What happens when dimethyl terephthalate (DMT) is mixed with monoethylene glycol (MEG)? It does not immediately become polyester fibre. First, the materials undergo transesterification, producing an intermediate mixture. This is followed by polycondensation, where the molecular chains grow to the high molecular weight required for fibre production.

This two-stage sequence is the key to understanding the manufacturing of polyester using the DMT route. In this article, we will follow the process from DMT and MEG to PET polymer and finally connect the polymer-production stage with textile fibre manufacturing.

Quick Answer: In the DMT route, DMT reacts with MEG by transesterification (ester interchange) to form diglycol terephthalate (DGT/BHET-containing intermediate) while methanol is removed. The intermediate is then subjected to polycondensation at elevated temperature under vacuum, removing MEG and increasing molecular weight. The resulting PET can be converted into chips or sent directly to melt spinning.

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What is the DMT route for polyester manufacturing?

Polyester used for textile fibres is commonly based on poly(ethylene terephthalate), or PET.

PET is a step-growth polymer. The DMT route uses two principal raw materials:

• Dimethyl terephthalate (DMT)
• Monoethylene glycol (MEG), also called ethylene glycol (EG)

The DMT route is an indirect route to PET. The first stage is transesterification, while the second stage is polycondensation.

The important distinction is:

DMT + MEG → transesterification → intermediate → polycondensation → PET

The alternative PTA route starts with terephthalic acid (TPA/PTA) and uses direct esterification instead of DMT transesterification. The supplied reference identifies both as industrial routes for PET production.

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Why is an intermediate formed before polycondensation?

This is one of the most important points for students.

To obtain high molecular weight PET, the two reacting functional groups need to be present in the appropriate stoichiometric relationship. The DMT–MEG reaction therefore first produces an intermediate in which the terephthalate and glycol components are brought into a suitable chemical form.

The supplied textile engineering reference describes diglycol terephthalate (DGT) as the intermediate and gives the simplified DMT reaction as:$$\mathrm{DMT + 2MEG \rightarrow DGT + 2CH_3OH}$$

In structural form:$$\mathrm{CH_3OOC-C_6H_4-COOCH_3 + 2HO-CH_2CH_2-OH}$$$$\rightarrow \mathrm{HO-CH_2CH_2-OOC-C_6H_4-COO-CH_2CH_2-OH + 2CH_3OH}$$

Here:

• DMT = dimethyl terephthalate
• MEG = monoethylene glycol
• DGT = diglycol terephthalate
• CH₃OH = methanol

The formation of this intermediate provides a more uniform starting material for the subsequent polycondensation stage. The reference lists advantages including fewer side reactions, greater reaction speed, better colour and improved drawability.

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Step-by-step manufacturing of polyester using the DMT route

The overall process can be represented as:

DMT + MEG
↓
Melting and mixing
↓
Transesterification / ester interchange
↓
Methanol removal
↓
DGT + oligomers
↓
Polycondensation
↓
MEG removal under vacuum
↓
High molecular weight PET
↓
Chips or direct melt spinning
↓
Polyester filament or staple fibre

The exact plant arrangement can vary, particularly between batch and continuous processes.

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1. Preparation of DMT and MEG

The quality of the starting materials matters because impurities can affect the polymerization reaction and the final polymer.

The supplied reference gives the following DMT and MEG specifications in its discussion of PET raw materials:

Raw material	Selected reference value
DMT melting point	141 °C
DMT boiling point	280 °C
DMT acid number	0.2
DMT saponification number	578
MEG boiling point	195–198 °C
MEG density	1.110–1.112 g cm⁻³ at 20 °C
MEG refractive index	1.4330–1.4340 at 20 °C
MEG water content	0.1%

These are values reported in the supplied 1997 textile engineering reference and should be treated as reference-process data rather than universal specifications for every modern PET plant.

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2. Melting and feeding of DMT

DMT is a crystalline solid, so it must be brought into a suitable molten or dissolved state before efficient reaction with MEG.

One described industrial arrangement melts DMT at approximately 170–180 °C. Another system maintains the DMT melter at about 150 °C and pumps the molten DMT through filtration before feeding it to transesterification vessels.

Filtration is useful because unmelted DMT should not be allowed to enter the downstream process.

Why is DMT handling important?

DMT can sublime. If temperature is increased too rapidly, sublimation can create material losses and may contribute to deposits in downstream equipment. The supplied material specifically notes that temperature should be increased slowly to avoid DMT sublimation.

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3. Transesterification of DMT with MEG

This is the first major reaction stage in the DMT route.

The reaction is also called:

• ester interchange
• ester exchange
• transesterification
• EI or TE

The methyl ester groups of DMT exchange with the hydroxyl groups of MEG.

Simplified reaction

$$\mathrm{DMT + 2MEG \rightarrow DGT + 2Methanol}$$

The important point is that methanol is formed as a by-product.

The reaction therefore needs an arrangement for removing methanol from the reaction system.

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What happens inside the transesterification reactor?

DMT and MEG are heated together in the presence of a suitable transesterification catalyst.

The supplied sources describe transesterification beginning above 150 °C. One described operating condition reaches approximately 197–198 °C, while another approach uses temperatures up to about 220 °C with methanol separation.

A slight excess of MEG may be used. During the reaction, methanol is removed through the overhead system.

In a continuous industrial arrangement, the reaction can be carried out around the boiling point of MEG, with a reflux column returning MEG while methanol is removed. The reference describes transesterification vessels arranged as a cascade, commonly using several prepolymer reactors.

A useful way to remember the stage

Transesterification removes methanol.

That single sentence is useful for examinations.

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Why is methanol removal important?

The transesterification reaction is an equilibrium reaction.

Removing one of the reaction products, methanol, helps drive the reaction toward formation of the glycol terephthalate intermediate.

The process therefore combines:

heat + catalyst + methanol removal → progression of transesterification

The supplied reference reports that in practice, after approximately 3 h at 197–200 °C, more than 98% of CH₃OH had been evaporated under the stated conditions. This is a reported reference value, not a universal operating target for every industrial plant.

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4. Catalysts used in the DMT route

DMT and MEG do not react sufficiently rapidly for practical production without catalytic assistance.

The textile engineering reference groups transesterification catalysts into:

1. metals,
2. metal oxides,
3. metal salts.

Metal oxides and salts of weak or volatile acids are commonly discussed for this purpose.

Examples of catalysts reported in the supplied material include compounds based on:

• zinc
• manganese
• cobalt

The reference flow diagram specifically identifies Zn(Ac)₂, Mn(Ac)₂ and Co(Ac)₂ in the DMT-route process.

However, catalyst selection is not simply a matter of making transesterification faster.

Catalyst → reaction rate → side reactions and degradation → polymer quality

A catalyst that performs well during transesterification may also influence subsequent PET degradation. Therefore, the catalyst system and its concentration have to be considered across the whole polymerization process.

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5. Formation of DGT, BHET and oligomers

A common source of confusion is the terminology used for the material leaving the first stage.

The first stage can be understood as producing a mixture containing glycol terephthalate intermediates and low-molecular-weight oligomers, rather than assuming that the reactor contains only one perfectly pure compound.

Another supplied source describes the first stage of PET polymerization as formation of BHET, usually together with various PET oligomers. The actual concentration of free BHET depends strongly on the glycol-to-terephthalate ratio.

So, for study purposes, it is better to think of the first-stage product as:

BHET/DGT + low molecular weight oligomers

rather than treating the process as a single isolated-compound reaction.

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6. Polycondensation: the stage where molecular weight increases

After transesterification, the intermediate mixture moves to the polycondensation stage.

This is where the PET chains grow to the molecular weight needed for fibre formation.

A simplified reaction can be written as:$$n\,\mathrm{HO-CH_2CH_2-OOC-C_6H_4-COO-CH_2CH_2-OH}$$$$\rightarrow \mathrm{PET\ polymer}+(n-1)\mathrm{MEG}$$

The important difference from transesterification is the volatile material being removed:

• Transesterification: methanol is removed.
• Polycondensation: mainly MEG is removed.

The supplied references describe polycondensation at approximately 270–300 °C under vacuum.

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Why is vacuum used?

Polycondensation is an equilibrium reaction.

As the PET chains grow, MEG is produced. Removing MEG shifts the reaction toward further polymer formation.

Therefore:

higher temperature + high vacuum + efficient removal of MEG → higher molecular weight

But there is an important limitation.

Excessive temperature or excessive residence time can increase thermal degradation. PET melt is thermally sensitive, and degradation can affect colour and polymer quality.

So the objective is not simply “maximum temperature” or “maximum vacuum.”

The real objective is controlled polymer growth with minimum degradation.

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7. The role of the finisher

As polycondensation proceeds, the PET melt becomes increasingly viscous.

A finisher is used in continuous PET production to carry out the final part of molecular-weight build-up.

The supplied industrial reference describes continuous PET equipment including a prepolymerizer and a disc or screw finisher.

A finisher provides a large effective surface area of polymer melt, helping volatile reaction products escape under vacuum.

This becomes increasingly important because the melt viscosity is high during the final stage.

The reference notes that fibre-grade PET melt at 285 °C can have a melt viscosity of the order of 3000 poise (300 Pa·s) under the specified shear conditions.

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8. Why molecular weight matters for textile fibres

A textile fibre cannot be made simply by having PET chemically present. The polymer must have suitable molecular characteristics for melt spinning and subsequent drawing.

As molecular weight increases appropriately:

• melt viscosity increases,
• chain entanglement becomes more significant,
• the polymer becomes more suitable for fibre formation,
• the melt can be processed into continuous filaments that can subsequently be oriented.

The DMT-route process therefore has a direct connection with textile engineering.

Polymerization conditions → molecular structure → melt behaviour → spinning behaviour → fibre properties

This is why polymer production and fibre spinning should not be studied as completely separate subjects.

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9. Side reactions during PET manufacture

PET production is not a perfectly clean single reaction.

Several side reactions can occur.

Formation of diethylene glycol

Two MEG molecules can undergo an ether-forming reaction:$$\mathrm{HO-CH_2CH_2-OH + HO-CH_2CH_2-OH}$$$$\rightarrow \mathrm{HO-CH_2CH_2-O-CH_2CH_2-OH + H_2O}$$

The product is diethylene glycol (DEG).

DEG units can become incorporated into the PET chain.

The supplied textile reference reports that DEG formation occurs during several stages of PET synthesis and that its presence influences the polymer’s structure and properties. It also reports that approximately 70% of DEG formation in the illustrated process occurred during preheating and low-vacuum polycondensation, with about 20% during transesterification and 5–10% during the final polycondensation stage.

These percentages belong to the specific process data reported in that reference and should not be generalized to every PET plant.

Why does DEG matter?

The reference reports that each mol% DEG lowers the PET melting point by about 2.5 °C and that DEG can increase dye affinity. It also notes that variation in DEG content can therefore influence dyeing behaviour.

Exam Tip: If asked about an important side reaction in PET manufacture, remember DEG formation.

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10. Thermal degradation of PET

PET is processed at high temperature, so thermal degradation must be controlled.

The supplied reference states that PET melt becomes susceptible to degradation at high polymerization temperatures, with discoloration being one possible consequence.

Several factors therefore need control:

• temperature
• residence time
• vacuum conditions
• catalyst system
• raw-material purity
• water content
• melt exposure to unsuitable thermal conditions

Another source specifically notes that hydrolytic or thermal damage to DMT and excess water can influence the transesterification stage. It also warns that a rapid increase in reaction temperature can increase the formation of diglycol by-products.

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11. Catalyst deactivation before polycondensation

There is an interesting process-control issue here.

Catalysts used for the transesterification stage can also promote unwanted reactions during the high-temperature polycondensation stage.

For this reason, the supplied reference describes the use of catalyst deactivators or sequestering agents after transesterification. Examples mentioned include:

• polyphosphoric acid
• triphenyl phosphite
• triphenyl phosphate

These can deactivate the ester-interchange catalyst.

This is a good example of why chemical-process design cannot be understood by looking at one reaction in isolation.

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12. Polycondensation catalysts

Catalysts used for the final polycondensation are different in function from the transesterification catalyst system.

The supplied references describe antimony trioxide (Sb₂O₃) and antimony triacetate among commonly used antimony-based PET catalysts.

The exact catalyst package depends on the process and product requirements.

For students, the main concept is:

The catalyst system influences reaction rate, side reactions, colour, thermal stability and the resulting PET structure.

Do not memorize a catalyst name without understanding which stage it belongs to.

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13. Batch and continuous DMT-route processes

PET can be produced using batch, semicontinuous or continuous process arrangements.

The supplied textile engineering reference describes three broad arrangements:

Process	Polymer handling before spinning	Main characteristic
Batch	Polymer is produced in batches and generally converted to chips before spinning	Flexible but greater batch-to-batch variation
Semicontinuous	Polymerization is continuous, followed by chip production and remelting	Intermediate arrangement
Continuous	Polymer is pumped directly from final polycondensation to spinning	Avoids chip production, handling, drying and remelting

The continuous route can provide more uniform product quality and avoids several additional thermal histories associated with chip handling and remelting. However, the reference also notes its lower flexibility and higher sensitivity to power failure.

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14. From PET polymer to polyester fibre

Polymer manufacturing is only the first part of textile polyester production.

The PET melt can follow different routes.

Route A: Polymer → chips → spinning

The polymer is:

1. extruded,
2. cooled,
3. cut into chips,
4. stored,
5. crystallized/dried where required,
6. remelted,
7. melt-spun.

Route B: Direct polymerization → spinning

In a continuous process, the polymer from the final polycondensation stage can be pumped directly to the spinning machines.

The second route eliminates:

• chip manufacture,
• chip handling,
• drying,
• remelting.

The supplied reference identifies direct spinning as a continuous process configuration.

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15. Why the DMT route matters in textile engineering

It may seem that DMT transesterification belongs to chemical engineering while spinning belongs to textile engineering.

In reality, they are strongly connected.

The polymer produced during the DMT route determines important characteristics of the melt entering the spinning process.

For example:

Polymer molecular weight
↓
Melt viscosity
↓
Spinning behaviour
↓
Filament formation and drawing
↓
Orientation and fibre properties

The manufactured-fibre reference treats PET polymer production, melt spinning, drawing, heat setting, structure and mechanical properties as interconnected parts of manufactured fibre technology.

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DMT route vs PTA route

Students often confuse these two PET manufacturing routes.

Feature	DMT route	PTA route
Main terephthalate raw material	DMT	PTA/TPA
First reaction	Transesterification / ester interchange	Direct esterification
Major volatile product in first stage	Methanol	Water
Intermediate	Glycol terephthalate-containing mixture	Glycol terephthalate-containing mixture
Second major stage	Polycondensation	Polycondensation
Main volatile product during polycondensation	MEG	MEG
Historical significance	Earlier major PET route	Became increasingly important after economical high-purity PTA became available

The supplied sources explain that DMT was historically important because sufficiently pure PTA was not initially available economically. With the development and commercialization of suitable PTA processes, the direct PTA route became increasingly important.

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Common mistakes students make

1. Saying that DMT directly polymerizes into PET

Not quite.

The DMT route has a transesterification stage followed by polycondensation.

2. Confusing DMT with PTA

DMT is an ester, while PTA/TPA is the corresponding dicarboxylic acid.

3. Forgetting the methanol

In DMT transesterification:$$\boxed{\text{Methanol is removed}}$$

4. Saying that vacuum is mainly used during transesterification

The major vacuum-driven molecular-weight build-up occurs during polycondensation.

5. Assuming higher temperature is always better

Higher temperature can accelerate reactions, but excessive temperature can also increase unwanted reactions and degradation.

6. Treating PET polymerization as a single reaction

For understanding the industrial process, divide it into:

transesterification → intermediate formation → polycondensation

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Industry Connection: why process control matters

A modern fibre plant does not simply produce polymer and then check it at the end.

The supplied literature emphasizes the relationship between process conditions and consistent fibre properties. PET manufacturing therefore involves control of factors such as raw-material quality, reaction conditions, molecular weight and melt behaviour.

This matters because variations in polymer quality can eventually appear as variations in spinning behaviour and fibre properties.

For a textile engineer, the important lesson is:

Fibre quality starts before the spinneret.

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A simple way to remember the complete DMT route

Use this sequence:

If you remember this sequence, you have the basic process logic.

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Visual recommendation

VISUAL TYPE: Process flowchart

TITLE: Manufacturing of Polyester Using the DMT Route

WHAT IT SHOULD SHOW: DMT and MEG feeding → DMT melting/feeding → transesterification reactor → methanol separation → DGT/BHET and oligomer mixture → prepolymerization → final polycondensation under vacuum → finisher → PET melt → chip production or direct spinning.

WHY IT HELPS: The DMT route contains several chemical and physical stages. A flowchart allows students to see where methanol and MEG are removed and where molecular weight increases.

SUGGESTED ALT TEXT: Flow diagram showing polyester PET manufacturing through DMT and MEG transesterification followed by vacuum polycondensation.

SUGGESTED CAPTION: Simplified process flow for PET production by the DMT route.

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Quick Revision

1. PET used for polyester fibres is poly(ethylene terephthalate).
2. The DMT route uses dimethyl terephthalate and monoethylene glycol.
3. The first major reaction is transesterification (ester interchange).
4. Methanol is the principal by-product removed during transesterification.
5. The first stage produces glycol terephthalate intermediates and oligomers.
6. The second major stage is polycondensation.
7. MEG is removed during polycondensation to drive molecular-weight growth.
8. Polycondensation is carried out at high temperature under vacuum.
9. DEG formation is an important side reaction that can influence PET structure and properties.
10. PET may be produced as chips or sent directly from continuous polymerization to melt spinning.

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5. QUICK REVISION

DMT route in one line

DMT + MEG → transesterification + methanol removal → DGT/BHET/oligomers → polycondensation + MEG removal → high molecular weight PET → spinning.

Exam Tip

If an examination question asks “Explain the manufacturing of polyester by DMT route,” do not write only the chemical equations. Include:

• raw materials,
• DMT melting/feeding,
• transesterification,
• catalyst,
• methanol removal,
• intermediate formation,
• polycondensation,
• vacuum,
• MEG removal,
• molecular-weight development,
• PET chips/direct spinning.

Recommended internal-link opportunities

Anchor text	Suggested related article	Purpose
PET fibre manufacturing	PET Fibre Manufacturing: Process, Properties and Applications	Moves from polymer production to fibre production
melt spinning	Melt Spinning of Synthetic Fibres: Working Principle and Process	Explains the next manufacturing stage
polyester fibre properties	Polyester Fibre: Properties, Advantages and Applications	Connects polymer structure with textile performance
transesterification	Transesterification in Polyester Manufacturing	Gives deeper chemistry background
polycondensation	Polycondensation: Principle, Reaction and Applications	Builds polymerization fundamentals
intrinsic viscosity	Intrinsic Viscosity of Polymer: Meaning and Measurement	Connects molecular characteristics with polymer quality
PET chips	PET Chips: Manufacturing, Properties and Uses	Explains the intermediate material before spinning
polyester drawing	Drawing of Polyester Fibres: Principle and Effect on Properties	Continues the fibre-processing journey

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6. FAQS

1. What is the DMT route for polyester manufacturing?

The DMT route is a method of producing PET using dimethyl terephthalate (DMT) and monoethylene glycol (MEG). It consists mainly of transesterification followed by polycondensation.

2. What is the first step in the DMT route?

The first major reaction is transesterification or ester interchange between DMT and MEG.

3. What is produced during DMT transesterification?

DMT reacts with MEG to form glycol terephthalate intermediates, including DGT/BHET-containing material, while methanol is released and removed.

4. Why is methanol removed during transesterification?

The reaction is an equilibrium reaction. Removing methanol helps shift the reaction toward formation of the glycol terephthalate intermediate.

5. What happens during PET polycondensation?

The glycol terephthalate intermediates and oligomers react to form longer PET chains. MEG is removed, usually under vacuum, allowing molecular weight to increase.

6. Why is vacuum used in PET polycondensation?

Vacuum helps remove volatile reaction products such as MEG. This supports continued polycondensation and molecular-weight development.

7. What is the difference between the DMT and PTA routes?

The DMT route uses DMT and MEG and begins with transesterification, producing methanol. The PTA route uses PTA/TPA and MEG and begins with direct esterification, producing water. Both then proceed toward polycondensation.

8. Why is DEG formation undesirable?

DEG can become incorporated into the PET chain and modify its structure and properties. Its level therefore needs to be controlled for consistent polymer and fibre performance.

9. Can PET be sent directly to spinning?

Yes. In a continuous polymerization/direct-spinning arrangement, PET from the final polycondensation vessel can be pumped directly to the spinning machines, avoiding chip manufacture, handling, drying and remelting.

10. Is the DMT route still the main PET manufacturing route?

The supplied historical references explain that DMT was once the dominant PET starting material, while the PTA route became increasingly important after economical production of sufficiently pure PTA was developed. Therefore, the historical importance of the DMT route is greater than its role in many later PET processes.