The Wonderful World of Polymers: An Article to Help You Understand Polymers

 

In our daily lives, from the clothes we wear and the plastic bags we use, to car tires and mobile phone casings, we cannot do without a miraculous substance—polymers, also known as macromolecules. While they appear incredibly diverse, the underlying chemical reaction logic and the fundamental nature of these materials follow a clear and consistent pattern.

I.The “Birth Story” of Polymers: Three Major Chemical Reactions

Polymers are giant molecules formed by thousands of small molecules (called “monomers”) linking together like hands holding hands. Based on how they “hold hands,” there are three main types of reactions:

01 Addition Polymerization: Direct “Hand-in-Hand” Linking

How does it polymerize? Like ethylene monomers, they have a reactive double bond (π bond). This double bond opens up like two hands to grab other monomers, forming a long carbon chain, such as the most common polyethylene (PE).

Caratteristiche:

The monomers are usually olefins, and the products are mostly pure hydrocarbon chains.

The chemical element composition of the product is exactly the same as that of the monomer; nothing else is released.

The molecular weight of the polymer is exactly an integer multiple of the monomer’s molecular weight.

02 Condensation Polymerization: Hand-in-Hand Linking, with “Byproducts” Released

How does it polymerize? These monomers have specific “functional groups” (such as -COOH carboxyl group, -OH hydroxyl group, -NH₂ amino group). When they react, they not only link together but also release a small molecule byproduct, such as water or alcohol. This is like the formation of nylon (polyamide).

Caratteristiche:

The reaction process always produces small molecules like water.

The chain of the product retains the “linking” characteristics of the functional groups (such as ester bonds -COO-, amide bonds -NHCO-), so the chain often contains heteroatoms such as oxygen and nitrogen.

Because “byproducts” are released, the structural unit of the polymer has fewer atoms than the monomer, and the molecular weight is not an integer multiple of the monomer’s molecular weight.

03 Ring-Opening Polymerization: Seemingly Condensation, but Actually Addition

This is a special reaction where the monomer itself is a cyclic structure (such as ethylene oxide). During the reaction, the ring is opened, and then the ends are linked together to form a long chain. In terms of elemental composition, it resembles addition polymerization (no byproducts released); but the structure of the final product resembles a condensation polymer (with ether bonds and other characteristics in the chain).

II. The “Rhythm” of Polymerization: Two Growth Mechanisms

Besides the reaction type, the “rhythm” of the polymerization process itself also varies significantly.

04 Chain Polymerization: A “100-Meter Sprint” with Instantaneous Burst

Process: Once the reaction is initiated, an active center (such as a free radical or ion) is generated. This active center then propagates rapidly like a domino effect, consuming monomers at an incredible speed (in fractions of a second to a few seconds), instantly growing into a large molecule. During the reaction, the system contains almost only monomers and already completed macromolecules.

Caratteristiche: The molecular weight quickly reaches a stable value and does not increase significantly over time. However, the conversion rate (the proportion of monomers converted into polymers) gradually increases over time.

Based on the active center, it can be divided into: free radical polymerization, cationic polymerization, anionic polymerization, and coordination polymerization (the latter is a key technology for producing high-performance plastics such as PP and PE).

05 Step-Growth Polymerization: A Steady and Gradual “Marathon”

Process: There is no such frantic active center. Monomers react with each other, small molecules react with each other, and small and large molecules react with each other through functional groups, steadily growing step by step. A large number of intermediate products with medium molecular weight are produced in the early stages of the reaction.

 

Caratteristiche: The molecular weight increases slowly and continuously with reaction time. However, the monomers are consumed quickly in the early stages of the reaction, so the conversion rate increases rapidly early on.

III. The “Essence” of Polymers: From Chemical Concept to Industrial Cornerstone

Chemically speaking, the essence of polymers lies in their structure as giant molecular chains connected by covalent bonds. This is the root of all their properties.

In the application of the rubber industry, the concept of “polymer” becomes more specific and practical. It refers specifically to the basic elastomer raw material in the formulation, which is the “cornerstone” or “base rubber” that determines the performance of rubber products.

Natural representative: Natural rubber (NR)

Synthetic family: Styrene-butadiene rubber (SBR), polybutadiene rubber (BR), nitrile butadiene rubber (NBR), ethylene propylene diene monomer rubber (EPDM), etc.

The choice of which “cornerstone” polymer to use largely determines the performance framework of the final rubber product:

Heat resistance, oil resistance, cold resistance, ozone resistance, etc.

Elasticity, strength, processing fluidity

Suitable cross-linking (vulcanization) methods

Cost and market supply situation

IV. The “Combination Technique” of Polymers: Alloying and Blending

Single polymers often fail to meet all requirements, so engineers have resorted to “alloying”—physically mixing two or more polymers together to combine their strengths and compensate for their weaknesses. For example:

NR + BR: Improves the wear resistance and low-temperature elasticity of natural rubber.

NBR + PVC: Enhances the flame retardancy and strength of nitrile rubber.

EPDM + NR: Significantly improves aging resistance while maintaining elasticity.

This “alloy” design requires careful consideration of the compatibility and co-vulcanization of the polymers, and is a core element of advanced formulation technology.

V. From “Form” to “Selection”: Polymers in Practice

Polymers on the market come in various forms, directly impacting processing:

Large chunks (e.g., NR): Require cutting and preheating, making processing more laborious.

Chips/Granules (e.g., some EPDM): Easy to weigh and mix/disperse.

Liquid form (e.g., liquid silicone rubber): Excellent fluidity, suitable for precision casting.

Special note on the misconception of “oil-extended rubber”: Factories pre-add oil to reduce viscosity (e.g., “oil-extended SBR”). When calculating your formula, you must account for this oil! Otherwise, the actual polymer content will be less than you expect, leading to an imbalance in filler and vulcanizing agent ratios, severely affecting performance.

How to choose your “foundation” polymer? A practical decision-making sequence:

Determine the application: Is it for tires, seals, or cables?

Consider the environment: What temperatures will it be exposed to? What types of oils? Is ozone present?

Control costs: Approximate cost ranking: NR < SBR/BR < EPDM < NBR < Fluororubber (FKM).

Select the vulcanization system: Do you prefer traditional sulfur vulcanization or peroxide vulcanization?

Consider blending: Is “alloying” necessary to balance performance?

Final design: After determining the “foundation,” begin designing the detailed formula including fillers, softeners, and vulcanizing agents.

Conclusione

From microscopic chemical reaction mechanisms to macroscopic material performance, and then to the selection and formulation in industrial practice, the world of polymers is both rigorous and full of creativity. Understanding the logic of this “foundation” is the key to unlocking the door to polymer science and applications.