Basic Polymerization Reaction

3 PREPARATION OF PROPYLENE POLYMER 3.3.1 Basic Polymerization Reaction 

Propylene is catalytically polymerized to form a linear, flexible-chain polymer according to the following equation  

The polymer contains a repeat unit of -CH2 HC(CH3) from the opening of its double bond, and the formation of a pendant methyl group. The reaction is exothermic. Its heat of reaction is in the order of 20.0 kcal/mol or 83.6 kJ/mol  

As will be discussed later, the polymerization of propylene is catalyzed by either Ziegler-Natta catalyst or metallocene catalyst. In the Ziegler-Natta catalyzed polymerization, the propylene monomer accesses the reaction site between the metal and carbon atoms in two ways the head-to-tail and tail-to-head insertion. The head-to-tail insertion occurs primarily in isotactic polymerization [8-10], while the tail-to-head insertion occurs in the low-temperature catalysis with VCl4-Et2AlCl catalyst [11]. There are mostly head-to-tail insertions and very few head-to-head or tail-to-tail inversions in the main chain of polypropylene [12,13]. 

There are three types of monomer insertions with respect to the pendent methyl groups the meso, racemic. [14]. Meso insertion produces a polymer with the methyl groups in the same spatial position, which is referred to as isotactic polymer racemic insertion produces a polymer with the methyl groups in alternating locations, referred to as the syndiotactic polymer. When the monomer insertion is random and nonstereospecific, a noncrystalline atactic polymer is produced. These three forms of polypropylene are schematically represented in the following chain configurations  

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The basic polymerization reaction is described by the foUowing equations. 

Natural rubber is synthesized by a wide variety of plants. The botanic rationale for this synthesis is still a mystery. The biosynthesis of natural rubber has been studied extensively in the past [47-50], and the basic polymerization reactions have been defined. However, the full mechanism of formation of the rubber particles has still not been elucidated, although some suggestions have been made [48,50,51],... 

The neat resin preparation for PPS is quite compHcated, despite the fact that the overall polymerization reaction appears to be simple. Several commercial PPS polymerization processes that feature some steps in common have been described (1,2). At least three different mechanisms have been pubUshed in an attempt to describe the basic reaction of a sodium sulfide equivalent and -dichlorobenzene these are S Ar (13,16,19), radical cation (20,21), and Buimett s (22) Sj l radical anion (23—25) mechanisms. The benzyne mechanism was ruled out (16) based on the observation that the para-substitution pattern of the monomer, -dichlorobenzene, is retained in the repeating unit of the polymer. Demonstration that the step-growth polymerization of sodium sulfide and /)-dichlorohenzene proceeds via the S Ar mechanism is fairly recent (1991) (26). Eurther complexity in the polymerization is the incorporation of comonomers that alter the polymer stmcture, thereby modifying the properties of the polymer. Additionally, post-polymerization treatments can be utilized, which modify the properties of the polymer. Preparation of the neat resin is an area of significant latitude and extreme importance for the end user. 

Aluminum chloride hydroxide [1327-41 -9] [10284-64-7], AlQ(OH)2 [14215-15-7], AlQ2(OH), products, commonly known as polyaluminum chlorides (PAG), are used for a wide variety of industrial appHcations. Other names for PAG are basic aluminum chloride, polybasic aluminum chloride, aluminum hydroxychloride, aluminum oxychloride, and aluminum chlorohydrate. The presence of polymeric, aluminum-containing cations, the distribution of which can differ gready, typifies PAG products. Although the formation of polynuclear aluminum species in solution has been studied for over a century, there is stiU much controversy concerning aluminum polymerization reactions and the resulting product compositions. 

Dilatometer Basically it is a pyrometer equipped with instruments to study density as a function of temperature and/or time. It can measure the thermal expansion or contraction of solids or liquids. They also study polymerization reactions it can measure the contraction in volume of unsaturated compounds. It basically is a technique in which a dimension of a material under negligible load is measured as a function of temperature while it is subjected to a controlled temperature program. 

The rate of the r-BuX + Me3Al M X > f-BuMe + Me2AlX reaction decreases as X = Cl > Br > I10. This decrease is explained by a decrease in the rate of displacement of MeX by r-BuX, which in turn is determined by the basicity and/or size of the halogen in f-BuX. Since the basicity decreases and size increases as X changes from Cl to Br to I, the rate of displacement, R, decreases. In isobutylene polymerization using f-BuX/Me3Al/MeX and r-BuX/Et2 AlCl/MeX (X = Cl, Br, I), the r-BuX reactivity decreases as f-BuCl > f-BuBr > r-BuI = 0. The similarity between initiator reactivity sequences in model and polymerization reactions indicates that the rate governing event is the same for both, i.e the rate of displacement, R1. 

This chapter will describe how we can apply an understanding of thermodynamic behavior to the processes associated with polymers. We will begin with a general description of the field, the laws of thermodynamics, the role of intermolecular forces, and the thermodynamics of polymerization reactions. We will then explore how statistical thermodynamics can be used to describe the molecules that make up polymers. Finally, we will learn the basics of heat transfer phenomena, which will allow us to understand the rate of heat movement during processing. 

Basic classification and definitions of polymerization reactions (lUPAC Recommendations 1994), Pure Appl. Chem. 66, 2483-2486 (1994). Reprinted as Chapter 4, this edition. 

Basic Classifications and Definitions of Polymerization Reactions, Pure Appl. Chem. 66,... 

The above review shows the progress that has been made in the last 30 years. The prebiotic synthesis of amino acids, purines, pyrimidines, and sugars is understood at a basic level, although more details of the reactions are needed. The polymerization processes are less well understood, and while some of them are plausible it is necessary to work them out in greater detail. The template polymerization reactions are an exciting beginning and may show how genetic information started to accumulate. So far the problem of nucleic acid directed enzyme synthesis has not been dealt with on an experimental level. The problems in this area, which are very difficult, are considered by other speakers in this symposium. 

The fact, that almost all basic functions in caprolactam or polyamide medium occur in the form of amidic anions [-CO-N-]- arose the idea that these particular anions are directly responsible for the polymerization reaction. In analogy to the transesterification mechanism an intermediate addition complex might be formed by combining the amidic group with the respective anion... 

Figure 9.4 Reaction conditions exert a strong influence on the course of a sol-gel polymerization reaction. Basic pH, higher temperatures, and greater dilutions favor the formation of rings and ring clusters, as shown in the pathway on the left. Acidic pH, lower temperatures, and higher concentrations favor the formation of chains and dendritic structures.

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