Principales limitantes en el desarrollo de la caña de azúcar

While the use of ROP to produce poly(esters) has mainly been seen as the dominant technique to produce polymers with degradable properties, an alternative approach has regained significant interest in the polymer materials field over the last decade: the radical Ring-Opening Polymerization (rROP).77-79 Indeed the rROP of cyclic monomers (mainly cyclic ketene acetals, CKAs) has been seen as an unconventional but successful approach for the production of polymers containing degradable ester repeat units in their backbone.78,79 Firstly introduced in the early 80s in various studies by Baileyet al., this approach uses the presence of an exo-methylene double bond functionality on the cyclic ketene acetal monomer which can undergo polymerization via radical addition and force the cyclic monomer to ring-open itself, hence producing a growing ester chain able to subsequently form a poly(ester) after further monomer addition.80-85CKA monomers can be synthesized in a two-step processes consisting of an acetal exchange reaction followed by a dehydrohalogenation.80 For example, 2-methylene-1,3-dioxepane (MDO, a 7 membered cyclic ester monomer, CKA 1) is obtained from the reaction between chloroacetaldehyde- dimethylacetal with 1,4-butanediol to form the 2-chloromethyl-1,3-dioxepane which is then treated with potassiumt-butoxide to produce, after purification using washes and distillation, the final CKA MDO (Scheme 1.7a).80 Similarly, the benzyl functional version, 5,6-benzo-2- methylene-1,3-dioxepane (BMDO, CKA 2) can be synthesized from the acetal exchange reaction of 1,2-benzene dimethanol followed by dehydrohalogenation (Scheme 1.7b).86

Scheme 1.7.Schematic representation of the synthetic formation of cyclic ketene acetals (a) 2-methylene-1,3-dioxepane (MDO,CKA 1) and (b) 5,6-Benzo-2-methylene-1,3-dioxepane (BMDO,

CKA 2).

Using this synthetic approach various CKA monomers have been produced where the ring sizes (e.g. 5, 6, 7 membered ring) and functional groups (e.g. phenyl, alkyl etc.) can be modified to produce a wide array of CKA structures, which can be polymerized by rROP to form polyesters with different properties (Figure 1.5).78,79 The synthesis of MDO (CKA 1) was of particular interest as it was found to produce, after polymerization, a polymer with a similar structure to conventional poly(ε-caprolactone) (PCL) (Scheme 1.8), hence providing an alternative radical production for this widely used poly(ester).80

Scheme 1.8.Schematic representation for the parallel synthesis of poly(esters) from the ROP ofε-CL and the rROP of 2-methylene-1,3-dioxepane.

Figure 1.5.Schematic representation of different cyclic ketene acetals (CKAs).

The first radical ring-opening polymerizations of CKAs were reported by Bailey et al. with the rROP of 3,9-dimethylene-1,5,7,11-tetraoxaspiro-[5,5] undecane (CKA 3) and 2- methylene-1,3-dioxepane (CKA 1), using conventional radical thermal initiators such as benzoyl peroxide, di-t-butylperoxide or 2,2’-azobis(isobutyronitrile) (AIBN) under various

conditions, where polymerizations were carried out in bulk, in solution and at different temperatures (60 to 150 °C).80-85 During the rROP of CKAs, Baileyet al. reported that the mechanism was similar to the conventional free radical polymerization technique where the process starts with (Scheme 1.9a) the decompositions of an initiator to produce radical species that will react with the double bond of the CKA.80 The cyclic radical intermediate then isomerizes by radical ring-opening polymerization to create the initial primary radical containing the ester repeat units (Scheme 1.9b). The reaction then repeats itself and the primary radical containing the ester repeat unit grows to produce poly(ester) chains (Scheme 1.9c).

Scheme 1.9.Schematic representation of the mechanism for the radical ring-opening polymerization (rROP) of MDO (CKA 1).

While Bailey and co-workers investigated the successful synthesis of poly(esters) from the rROP of CKA monomers such as MDO, they also reported the occurrence of a side reaction in which ring-retention of the CKA could be observed during the polymerization.80 Indeed, the formation of the cyclic intermediate produces a tertiary radical (Scheme 1.9b) which can also react and produce a poly(acetal) structure within the poly(ester) backbone (Scheme 1.10). The occurrence of such ring-retention reactions during the rROP have been shown to be dependent on various parameters including: the polymerization temperature, CKA

monomer structure (ring sizes, substituents), and the monomer and initiator concentrations.81,87-89 R R O O O O R O Ox _and/or O O n n _n n

Poly(acetal) Poly(acetal)-co-poly(ester)

R O

y O Ox

n n

O

Scheme 1.10.Schematic representation of the ring-retention reaction occurring during the rROP of CKA monomers and formation of either poly(acetal) and poly(acetal)-co-poly(ester) structures.

For example, the rROP of the 7-membered non-functional CKA, MDO (CKA 1) and phenyl- functionalized BMDO (CKA 2) was found to occur via quantitative ring-opening of the monomers for polymerizations carried out at various temperatures from 50 to 120 °C.80,81,90,91 Conversely, the 5-membered ring CKA, 2-methylene-1,3-dioxalane (MDL, CKA 5) was found to produce poly(esters) containing ring-retained units throughout the polymer backbone as a consequence of the poor stability of the primary radical obtained during the rROP.87 The ratio of ring-opened and ring-retained units within the polymers was found to vary when the temperature of the polymerization was changed. Indeed, for a polymerization carried out at 60 °C, only 50% of the polymer chains were composed of ring-opened unitsvs.

83% reached when the same polymerization was carried out at 125 °C. A similar effect was observed when the concentration of the monomer in the polymerization mixture was decreased leading to more ring-retained polymer structures.87 _{For these CKAs, the}

introduction of a phenyl or hexyl functional groups on the ring, to create 2-methylene-4- phenyl-1,3-dioxolane (MPDL, CKA 7) and 2-methylene-4-hexyl-1,3-dioxalane (MHDL, CKA 6), was able to increase the stability of the primary radical and produced polymers with full ring-opening of the monomer across a wide range of temperatures from 60 to 150 °C.87,88,92

Although Bailey and co-workers reported the radical ring-opening polymerization of MDO (CKA 1) as an interesting approach to produce a poly(ester) with the same structure as the poly(ester) obtained from the conventional ROP of poly(ε-caprolactone) (PCL), their report was lacking in the characterization of the poly(MDO) structure.80 Hence, Gonsalves et al. later further investigated the polymerization of MDO for 72 h using AIBN as the initiator at 50 °C and highlighted the presence of side reactions arising from the instability of the growing primary radical polymer chains.91 Indeed, they observed that some side branches were formed along the polymer backbone and were resulting from the 1,4- and 1,7-hydrogen transfer reaction occurring during the polymerization (Scheme 1.11). Under their conditions, the amount of hydrogen transfer reactions and hence the amount of branches in the poly(MDO) backbone was found to reach 20% revealing that the structure of the final polymers was in fact a branched analogue to the structure obtained from the ROP of poly(ε- caprolactone). Further recent studies, mainly by Agarwal and co-workers, also investigated the effect of the 1,4- and 1,7-hydrogen transfer reactions and showed that the extent of branches in the radical ring-opening polymerization of MDO was dependent on the temperature at which the reactions were carried out87,90_{Indeed, for polymerizations carried}

out at temperatures above 100 °C, the amount of branches was found to be significantly lower.

Scheme 1.11.Schematic representation of the possible branching occurring during the rROP of MDO via1,4- and 1,7-hydrogen transfer.