1. Introduction and Fundamental Properties
Vinyl-terminated dimethyl methylvinyl siloxane, a pivotal functional polymer in silicone industry, is registered under CAS No. 68083-18-1. It is also commercially known as polysiloxanes, di-Me, Me vinyl, vinyl group-terminated, or methyl vinyl silicone gum. This compound is characterized by reactive vinyl groups at both ends of its molecular chains, making it an essential cross-linker and base polymer for producing high-performance silicone rubbers via addition-cure systems.

The substance typically appears as a clear, colorless viscous oil. Its fundamental physical properties include a density of approximately 0.93 g/cm³, a remarkably low melting point (below -60°C), a high boiling point (exceeding 205°C), and a flash point above 110°C. It has a molecular formula of C13H30O3Si4 and a molecular weight of about 346.72. To preserve the reactivity of the terminal vinyl groups during storage, it is recommended to keep the material sealed in a dry environment at a controlled temperature of 2-8°C.

2. Synthesis and Structural Control
Industrially, this vinyl silicone oil is primarily synthesized via anionic ring-opening polymerization. The process involves cyclic siloxanes (such as a mixture of octamethylcyclotetrasiloxane and methylvinylcyclosiloxanes) as monomers and divinyltetramethyldisiloxane as the end-capping agent, catalyzed by a basic catalyst like tetramethylammonium hydroxide. The polymerization is typically conducted at temperatures between 100-120°C for 4 to 8 hours.

A key advantage of this synthetic route is the precise control it offers over the final product's architecture. By adjusting the monomer ratio, the amount of end-capper, and reaction conditions, manufacturers can tailor critical parameters including the degree of polymerization (chain length), vinyl content (both at chain ends and within the backbone), and molecular weight distribution. This enables the production of materials with specific viscosities, such as the common 800-1200 cSt grade, designed for different processing and performance requirements.

3. Primary Application Mechanism: Addition Cure
The core utility of this material lies in its role in platinum-catalyzed hydrosilylation, also known as addition curing. In this reaction, the silicon-hydrogen (Si-H) bonds of a cross-linker (methylhydrogen siloxane) add across the vinyl (CH2=CH-) groups of the vinyl-terminated oil. This reaction, efficiently catalyzed by platinum complexes (typically used at 5-20 ppm), forms stable silicon-carbon bonds, creating a three-dimensional, cross-linked elastomer network.

In this system, the vinyl silicone oil serves as the backbone polymer. The concentration and distribution of the vinyl functionalities directly govern the cross-link density of the resulting rubber, which in turn determines its final mechanical properties, such as tensile strength, elongation at break, and hardness. The cure rate can be finely modulated by the type and amount of catalyst used.

4. Key Application Areas

• High-Performance Silicone Rubbers: It is the fundamental building block for Heat-Cured Rubbers (HCR) and Liquid Silicone Rubbers (LSR). These rubbers are used in demanding applications like automotive seals, gaskets, keyboard pads, and medical device components due to their durability, thermal stability, and chemical resistance.

• Electronics and Optoelectronics: Its cured form offers excellent electrical insulation, moisture resistance, and broad service temperature range. It is extensively used for potting and encapsulating sensitive electronic components, circuit boards, and transformers. When modified with phenyl groups, its refractive index can be adjusted to 1.52-1.54 with high transmittance (85-92% at 450nm), making it ideal for encapsulating LED chips and optical lenses.

• Food-Contact Materials: An important aspect of its regulatory status is its approval for use in food-contact applications. According to the Chinese national standard GB 9685-2016, this substance is permitted for use in silicone rubbers, coatings, and paper/board materials intended for food contact, and it may be used according to good manufacturing practice. This allows its use in products like silicone baking molds, spatulas, and seals for food containers.

5. Safety and Handling
From a safety perspective, the material is generally considered stable but combustible. Safety Data Sheets indicate it is not classified as hazardous, though it may cause mild transient irritation to eye mucosa. Proper industrial hygiene practices, including adequate ventilation and the use of personal protective equipment like gloves and safety glasses, are recommended during handling.

6. Conclusion
In summary, vinyl-terminated dimethyl methylvinyl siloxane (CAS 68083-18-1) is a versatile and indispensable workhorse in silicone chemistry. Its unique structure, offering controllable reactivity, bridges molecular design and macroscopic material properties. From enabling advanced technologies in electronics and optics to ensuring safety in everyday food-contact items, this material continues to underpin innovation across a vast spectrum of industries.