1.3- dioxepane, polymerization

As indicated by the direction of the arrows the isomeric 7-membered oxonium ion dominates in the polymerization of the 5-membered 1,3-dioxolane whereas in the polymerization of the 7-membered 1,3-dioxepane cationated monomer dominates. This is apparently due to the differences in strain of the involved rings. Kinetic analysis of the polymerization of these two monomers has shown that the isomeric (enlarged) oxonium ions can be treated as the kinetically dormant species propagation and depropagation on these species proceed with almost identical rates. This explains why for the same starting concentration of initiator, as observed by Plesch (19), 1,3-dioxepane polymerizes over 100 times faster than T,3-dioxolane. This is because the proportion of the productively active species is higher for the former than for the latter monomer. 

Ring-opening polymerization of 2-methylene-l,3-dioxepane (Fig. 6) represents the single example of a free radical polymerization route to PCL (51). Initiation with AIBN at SO C afforded PCL with a of 42,000 in 59% yield. While this monomer is not commercially available, the advantage of this method is that it may be used to obtain otherwise inaccessible copolymers. As an example, copolymerization with vinyl monomers has afforded copolymers of e-caprolactone with styrene, 4-vinylanisole, methyl methacrylate, and vinyl acetate. 

FIGURE 6 Synthesis of PCL by the free radical polymerization of 2-methylene-l,3-dioxepane. (From Ref. 51.)... 

Bailey, W, J., Ni, Z., and Wu, S.-R., Synthesis of poly-e-capralactone via a free radical mechanism. Free radical ringopening polymerization of 2-methylene-l, 3-dioxepane, J. Polym. Sci., Polym. Chem. Ed.. 3021-3030, 1982. 

By utilizing a combination of RAFT and cationic ROP, the synthesis of [poly(methyl methacrylate)][poly(l,3-dioxepane)][polystyrene] miktoarm star terpolymers was achieved [182], The approach involved the synthesis of PS functionalized with a dithiobenzoate group by RAFT polymerization and subsequent reaction with hydroxyethylene cinnamate (Scheme 98). The newly created hydroxyl group was then used for the cationic ring opening polymerization of 1,3-dioxepane (DOP). The remaining dithiobenzoate group was used for the RAFT polymerization of methyl methacrylate. 

A third example combines cationic ROP and ATRP for the synthesis of (polytetrahydrofurane)(poly-l,3-dioxepane)(PS) miktoarm stars (Scheme 99). The initiating sites for the above polymerization were created step-by-step from amino-succinic acid (Scheme 99). 

This indicates the possibility of making addition polymers biodegradable by the introduction of ester linkages in to the backbone. Since the free radical ring-opening polymerization of cyclic ketene acetals, such as 2-methylene-1,3-dioxepane (1, Scheme I), made possible the introduction of ester groups into the backbone of addition polymers, this appeared to be an attractive method for the synthesis of biodegradable addition polymers. 

Albertsson A-C, Palmgren R (1996) Cationic Polymerization of 1,5-dioxepan-2-one with Lewis acids in bulk and solution. J Macromol Sci Pure Appl Chem A33 747-758... 

Mathisen T, Albertsson A-C (1989) Polymerization of l,5-dioxepan-2-one. 1. Synthesis and characterization of the monomer l,5-dioxepan-2-one and its cyclic dimer 1,5,8, 12-tetraoxacyclotetradecane-2,9-dione. Macromolecules 22 3838-3842... 

Kricheldorf and coworkers have found that some heterocyclic compounds containing two reactive bonds behave as bifunctional reactants in a ROP that proceeds by a step polymerization mechanism [Kricheldorf, 2000]. An example is the polymerization of 2,2-dibutyl-2-stanna-l,3-dioxepane with an aliphatic diacid chloride ... 

Solvent effects including 2-methyl-l,3-dioxepane (MDOP), as a solvent, on the propagation kinetics of methyl acrylate (MMA) have been investigated using the PLP-SEC technique (PLP = pulse laser polymerization) <2005MI267>, and the composition of dioxolane-dioxepane copolymers has been studied by IR and differential scanning calorimetry (DSC) <2004PB349>. 

Kricheldorf et al. reported an anionic polymerization of y-D,L-butyro-lactone or D,L-lactide with cyclic dibutyltin initiators, such as 2,2-dibutyl-2-stanna-l,3-dioxepane, to give cyclic polymers [ 138-140]. Figure 41 shows the ring expansion polymerization of lactone for synthesizing a cyclic polymer as an example. They also synthesized the cyclic polymer with a living mechanism in the polymerization of e-caprolactone [141]. 

The following mechanism has been suggested for the polymerization of 1,5-dioxepan-2-one in bulk or in THF or toluene solution using aluminum isopro-poxide as an initiator (Scheme 15). 

Scheme 15. Aluminum isopropoxide initiated polymerization of l,5-dioxepan-2-one...

Recent studies of the tetrabutyltin-initiated polymerization of e-CL have indicated that Bu2SnO dissolved in Bu4Sn is the main initiator [123]. Almost 100% conversions are observed in the living macrocyclic polymerization of e-CL in bulk at 80 °C with 2,2-dibutyl-2-stanna-l,3-dioxepane as initiator [124]. 

The cationic polymerization is difficult to control and often only low-molecular weight polymers are formed. When the bulk and solution polymerization of l,5-dioxepan-2-one (DXO) with cationic initiators were studied, the highest molecular weight achieved was about 10,000 [23]. More detailed reviews on cationic ROP have been published by Penczek and coworkers [26,27]. 

The cyclic tin alkoxides have the additional advantage of offering a convenient synthetic pathway for the synthesis of macromers, triblock, and multiblock copolymers [81,82]. Macromers from l-LA [83],e-CL [84], and l,5-dioxepan-2-one (DXO) [85] have been synthesized as well as triblock poly(L-LA-b-DXO-b-L-LA) [86] and multiblock copoly(ether-ester) from poly(THF) and e-CL [87]. The polymerization proceeds by ring expansion and the cyclic structure is preserved until the polymerization is quenched by precipitation. 

We have spent much effort in establishing methods for synthesizing a specific lactone deriving from 1,3-propanediol, i.e., l,4-dioxepan-2-one and for polymerizing it according to standard ROP technologies (see Scheme 6). 

Ring opening polymerization of monocyclic acetals such as 1,3-dioxolane and 1,3-dioxepane has been extensively investigated by several research groups for many years. [6] However, there still remains something to be clarified as to the polymerization mechanisms, particularly the structure of growing species. One of the several reasons that have prevented the elucidation of the polymerization... 

The same authors reported that during the polymerization of 1,3-dioxepane with perchloric acid appreciable quantities of dimer are produced (30). Yamashita (31) who studied the kinetics of the same polymerization, reports that the molecular weight of the polymer increases with conversion until a maximum value is attained and then decreases drastically at higher conversions. Possibly, this phenomenon is to be attributed to a degradation of the polymer to form the dimer or higher oligomers. 

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