermal conversion of butadiene to polymer requires removal of 74.9 kJ/mol, whereas conversion of cyclopentene to polymer requires removal of only 18.4 kJ/mol. A commercial polyalkenamer and other polymers made from metathesis and ringopening metathesis polymerization are described in Section 16.4. 

Formation of an epoxy resin. The cross-links shown in the last structure indicate bonding arising from nucleophilic opening of an epoxide on another bis-epoxide. 

Macrocyclic oligomer precursors of polycarbonates, PET, polymers of amides, etherketones, and ethersulfones are candidates for further study of macrocyclic oligomer polymerization thermodynamics [6]. Research extends to cyclic arylates (cyclic aryl-aryl esters) and cyclic alkyl aryl esters going back to isolating a cyclic trimer of poly(ethylene-terephthalate) in 1954 [13]. Much of the work on macrocyclic oligomers as precursors to high-MW macromolecules starts with spiro(bis)indane (SBI) biphenyl monomer to produce macrocyclic carbonates. 

Reaction conditions needed to form selective cycUcs include maintenance of the right hydrolysis, condensation ratio, and the right amine catalyst [13]. 

Structural characterizations of macrocycUc aromatic sulfide oUgomers were studied jointly at Guangzhou Institute of Chemistry and McGUl University Department of Chemistry. The studies used matrix-assisted laser desorption and ionization time of flight mass spectroscopy (MALD ITF-MS), which the researchers said was a powerful tool to analyze the macrocyclics [13]. 

Several cyclic organic molecules can be polymerized by the ring-opening polymerization method [1-5]. Although the metathesis polymerization that we discussed earlier also is a ring-opening polymerization, it was a special 

Trioxane can be polymerized into poly(formaldehyde) by the action of Lewis acids (see Eq. 2.43). Notice that in this method of polymerization there is no change in the chemical composition as the monomer gets converted into the polymer. Caprolactam can be polymerized to nylon-6 by the ring-opening method (see Eq. 2.44). 

Three-membered rings such as epoxides and aziridines can also be polymerized to their polymeric analogues. The release of ring-strain is an important factor in the polymerization of the three-membered rings. Thus, ethylene oxide can be polymerized to poly(ethylene oxide) by anionic or cationic initiators (see Eq. 2.45). 

Similarly, ethylene imine polymerizes quite readily in the presence of cationic initiators (see Eq. 2.46). 

Subsequent chain propagation reactions can occur by the electrophilic attack of the reactive carbocation (see Eq. 2.48). 

In general, it is possible to obtain higher degrees of polymerization by ring-opening polymerization than with polycondensation. Two types are used in oligo and polysaccharide syntheses orthoester synthesis and cationic anhydrosugar polymerization. 

In the orthoester synthesis, using HgBr2 as catalyst, a cyclic orthoester with no free hydroxyl groups is converted into products with degrees of polymerization of up to 50 and yields of up to 50%  

This must be a case of polymerization via activated monomers (and not a polycondensation) since the molecular weight is determined by the 

Amylose, amylopectin, glycogen, and dextrin belong to the amylose group. Amylose ( -20%) and amylopectin ( 80%) are the components of starch. 

These generally differ from the more conventional processes proceeding on multiple bonds. As indicated by their name, they occur by the breaking of some (usually polar) bond between two atoms of a cyclic monomer, and the product of this opening, original cyclic but now linear, forms a part of the growing chain 

By ring-opening polymerization, polymers can formed from cycloalkenes, but mostly from simple and more complicated heterocycles containing heteroatoms such as O, N, S, P and Si. Ring-opening polymerizations are mostly initiated by the ionic mechanism. 

Metathesis is a special kind of ROP and also of disproportionation. The monomers are cycloalkenes, i. e. compounds which are both cyclic and contain a double bond. Both linear and cyclic, but always unsaturated, macromolecules are generated. The similarity of disproportionation and metathesis is indicated by the scheme 

Metatheses mostly proceed by a coordination mechanism on catalysts of the Ziegler-Natta type. 

Cyclic compounds are potentially polymerizable as the difunctionality criterion for polymerizability is achieved by a ring-opening process as shown below for ethylene oxide  

Monomer type Monomer structure Repeating unit Polymer type 

Thus the most reactive (i.e., thermodynamically least stable) monomers are those containing 3- or 4-membered rings. The data in Table 10.1 further show that cyclohexane is the most resistant to polymerization, since AG is positive. For cyclopropane, cyclobutane, cyclopentane, cycloheptane, and cyclooctane, AG for polymerization is negative, indicating that the polymerization is feasible. However, thermodynamic feasibility does not always guarantee realization in practice and no high polymers of cyclopropane and cyclobutane are known (Sawada, 1976). 

Considering the thermodynamic relation given above, since AH and A are negative for polymerization, AG becomes less negative as the temperature in- 

Source Adapted from Sawada (1976) by conversion of units. 

Small changes in the physical conditions and chemical structure can have a large effect on the polymerizability of a cyclic monomer. Thus, whereas 5-membered cyclic ethers such as tetrahy-drofuran have negative free-energy change and so are polymerizable, the ve-membered cyclic 

Besides the thermodynamic feasibility, there should also be a kinetie pathway for the ring to open, faeilitating polymerization. Cycloalkanes, for example, have no bond in the ring structure that is prone to attaek and thus laek a kinetic pathway. This is in marked contrast to the cyclic monomers sueh as laetones, lactams, cyclic ethers, acetals, and many other cyclic monomers that have a heteroatom in the ring where a nucleophilic or electrophilic attack by an initiator species can take plaee to open the ring and initiate polymerization. Both thermodynamic and kinetic factors are thus favorable for these monomers to polymerize (Odian, 1991). 

Manners and coworkers reported the synthesis and ring opening of unsym-metrical [2]ferrocenophanes. Cationic initiators could be used for the polymerization of the [2]carbathioferrocenophane. Furthermore, thermal and anionic ROP have been useful for the polymerization of [IJthiaferrocenophane and [l]sele-naferrocenophanes. Poly(ferrocenyl sulhdes) have been foimd to posses strong metal-metal interactions, as indicated by the presence of two reversible oxidation processes observed in their cyclic voltanunograms. 

The research groups of Maimers and Pannell reported the ROP of ferroceno-phanes with tin bridges. High molecular weight polymers could be isolated through thermal ROP, and the ferrocenophanes could be opened in solution at room temperature.  

In addition to step and chain polymerizations, another mode of polymerization is of importance. This is the ring-opening polymerization of cyclic monomers such as cyclic ethers, esters (lactones), amides (lactams), and siloxanes. Examples of commercially important types are given in Table 10.1. Of those listed, only the polyalkenes are composed solely of carbon chains. Those that have enjoyed the longest history of commercial exploitation are polyethers prepared from three-membered ring cyclic ethers (epoxides), polyamides from cyclic amides (lactams), and polysiloxanes from cyclic siloxanes. 

Cyclic monomers should, therefore, be capable of being polymerized provided a suitable mechanism for opening the ring is available (kinetic factor). The ease of polymerization of a cyclic monomer, however, depends on both thermodynamic and kinetic factors. 

The single most important factor that determines whether a cyclic monomer can be converted to linear polymer is the thermodynamic factor, that is, the relative stabilities of the cyclic monomer and linear polymer structure [1,2]. Table 10.2 shows the semiempirical enthalpy, entropy, and free-energy changes for the conversion of cycloalkanes to polymethylene in all cases. The Ic (denoting liquid-crystalline) subscripts of AH, AS, and AG indicate that the values are those for the polymerization of liquid monomer to crystalline polymer. 

The data in Table 10.2 show that polymerization is favored thermodynamically (AG is negative) for all except the 6-membered ring. Ring-opening 

ROP reactions of cyclic metal-containing monomers have been utilized to prepare polymeric materials. This method has led to seminal advances in preparing high MW polymers with ferrocene in the mainchain. Ring-opening methods have led to enhanced control of polymer architecture. As with the other methods of polymerization, the metal can either be part of the cyclic structure or attached to an organic ligand bonded to the cychc system. 

Vast arrays of metal-containing polymers have been produced that offer a wide variety of properties. Key milestones in the history of this diverse topic and a sense of its growth and importance were discussed in this chapter. While initial efforts focused on polysiloxanes, today s efforts are quite diverse and include the production of multisite catalysts, variable oxidation state materials, and smart materials where the precise structure can be changed through the introduction of different counterions. These polymers have been produced by all of the well-established polymerization methodologies. The metal atoms reside as part of the macromolecular backbone, in sidechains, coordinated to the backbone, and as integral parts of dendrites, stars, and rods. Truly, many of tomorrow s critically important materials will have metal atoms as an integral part of the polymer framework, which will allow the materials to function as demanded. 

Carraher, Jr., J. E. Sheats, C. U. Pittman, Jr., eds.. Advances in OrganometalHc and Inorganic Polymers Science, Marcel Dekker, New York, 1982. 

Of special interest is the heterogeneous catalytic coordination ROP process recently proposed by Hamaide et al. [117-119]. The catalytic system was ob- 

The trithiaferrocenophanes monomers (fBuC5H3)Fe(C5H4)S3 41 and (tBuC5H3)2-FeS3 42 were prepared from the tert-Bu-substituted ferrocene by lithiation (wBuLi), followed by treatment with elemental sulfur [50]. These monomers polymerized in the presence of BU3P at room temperature in 48 h to yield polymers 43 with M 3700, 26000 and M 2800, 250000, respectively. The broad MWD is 

The versatility of polyphosphazenes is evident by the scope of applications that have been reported for these materials [55]. If the phosphazene ring is subjected to ring strain by the presence of a transannular ferrocenyl group, as in structure 46, under these circumstances polymerization takes place to give 47 (Fig. 10-4). In this case reaction occurs, although no halogen atoms are attached to the phosphorus atoms. However, the reaction is accelerated by the presence of catalytic quantities of (NPCl2)3 [56]. 

Hocker, L. Reif, W. Reimann, and K. Riebel, Reel. Trav. Chim. Pays-Bas, 1977, 

Yamaguchi, K. Tanabe, T. Ogura, and M. Yagi, Bull Chem. Soc. Jpn, 

Results of investigations related to the mechanistic aspects of ring-opening reactions of cyclo-olefins have been discussed by Dolgoplosk et al the individual stages of the process were considered. Formation of stable complexes of carbenes of the type RQCH= (Q = O, S, or Si) inhibit the chain process of metathetical polymerization of cyclo-olefins.  

Analogous ferrocenophanes containing a single germanium atom, prepared by reaction of dilithioferrocene with dichlorodialkyl germane have also been reported 

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The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Even at the qualitative level of the discussion above, it is difficult to make predictions regarding the spontaneity of the ring-opening polymerization reaction (5.FF) ... 

Table 5.8 Values for AH and AS for the Ring-Opening Polymerization, Reaction (5.FF), for Monomers with the Indicated Values of 1...

We conclude this section by citing some examples of ring-opening polymerizations. Table 5.9 lists several examples of ring-opening polymerizations. In addition to the reactions listed, we recall the polymerizations of lactones and lactams exemplified by equations in Table 5.3 and 5.4, respectively. 

Ring-opening polymerizations are catalyzed by a wide variety of substances, including the bases OH and RO and the acids H and BF3 water is also used as a catalyst. The reactions proceed by the opening of the ring by the catalyst to form an active species. 

Table 5.9 Some Typical Ring-Opening Polymerization Reactions...

One product is poly(2-ethyl-2-oxa2oline) (PEOX). It is prepared by the ring-opening polymerization of 2-ethyl-2-oxazoline (19) with a cationic initiator (48) (eq. 6). 

Cyclic ether and acetal polymerizations are also important commercially. Polymerization of tetrahydrofuran is used to produce polyether diol, and polyoxymethylene, an excellent engineering plastic, is obtained by the ring-opening polymerization of trioxane with a small amount of cycHc ether or acetal comonomer to prevent depolymerization (see Acetal resins Polyethers, tetrahydrofuran). 

Synthesis. The synthesis of poly(dichlotophosphazene) [25034-79-17, (N=PCl2) (4), the patent polymer to over 300 macromolecules of types (1) and (2), is carried out via controlled, ring-opening polymerization of the corresponding cycHc trimer, (N=PCl2)3 [940-71 -6]. 

The first phosphazene polymers containing carbon (79), sulfur (80,81), and even metal atoms (82) in the backbone have been reported. These were all prepared by the ring-opening polymerization of partially or fully chloro-substituted (or fluoro-substituted) trimers containing one hetero atom substituting for a ring-phosphoms atom in a cyclotriphosphazene-type ring. 

Ring-Opening Polymerization. As with most other inorganic polymers, ring-opening polymerization of cyclotetrasilanes has been used to make polysilanes (109,110). This method, however, has so far only been used for polymethylphenylsilane (eq. 12). Molecular weights (up to 100,000) are higher than from transition-metal catalyzed polymerization of primary silanes. 

Ring-Opening Polymerization. Ring-opening polymerization of cycloolefins in the presence of tungsten- or molybdenum-based catalysts proceeds by a metathesis mechanism (67,68). 

Nylon-6 is the polyamide formed by the ring-opening polymerization of S-caprolactam. The polymerization of S-caprolactam can be initiated by acids, bases, or water. Hydrolytic polymerization initiated by water is often used in industry. The polymerization is carried out commercially in both batch and continuous processes by heating the monomer in the presence of 5—10% water to temperatures of 250—280°C for periods of 12 to more than 24 h. The chemistry of the polymerization is shown by the following reaction sequence. 

The polymerizations of tetrahydrofuran [1693-74-9] (THF) and of oxetane [503-30-0] (OX) are classic examples of cationic ring-opening polymerizations. Under ideal conditions, the polymerization of the five-membered tetrahydrofuran ring is a reversible equiUbtium polymerization, whereas the polymerization of the strained four-membered oxetane ring is irreversible (1,2). 

The four-membered oxetane ring (trimethylene oxide [503-30-0]) has much higher ring strain, and irreversible ring-opening polymerization can occur rapidly to form polyoxetane [25722-06-9] ... 

Cationic ring-opening polymerization is the only polymerization mechanism available to tetrahydrofuran (5,6,8). The propagating species is a tertiary oxonium ion associated with a negatively charged counterion ... 

The chemistry of polymerization of the oxetanes is much the same as for THE polymerization. The ring-opening polymerization of oxetanes is primarily accompHshed by cationic polymerization methods (283,313—318), but because of the added ring strain, other polymerization techniques, eg, iasertion polymerization (319), anionic polymerization (320), and free-radical ring-opening polymerization (321), have been successful with certain special oxetanes. 

HO—R—COOH, or an amino acid, H2N—R—COOH. In some cases, such monomers self-condense to a cycHc stmcture, which is what actually polymerizes. For example, S-caprolactam (1) can be thought of as the self-condensation product of an amino acid. Caprolactam undergoes a ring-opening polymerization to form another... 

Fig. 9. Initiation of epoxy cure. Irradiation of a triaryl sulfonium salt produces a radical cation that reacts with an organic substrate RH to produce a cation capable of releasing a proton. The proton initiates ring-opening polymerization. X = BF , PFg, AsFg, and SgFg. ...

The ring-opening polymerization of is controUed by entropy, because thermodynamically all bonds in the monomer and polymer are approximately the same (21). The molar cycHzation equihbrium constants of dimethyl siloxane rings have been predicted by the Jacobson-Stockmayer theory (85). The ring—chain equihbrium for siloxane polymers has been studied in detail and is the subject of several reviews (82,83,86—89). The equihbrium constant of the formation of each cycHc is approximately equal to the equihbrium concentration of this cycHc, [(SiR O) Thus the total... 

Braided Synthetic Absorbable Sutures. Suture manufacturers have searched for many years to find a synthetic alternative to surgical gut. The first successful attempt to make a synthetic absorbable suture was the invention of polylactic acid [26023-30-3] suture (15). The polymer was made by the ring-opening polymerization of L-lactide [95-96-5] (1), the cycUc dimer of L-lactic acid. 

In 1967, a polyglycoUc acid [26124-68-5] suture (17) made by the ring-opening polymerization of glycoUde (2), the cycUc dimer of glycoUc acid, was invented. PolyglycoUc acid sutures are degraded by hydrolysis more rapidly than polylactic acid. 

Ring-Opening Polymerization. Examples of the formation of copolymers by ring-opening reactions are shown in equations 33 and 34... 

The ring system 13.18 (R = Cl) is a source of hybrid PN/SN polymers containing three-coordinate sulfur via a ring-opening polymerization process. This polymerization occurs upon mild thermolysis at 90°C (Section 14.4)." ... 

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