Polymerisation of Cyclic Carbonates

The ring-opening polymerisation of cyclic carbonates with coordination catalysts concerns monomers containing a five-membered ring or a six-membered ring in the molecule. 

One of the mechanisms taken into consideration for the polymerisation of propylene carbonate was that this proceeded via orthocarbonate species. The likelihood of such a mechanism might be ascertained by the polymerisation of bispropylene spiroorthocarbonate with diethylzinc as the catalyst at 160 °C, which was found to yield poly(propylene ether-carbonate) [143], 

It seems that the alkylene carbonate polymerisation proceeds via monomer decarboxylation in the first reaction step. The decarboxylation most probably involves metal carbonate species owing to ring opening of the 1,3-dioxolan-2-one via Cp—O bond cleavage [146,147]. A possible reaction scheme in the presence of zinc-based coordination catalysts is presented by scheme (14), in which, for the sake of clarity, participation of the adjacent zinc atom as the nucleophilic attack carrier is omitted  

The polymerisation of propylene carbonate can be proposed to proceed via C(0)-0 bond cleavage which is presented in a simplified way, omitting the participation of the adjacent zinc atom, by the following scheme  

The polymerisation of the bispropylene spiroorthocarbonate with zinc-based coordination catalysts probably involves zinc alcoholate propagating species, which is shown schematically as follows  

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The coordination polymerisation of cyclic carbonates with a six-membered ring in the molecule, such as trimethylene carbonate (l,3-dioxan-2-one) and 2,2-dimethyltrimethylene carbonate (5,5-dimethyl-l,3-dioxan-2-one) [148-150], carried out in the presence of metal carboxylates e.g. zinc stearate, tin-based catalysts such as the di(w-butyl)stannic diiodide-triphenylphosphine system [151] or porphinatoaluminium compounds such as (tpp)AlOR [149] is not accompanied with decarboxylation and yields the respective polycarbonates (Table 9.2). The ring cleavage during the polymerisation of trimethylene carbonate and 2,2-dimethyltrimethylene carbonate in the presence of the above catalysts has been found [148,149,151] to occur at the C(0)-0 bond, resulting... 

Ring opening polymerisation of cyclic carbonates, initiated by glycols, leads to polycarbonate diols (reaction 8.5). 

A second method for polycarbonate polyol synthesis is the ring opening polymerisation of cyclic carbonates of 5-6 members, initiated by various polyols as starters (reaction 8.38) [68-76]. 

Oxacyclic monomers constitute the most widely investigated class of heterocyclic monomers regarding both academic and industrial interest. In particular, the coordination polymerisation of cyclic ethers such as epoxides (oxiranes) and of cyclic esters such as lactones, lactides and cyclic carbonates has been considered. 

The coordination polymerisation of cyclic esters concerns mostly lactones, especially those containing a four-membered ring in the molecule. There is, however, an interest in the coordination polymerisation of such oxacyclic ester monomers as lactide and alkylene carbonate and, to a lesser extent, in the... 

The polyester polyols are obtained by the polycondensation reactions between dicarboxylic acids (or derivatives such as esters or anhydrides) and diols (or polyols), or by the ring opening polymerisation of cyclic esters (lactones, cyclic carbonates). 

The third type of reaction for polyester polyols synthesis is the ring opening polymerisation of cyclic esters, such as -caprolactone (reaction 8.4) or cyclic carbonates, such as ethylene glycol carbonate, propylene glycol carbonate, neopentyl glycol carbonate, etc., (reaction 8.5) initiated by diols (or polyols) and catalysed by specific catalysts [7, 16]. 

PTMC synthesis is realised either by copolymerisation of epoxides with carbon dioxide or by the ring-opening polymerisation (ROP) of cyclic carbonate monomers. It is also possible to obtain aliphatic carbonates via polycondensation of dialkyl or diphenyl carbonate or chlorophormates and aliphatic diols. However, polycondensation usually leads to polymers with rather low molar masses (Hyon et al., 1997). Besides, side reactions often occur during polycondensation (Jerome and Lecomte, 2008). 

Agarwal, S., Puchner, M., 2002. Ring opening polymerisations of cyclic esters and carbonate by rare-earth LnCp3. European Polymer Journal 38, 2365—2371. 

As far as the polymerisation of heterocyclic compounds with one hetero-atom is concerned (cyclic ethers and their analogues) there seems little doubt at present that the propagation involves a displacement at the positive propagating centre. The ring which is part of this -onium ion is opened between the charged atom and a carbon atom next to it, and this becomes attached to the hetero-atom of the monomer ... 

Coordination polymerisation of heterocyclic monomers comprises polymerisation and copolymerisation processes of such monomers as oxacyclic monomers, especially epoxides [2,61-71], thiacyclic monomers like episulphides [72-76], azacyclic monomers [77,78] and phosphacyclic monomers [79]. Monomers with an exocyclic oxygen atom, such as cyclic esters like lactones [80-90] and lactide [90-92], cyclic acid anhydrides [93-98], cyclic carbonates [99,100] and related monomers, belong to oxacyclic monomers undergoing coordination polymerisation or copolymerisation. 

The coordination polymerisation of heterounsaturated monomers, such as aldehydes [101-103] and ketones [104], isocyanates [105] and ketenes [106,107], in homopolymerisation systems has not been widely described in the literature. However, the coordination copolymerisation of heterounsaturated monomers not susceptible to homopropagation, such as carbon dioxide [71,108-113], with heterounsaturated monomers such as cyclic ethers has been successfully carried out and is of increasing interest. 

Heterocyclic monomers containing both endocyclic and exocyclic heteroatoms such as cyclic esters (lactones, lactide, carbonates) and cyclic anhydrides undergo coordination polymerisation or copolymerisation involving complex formation between the metal atom and the exocyclic heteroatom [100,124]. Polymerisation of /1-lactones is representative of such coordination polymerisations with catalysts containing an Mt-X active bond the initiation and propagation steps are as follows ... 

Note that some general considerations of ring-opening polymerisation originate from studies of the coordination polymerisation of tiiranes. Other thiacyclic monomers that contain an endocyclic sulphur atom and an exocyclic oxygen atom, such as monothiocarbonates, are instead analogous to five-membered cyclic carbonates. 

Since oxiranes are representative heterocyclic monomers containing an endo-cyclic heteroatom, and the most commonly polymerised of such monomers, they have been subjected to copolymerisations with heterocyclic monomers containing both an endocyclic and an exocyclic heteroatom. Coordination copolymerisations of heterocyclic monomers with different functions are focused on oxirane copolymerisation with cyclic dicarboxylic acid anhydride and cyclic carbonate. However, the statistical copolymerisation of heterocyclic monomers with an endocyclic heteroatom and monomers with both endocyclic and exocyclic heteroatoms have only a limited importance. Also, the block copolymerisation of oxirane with lactone or cyclic dicarboxylic acid anhydride is of interest both from the synthetic and from the mechanistic point of view. Block copolymerisation deserves special interest in terms of the exceptionally wide potential utility of block copolymers obtained from comonomers with various functions. It should be noted, however, that the variety of comonomers that might be subjected to a random, alternating and block polymerisation involving a nucleophilic attack on the coordinating monomer is rather small. 

At the end of considerations concerning the coordination polymerisation of heteroatom-containing cyclic and acyclic monomers, it is obvious that future development (beyond carbon monoxide copolymerisations) is to be anticipated, especially concerning new catalytic processes, including both new... 

In the cationic polymerisation of THF, very small quantities of cyclic compounds (cyclic oligomers of THF) are formed (less than 3%), this is much lower than in alkyleneoxide cationic polymerisation [10, 11]. The cyclic oligomers are formed by the intramolecular nucleophilic attack of the etheric oxygen of the polymeric chains on the carbon atom from a position of the trialkyloxaonium chain end (reactions 7.2). 

The catalysts of this ring opening polymerisation reactions are pyridinium salts (for example N-benzyl pyridinium p-toluene sulfonate [75]), p-toluene sulfonic acid [75], stannium or titanium compounds [68-74] etc. Other cyclic polymerisable cyclic carbonates are ethylene carbonate and propylene carbonate [68-74]. 

Various biodegradable polycarbonate(s) (PC) polymers have been fabricated via the organocatalytic ring-opening polymerisation of functional cyclic carbonate monomers, which were quarternised to create cationic polymers with various pendent structures such as alkyl, aromatic and imidazolinium (Figure 8.4). These polymers have shown excellent antimicrobial properties and haemolytic characteristics when assayed using rat red blood cells [98]. 

In all cases there was a small amount of the cyclic carbonate formed. It was observed that the monometallic complex Zr(L )(0 Pr)2 was inactive for the polymerisation. This suggests that the bimetallic nature of the complex is important, which has previously been shown for zinc(ii) complexes. Ko has also screened a series of benzotriazole phenolate zirconium complexes. Figure 8.10. Zr(L)2(0 Pr)2, where R=H was by far the most active (pC02 = 600 psi, T= 100 °C, 16 h) giving an 87% conversion of CHO, with a 99% selectivity to polymer with 90% carbonate linkages. Although these results are inferior to the more established systems they do show promise and this is an area of future investigation. 

Patents claiming the use of hindered phenols in aromatic polyesters along with specific co-additives include cyclic carbonates and phenols, e.g., ethylene carbonate and l,3,5-trimethyl-2,4,6-tris(3,5-di- -butyl-4-hydroxybenzyl)benzene (Irganox 1330 Ciba) [54] combinations with oxetane-substituted phosphites [55] addition at the polymerisation stage of a synergistic mix of... 

Haba O., Tomizuka H., Endo T, Anionic ring-opening polymerization of methyl 4,6-0-benzylidene-2,3-0-carbonyl-a-D-glucopyranoside A first example of anionic ring-opening polymerisation of five-membered cyclic carbonate without elimination of CO2, Macromolecules, 38, 2005, 3562-3563. 

Scheme 4.3 Proposed mechanism of cationic ring-opening polymerisation of six-membered cyclic carbonates.

Cyclic dicarbonates, cyclo /s(hexamethylene carbonate) and cyclo /s(diethylene glycol carbonate) were polymerised by lipase from C. antarctica and P. fluorescens [62]. CALB was used for the ROP of cyclo /s(decamethylene carbonate) (DMC2) giving a polymer with a MW of 5.4 x 10 and 99% yield, and an ultralow enzyme/substrate weight ratio of 1/200. Compared with six-membered trimethylene carbonate, a much lower reaction activity of large-sized DMC2 vvas observed, the opposite of the enzymatic polymerisation of lactones with different ring sizes [63]. 

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