Things about Organic Polysiloxane Macromolecular Compounds

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When we refer to organic polysiloxane, we usually mean the macromolecular compounds have main chemical chains in the form of Si-O-Si whose silicon atoms are directly linked to organic groups. Its common formula is (RmSiO4-m/2)n, The ‘R’ means the organic group like methyl and others. The ‘n’ means the degree of polymerization. It is just like normal macromolecular compounds, the polysiloxane could be divided into linear, cyclo and many other types based on their chemical structures. Also, organic polysiloxane has many features which are absolutely different from other macromolecular compounds.

Synthesis of Organic Polysiloxane Macromolecular Compounds

There are generally two types of making organic polysiloxane, the polycondensation and ring-openingpolymerisation. The former one refers to the chemical reaction which is that two or more monomers who have functional groups mutually react to produce macromolecular compounds with some smaller molecules. The latter one is one kind of polymerization which does not produce ant small molecules during the reaction process thus the chemical components are the same as the monomers used in the beginning.

Polycondensation

The hydrolysis & condensation of chlorosilane is the key method to collect polysiloxane in industry.

The hydrolysis of chlorosilane contains two reactional procedures- firstly it is hydrolyzed and produces silanols and silanols then polymerize with other chemicals to produce siloxanes. The linear polysiloxane is made from dichlorosilane’s hydrolysis. This reaction has a high requirement on dichlorosilane’s purity or the quality of the mature products will be influenced. It is in order to transform the dichlorosilane to linear polysiloxane. In industry, the silicone oil manufacturers use continuous hydrolysis to produce polysiloxane. This method uses the theory that Si-O key will be broken and produce balance system during the condition where there are agents.

Besides the conditions mentioned above, the structure of hydrolysis will also be affected by the chemical characteristics of the acid, alkaline and liquids used during the reaction processes. For instance, when use HCl who is 6mol/L to alternative water to be used during the hydrolysis, the production rate of low polysiloxane could reach 70%; however, if we use 50% ~ 80% H2SO4, we will get some polysiloxane who has high molecular quantity. The existence of organic liquids during the hydrolysis process will also influence the structure of mature products. Normally, in this case, there will be a higher production rate.

Ring-Openingpolymerisation

The ring-openingpolymerisation of polysiloxane refers to the procedures that cyclosiloxane monomer breaks then rearranges to become linear siloxane within adding corresponding agents. To compare with polycondensation, ring-openingpolymerisation could control the structures & molecular quantity of matured products better and could get compounds that have higher molecular quantity. The ring-openingpolymerisation of cyclosiloxane is today’s most important method to produce polysiloxane. The matters like high-temperature rubber & PDMS silicone oils are both made via this method. Among the large variants of cyclosiloxane monomers, D3 and D4 are the two most important polymerized linear polysiloxane monomers.

Some initiators of cyclosiloxane are mainly the inorganic or organic strong alkaline. There are many types and some common types include R4NOH, R4POH, MOH, ROM, ≡SiOM, R4NOSi≡ and R4POSi≡.

The structures of polysiloxane have influences on the speed rate of polymerization. Generally, the circles are bigger; the reaction speed will be faster. However, D3 is an exception, it is active feature of its big circles, thus its reaction speed is higher than D4.

There are two general types of ring-openingpolymerisation of cyclosiloxane, the more frequently-used one is the balanced polymerization, but it is only limited to be used in the system whose balanced polymerized production rate is at a high level. The other one is called imbalanced polymerization which is to eliminate the actively growing center of the polymerization before the balance occurs. The balanced polymerization is also called thermodynamic control polymerization.

Properties of Organic Polysiloxane Macromolecular Compounds

Organic polysiloxane is one special type of macromolecular compounds which have special structures. Their main chains could be regarded as the one-dimensional similarity with inorganic silicon dioxide while its side chains have organic groups. This is why their properties are between organic and inorganic materials.

The Si-O key of organic polysiloxane is 0.164nm, which is shorter than covalent radius (0.176nm). This is due to Si-O key has a stronger ionic feature and some two-key features. But if we compare them with the normal C-C keys and C-O keys of other macromolecular compounds, they are way longer. Si-O-Si key’s angle is also ‘soft’ and it is according to the differences in structures, the angles could be switched from 104° to 180°. Therefore, the chain of polysiloxane is very soft.

Linear polydimethylsiloxane is the most important organic polysiloxane. There are two nonpolar methyl on each of its silicon atom. These groups could circle around Si-C key freely and isolate the Si-O main chain who has bigger energy effectively. It is due to the repulsion between methyl and the main chain, polydimethylsiloxane chain could form regular helix structure in low temperature.

The key energy of Si-O key is very high and even higher than C-O, C-C and Si-C keys. This leads to the stability of polysiloxane in high temperature which is better than other macromolecular compounds. In a higher temperature, polysiloxane chain will have splitting and produce volatile low cyclopolysiloxane like D3. This is why the thermal stability is lower.

It is in order to improve the inner thermal stability of organic polysiloxane, this could be done by adding other chemical groups. The reason is that these groups not only curb the producing of cyclosiloxane, but also lower the softness of high-molecular chains.

Organic polysiloxane could be pretty stable when there are no agents even if the temperature is higher. However, when adding some acid or alkaline agents, the polysiloxane will be degraded by water. The water degradation is a revered reaction to the condensation, its feature is that the molecular chains are broken irregularly, and then produces a new balance system to lower the molecule quantity but to increase its scope of distribution. This is why the corresponding molecular quantity will not be higher if there is water during the polymerization- The water becomes the MDM during the reaction.   

Summary

To be different from the silicone monomers we introduced in the last article, the silicone macromolecular compounds are more well-known and are produced & sold in the larger quantity. These chemicals play important roles not only in industry, but also in our daily life.