Polymer Synthesis

Polymer Synthesis (Ring opening)

Graded Seminar

Submitted By-Tejas Chandrakant JagtapM.Pharm (Pharamceutics)

1st Sem

Guided by-Dr.(Mrs.) Shilpa Chaudhari

Dr. D. Y. Patil College of Pharmacy, Akurdi, Pune-44

polymer

Ring Opening Polymerization

Contents: Introduction to Polymer Synthesis

Classification of Polymer


I. Radical ROPII. Cationic ROPIII. Anionic ROP

History

Mechanism of ROP

References


Polymers are extremely large molecules that are essential to our every existence. They are main constituents of food (starch, protein, etc.), clothes (polyester, nylons, etc.), houses (wood cellulose, alkyl, paints, etc.), and bodies (poly(nucleic acids), proteins, etc.).

Polymer Synthesis

There are two major types of polymerization methods used to convert small molecules (monomers) into polymers. These methods were originally referred to as addition and condensation polymerization.

Depending on the author, Addition polymerization is called as chain, chain-growth, or chain

reaction polymerization. Condensation polymerization is now referred to as step-growth or step-

reaction polymerization.

Introduction to Polymer Synthesis

1


Based on Origin

• Natural• Eg. Cellulose, natural

rubber, nucleic Acid

• Synthetic• Eg. fibers, elastomers,

plastics, adhesives, etc.

Based on Structure

• Linear, Branched or Cross-linked, Ladder vs. Functionality

• Amorphous or Crystalline

• Homopolymer or Copolymer• Random• Alternating• Block• Graft

• Fibers, Plastics, or Elastomers

Based on POLYMERIZATION MECHANISM

• Condensation

• Addition

• Ring Opening

Based on Thermal Behavior

• Thermo Plastic

• Thermosetting Plastic

Based on Mechanical Behavior• Soluble polymers• Hydrogels• Nano- and micro

particles• Foams for tissue

engineering• Fibers• Lenses• Adhesive• Surface modifications• Polymer brushes• Interpenetrating

networks• Shape memory materials• Composites/additives

Classification of Polymers

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Ring-opening polymerization (ROP) is a form of chain-growth polymerization, in which the terminal end of a polymer chain acts as a reactive center where further cyclic monomers can react by opening its ring system and form a longer polymer chain. (Fig.1)

A Polymerization in which a cyclic monomer yields a monomeric unit which is acyclic or contains fewer cycles than the monomer.

If the monomer is polycyclic, the opening of a single ring is sufficient to classify the reaction as ring-opening polymerization.

The propagating center can be radical, anionic or cationic.

ROP continues to be the most versatile method of synthesis of major groups of biopolymers, particularly when they are required in quantity.

Polymer

Fig.1 Ring opening polymerization3


Cyclic monomers that are polymerized using ROP encompass a variety of structures, such as: alkanes, alkenes, etc.

Compounds containing heteroatoms in the ring:

Oxygen: ethers, acetals, esters (lactones, lactides, and carbonates) and anhydrides.

Sulfur : polysulfur, sulfides and polysulfide.

Nitrogen : amines, amides (lactames), imides, N-carboxyanhydride and 1,3-oxaza derivatives.

Phosphorus : Phosphates, phosphonates, phosphites, phosphines and phosphazenes.

Silicon: siloxanes, silathers, carbosilanes and silanes.


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HistoryRing-opening polymerization (ROP) has been used since the beginning of the

1900s in order to synthesize polymers. Synthesis of polypeptides which has the oldest history of ROP, dates back to the work in 1906 by Leuchs.

Many years later came the method of the ROP of anhydro sugars, providing polysaccharides, including synthetic dextran, Xanthan gum, welan gum, gellan gum, diutan gum, and pullulan.

Mechanisms and thermodynamics of ring-opening polymerization was further established in the 1950s.

The first high-molecular weight polymers (Mn up to 105) with a repeating unit were prepared by ROP as early as in 1976.

Nowadays, ROP plays an important role in industry such as production of nylon-6. ROP can introduce functional groups such as ether, ester, amide, and carbonate into the polymer main chain, which cannot be achieved by vinyl polymerization affording polymers only with C-C main chain.

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History Polymers obtained by ROP can be also prepared by polycondensation in most cases,

but following controlled radical polymerization is possible in ROP, which is difficult in polycondensation.

Recently, development of novel monomers and catalysts has enabled polymer chemists to control molecular weights, structure, and configuration of the polymers precisely.

Cyclic carbonates undergo both cationic polymerization and anionic polymerization to afford the corresponding polycarbonates, which are expected as biocompatible and biodegradable polymers.

Recently, ultra high molecular weight bisphenol A polycarbonate (> 2,000 kDa) has been synthesized by ROP of a large-membered bisphenol A-based cyclic carbonate.

The resulted polymer can be used as engineering plastics due to its thermal stability and high impact resistance. When the reactive center of the propagating chain is a carbocation, the polymerization is called cationic ring-opening polymerization.

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Mechanism Of ROP

Ring-opening polymerization can proceed via

1. Radical Polymerization

2. Anionic Polymerization

3. Cationic Polymerization

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With radical ring-opening polymerization, it is possible to produce polymers of the same or lower density than the monomers. This is important for applications that require constant volume after polymerization, such as tooth fillings, coatings, and the molding of electrical and electronic components.

MechanismIn free radical ROP, the cyclic structure will undergo homolytic dissociation rather than

undergoing heterolytic dissociation (as is the case for any ionic ROP). There are two typical mechanistic schemes in radical ROP.

Scheme1: The terminal vinyl group accepts a radical. The radical will be transformed into a carbon radical stabilized by functional groups (i.e. halogen, aromatic, or ester groups). This will lead to the generation of an internal olefin.

1. Radical ROP

Fig.2 Radical ROP of Vinyl cyclopropane 8


Scheme 2: In this case, the exo-methylene group is the radical acceptor. The ring-opening reaction will form an ester bond, and the radical produced is stabilized by a phenyl group.

1. Radical ROP

Fig.3 Radical ROP of Ketene Acetal

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2. Anionic ROPAnionic ring-opening polymerizations (AROP) are ring-opening polymerizations that

involve nucleophilic reagents as initiators.

Monomers with a three-member ring structure - such as epoxide, aziridine, and episulfide - are able to undergo anionic ROP due to the ring-distortion, despite having a less electrophilic functional group (e.g. ether, amine, and thioether).

These cyclic monomers are important for many practical applications. The polarized functional group in cyclic monomers is characterized by one atom (usually a carbon) that is electron-deficient due to an adjacent atom that is highly electron-withdrawing (e.g. oxygen, nitrogen, sulfur etc.)

Ring-opening will be triggered by the nucleophilic attack of the initiator to the carbon, forming a new species that will act as a nucleophile. The sequence will repeat until the polymer is formed.

Fig.4 General Mechanism of Anionic Ring opening ROP 10

Ring Opening Polymerization11

2. Anionic ROP A typical example of anionic ROP is that of ε-caprolactone, initiated by an alkoxide functional group.

Fig.5 The anionic ring-opening polymerization of ε - caprolactone, initiated by alkoxide function.


I. InitiationCommon nucleophilic reagents used for the initiation of AROP usually will include

organometals (e.g. alkyl lithium, alkyl magnesium bromide, alkyl aluminum, etc.), metal amides, alkoxides, phosphines, amines, alcohols and water.

The monomers that undergo AROP will contain polarized bonds (ester carbonate, amide, urethane, and phosphate) which respectively leads to the production of the corresponding polyester, polycarbonate, polyamide, polyurethane and polyphosphate. Monomer rings that are asymmetrically substituted will open with nucleophilic attack on the least substituted carbon atom.

II. PropagationThe general mechanism of propagation for anionic ROP relies on the nucleophilic attack

of a propagating chain end to a monomer. Another possible mechanism for propagation is the nucleophilic attack of an activated monomer to the growing chain end. ε-caprolactam and N-carboxy-anhydride undergo this kind of mechanism.

III. Transfer and terminationTermination in AROP can be described as chain transfer reactions to monomer that is

available. The active centers of AROP monomers are nucleophilic and also act as bases to abstract protons from the monomer, initiating new chains.

Thus, AROP often results in low molecular-weight polymers.

2. Anionic ROP


3. Cationic ROP Cationic ring-opening polymerization (CROP) is characterized by having a cationic initiator and

intermediate. Examples of cyclic monomers that polymerize through this mechanism include lactones, lactams, amines and ethers.

CROP proceeds through an SN1 or SN2 propagation, chain-growth process. The predominance of one mechanism over the other depends on the stability of the resulting cationic species.

For example, if the atom bearing the positive charge is stabilized by electron-donating groups, polymerization will proceed by the SN1 mechanism.

The cationic species is an heteroatom and the chain grows by the addition of cyclic monomers thereby opening the ring system.

Fig.6 SN1 and SN2 mechanisms of CROP.


3. Cationic ROP

Fig.7 Typical Example of CROP.

Cationic polymerization is used in the production of polyisobutylene (used in inner tubes) and poly(N-vinylcarbazole) (PVK).


I. InitiationThe monomers can be activated by Bronsted acids, carbenium ions, onium ions, photo -

initiators, and covalent initiators.

II. PropagationThe cationic species is an heteroatom and the chain grows by the addition of cyclic

monomers thereby opening the ring system. In CROP, three mechanisms are distinguished by the propagating species.

When the cationic species is a secondary ion, polymerization proceeds by ring expansion. This mechanism is observed when the monomer is in low concentration. When it is a tertiary ion, polymerization proceeds by linear growth.

The monomer can likewise be activated (i.e. cationic) and the propagation step will proceed via electrophilic addition of the activated monomer to the growing chain.

III. TerminationCROP can be considered as a living polymerization and can be terminated by

intentionally adding termination reagents such as phenoxy anions, phosphines or polyanions. When the amount of monomers becomes depleted, termination can occur intra or intermolecular.

The active end can "backbite" the chain, forming a macrocycle. Alkyl chain transfer is also possible, where the active end is quenched by transferring an alkyl chain to another polymer.

3. Cationic ROP


References1. IUPAC, "Ring-opening polymerization“, Compendium of Chemical Terminology, 2nd ed. (the "Gold

Book") (1997). Online corrected version: (2006).

2. Jenkins, A. D.; Kratochvíl, P.; Stepto, R. F. T.; Suter, U. W., "Glossary of basic terms in polymer science (IUPAC Recommendations 1996)". Pure and Applied Chemistry. Pg. no. 68(12): 2287–2311.

3. Dainton, F. S.; Devlin, T. R. E.; Small, P. A, "The thermodynamics of polymerization of cyclic compounds by ring opening“, (1955), Transactions of the Faraday Society, Pg. no. 51: 1710.

4. Conix, André; Smets, G., "Ring opening in lactam polymers". Journal of Polymer Science, (January 1955), Science Direct, Pg. no. 15: 221–229.

5. www.wikipedia.com, The Free Encyclopedia.

http://www.wikipedia/

Ring Opening Polymerization 19

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