Name: polymer tacticity and chain properties
Uploaded: 2023-04-30T11:24:03.000Z
Duration: 1 min 26 s
Channel: JoVE

Polymerization of a substituted vinyl monomer leads to a polymer chain with numerous chiral carbons. Based on the relative configurations of these chiral centers, a polymer can be classified as isotactic, syndiotactic, or atactic.
In the isotactic configuration, all substituents are aligned in the polymer chain backbone.
In the syndiotactic configuration, the substituents periodically alternate on either side of the polymer chain.
In an atactic polymer, the substituents are randomly oriented.
Unlike the atactic configuration, the stereoregular arrangement of substituents in isotactic and syndiotactic configurations enables the close-packing of polymer chains.
So, the crystallinity and melt transition temperature of a polymer increase with an increase in the stereospecificity of polymer chains, while an increase in random orientations makes the polymer less crystalline.
For example, isotactic polypropylene has a melting point between 160 and 170 degrees Celsius, which drops to 125 to 131 degrees in syndiotactic polymers. Conversely, atactic polypropylene is a rubbery material without a sharp melting point.

polymer classification: stereospecificity

Polymer Classification: Stereospecificity

Polymerization generates chiral centers along the entire backbone of a polymer chain. Accordingly, the stereochemistry of the substituent group has a significant effect on polymer properties. Polymers formed from monosubstituted alkene monomers feature chiral carbons at every alternate position in the polymer backbone. Relative to the predominant orientation of substituents at the adjacent chiral carbons, the polymer can exist in three different configurations: isotactic, syndiotactic, and atactic.
In the isotactic configuration, the substituents are generally positioned on the same side of the polymer backbone. In the syndiotactic configuration, the substituents periodically alternate on both sides of the polymer chain. In the atactic configuration, the substituents orient randomly. Figure 1 depicts the comparison of substituents&#8217; arrangement in the isotactic, syndiotactic, and atactic polypropylene polymer chains.
<img alt="Polymers molecular structure diagram: isotactic, syndiotactic, atactic polypropylene configurations." src="/files/ftp_upload/12918/12918_PT_Figure_1.svg" style="text-align: center; height: 250px;" data-altupdatedat="1761830439000" data-alt="Figure1" />
Figure 1: Structural configurations of isotactic polypropylene (top), syndiotactic polypropylene (middle), and atactic polypropylene (bottom) chains. 
The more regular arrangement of substituents in isotactic and syndiotactic configurations facilitates the close-packing of polymer chains and increases the polymer's density, crystallinity, and melt transition temperature. On the other hand, an increase in the fraction of the atactic configuration makes loosely bound polymer chains, which reduces the density and crystallinity of the polymer.
For example, the melting temperature for commercial isotactic polypropylene is 160 to 170 &#176;C, depending on the quantity of atactic traces present, while for syndiotactic polypropylene, it is 125 to 131 &#176;C. In contrast, atactic polypropylene is an amorphous rubbery material without a sharp melting point. So, control over the stereospecificity of polymer chains is important while synthesizing polypropylene for commercial applications, such as temperature-resistant tubes and bottles.

Polymerization generates chiral centers along the entire backbone of a polymer chain. Accordingly, the stereochemistry of the substituent group has a ...

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Organic Chemistry

Unit 1

Synthetic Polymers

KEY TERMS AND CONCEPTS

polymer tacticity and chain properties

Polymer tacticity describes how side groups are arranged along a polymer chain. In polymers made from monosubstituted alkene monomers, polymerization creates chiral centers at every other carbon in the backbone. That stereochemistry can strongly affect the final properties of the polymer.

There are three main configurations: isotactic, syndiotactic, and atactic. In isotactic polymers, the substituents are mostly on the same side of the backbone. In syndiotactic polymers, the substituents alternate from one side to the other. In atactic polymers, the substituents are arranged randomly.

These arrangements change how tightly polymer chains pack together. Isotactic and syndiotactic chains can pack closely, which increases density, crystallinity, and melt transition temperature. Atactic chains are more loosely packed, so they have lower density and crystallinity.

Polypropylene shows these differences clearly. Commercial isotactic polypropylene melts at about 160 to 170 °C, depending on how much atactic material is present. Syndiotactic polypropylene melts at about 125 to 131 °C. Atactic polypropylene does not have a sharp melting point and behaves like an amorphous rubbery material. Because of this, controlling stereospecificity matters when making polypropylene for products such as temperature-resistant tubes and bottles.

Polymer Tacticity and Chain Properties

characteristics and nomenclature of homopolymers

characteristics-and-nomenclature-of-homopolymers

Naming Homopolymers from Their Monomers

Homopolymers are polymers made from identical monomer units. They contain only one repeating unit in the polymer chain. A clear example is polypropylene, which is built from propylene monomers. The name of a homopolymer is usually formed by adding the prefix poly to the monomer name. That is why the polymer from propylene is called polypropylene. When the monomer has multiple parts or substituents, the monomer name may be placed in parentheses after poly. Examples include...

characteristics and nomenclature of copolymers

characteristics-and-nomenclature-of-copolymers

Copolymers: Types, Structures, and Naming

Copolymers are polymers made from more than one kind of monomer. When different monomer species are polymerized together, the chain can contain multiple repeating units. This process is called copolymerization. Copolymers can be named by the number of monomers involved. A polymer made from two monomers is a bipolymer. Polymers made from three monomers are called terpolymers, and those made from four monomers are quaterpolymers. A common example is poly(styrene-co-acrylonitrile), also called...

polymers: defining molecular weight

Polymer Molecular Weight Averages

Polymer molecular weight is often described with averages because a polymer sample contains chains of many different lengths. Unlike a small molecule, which has one definite molecular weight, each polymer chain can have a unique molecular weight. The result is a sample made up of chains with varying sizes. The number average molecular weight, or Mn, uses the number fraction of each unique chain. A number fraction is the number of chains with a given molecular weight divided by the total number...

polymers: molecular weight distribution

Polymer Chain Size and Dispersity

Polymer chain size and dispersity describe how uniform the chains are in a polymer sample. The weight average molecular weight, Mw, is usually higher than or equal to the number average molecular weight, Mn. These two values are equal only when every chain has the same molecular weight. That exact match is rare in synthetic polymers. It is difficult to control polymerization at the molecular level with complete accuracy. Because of this, the ratio Mw to Mn is used to describe the molecular...

polymer classification: architecture

Polymer Chains: Branching and Cross-Linking

Polymer chains can be classified by their shape and by how their chains connect. Linear polymers have long chain-like structures with little or no branching. Even when a monomer has large substituent groups that look like branches, the polymer is not considered branched if those groups are part of the main chain structure. Branched polymers contain secondary chains that grow out from the main polymer chain. This branching happens when polymer growth shifts away from the edge of the growing...

polymer classification: crystallinity

Crystallinity in Polymer Structure

Crystallinity in polymer structure depends on how polymer chains are packed. Unlike ionic solids or small covalent molecules, polymers usually do not form fully crystalline solids because their long chains move and diffuse less easily. Instead, many polymers contain microscopic crystalline domains separated by amorphous domains. Crystalline domains are the ordered regions of a polymer. In these areas, the chains line up closely and stay near each other because of intermolecular forces. In...

Polymer tacticity describes how side groups are arranged along a polymer chain. In polymers made from monosubstituted alkene monomers, polymerization creates chiral centers at every other carbon in the backbone. That stereochemistry can strongly affect the final properties of the polymer. There are three main configurations: isotactic, syndiotactic, and atactic. In isotactic polymers, the substituents are mostly on the same side of the backbone. In syndiotactic polymers, the substituents...

radical chain-growth polymerization: overview

radical-chain-growth-polymerization-overview

Radical Chain-Growth Polymerization Steps

Radical chain-growth polymerization builds a polymer by repeated addition of monomers to a growing chain. It uses a free-radical intermediate, which is the reactive species that keeps the chain moving forward. The process starts with a radical initiator. These initiators generate free radicals on their own by homolytic fission, or splitting a bond so each atom keeps one electron. Common initiators include organic peroxides such as dibenzoyl peroxide and azo compounds. A low...

radical chain-growth polymerization: mechanism

radical-chain-growth-polymerization-mechanism

Radical Polymer Chain Growth and Ending

Radical chain-growth polymerization builds polymers through initiation, propagation, and termination. A free radical from the initiator adds to the unsaturated bond in a monomer. The unpaired electron from the radical and one pi electron from the double bond form a sigma bond. The other pi electron then becomes a new free radical on the growing molecule. The initiation step can be shown with a phenyl free radical reacting with vinyl chloride. After initiation, the new radical adds to another...

radical chain-growth polymerization: chain branching

radical-chain-growth-polymerization-chain-branching

How Radical Polymer Chains Branch

Radical polymerization can create branched polymer chains. A clear example is ethylene polymerized by a radical route, which forms a low-density type of polyethylene with a heavily branched skeletal structure. During growth, the radical site can remove a hydrogen atom from the chain and move from the end of the chain, a primary carbon center, to a point within the chain, a secondary carbon center. When that happens, the part of the chain from the hydrogen-removal site to the end becomes a...

anionic chain-growth polymerization: overview

anionic-chain-growth-polymerization-overview

Anionic Polymerization and Everyday Plastics

Anionic polymerization is a chain-growth process that uses a carbanion as an intermediate. It begins when a strong nucleophile, such as an organolithium compound or a Grignard reagent, starts the reaction. Butyl lithium is the most commonly used initiator for this process. The monomers used in anionic polymerization must have a vinyl group attached to one or two electron-withdrawing groups. Acrylonitrile, vinyl chloride, and styrene each have one electron-withdrawing group and can form anionic...

anionic chain-growth polymerization: mechanism

anionic-chain-growth-polymerization-mechanism

Living Polymers in Anionic Polymerization

Anionic chain-growth polymerization builds a polymer through initiation, propagation, and termination. It begins when a nucleophilic anion, such as butyl lithium, attacks the pi bond of a vinylic monomer. This creates a carbanion, which is a carbon atom with a negative charge. The electron-withdrawing group on the monomer helps stabilize that carbanion. The carbanion then drives the propagation step. It acts as a Michael donor and attacks a second vinylic monomer, which serves as the Michael...

cationic chain-growth polymerization: mechanism

cationic-chain-growth-polymerization-mechanism

Cationic Polymerization Steps in Chain Growth

Cationic polymerization builds a polymer chain through three stages: initiation, propagation, and termination. It starts when a monomer’s pi bond is protonated by a Lewis acid catalyst formed from boron trifluoride and water. This protonation creates a carbocation, which is stabilized by an electron-donating group. During propagation, the pi bond of a second monomer acts as a nucleophile, meaning it donates electrons. It attacks the carbocation and forms a dimer, which becomes a new...

ziegler&ndash;natta chain-growth polymerization: overview

zieglernatta-chain-growth-polymerization-overview

Ziegler-Natta Catalysts and Linear Polymers

Ziegler-Natta polymerization uses a special catalyst to make linear polymers. It is a type of addition polymerization, also called chain-growth polymerization. This method favors straight polymer chains over branched ones. The catalyst is called a Ziegler-Natta catalyst. It is named after Karl Ziegler and Giulio Natta, who developed it in 1953. The catalyst is an organometallic complex made from titanium tetrachloride and triethyl aluminum. Its active form is an alkyl titanium compound. With...

step-growth polymerization: overview

Step-Growth Polymerization in Common Polymers

Step-growth polymerization is a condensation process that builds long-chain polymers from bi- or multifunctional monomers. In this reaction, the monomers are reactive from the start. Early in the process, most monomers form short oligomers, which are small reactive chains. These oligomers then link together later to make long polymer chains. Because the chains grow step by step, the reaction must run for a long time to reach high molecular weight. Many natural and synthetic polymers come from...

molecular weight of step-growth polymers

Step-Growth Polymerization and Chain Length

Step-growth polymerization builds polymers by linking bi- or multifunctional monomers. Bifunctional monomers form linear step-growth polymers. Multifunctional monomers can form non-linear or branched polymers. As the reaction continues, the molecular weight keeps building up step by step. High molecular weight polymers usually appear at the late stages of the polymerization. By that point, about 99% of the monomers have been consumed. The extent of the reaction can be found with the Carothers...

types of step-growth polymers: polyesters

Polyester Uses and PET Production

Polyesters are widely used step-growth polymers, and PET is one of the most important examples. These materials helped transform the textile industry because polyester blends resist wrinkles. That reduced the need for starching and ironing clothes. Industrial PET is made from dimethyl terephthalate and ethylene glycol by transesterification at 150 °C. In this process, methanol is the by-product. It vaporizes at the reaction temperature, which helps drive the reaction to completion. PET can...

olefin metathesis polymerization: overview

olefin-metathesis-polymerization-overview

Olefin Metathesis Polymerization Pathways

Olefin metathesis polymerization creates polymers by rearranging double bonds in olefins with a catalyst. The metathesis reaction moves substituents between two alkene molecules, and that same chemistry can be used to build polymer chains. Ruthenium-based Grubbs catalyst is the most common catalyst for this process. Grubbs catalyst contains a carbon-metal double bond called a carbene. The reaction is reversible and starts when the catalyst reacts with an alkene in a [2+2] cycloaddition. This...

olefin metathesis polymerization: ring-opening metathesis polymerization (romp)

olefin-metathesis-polymerization-ring-opening-metathesis

ROMP Polymerization Driven by Ring Strain

Ring-opening metathesis polymerization, or ROMP, is a polymer method that starts with strained cycloalkenes. A cycloalkene is a ring-shaped alkene, which means it contains a carbon-carbon double bond inside a ring. In ROMP, the ring strain helps push the reaction forward. The reaction begins when a cycloalkene reacts with Grubbs catalyst. This forms a metallacyclobutane intermediate, which then opens to make a new carbene. That new carbene can react with another cycloalkene molecule, and the...

olefin metathesis polymerization: acyclic diene metathesis (admet)

olefin-metathesis-polymerization-acyclic-diene-metathesis-admet

ADMET Polymerization for Linear Polymers

ADMET polymerization is a type of olefin metathesis used to build linear unsaturated polymers from terminal dienes. In this reaction, molecules such as 1,8-nonadiene react with each other and release ethylene gas. The process is reversible, so the ethylene must be removed from the reaction mixture for polymerization to continue. ADMET works through a metallacyclobutane intermediate, which forms when one double bond in a terminal diene undergoes [2+2] cycloaddition with a Grubbs catalyst. The...

key terms and concepts

Learn how isotactic, syndiotactic, and atactic polymer chains differ, and how tacticity changes density, crystallinity, and melting behavior in polypropylene.

Polymer Tacticity and Chain Properties | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Polymer Tacticity and Chain Properties

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