Sequence-controlled polymers anionic polymerization

In summary the hydrogenation of styrene-diene block polymers is a practical route to tough, clear heat-resistant plastics. Variations in overall physical properties can be obtained by controlling composition, microstructure, and the nature of the block sequence using conventional anionic polymerization techniques. 

It has been shown recently (10) that such block structures could be tailored precisely by the general method summarized hereabove. It is indeed possible to convert the hydroxyl end-group of a vinyl polymer PA (f.i. polystyrene, or polybutadiene obtained by anionic polymerization terminated with ethylene oxide),into an aluminum alcoholate structure since it is well known that CL polymerizes in a perfectly "living" manner by ring-opening insertion into the Al-0 bond (11), the following reaction sequence provides a direct access to the desired copolymers, with an accurate control of the molecular parameters of the two blocks ... 

The phenomenal growth in commercial production of polymers by anionic polymerization can be attributed to the unprecedented control the process provides over the polymer properties. This control is most extensive in organolithium initiated polymerizations and includes polymer composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even monomer sequence distribution in copolymers. Furthermore, a judicious choice of process conditions affords termination and transfer free polymerization which leads to very efficient methods of block polymer synthesis. 

Anionic polymerizations initiated with alkyllithium compounds enable us to prepare homopolymers as well as copolymers from diene and vinylaromatic monomers. These polymerization systems are unique in that they have precise control over such polymer properties as composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even copolymer monomer sequence distribution. Attempts have been made in this paper to survey these salient features with respect to their chemistry and commercial applications. 

Polymers. The polymers used in the blending experiments were prepared by anionic polymerization using an alkyllithium initiator and a chemical randomizing agent to control monomer sequence, in the manner described by Hsieh and Wofford (3). Randomness was checked in each case by measuring the styrene content as a function of conversion. Table I gives descriptive data for these polymers. 

Polymer made by anionically initiated polymerization with MW, chain-end and chain-sequence control with defect-free chains... 

To control the molecular weight (MW) and MWD, Sakurai and coworkers developed an anionic polymerization method with masked disilene, a highly strained precursor with biphenyl (Chart 13.4). Although the substituents are limited to alkyl and certain dialkylamino groups, this method made it possible to design well-defined Si Si sequences, leading to diblock polymers with organic polymer blocks such as poly(triphenylmethylmethacrylate) by means of chiral anionic catalysts. 

The broad range of control of solution polymer structure and macrostructure of styrene-butadiene rubbers that is only possible using lithium catalysts was discussed in Section 2. We have seen how the microstructure of the butadiene units in the chain and comonomer sequence distribution can be controlled with the addition of polar modifiers and/or variations in pol5onerization process variables. Additionally, the unique control of macrostructure features and the new possibilities offered by reactive functional groups were discussed as part of the molecular engineering capabilities of solution anionic polymerizations. 

SOLUTION POLYMERIZATION Solution SBR typically made in hydrocarbon solution with alkyl lithium-based inihator. In this stereo-specific catalyst system, in principle, every polymer molecule remains live until a deactivator or some other agent capable of reacting with the anion intervenes. Able to control molecular weight, molecular weight distribution, and branching. Able to make random and block copolymers with designed chain sequence. Able to make copolymer with controlled styrene content. Able to control the butadiene structure of vinyl/ ds/ trans. Higher purity due to no addition of soap. 

When a difunctional trityl ion salt was used to initiate the living cationic polymerization of cyclopentadiene, block copolymers were formed through charge neutralization with living anionic ct-MS. By varying the functionality of the a-MS polymers, controlled block sequences were generated in the copolymers. 

---