Polycarbonate used to be a phosgene business. The interfacial process reacted bisphenol A with phosgene in a methylene chloride-water system, and every plant that ran it carried the associated inventory, containment and community-perception burden. The melt transesterification route replaced the phosgene with a carbonate ester, and the carbonate ester of choice is made from dimethyl carbonate. This article sets out the two-stage chemistry – DMC to diphenyl carbonate, diphenyl carbonate to polycarbonate – with the stoichiometry, the catalysts, the specification values that matter at each stage, and the properties the non-phosgene route delivers that the older one could not.
For the parent product and its three grades, see Dimethyl Carbonate: Properties, Grades and Manufacture; the material is catalogued on the DMC product page.
1. Stage one: DMC to diphenyl carbonate
(CH3O)2CO + 2 C6H5OH -> (C6H5O)2CO + 2 CH3OH
The stoichiometry is two phenol per carbonate and two methanol released. The mass balance closes exactly:
| Species | MW | Mass in | Mass out |
|---|---|---|---|
| Dimethyl carbonate | 90.08 | 90.08 | – |
| Phenol (2 mol) | 94.11 each | 188.22 | – |
| Diphenyl carbonate | 214.22 | – | 214.22 |
| Methanol (2 mol) | 32.04 each | – | 64.08 |
| Total | 278.30 | 278.30 |
The reaction proceeds through methyl phenyl carbonate as an intermediate, and the second transesterification is the slower one, so the process is run with methanol removal to drive both steps. Industrially it is a catalytic distillation problem: the methanol-DMC azeotrope has to be broken to recycle the methanol and return the DMC, while phenol is kept in excess and returned from the downstream stage.
Catalysts are the classic transesterification set – titanium alkoxides, organotin compounds such as dibutyltin oxide, and alkali or alkaline-earth compounds on supports. Each brings a residue problem: titanium leaves haze, tin is regulated in some markets, and alkali metals degrade the final polymer’s colour and hydrolytic stability. Catalyst choice is therefore a product-quality decision, not only a rate decision.
2. Stage two: diphenyl carbonate to polycarbonate
n (C6H5O)2CO + n HO-C6H4-C(CH3)2-C6H4-OH -> polycarbonate + 2n C6H5OH
Melt transesterification of diphenyl carbonate with bisphenol A releases phenol, which is removed under vacuum and recycled to stage one. The two stages therefore form a phenol loop: phenol is the carrier that transports the carbonate group from methanol-derived DMC into the polymer, and it returns.
| Parameter | Typical range | Why |
|---|---|---|
| Temperature | 180 to 300 degC, rising through the train | melt viscosity climbs by orders of magnitude as molecular weight builds |
| Pressure | atmospheric to below 1 mbar in the finisher | phenol removal drives conversion |
| Catalyst | alkali or alkaline-earth salts, or quaternary ammonium/phosphonium | activity versus colour and hydrolytic stability |
| Stoichiometry | DPC in slight excess early, balanced at the end | end-group control determines molecular weight |
| Residual phenol | ppm level in the product | odour, taste and food-contact compliance |
3. Why DMC quality matters at the first stage
DMC is a reagent here, not a solvent, and its impurities enter the phenol loop.
| DMC parameter | Effect on the DPC stage | Practical limit |
|---|---|---|
| Methanol | shifts the transesterification equilibrium backwards and consumes catalyst | high-purity grade, 200 ppm max, better if lower |
| Water | hydrolyses DMC to methanol and CO2, and hydrolyses DPC to phenol and CO2 | 100 ppm max, lower preferred |
| Colour bodies | carry through into DPC and then into polymer colour | specify APHA even for non-battery use |
| Chloride | catalyst poison and corrosion risk in the train | specify; the non-phosgene route’s advantage is lost if chloride enters with the raw material |
| Glycol / cyclic carbonate | fingerprint of the transesterification manufacturing route; can act as a chain terminator | ask for the figure explicitly |
That last row is the one buyers forget. A non-phosgene polycarbonate plant advertises the absence of chlorine, and then discovers chloride arriving in the carbonate feedstock or glycol arriving in the same drum. Both are manufacturing-route fingerprints of the DMC plant, which is why the previous article in this series recommends asking which route your supplier runs.
4. The comparison with phosgene
| Aspect | Interfacial phosgene route | Melt DMC route |
|---|---|---|
| Carbonyl source | phosgene, COCl2, 98.92 | dimethyl carbonate, 90.08 |
| Chlorine in the process | yes, stoichiometric | none required |
| Solvent | methylene chloride, recovered and vent-treated | none – melt process |
| By-product | sodium chloride brine from the acid acceptor | phenol, recycled internally |
| Colour and optical quality | good, with washing | excellent if catalyst and phenol removal are right |
| Hydrolytic stability | sensitive to residual catalyst and chloride | sensitive to residual alkali catalyst |
| Capital intensity | high: phosgene generation and containment | high: vacuum train and melt finishing |
The environmental argument is straightforward – no phosgene inventory, no chlorinated solvent, no brine – and the product argument is that the melt route can deliver a polymer with no chlorine at all, which matters for optical media, medical devices and food-contact applications where extractables are scrutinised.
5. Where the DMC-polycarbonate chain is heading
Two forces keep the route expanding. The first is regulation of phosgene handling, which makes new phosgene capacity expensive to permit in most jurisdictions. The second is that the same DMC supply chain serves the electrolyte market: a plant that can make battery-grade DMC can also make high-purity DMC for DPC, so the two industries increasingly draw on the same distillation trains and the same drying technology. For a buyer this has a practical implication – DMC availability for polycarbonate can tighten when electrolyte demand spikes, because the same tonnes are contested.
Adjacent chemistry worth knowing: the same transesterification logic converts cyclic carbonates into DMC and glycols, as described in our main DMC article, and the carbonate functionality also appears in glycerol carbonate, where the bio-based feedstock changes the economics rather than the chemistry.
6. Specification sheet for the two purchases
6.1 Buying DMC for DPC
- High-purity grade, GC 99.90% minimum, carbonate 99.97% minimum.
- Methanol at or below 200 ppm; water at or below 100 ppm.
- Colour specified (APHA), because it carries straight through to polymer colour.
- Chloride and glycol figures stated on the certificate, not on request.
- Nitrogen-blanketed packaging, since a drum that absorbs water becomes a methanol source in the reactor.
6.2 What to ask about the DPC you receive
- Residual phenol and residual methyl phenyl carbonate.
- Chloride and total halide, if the polymer is destined for optical or medical use.
- Catalyst residues – which metal, and at what level.
- Colour of a standard moulded plaque, not just the powder.
7. Frequently asked questions
Is the DMC route polycarbonate the same polymer as the phosgene one?
Chemically yes – the repeat unit is identical. Differences come from end groups, catalyst residues and the absence of chlorine, which is why the melt route’s product is often preferred for optical and medical grades and why the two are not always interchangeable in a qualified application.
Why is phenol recycled rather than consumed?
Because phenol is the leaving group in stage one and the leaving group in stage two, in the opposite direction. The loop is the reason the process is economic: the only net consumptions are DMC and bisphenol A.
Can I use technical-grade DMC to make DPC?
Not without accepting a quality penalty. Technical grade’s 500 ppm methanol and 200 ppm water both push the equilibrium the wrong way and both end up as colour and as hydrolytic instability in the polymer.
References and scope note
All stoichiometry in this article is calculated from standard atomic weights and is reproducible from the molecular weights shown. Process descriptions are the established commercial configuration for orientation, not validated plant instructions. No claim is made about the performance of any specific polymer grade; qualification testing remains the customer’s responsibility.
Technical and commercial orientation only. Not a substitute for engineering design, hazard assessment or regulatory advice.
PolyBlueChem supplies high-purity dimethyl carbonate for DPC and polycarbonate production, with certificates that name methanol, water, colour and chloride. Contact polybluechem@163.com for pricing and specification discussion.