Silicone oil presents a linear structure, while silicone resin has a three-dimensional structure. The reason behind this is closely related to their monomer structure. For example, silicone oil made with D structure as a monomer can only produce linear results. But if T or Q structures are used as monomers, these monomers will undergo crosslinking reactions under normal conditions. Specifically, when T or Q coexist with the capping agent M, especially when the amount of M is sufficient, these monomers will crosslink in a limited space, ultimately forming a spherical structure (theoretically).

Taking a specific reaction as an example, when 4 M react with 1 Q, the simplest product obtained is MQ silicone resin.
The core silicon atom is surrounded by four oxygen atoms. However, since each oxygen atom needs to share with two silicon atoms, these four oxygen atoms actually only connect two silicon atoms, forming the core structure of silicon dioxide. As the proportion of Q gradually increases and the proportion of M correspondingly decreases, the structure of this core silica will gradually increase. When the proportion of M drops to zero, the product is transformed into sand.
In the above molecules, with the condensation of Q, the core of silica is formed. As a capping agent, M not only limits the molecular weight but also regulates the types of surface functional groups. This substance with a silicon dioxide core, adjustable size, and surface covered with organic groups is exactly what we call silicone resin. Unlike silicone oil, its core does not carry substituents, and the alkyl structure is entirely located on the surface, with the most common being methyl, vinyl, and hydrogen containing groups, as these are the most common in the M structure. If other functional groups need to be introduced, they can be achieved by adding a T-coupling agent, thus obtaining MTQ silicone resin, whose surface can contain various functional groups such as amine groups, epoxy groups, thiol groups, acryloyl groups, etc. In addition, this structure is quite similar to the POSS that was hotly discussed in the academic community a few years ago.
POSS molecules, as molecules with specific spatial structures synthesized solely from T monomers, can flexibly change their active groups by replacing different types of coupling agents. Meanwhile, due to different combinations, the size of POSS molecules obtained may also vary.
But as long as we have a deep understanding of the synthesis principles of MQ and MTQ, it is not difficult to understand why POSS molecules are highly regarded in academia but still difficult to find in industry. Because the polymerization process of T structure raw materials is difficult to accurately control, it is often easy to form gel. Even in the low concentration environment, it is difficult to obtain a single structure POSS molecule. And the scarcity of single functional POSS molecules is no less than searching for Higgs bosons in particle accelerators.
Can you try to calculate the probability of forming a single molecule structure assuming that all product structures are T8 and that the two silicon monomers have the same reactivity. Here, we can use a simple model for derivation. Assuming that 1 mol of monofunctional group A reacts with 1 mol of bifunctional group B, according to the reaction mechanism, we will obtain 0.25 mol of AB, 0.25 mol of BA, 0.25 mol of ABA, and 0.25 mol of B. It is worth noting that AB and BA are symmetrical structures, so the probability of forming AB is actually only 50%. For more complex POSS molecules, their three-dimensional structure and multifunctional properties will significantly increase the types of products, specifically in terms of the content of each product... If elaborated in detail, it may be enough to write a master's thesis.
Furthermore, to achieve the outstanding performance demonstrated by POSS, silicone resin has become a mature and reliable choice in the industrial field. The unique feature of silicone resin lies in its core structure of silica, which introduces silicone resin at the molecular level, significantly changing the characteristics of carbon based polymers and making their properties closer to silica. This greatly enhances the performance of conventional carbon based polymers in terms of heat resistance, refractive index, and resistance to yellowing.
Although silicone resin is currently mainly used in the silicone industry, playing a reinforcing role, its application areas are constantly expanding. In the rubber industry, reinforcement is a key concept aimed at enhancing the strength of rubber. Imagine strengthening rubber bands to enhance the toughness of car tires, which is a direct manifestation of the reinforcement effect. The similarity between silicone and rubber makes it necessary to reinforce in certain situations, and silicone resin is an ideal choice for transparent reinforcement.

However, silicone resins with functions such as epoxy and amino are less commonly used in the industry. This is mainly due to its relatively high price and the scarcity of talents with relevant synthesis skills. When it comes to organosilicon, we often find it incompatible with other systems such as epoxy, PU, UV, etc. This is because the reaction characteristics of organosilicon are unique, mainly relying on hydrosilylation and silanol condensation reactions, which are difficult to combine with reactions in other systems.
Can we consider introducing functional groups such as epoxy, amino, NCO into organosilicon? However, this is not always feasible. The commonly used structure in organosilicon is silicone oil, while dimethyl siloxane has extremely low polarity and is physically incompatible with epoxy, PU, etc., resulting in delamination when mixed. Do we still need to add surfactants to these two organic phases?

Therefore, organosilicon has always existed in a unique identity, just like when a group of people gather for a meal, it carries a plate alone to the corner of the wall for dining. Although similar situations may occur in organic phosphorus and organic boron, they are not as significant as in organic silicon. The only thing that can link organic silicon with carbon based polymers is silicone resin. However, once it comes to binding with carbon, the addition condensation reaction with organosilicon becomes irrelevant. Perhaps from the perspective of organosilicon, silicone resin can be seen as a rebellious existence.